Indian Patents. 199048:AN IMPROVED LIGHT DETECTOR

Title of Invention	AN IMPROVED LIGHT DETECTOR
Abstract	Abstract Of The Invention An organic luminescent coating such as Aluminum-Tris-Quinolate is applied to a light detector for converting UV to green light to improve the efficiency of the light detector. The coating also provides anti-reflection properties when of sufficient thickness.

Full Text	This invention was made with Government support under Contract No. XAI-3-11167-03, awarded by the Air Force. The government has certain rights in this invention. Related Invention: This co-pending invention is related to the invention covered by provisional Application Serial No. 60/010,013, filed January 11, 1996, entitled "Organic Luminescent Coating For Light Detectors". Field Of The Invention: The field of this invention relates generally to photodetectors, including solar cells, and more particularly to anti-reflection (AR) coatings for photodetectors. Background Of The Invention: Light detectors include solar cells, used to convert light to electrical energy, and photocells and photodiodes used to measure light intensity. Materials for light detectors include semiconductors such as silicon, gallium arsenide, germanium, selenium, and the like. Typically, the light gathering efficiency of these devices is improved by providing them with an Anti-Reflection (AR) coating. If less light is reflected, then more is available for absorption and conversion to an electrical signal. These coatings can be a single layer thick, in which case they are typically a 1/4 wavelength of light thick, or they may be multilayer coatings for even greater efficiency. Such coatings are typically composed of glassy insulators such as aluminum oxide, titanium oxide, silicon dioxide, silicon nitride or magnesium fluoride. Furthermore, high sensitivity in the blue and ultraviolet (UV) region of the spectrum is difficult to achieve with conventional semiconductor detectors, since reflectance of semiconductor materials strongly increases in this spectral range. Also, a typical AR-coatings is typically optimized for the green spectral range, becoming less effective in the blue and UV spectral regions. Summary Of The Invention: An object of the invention is to provide high efficiency solar cells and photodiodes for a broad range of wide spectral bandwidth applications. Another object of the invention is to provide photocells and photodiodes with increased sensitivity, especially in the blue and UV regions of the spectrum. Another object of the invention is to provide an apparatus for measuring film properties such as internal and external luminescence quantum efficiency with an accuracy heretofore unobtained with traditional measurement methods such as the integrating spheres. This apparatus can be utilized for identifying candidate coatings for use in high efficiency solar cells, and high sensitivity photocells and photodiodes. These and other objects of the invention are provided by an improved light detector, which consists of a coating of an organic luminescent material, such as Aluminum-Tris-Quinolate (Alqj also known as 8-hydroxyquinoline aluminum, Alq), or TPD {H,N'-diphenyl-N, N'-bis(3-methylphenyl) 1, l'-biphenyl-4, 4' diamine}, or Gag;CI {bis-(8-hydroxyquinaldine)-chlorogallium, on a light detector, preferably as a substitute for a conventional AR coating. When organic film is substituted for the AR coating, the film also acts as an AR coating, if it is thick enough, since it does not absorb appreciably in its own luminescent region of the spectrum. If the coating has a 100% internal luminescence efficiency (efficiency of conversion of the absorbed light into luminescent light), the power efficiency for an amorphous silicon solar cell with such a coating is improved by 11%. Conventional AR coatings can be retained for scratch resistance, at slightly less gain. Alternatively, the organic film can be intentionally designed as a final layer of a multi¬layer AR coating. Brief Description Of The Drawings: Various embodiments of the invention are illustrated and described below with reference to the accompanying drawings, in which like items are identified by the same reference designation, wherein: FIG. la shows a conventional photocell or solar cell of the prior art, the UV portion of the spectrum being partially reflected by the AR coated device surface. FIG. lb shows a preferred embodiment of the invention, in which a photocell or solar cell is coated with an organic luminescent coating for converting UV to green light, and for acting as an AR coating; FIG. Ic shows another embodiment, in which a luminescent organic material is coated over a conventional AR coating. FIG. 2 shows curves illustrating the comparative performance of a conventional solar cell, and one coated with 100% quantum efficiency (Q.E.) organic material; FIG. 3 shows curves illustrating the calculated quantum efficiency improvement of 10.9% when comparing a cell coated with 100% QE organic material to a conventional cell; FIG. 4 shows an apparatus for measuring film spectral and luminescence properties; FIG. 5 shows curves illustrating the absorption spectra of Alq3 films obtained from three different film thicknesses, while the inset shows light attenuation vs. film thicknesses at X=370 nm, the slope giving the absorption coefficient; FIG. 6 shows curves illustrating the photoluminescence (PL) excitation spectra for film thicknesses of 150 A and 1.35 /xm, while the inset is a schematic of a Alq,-excited state relaxation process; FIG. 7 shows a curve illustrating the dependence of the internal luminescence efficiency for the \=530 nm band vs. film thickness for films grown on glass, the excitation wavelength for these data being 370 nm; and FIG. 8 shows an end view of a simple apparatus set-up and method for measuring film luminescence properties for another embodiment of the invention. Detailed Description Of The Invention: In FIG. la, a photocell 4 is shown with a conventional monolayer AR coating 3 optimized for \ = 500 nm. Device with such coatings undesirably reflect away substantial fractions of UV light with X convert the absorbed near-OV light into green light of 530 nm wavelength. Therefore, wavelengths greater than 400 nm are transmitted through the film with low loss, while more than one-half of near-UV light is absorbed in a 300-400 A thick Alq, film. An organic thin film 2 can thus act as a UV-to-green light converter and at the same time serve as AR coating 2 for X> 400 nm, as shown in FIG. lb. It is well known that the efficiency of conventional photodiodes and solar cells decreases rapidly in the near-UV range. The dashed line in FIG. 2 (curve 16) illustrates the energy distribution of photons in the solar AMO (atmospheric mass zero, the standard of intensity and wavelength distribution of sunlight in space) spectrum. Also plotted in Fig. 2 is the quantum efficiency of a typical conventional solar cell 4 with an AR coating 3, shown by curve 12 plotted between solid circles (R.A. Street, "Hydrogenated Amorphous Silicon", Cambridge University Press, 1991). It is clear that for wavelengths shorter than 400 nm, conventional cells (see FIG. la) can not provide efficient conversion of solar radiation. Also, curve 14 plotted between square points in FIG. 2 shows the calculated quantum efficiency for conventional cells coated with a 100% Q.E. organic material. Here, it was assumed that all the solar radiation with wavelengths less than 420 nm is absorbed by Alq, and converted into the 530 nm light (100% internal efficiency of luminescence). It is also assumed that all the photons emitted by the Algj coating 2 are coupled into the solar cell 4 (see FIG. lb), which is reasonable since the refraction index of Si (x.xx) is much higher than that of Alq^ (1-73) or air (-1.00). /IG. 3 illustrates photocurrent vs. wavelength curves for conventional solar cells (taken from M.A. Green," High Efficiency Silicon Solar Cells", Trans Tech Publishing, p.416, 1987) represented by curve 18 plotted between solid circles, and the calculated spectrum (curve 20, plotted between open squares), for the same cell coated with a 100% Q.E. organic layer thick enough to absorb all the incident UV light (the exact film thickness should also satisfy the anti reflection condition for the solar radiation spectral maximum). The difference in area under these two curves, 18 and 20, corresponds to the expected solar cell power efficiency Increase and is close to 11%. It should also be noted that design of the photocells 2,4 themselves may now be optimized for 530 nm, allowing for the use of more efficient, deep p-n junctions for solar gells. It is apparent that a similar approach can be used to increase the UV sensitivity of photodiodes. Low reflectance in the UV range is provided by the low reflectance of the organic thin film. In this case, however, the film thickness should satisfy the antireflection condition for the .UV light. For 250 nm wavelength UV, the quarter-wavelength AR coating thickness should be about 350 A. This thickness of film is sufficient to fully absorb incident UV light, as discussed above. It should also be noted that, according to the measurements made by the inventors, the quantum efficiency of luminescence is independent of the film thickness for layers thicker than 100 A, due to the relatively short diffusion length of excited states in amorphous organic compounds. There is, therefore, no obstacle to the utilization of extremely thin films of these compounds as AI luminescent coatings. There are two additional advantages of UV-photodiodes with organic coatings. The first is the flatness of the coated photQdiode response at wavelengths shorter than the absorption edge of the organic material coating. This is because the coating luminescence quantum efficiency does not depend on the excitation wavelength. This property facilitates photodiode calibration and simplifies analysis of the experimental results. The strong dependence of the uncoated photodiode photoresponse on the excitation wavelength is caused by intrinsic properties and can not be eliminated in the uncoated devices. The second advantage is the visualization of UV radiation which is converted into the visible spectrum on the coated photodiode surface. It considerably simplifies positioning and alignment of the photodiode with respect to the incident light. The ability to measure the luminescent properties of thin films, and of Alq^ in particular, was critical in discovering Alq^ as a material to be utilized in organic LED devices. Other materials may also be discovered via use of an innovative measuring technique based on the embodiment of the present invention, described in detail below. Most studies to date have focused on understanding the mechanism of charge transport, and the specifics of electroluminescence of Alq^^-based OLEDs. However, data on the optical properties of solid films of Alq,, which are crucial for the in-depth understanding of the operation of OLEDs and other optoelectronic devices, such as light detectors, are not available. For example, the data on optical absorption are apparently limited to the work of p.E. Burrows et al. (See Appl. Phys. Lett. 64, pg. 2285, 1993), where only a narrow spectral range was studied. The inventors believe that no previous data has been published on the photoexcitation spectra of Alg,. The absorption, photoexcitation, external and internal quantum efficiencies of Alq^ thin films grown by thermal evaporation in vacuum have been successfully measured by the inventors. The absorption measurements determined absorption coefficients (a ) in solid films higher than lO* cm"' in the spectral range between X= 250 nm and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external Quantum Efficiency 17^ for the prominent green luminescence band (with a center wavelength at X=530 nm) does not depend on the excitation radiation wavelength. Direct measurements of tj^ suggest that the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed, exceeding 30% at room temperature. The discovery of this information, made possible via the inventors novel measurement methods, explains the lack of past interest in using hlq^ in the manufacture of light detectors. Measurement of Alq, optical characteristics was performed on Alqj films grown on glass, sapphire and silicon substrates. Prepurified Alq, films with thicknesses ranging from 50A to 1.35 ftm were deposited under vacuum by thermal evaporation as described by P.E. Burrows et al., in "Electroluminescence from Trap-Limited Current Transport in Vacuum deposited Organic Light Emitting Devices", Appl. Phys. Lett. 64, p. 2285 (1994). Prior Lu growtn, glass and sapphire substrates were cleaned by three successive rinses in boiling 1,1,l-trichloromethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with -20 A-thick native oxide layer on the surface. Alqj films grown on Si substrates were used for ellipsometric determination of the layer thickness and refractive index (n) , which was determined to be n=1.73±0.05 at X=633 nm. As shown in FIG. 4, the experimental apparatus for measuring the film spectral properties consists of a 0.25 m monochromator 24 used to select light with a full width at half maximum of AX = 15 nm from the broad spectral band of a Xe-arc lamp 22. After passing through a 165 Hz chopper 26, the excitation light is focused to a 1x2 mm spot using a CaF^ lens system 28, 34, and a 45° angle tilted mirror 32. Calibrated, UV-enhanced 1 cm diameter Si-ptiotodiode 50, and 4x2 cm Si solar cell 44 were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier 51. All measurements were made at room temperature. For absorption measurements, a bandpass filter 36 with transmission between \=250 nm and 420 nm was introduced between the sample and the Si-detector to eliminate the Alqj photoluminescence signal, as well as scattered light at X?420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp 22 as well as the low sensitivity of the si-detector 50 at X5000A), and hence thinner samples (51oA and 240oA) were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thickness are given in FIG. 5 (see curves 52, 54 and 56) . The absolute value of the absorption coefficient is calibrated at X=370 nm by measuring transmission through several samples of different thicknesses (see inset, FIG. 5, curve 58), and is a=(4.4± 0.1)xlO* cm'. Figure 5 shows that the overlap of two broad absorption bands with maxima at X=385 nm and X=260 nm (indicated by arrows) results in a>lO' cm' over the entire spectral range from X=425 nm to X=250 nm, which is the short wavelength detection limit of the apparatus. In FIG. 6, the X=530 nm PLE (photoluminensce excitation) spectra for thin (150 A) Alq, is shown by curve 60, and for thick (1.35 /zm) Alqj films is shown by curve 62. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at X=260 nm. The 1.35 fim thick sample, however, absorbs 95% of the incident light at all wavelengths shorter than X=425 nm. The plateau at X Determination of the absolute value of the external PL efficiency, T}^, was accomplished using a bandpass filter 36 with peak transmission at X=370 nm placed immediately above the sample (see FIG. 4) to reduce scattered light from the monochromator. A 1.35 fim thick organic film 48, such as Alq, film, on a glass substrate 38 was set directly onto the surface of the silicon solar cell 44. Using the values obtained for a, (FIG. 6), and taking into account the decreased UV photoresponse of the Si solar cell 44 as compared to its response at longer(-500 nm) wavelengths (see FIGS. 2 through 12), it was found that X=370 nm radiation transmitted through the 1.35 /tm thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected. The PL efficiency for X=370 nm, integrated over a 2Tr solid angle (estimated as discussed by Garbuzov in j. Luminescence 27, p. 109, (1982)) , gives »/^ from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, the thickness of the organic film and area of the substrate, indicating that the measured value of ij^ is strongly affected by scattering losses and waveguiding of PL in the film and the glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of v^ measurements using an integrating sphere. In the present invention one embodiment includes an apparatus to measure film spectral properties, which provides improvements that overcome these limitations of the prior art. The film 40 measured by the present apparatus is deposited on a glass substrate 38 and is thick enough to absorb all the excitation radiation. The substrate is placed film side down on the surface of a calibrated Si solar cell 44. The excitation radiation is focused at the film 40 through the glass 38. The film luminescence power is measured by the solar cell 44 (all the radiation (P) coupled out from the sample in 27r solid angle) and by the second photodetector 50, located over the sample (which signal with amplitude A, is proportional to the intensity of radiation that is coupled out in the direction opposite to tne solar cell 44). Then refractive index matching fluid 42 is placed between the film or coating 40 and solar cell 44, and measurements are repeated under the same excitation level. The lower limit to the internal quantum efficiency of the film is calculated as: V, = fP,+PB/A)/P„, where: (1) P represents the film luminescence power coupled out from the sample in a 2TT solid angle and measured by the solar cell 44 under the condition of no ininersion; P] is the luminescence power measured by the solar cell 44 in the case of immersion; Po is the power of the excitation radiation; A is the signal of the second photodetector 50 under the condition of no immersion; and B is the signal of the second photodetector 50 under the condition of immersion, B EXAMPLE 1 In the first embodiment, the samples comprised of the combination of substrate 38 and Alq, coating 40 are placed film-side-down in index of refraction-matching fluid 42 (with n=1.524) dropped onto the surface of the solar cell 44. Under these conditions ^^=ZB% for the radiation emitted towards the solar cell 44 was obtained. In order to evaluate the PL intensity in the opposite direction (through the glass slide 38), an identical sample was placed on the solar cell 44 without the immersion oil 42, and the PL signals of the two samples were compared using a second photodetector 50 placed above the samples (see FJG. 4). The PL efficiency, as recorded by the second photodetector 50, of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose efficiency was measured by the solar cell 44 to be -3.6%. Therefore, the total efficiency of the immersed samples was 28%+(0.8)x3.6%^31%. This number can be considered as a lower limit for i\, since the refractive index of the immersion fluid is less than n for Alqj, and hence the waveguiding losses to the Alq^ film, although improved, are not completely eliminated. EXAMPLE 2 In the second, preferred embodiment, the luminescence intensities of the 1.35 ^m thick film grown on Si substrate is compared with that of the film grown on a glass substrate, since the refractive index of Si (4.16 at X=530 nm) is much higher than that of Alq,, waveguide losses are eliminated and only photons emitted at angles smaller than the total internal reflection angle (35'w.r.t. normal) can contribute to TJ^. In this case, a good approximation to t\^ is given by Tj,=jji(l-R,) (l+Rj) (n-{n'-l)'^)(2n)'. Where n is the luminescent film refractive index (in this case the luminescent film is Alqj) . R, and R^ are the Fresnel reflection coefficients for the PL radiation at the luminescent film-air interface and semiconductor-luminescent film interfaces, respectively, averaged over angles J}; caused by the total internal reflection at the Alqj-air interface, perhaps explaining why past efforts that did not use the improvement of index matching fluid coupling failed to anticipate how high the efficiency of Alqj actually is. Using the previous calibration efficiency of the film on glass (see above) , it is found that the value of 7;, for the film on the Si substrate equals to (3.2+0.2%). The inventors, by using the above equation, calculated that the internal efficiency for the green luminescent band centered at X= 530 nm is jji=(32±2)%. Note that this is lOx the (3.2±0.2%) absolute value of T;^ for the film on the Si substrate, which requires using the previous calibration efficiency of the film on glass , which in turn requires the immersion oil improvement of example 1. The difference between the calculated jj, and the measured TJ, of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alqj film 4 0 sandwiched between the immersion oil 42 and the glass 38. However, a considerable fraction of this waveguided radiation is also collected by the solar cell 44 due to total internal reflection at the oil-air interface, as shown in FIG. 4. Thus, the results obtained by both embodiments, as shown in examples 1 and 2, are in good agreement. EXAMPLE 3 The dependence of the internal luminescence efficiency on film thickness for samples grown on glass substrate is shown by curve 64 in FIG.7. To obtain these data, the luminescence from both the thin and previously calibrated thick samples is measured with a low pass filter (passing X>42 0 nm) placed over the photodetector 44 to eliminate any non-absorbed excitation and scattered light from the monochromator 24. The amount of absorbed excitation light is directly measured for films with thickness >50oA, and is calculated for thinner films using the data in the inset of Fig. 5. Data shown in FIG. 7 correspond to freshly prepared samples, with air exposure times coating. Although various embodiments of the invention are shown and illustrated above, they are not meant to be limiting. Those skilled in the art may recognize certain modifications to these embodiments, which are meant to be covered by the spirit and scope of the appended claims. For example, C.W. Tang et al., "Electroluminescence of Doped Organic Thin Films", J. Appl. Phys. 65, No.9 3610 (1989), teaches that Alq, may be doped with traces I i of coumarin 540 and DCMl to' increase its efficiency and to shift i I ■ its emission from the gre^n to anywhere between the blue-green to orange-red regions of the spectrum. As a further example of another embodiment of the invention, for estimation of the internal quantum efficiency yj^ of a film, reference is made to FtG. 8. In this embodiment, the material of interest is deposited directly at a Si-photodiode 70 surface as a film 74. The photodiode includes a p layer 71 and an n layer 72. In this case, the value of ij, c^n be estimated from simple equations and the measurement of the magnitude of current flowing through photodiode 70 via a current measuring device 7 6: where J^ is the photodiode photocurrent (in electrons/sec.) measured by current measuring device 76, I^ is the intensity of the UV excitation radiation 78 (in photons/sec.) , and /3ph (in electrons/photon) is the photodiode quantum efficiency at a wavelength corresponding to the peak of film 74 photo-luminescence. Also, D.Z Garbuzov et al., "Organic film deposited on Si p-n junctions: Accurate measurements of florescence internal efficiency, and application to luminescent antireflection coatings", Journal of Applied Phvsics, Vol. 80, No. 8, October 15, 1996, pages 4644-4648, shows and describes the results obtained by the present inventors for testing materials other than Alq,. More specifically, through use of the present invention, coatings of TPD {N,N'-diphenyl-N, N'-bis-(3-methylphenyl)-!, l'-biphenyl-4, 4' diamine}, and of GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallinm} were tested, resulting in the measurement of internal fluorescence efficiency 17, of 0.35 + 0.3, and 0.36 ± 0.3, respectively. other efficient chromophores have also been j-ucjiiLj-riea oy the inventors for use as high efficiency florescent anti-reflection (AR) coatings. These materials include poly¬pheny lenevinylene, which can be prepared as a very uniform and flat film via wet chemical growth; and distyriarelene derivatives, which are vacuum depositable (see C. Hosokawa, et al. , ADPI. Phvs. Lett.. Vol. 67, pages 3853-3855 (1195)). Another material is Al (acetylacetonate) 3[A1 (acac) 3] , a compound that absorbs in the ultraviolet spectral region, and emits blue or purple spectral region. We Claim; 1. A combination comprising: a light detector selected from the group consisting of photocells, photodiodes and solar cells; and an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector. 2. A combination comprising: a light detector; and an organic luminescent coating capable of converting UV to visible light in the range from blue-green to red at high quantum efficiencies as light absorbs in passing through said coating into said light detector. 3. The combination of Claim 2, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, GaqJCl, TPD, polyphenylenevinylene, and distyriarelene derivatives. 4. The combination of Claim 2, wherein the light aetector is selected from the group consisting of photocells, photodiodes and solar cells. 5. A combination comprising: a light detector; and an organic luminescent coating capable of conveptln^, UV to green light at high quantum efficiencies as light passes through said coating into said light detector. 6. The combination of claim 5, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq;ci, TPD, polyphenylenevinylene, and distyriarelene derivatives. 7. The combination of Claim 5, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells. 8. The combination of Claim 7, wherein the light detector is made from silicon. 9. The combination of Claim 7, wherein the light detector is made from gallium arsenide. 10. The combination of Claim 5, wherein the light detector further includes a conventional anti-reflection coating- 11. The combination of claim 6, wherein the Aluminum-Tris-Quinolate coating is from about lOoA to about 1.35 fiv\ in thickness. 12. The combination of Claim 6, wherein the Aluminum-Tris-Quinolate coating is from about 30oA to about 40oA in thickness. 13. An apparatus for measuring film spectral properties', comprising: means for providing monochromatic light; means for chopping the monochromatic light; means for focusing the monochromatic light onto a sample, the sample being a substrate with a film to be analyzed on its surface; coupling the film side of the sample to a first light detector with Immersion oil; means for focusing light emitted by the sample onto a second light detector; and means for analyzing the signals from the first and second light detectors to provide the desired film spectral properties. 14. An apparatus as in Claim 13, further comprising a UV bandpass filter between the monochromatic light and the sample- 15. An apparatus as in Claim 13, further comprising a bandpass filter between the sample and the second light detector. 16. An apparatus as in Claiin 15, wherein the bandpass filter has light transmission between about 250 and 420 nm. 17. An apparatus as in Claim 13, further comprising a bandpass filter above the sample. 18. As in Claim 17, wherein the bandpass filter has peak transmission at about 370 nm. 19. An apparatus as in Claim 13, wherein the means for providing a monochromatic light comprises a Xenon lamp and a monochromator. 20. An apparatus as in Claim 13, wherein the means for focusing the light onto the sample is a lens. 21. An apparatus as in Claim 20, further comprising a mirror. 22. An apparatus as in Claim 13, wherein the substrate is transparent. 23. An apparatus as in Claim 13, wherein the substrate is selected from the group consisting of glass and sapphire. 24. An apparatus as in Claim 13, wherein the means for analyzing the signals from the first and second light detectors consists of a current and lock-in amplifier. 25. An apparatus as in Claim 13, wherein said first and second light detectors are selected from the group consisting of silicon solar cells, silicon photodetectors and UV enhanced photodiodes. 26. An apparatus as in Claim 13, further comprising an improved method of measuring the internal quantum efficiency of a luminescent film, comprising the steps of: using the apparatus to obtain the calibration efficiency of the luminescent film on a transparent substrate; determining the absolute value of the external quantum efficiency for the luminescent film on a semiconductor substrate from the obtained calibration efficiency on glass; and dividing the absolute value of the external quantum efficiency for the luminescent film on a semiconductor substrate by (1-R,) (1+R,) (n-(n'-l)'^) (2n)' to obtain the internal efficiency of the luminescent film, substantially free of measurement losses. 27. The method as in Claim 26, wherein the semiconductor substrate is silicon. 28. An apparatus for measuring film spectral properties including a light source, a monochromator, a chopper, lenses, filters, an optional mirror and at least one light detector, wherein the improvement comprises the combination of the light detector coupled by immersion oil to an organic luminescent film sample deposited on a substrate- semiconductors such as silicon, gallium arsenide, germanium, selenium, and the like. Typically, the light gathering ( efficiency of these devices is improved by providing them with an Anti-Reflection (AR) coating. If less light is reflected, then more is available for absorption and conversion to an electrical signal. These coatings can be a single layer thick, in which case they are typically a 1/4 wavelength of light thick, or they may be multilayer coatings for even greater efficiency. Such coatings are typically composed of glassy insulators such as aluminum oxide, titanium oxide, silicon dioxide, silicon nitride or magnesium fluoride. Furthermore, high sensitivity in the blue and ultraviolet (UV) region of the spectrum is difficult to achieve with conventional semiconductor detectors, since reflectance of semiconductor materials strongly increases in this spectral range. Also, a typical AR-coatings is typically optimized for the green spectral range, becoming less effective in the blue and UV spectral regions. Summary Of The invention: An object of the invention is to provide high efficiency solar cells and photodiodes for a broad range of wide spectral bandwidth applications. Another object of the invention is to provide photocells and photodiodes with increased sensitivity, especially in the blue and UV regions of the spectrum. Another object of the invention is to provide an apparatus for measuring film properties such as internal and external luminescence quantum efficiency with an accuracy heretofore unobtained with traditional measurement methods such as the integrating spheres. This apparatus can be utilized for identifying candidate coatings for use in high efficiency solar cells, and high sensitivity photocells and photodiodes. These and other objects of the invention are provided by an improved light detector, which consists of a coating of an organic luminescent material, such as Aluminum-Tris-Quinolate (Alq^ also known as 8-hydroxyquinoline aluminum, Alq), or TPD {N,N'-diphenyl-N, N'-bis(3-methylphenyl) 1, l'-biphenyl-4, 4' diamine}, or GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallium, on a light detector, preferably as a substitute for a conventional AR coating. When organic film is substituted for the AR coating, the film also acts as an AR coating, if it is thick enough, since it does not absorb appreciably in its own luminescent region of the spectrum. If the coating has a 100% internal luminescence efficiency (efficiency of conversion of the absorbed light into luminescent light), the power efficiency for an amorphous silicon solar cell with such a coating is improved by 11%. Conventional AR coatings can be retained for scratch resistance, at slightly less gain. Alternatively, the organic film can be intentionally designed as a final layer of a multi¬layer AR coating. Accordingly, the present invention provides an improved light detectc comprising a light detector selected from the group consisting of photocells, photodiodes and solar cells; and an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector. Various embodiments of the invention are illustrated and described below with reference to the accompanying drawings, in which like items are identified by the same reference designation, wherein FIG. la shows a conventional photocell or solar cell of the prior art, the UV portion of the spectrum being partially -reflected by the AR coated device surface. FIG. lb shows a preferred embodiment of the invention, in which a photocell or solar cell is coated with an organic luminescent coating for converting UV to green light, and for acting as an AR coating; FIG. lc shows another embodiment, in which a luminescent organic material is coated over a conventional AR coating. FIG. 2 shows curves illustrating the comparative performance of a conventional solar cell, and one coated with 100% quantum efficiency (Q.E.) organic material; FIG. 3 shows curves illustrating the calculated quantum efficiency improvement of 10.9% when comparing a cell coated with 100% QE organic material to a conventional cell; FIG. 4 shows an apparatus for measuring film spectral and luminescence properties; FIG. 5 shows curves illustrating the absorption spectra of Alq3 films obtained from three different film thicknesses, while the inset shows light attenuation vs. film thicknesses at X=370 nm, the slope giving the absorption coefficient; FIG. 6 shows curves illustrating the photoluminescence (PL) excitation spectra for film thicknesses of 150 A and 1.35 μ m, while the inset is a schematic of a Alq3-excited state relaxation process; FIG. 7 shows a curve illustrating the dependence of the internal luminescence efficiency for the \=530 nm band vs. film thickness for films grown on glass, the excitation wavelength for these data being 370 nm; and FIG. 8 shows an end view of a simple apparatus set-up and' method for measuring film luminescence properties for another embodiment of the invention. Detailed Description Of The Invention: In FIG. la, a photocell 4 is shown with a conventional monolayer AR coating 3 optimized for X = 500 nm. Device with such coatings undesirably reflect away substantial fractions of UV light with \ ^ convert the absorbed near-UV light into green light of 530 mo-wavelength. Therefore, wavelengths greater than 400 nm are transmitted through the film with low loss, while more than one-half of near-UV light is absorbed in a 300-400 A thick Alq3 film. An organic thin film 2 can thus act as a UV-to-green light converter and at the same time serve as AR coating 2 for X> 400 nm, as shown in FIG. lb. It is well known that the efficiency of conventional photodiodes and solar cells decreases rapidly in the near-UV range. The dashed line in FIG. 2 (curve 16) illustrates the energy distribution of photons in the solar AM0 (atmospheric mass zero, the standard of intensity and wavelength distribution of sunlight in space) spectrum. Also plotted in Fig. 2 is the quantum efficiency of a typical conventional solar cell 4 with an AR coating 3, shown by curve 12 plotted between solid circles (R.A. Street, "Hydrogenated Amorphous Si1icon", Cambridge University Press, 1991). It is clear that for wavelengths shorter than 400 nm, conventional cells (see FIG. la) can not provide efficient conversion of solar radiation. Also, curve 14 plotted between square points in FIG. 2 shows the calculated quantum efficiency for conventional cells coated with a 100% Q.E. organic material. Here, it was assumed that all the solar radiation with wavelengths less than 420 nm is absorbed by Alq^ and converted into the 530 nm light (100% internal efficiency of luminescence). It is also assumed that all the photons emitted by the Alq3 coating 2 are coupled into the solar cell 4 (see FIG. lb), which is reasonable since the refraction index of Si (x.xx) is much higher than that of Alq, (1.73) or air (-1.00). FIG. 3 illustrates photocurrent vs. wavelength curves fdr conventional solar cells (taken from M.A. Green," High Efficiency Silicon Solar Cells", Trans Tech Publishing, p.416, 1987) represented by curve 18 plotted between solid circles, and the calculated spectrum (curve 20, plotted between open squares) , for the same cell coated with a 100% Q.E. organic layer thick enough to absorb all the incident UV light (the exact film thickness should also satisfy the anti reflection condition for the solar radiation spectral maximum). The difference in area under these two curves, 18 and 20, corresponds to the expected solar cell power efficiency increase and is close to 11%. It should also be noted that design of the photocells 2,4 themselves may now be optimized for 530 nm, allowing for the use of more efficient, deep p-n junctions for solar cells. It is apparent that a similar approach can be used to increase the UV sensitivity of photodiodes. Low reflectance in the UV range is provided by the low reflectance of the organic thin film. In this case, however, the film thickness should satisfy the antireflection condition for the UV light. For 250 nm wavelength UV, the quarter-wavelength AR coating thickness should be about 350 A. This thickness of film is sufficient to fully absorb incident UV light, as discussed above. It should also be noted that, according to the measurements made by the inventors, the quantum efficiency of luminescence is independent of the film thickness for layers thicker than 100 A, due to the relatively short diffusion length of excited states in amorphous organic compounds. There is, therefore, no obstacle to the available. For example, the data on optical absorption are apparently limited to the work of P.E. Burrows et al. (See APDI. Phys. Lett. 64, pg. 2285, 1993), where only a narrow spectral range was studied. The inventors believe that no previous data has been published on the photoexcitation spectra of Alg3. The absorption, photoexcitation, external and internal quantum efficiencies of Alq3 thin films grown by thermal evaporation in vacuum have been successfully measured by the inventors. The absorption measurements determined absorption coefficients (a ) in solid films higher than 104 cm1 in the spectral range between \= 250 nm and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external Quantum Efficiency IJC for the prominent green luminescence band (with a center wavelength at X=530 nm) does not depend on the excitation radiation wavelength. Direct measurements of TJ„ suggest that the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed, exceeding 30% at room temperature. The discovery of this information, made possible via the inventors novel measurement methods, explains the lack of past interest in using Alg3 in the manufacture of light detectors. Measurement of Alq3 optical characteristics was performed on Alq3 films grown on glass, sapphire and silicon substrates. Prepurified Alo^ films with thicknesses ranging from 50A to 1.35 μ m were deposited under vacuum by thermal evaporation as described by P.E. Burrows et al., in "Electroluminescence from Trap-Limited Current Transport in Vacuum deposited Organic Light Emitting Devices", Appl. Phys. Lett. 64, p. 2285 (1994). Prior to growth, glass and sapphire substrates were cleaned by three^ successive rinses in boiling 1,1,1-trichloromethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with -20 A-thick native oxide layer on the surface. Alq3 films grown on Si substrates were used for ellipsometric determination of the layer thickness and refractive index (n) , which was determined to be n=1.73±0.05 at X=633 nm. As shown in FIG. 4, the experimental apparatus for measuring the film spectral properties consists of a 0.25 m monochromator 24 used to select light with a full width at half maximum of AX = 15 nm from the broad spectral band of a Xe-arc lamp 22. After passing through a 165 Hz chopper 26, the excitation light is focused to a 1x2 mm spot using a CaF2 lens system 28, 34, and a 45° angle tilted mirror 32. Calibrated, UV-enhanced 1 cm diameter Si-photodiode 50, and 4x2 cm Si solar cell 44 were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier 51. All measurements were made at room temperature. For absorption measurements, a bandpass filter 36 with transmission between X=250 nm and 420 nm was introduced between the sample and the Si-detector to eliminate the Alc^ photoluminescence signal, as well as scattered light at \>420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp 22 as well as the low sensitivity of the Si-detector 50 at \500oA) , and hence thinner samples (510A and 2400A) were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thickness are given in FIG. 5 (see curves 52, 54 and 56) . The absolute value of the absorption coefficient is calibrated at X=370 nm by measuring transmission through several samples of different thicknesses (see inset, FIG. 5, curve 58), and is a=(4.4± 0.1)xl04 cm"1. Figure 5 shows that the overlap of two broad absorption bands with maxima at X=385 nm and X=260 nm (indicated by arrows) results in a>104 cm"1 over the entire spectral range from X=425 nm to X=250 nm, which is the short wavelength detection limit of the apparatus. In FIG. 6, the X=530 nm PLE (photoluminensce excitation) spectra for thin (150 A) Alq3 is shown by curve 60, and for thick (1.35 pm) Alq3 films is shown by curve 62. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at X=260 nm. The 1.35 pm thick sample, however, absorbs 95% of the incident light at all wavelengths shorter than X=4 2 5 nm. The plateau at X Determination of the absolute value of the external PL efficiency, ije, was accomplished using a bandpass filter 36 with peak transmission at X=370 nm placed immediately above the sample (see FIG. 4) to reduce scattered light from the monochromator. A 1.35 μ m thick organic film 48, such as Alq3 film, on a glass substrate 38 was set directly onto the surface of the silicon solar cell 44. Using the values obtained for a, (FIG. 6), and taking into account the decreased UV photoresponse of the Si solar cell 44 as compared to its response at longer (~500 nm) wavelengths (see FIGS. 2 through 12), it was found that X=370 nm radiation transmitted through the 1.35 fan. thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected. The PL efficiency for X=370 nm, integrated over a 2TT solid angle (estimated as discussed by Garbuzov in J. Luminescence 27, p.109, (1982)), gives rje from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, the thickness of the organic film and area of the substrate, indicating that the measured value of ije is strongly affected by scattering losses and waveguiding of PL in the film and the glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of ije measurements using an integrating sphere. In the present invention one embodiment includes an apparatus to measure film spectral properties, which provides improvements that overcome these limitations of the prior art. The film 40 measured by the present apparatus is deposited on a glass substrate 38 and is thick enough to absorb all the excitation radiation. The substrate is placed film side down on the surface of a calibrated Si solar cell 44. The excitation radiation is focused at the film 40 through the glass 38. The film luminescence power is measured by the solar cell 44 (all the radiation (P) coupled out from the sample in 2TT solid angle) and by the second photodetector 50, located over the sample (which signal with amplitude A, is proportional to the intensity of 42, and the PL signals of the two samples were compared using a second photodetector 50 placed above the samples (see FIG. 4). The PL efficiency, as recorded by the second photodetector 50, of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose efficiency was measured by the solar cell 44 to be -3.6%. Therefore, the total efficiency of the immersed samples was 28%+(0.8)x3.6%*31%. This number can be considered as a lower limit for rje since the refractive index of the immersion fluid is less than n for Alq3, and hence the waveguiding losses to the Alq3 film, although improved, are not completely eliminated. EXAMPLE 2 In the second, preferred embodiment, the luminescence intensities of the 1.35 /un thick film grown on Si substrate is compared with that of the film grown on a glass substrate. Since the refractive index of Si (4.16 at X=530 nm) is much higher than that of Alqj, waveguide losses are eliminated and only photons emitted at angles smaller than the total internal reflection angle (35°w.r.t. normal) can contribute to ije. In this case, a good approximation to tjc is given by ijt;=iii(l-'Rl) (I+R3) (n-(n2-l)w)(2n)"1. Where n is the luminescent film refractive index (in this case the luminescent film is Alg3) . R, and R2 are the Fresnel reflection coefficients for the PL radiation at the luminescent film-air interface and semiconductor-luminescent film interfaces, respectively, averaged over angles caused by the total internal reflection at the Alq,-air interface, perhaps explaining why past efforts that did not use the improvement of index matching fluid coupling failed to anticipate how high the efficiency of Alq3 actually is. Using the previous calibration efficiency of the film on glass (see above) , it is found that the value of 7}e for the film on the Si substrate equals to (3.2±0.2%). The inventors, by using the above equation, calculated that the internal efficiency for the green luminescent band centered at X= 530 nm is IJ~(32±2)%. Note that this is lOx the (3.2±0.2%) absolute value of rje for the film on the Si substrate, which requires using the previous calibration efficiency of the film on glass , which in turn requires the immersion oil improvement of example 1. The difference between the calculated TJ-, and the measured ije of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alq3 film 40 sandwiched between the immersion oil 42 and the glass 38. However, a considerable fraction of this waveguided radiation is also collected by the solar cell 44 due to total internal reflection at the oil-air interface, as shown in FIG. 4. Thus, the results obtained by both embodiments, as shown in examples 1 and 2, are in good agreement. EXAMPLE 3 The dependence of the internal luminescence efficiency on film thickness for samples grown on glass substrate is shown by curve 64 in FIG.7. To obtain these data, the luminescence from both the thin and previously calibrated thick samples is measured with a low pass filter (passing \>420 nm) placed over the photodetector 44 to eliminate any non-absorbed excitation and scattered light from the monochromator 24. The amount of absorbed excitation light is directly measured for films with thickness >500A, and is calculated for thinner films using the data in the inset of Fig. 5. Data shown in FIG. 7 correspond to freshly prepared samples, with air exposure times Although various embodiments of the invention are shown and i1lustrated above, they are not meant to be 1imiting. Those skilled in the art may recognize certain modifications to these embodiments, which are meant to be covered by the spirit and scope of the appended claims. For example, C.W. Tang et al., "Electroluminescence of Doped Organic Thin Films", J. Appl. Phys. 65, No.9 3610 (1989), teaches that Alg3 may be doped with traces of coumarin 540 and DCM1 to increase its efficiency and to shift its emission from the green to anywhere between the blue-green to orange-red regions of the spectrum. As a further example of another embodiment of the invention, for estimation of the internal quantum efficiency r}t of a film, reference is made to FIG. 8. In this embodiment, the material of interest is deposited directly at a Si-photodiode 70 surface as a film 74. The photodiode includes a p layer 71 and an n layer 72. In this case, the value of J/J can be estimated from simple equations and the measurement of the magnitude of current flowing through photodiode 70 via a current measuring device 76: where J9 is the photodiode photocurrent (in electrons/sec.) measured by current measuring device 76, 1^ is the intensity of the UV excitation radiation 78 (in photons/sec.), and £„, (in electrons/photon) is the photodiode quantum efficiency at a wavelength corresponding to the peak of film 74 photo-luminescence . Also, D.Z Garbuzov' et al., "Organic film deposited on Si p-n junctions: Accurate measurements of florescence internal efficiency, and application to luminescent antireflection coatings", Journal of Applied Phvsics. Vol. 80, No. 8, October 15, 1996, pages 4644-4648, shows and describes the results obtained by the present inventors for testing materials other than Alq3. More specifically, through use of the present invention, coatings of TPD {N,N'-diphenyl-N, N'-bis-(3-methylphenyl)-l, l'-biphenyl-4, 4' diamine}, and of GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallinm} were tested, resulting in the measurement of internal fluorescence efficiency rj, of 0.35 + 0.3, and 0.36 ± 0.3, respectively. ■ Other efficient chromophores have also been identified by the inventors for use as high efficiency f lorescent anti-reflection (AR) coatings. These materials include poly¬pheny lenevinylene, which can be prepared as a very uniform and flat film via wet chemical growth; and distyriarelene derivatives, which are vacuum depositable (see C. Hosokawa, et al. , Appl, Phvs. Lett.. Vol. 67, pages 3853-3855 (1195)). Another material is Al (acetylacetonate)3[Al(acac)3], a compound that absorbs in the ultraviolet spectral region, and emits blue or purple spectral region - WE CLAIM : 1. An improved light detector comprising a light detector selected from the group consisting of photocells, photodiodes and solar cells; and an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector. 2. A light detector comprising a light detector; and an organic luminescent coating capable of converting UV to visible light in the range from blue-green to red at high quantum efficiencies as light absorbs in passing through said coating into said light detector. 3. The light detector as claimed in claim 2, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq2Cl, TPD, polyphenylenevinylene, and distyriarelene derivatives. 4. The light detector as claimed in claim 2, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells. 5. A light detector comprising a light detector; and an organic luminescent coating capable of converting UV to green light at high quantum efficiencies as light passes through said coating into said light detector. 6. The light detector as claimed in claim 5, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq2Cl, TPD, polyphenylenevinylene, and distyriarelene derivatives. 7. The light detector as claimed in claim 5, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells. 8. The light detector as claimed in claim 7, wherein the light detector is made from silicon. 9. The light detector as claimed in claim 7, wherein the light detector is made from gallium arsenide. 10. The light detector as claimed in claim 5, wherein the light detector has a conventional anti-reflection coating. 11. The light detector as claimed in claim 6, wherein the Aluminum-Tris-Quinolate coating is from 100A to 1.35 urn in thickness. 12. The light detector as claimed in claim 6, wherein the AIuminum-Tris-Quinolate coating is from 300A to 400A in thickness. 13. An apparatus for measuring film spectral properties, comprising means for providing monochromatic light; means for chopping the monochromatic light; means for focusing the monochromatic light onto a sample, the sample being a substrate with a film to be analyzed on its surface; coupling the film side of the sample to a first light detector with immersion oil; means for focusing light emitted by the sample onto a second light detector; and means for analyzing the signals from the first and second light detectors to provide the desired film spectral properties. 14. The apparatus as claimed in claim 13, wherein a UV bandpass filter provided between the monochromatic light and the sample. 15. The apparatus as claimed in claim 13, wherein a bandpass filter is provided between the sample and the second light detector. 16. The apparatus as claimed in claim 15, wherein the bandpass filter has light transmission between about 250 and 420 nm. 17. The apparatus as claimed in claim 13, comprises a bandpass filter above the sample. 18. The apparatus as claimed in claim 17, wherein the bandpass filter has peak transmission at about 370 nm. 19. The apparatus as claimed in claim 13, wherein the means for providing a monochromatic light comprises a Xenon lamp and a monochromator. 20. The apparatus as claimed in claim 13, wherein the means for focusing the light onto the sample is a lens. 21. The apparatus as claimed in claim 20, comprises a mirror. 22. The apparatus as claimed in claim 13, wherein the substrate is transparent. 23. The apparatus as claimed in claim 13, wherein the substrate is selected from the group consisting of glass and sapphire. 24. The apparatus as claimed in claim 13, wherein the means for analyzing the signals from the first and second light detectors consists of a current and lock-in amplifier. 25. The apparatus as claimed in claim 13, wherein said first and second light detectors are selected from the group consisting of silicon solar cells, silicon photodetectors and UV enhanced photodiodes. 26. An improved light detector substantially as herein described with reference to the accompanying drawings.

Full Text

This invention was made with Government support under Contract No. XAI-3-11167-03, awarded by the Air Force. The government has certain rights in this invention.
Related Invention:
This co-pending invention is related to the invention covered by provisional Application Serial No. 60/010,013, filed January 11, 1996, entitled "Organic Luminescent Coating For Light Detectors".
Field Of The Invention:
The field of this invention relates generally to photodetectors, including solar cells, and more particularly to anti-reflection (AR) coatings for photodetectors.
Background Of The Invention:
Light detectors include solar cells, used to convert light to electrical energy, and photocells and photodiodes used to measure light intensity. Materials for light detectors include

semiconductors such as silicon, gallium arsenide, germanium, selenium, and the like. Typically, the light gathering efficiency of these devices is improved by providing them with an Anti-Reflection (AR) coating. If less light is reflected, then more is available for absorption and conversion to an electrical signal. These coatings can be a single layer thick, in which case they are typically a 1/4 wavelength of light thick, or they may be multilayer coatings for even greater efficiency. Such coatings are typically composed of glassy insulators such as aluminum oxide, titanium oxide, silicon dioxide, silicon nitride or magnesium fluoride. Furthermore, high sensitivity in the blue and ultraviolet (UV) region of the spectrum is difficult to achieve with conventional semiconductor detectors, since reflectance of semiconductor materials strongly increases in this spectral range. Also, a typical AR-coatings is typically optimized for the green spectral range, becoming less effective in the blue and UV spectral regions.
Summary Of The Invention:
An object of the invention is to provide high efficiency solar cells and photodiodes for a broad range of wide spectral bandwidth applications.
Another object of the invention is to provide photocells and photodiodes with increased sensitivity, especially in the blue and UV regions of the spectrum.
Another object of the invention is to provide an apparatus for measuring film properties such as internal and external luminescence quantum efficiency with an accuracy heretofore

unobtained with traditional measurement methods such as the integrating spheres. This apparatus can be utilized for identifying candidate coatings for use in high efficiency solar cells, and high sensitivity photocells and photodiodes.
These and other objects of the invention are provided by an improved light detector, which consists of a coating of an organic luminescent material, such as Aluminum-Tris-Quinolate (Alqj also known as 8-hydroxyquinoline aluminum, Alq), or TPD {H,N'-diphenyl-N, N'-bis(3-methylphenyl) 1, l'-biphenyl-4, 4' diamine}, or Gag;CI {bis-(8-hydroxyquinaldine)-chlorogallium, on a light detector, preferably as a substitute for a conventional AR coating. When organic film is substituted for the AR coating, the film also acts as an AR coating, if it is thick enough, since it does not absorb appreciably in its own luminescent region of the spectrum. If the coating has a 100% internal luminescence efficiency (efficiency of conversion of the absorbed light into luminescent light), the power efficiency for an amorphous silicon solar cell with such a coating is improved by 11%. Conventional AR coatings can be retained for scratch resistance, at slightly less gain. Alternatively, the organic film can be intentionally designed as a final layer of a multi¬layer AR coating.
Brief Description Of The Drawings:
Various embodiments of the invention are illustrated and described below with reference to the accompanying drawings, in which like items are identified by the same reference designation, wherein:

FIG. la shows a conventional photocell or solar cell of the prior art, the UV portion of the spectrum being partially reflected by the AR coated device surface.
FIG. lb shows a preferred embodiment of the invention, in which a photocell or solar cell is coated with an organic luminescent coating for converting UV to green light, and for acting as an AR coating;
FIG. Ic shows another embodiment, in which a luminescent organic material is coated over a conventional AR coating.
FIG. 2 shows curves illustrating the comparative performance of a conventional solar cell, and one coated with 100% quantum efficiency (Q.E.) organic material;
FIG. 3 shows curves illustrating the calculated quantum efficiency improvement of 10.9% when comparing a cell coated with 100% QE organic material to a conventional cell;
FIG. 4 shows an apparatus for measuring film spectral and luminescence properties;
FIG. 5 shows curves illustrating the absorption spectra of Alq3 films obtained from three different film thicknesses, while the inset shows light attenuation vs. film thicknesses at X=370 nm, the slope giving the absorption coefficient;
FIG. 6 shows curves illustrating the photoluminescence (PL) excitation spectra for film thicknesses of 150 A and 1.35 /xm, while the inset is a schematic of a Alq,-excited state relaxation
process;
FIG. 7 shows a curve illustrating the dependence of the internal luminescence efficiency for the \=530 nm band vs. film

thickness for films grown on glass, the excitation wavelength for these data being 370 nm; and
FIG. 8 shows an end view of a simple apparatus set-up and method for measuring film luminescence properties for another embodiment of the invention.

Detailed Description Of The Invention:
In FIG. la, a photocell 4 is shown with a conventional monolayer AR coating 3 optimized for \ = 500 nm. Device with such coatings undesirably reflect away substantial fractions of UV light with X
convert the absorbed near-OV light into green light of 530 nm wavelength. Therefore, wavelengths greater than 400 nm are transmitted through the film with low loss, while more than one-half of near-UV light is absorbed in a 300-400 A thick Alq, film. An organic thin film 2 can thus act as a UV-to-green light converter and at the same time serve as AR coating 2 for X> 400 nm, as shown in FIG. lb.
It is well known that the efficiency of conventional photodiodes and solar cells decreases rapidly in the near-UV range. The dashed line in FIG. 2 (curve 16) illustrates the energy distribution of photons in the solar AMO (atmospheric mass zero, the standard of intensity and wavelength distribution of sunlight in space) spectrum. Also plotted in Fig. 2 is the quantum efficiency of a typical conventional solar cell 4 with an AR coating 3, shown by curve 12 plotted between solid circles (R.A. Street, "Hydrogenated Amorphous Silicon", Cambridge University Press, 1991). It is clear that for wavelengths shorter than 400 nm, conventional cells (see FIG. la) can not provide efficient conversion of solar radiation. Also, curve 14 plotted between square points in FIG. 2 shows the calculated quantum efficiency for conventional cells coated with a 100% Q.E. organic material. Here, it was assumed that all the solar radiation with wavelengths less than 420 nm is absorbed by Alq, and converted into the 530 nm light (100% internal efficiency of luminescence). It is also assumed that all the photons emitted by the Algj coating 2 are coupled into the solar cell 4 (see FIG. lb), which is reasonable since the refraction index of Si (x.xx) is much higher than that of Alq^ (1-73) or air (-1.00).

/IG. 3 illustrates photocurrent vs. wavelength curves for conventional solar cells (taken from M.A. Green," High Efficiency Silicon Solar Cells", Trans Tech Publishing, p.416, 1987) represented by curve 18 plotted between solid circles, and the calculated spectrum (curve 20, plotted between open squares), for the same cell coated with a 100% Q.E. organic layer thick enough to absorb all the incident UV light (the exact film thickness should also satisfy the anti reflection condition for the solar radiation spectral maximum). The difference in area under these two curves, 18 and 20, corresponds to the expected solar cell power efficiency Increase and is close to 11%. It should also be noted that design of the photocells 2,4 themselves may now be optimized for 530 nm, allowing for the use of more efficient, deep p-n junctions for solar gells.
It is apparent that a similar approach can be used to increase the UV sensitivity of photodiodes. Low reflectance in the UV range is provided by the low reflectance of the organic thin film. In this case, however, the film thickness should satisfy the antireflection condition for the .UV light. For 250 nm wavelength UV, the quarter-wavelength AR coating thickness should be about 350 A. This thickness of film is sufficient to fully absorb incident UV light, as discussed above. It should also be noted that, according to the measurements made by the inventors, the quantum efficiency of luminescence is independent of the film thickness for layers thicker than 100 A, due to the relatively short diffusion length of excited states in amorphous organic compounds. There is, therefore, no obstacle to the

utilization of extremely thin films of these compounds as AI luminescent coatings.
There are two additional advantages of UV-photodiodes with organic coatings. The first is the flatness of the coated photQdiode response at wavelengths shorter than the absorption edge of the organic material coating. This is because the coating luminescence quantum efficiency does not depend on the excitation wavelength. This property facilitates photodiode calibration and simplifies analysis of the experimental results. The strong dependence of the uncoated photodiode photoresponse on the excitation wavelength is caused by intrinsic properties and can not be eliminated in the uncoated devices.
The second advantage is the visualization of UV radiation which is converted into the visible spectrum on the coated photodiode surface. It considerably simplifies positioning and alignment of the photodiode with respect to the incident light.
The ability to measure the luminescent properties of thin films, and of Alq^ in particular, was critical in discovering Alq^ as a material to be utilized in organic LED devices. Other materials may also be discovered via use of an innovative measuring technique based on the embodiment of the present invention, described in detail below.
Most studies to date have focused on understanding the mechanism of charge transport, and the specifics of electroluminescence of Alq^^-based OLEDs. However, data on the optical properties of solid films of Alq,, which are crucial for the in-depth understanding of the operation of OLEDs and other optoelectronic devices, such as light detectors, are not

available. For example, the data on optical absorption are apparently limited to the work of p.E. Burrows et al. (See Appl. Phys. Lett. 64, pg. 2285, 1993), where only a narrow spectral range was studied. The inventors believe that no previous data has been published on the photoexcitation spectra of Alg,.
The absorption, photoexcitation, external and internal quantum efficiencies of Alq^ thin films grown by thermal evaporation in vacuum have been successfully measured by the inventors. The absorption measurements determined absorption coefficients (a ) in solid films higher than lO* cm"' in the spectral range between X= 250 nm and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external Quantum Efficiency 17^ for the prominent green luminescence band (with a center wavelength at X=530 nm) does not depend on the excitation radiation wavelength. Direct measurements of tj^ suggest that the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed, exceeding 30% at room temperature. The discovery of this information, made possible via the inventors novel measurement methods, explains the lack of past interest in using hlq^ in the manufacture of light detectors.
Measurement of Alq, optical characteristics was performed on Alqj films grown on glass, sapphire and silicon substrates. Prepurified Alq, films with thicknesses ranging from 50A to 1.35 ftm were deposited under vacuum by thermal evaporation as described by P.E. Burrows et al., in "Electroluminescence from Trap-Limited Current Transport in Vacuum deposited Organic Light Emitting Devices", Appl. Phys. Lett. 64, p. 2285 (1994). Prior

Lu growtn, glass and sapphire substrates were cleaned by three successive rinses in boiling 1,1,l-trichloromethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with -20 A-thick native oxide layer on the surface. Alqj films grown on Si substrates were used for ellipsometric determination of the layer thickness and refractive index (n) , which was determined to be n=1.73±0.05 at X=633 nm.
As shown in FIG. 4, the experimental apparatus for measuring the film spectral properties consists of a 0.25 m monochromator 24 used to select light with a full width at half maximum of AX = 15 nm from the broad spectral band of a Xe-arc lamp 22. After passing through a 165 Hz chopper 26, the excitation light is focused to a 1x2 mm spot using a CaF^ lens system 28, 34, and a 45° angle tilted mirror 32. Calibrated, UV-enhanced 1 cm diameter Si-ptiotodiode 50, and 4x2 cm Si solar cell 44 were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier 51. All measurements were made at room temperature.
For absorption measurements, a bandpass filter 36 with transmission between \=250 nm and 420 nm was introduced between the sample and the Si-detector to eliminate the Alqj photoluminescence signal, as well as scattered light at X?420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp 22 as well as the low sensitivity of the si-detector 50 at X5000A), and hence thinner samples (51oA and 240oA)

were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thickness are given in FIG. 5 (see curves 52, 54 and 56) . The absolute value of the absorption coefficient is calibrated at X=370 nm by measuring transmission through several samples of different thicknesses (see inset, FIG. 5, curve 58), and is a=(4.4± 0.1)xlO* cm'. Figure 5 shows that the overlap of two broad absorption bands with maxima at X=385 nm and X=260 nm (indicated by arrows) results in a>lO' cm' over the entire spectral range from X=425 nm to X=250 nm, which is the short wavelength detection limit of the apparatus.
In FIG. 6, the X=530 nm PLE (photoluminensce excitation) spectra for thin (150 A) Alq, is shown by curve 60, and for thick (1.35 /zm) Alqj films is shown by curve 62. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at X=260 nm. The 1.35 fim thick sample, however, absorbs 95% of the incident light at all wavelengths shorter than X=425 nm. The plateau at X Determination of the absolute value of the external PL efficiency, T}^, was accomplished using a bandpass filter 36 with peak transmission at X=370 nm placed immediately above the sample (see FIG. 4) to reduce scattered light from the monochromator. A 1.35 fim thick organic film 48, such as Alq, film, on a glass substrate 38 was set directly onto the surface of the silicon

solar cell 44. Using the values obtained for a, (FIG. 6), and taking into account the decreased UV photoresponse of the Si solar cell 44 as compared to its response at longer(-500 nm) wavelengths (see FIGS. 2 through 12), it was found that X=370 nm radiation transmitted through the 1.35 /tm thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected. The PL efficiency for X=370 nm, integrated over a 2Tr solid angle (estimated as discussed by Garbuzov in j. Luminescence 27, p. 109, (1982)) , gives »/^ from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, the thickness of the organic film and area of the substrate, indicating that the measured value of ij^ is strongly affected by scattering losses and waveguiding of PL in the film and the glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of v^ measurements using an integrating sphere. In the present invention one embodiment includes an apparatus to measure film spectral properties, which provides improvements that overcome these limitations of the prior art.
The film 40 measured by the present apparatus is deposited on a glass substrate 38 and is thick enough to absorb all the excitation radiation. The substrate is placed film side down on the surface of a calibrated Si solar cell 44. The excitation radiation is focused at the film 40 through the glass 38. The film luminescence power is measured by the solar cell 44 (all the radiation (P) coupled out from the sample in 27r solid angle) and by the second photodetector 50, located over the sample (which signal with amplitude A, is proportional to the intensity of

radiation that is coupled out in the direction opposite to tne solar cell 44).
Then refractive index matching fluid 42 is placed between
the film or coating 40 and solar cell 44, and measurements are
repeated under the same excitation level. The lower limit to the
internal quantum efficiency of the film is calculated as:
V, = fP,+PB/A)/P„, where: (1)
P represents the film luminescence power coupled out from the sample in a 2TT solid angle and measured by the solar cell 44 under the condition of no ininersion;
P] is the luminescence power measured by the solar cell 44 in the case of immersion;
Po is the power of the excitation radiation;
A is the signal of the second photodetector 50 under the condition of no immersion; and
B is the signal of the second photodetector 50 under the condition of immersion, B EXAMPLE 1 In the first embodiment, the samples comprised of the combination of substrate 38 and Alq, coating 40 are placed film-side-down in index of refraction-matching fluid 42 (with n=1.524) dropped onto the surface of the solar cell 44. Under these conditions ^^=ZB% for the radiation emitted towards the solar cell 44 was obtained. In order to evaluate the PL intensity in the opposite direction (through the glass slide 38), an identical sample was placed on the solar cell 44 without the immersion oil

42, and the PL signals of the two samples were compared using a second photodetector 50 placed above the samples (see FJG. 4). The PL efficiency, as recorded by the second photodetector 50, of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose efficiency was measured by the solar cell 44 to be -3.6%. Therefore, the total efficiency of the immersed samples was 28%+(0.8)x3.6%^31%. This number can be considered as a lower limit for i\, since the refractive index of the immersion fluid is less than n for Alqj, and hence the waveguiding losses to the Alq^ film, although improved, are not completely eliminated.
EXAMPLE 2 In the second, preferred embodiment, the luminescence intensities of the 1.35 ^m thick film grown on Si substrate is compared with that of the film grown on a glass substrate, since the refractive index of Si (4.16 at X=530 nm) is much higher than that of Alq,, waveguide losses are eliminated and only photons emitted at angles smaller than the total internal reflection angle (35'w.r.t. normal) can contribute to TJ^. In this case, a good approximation to t\^ is given by Tj,=jji(l-R,) (l+Rj) (n-{n'-l)'^)(2n)'. Where n is the luminescent film refractive index (in this case the luminescent film is Alqj) . R, and R^ are the Fresnel reflection coefficients for the PL radiation at the luminescent film-air interface and semiconductor-luminescent film interfaces, respectively, averaged over angles
J}; caused by the total internal reflection at the Alqj-air interface, perhaps explaining why past efforts that did not use the improvement of index matching fluid coupling failed to anticipate how high the efficiency of Alqj actually is. Using the previous calibration efficiency of the film on glass (see above) , it is found that the value of 7;, for the film on the Si substrate equals to (3.2+0.2%). The inventors, by using the above equation, calculated that the internal efficiency for the green luminescent band centered at X= 530 nm is jji=(32±2)%. Note that this is lOx the (3.2±0.2%) absolute value of T;^ for the film on the Si substrate, which requires using the previous calibration efficiency of the film on glass , which in turn requires the immersion oil improvement of example 1.
The difference between the calculated jj, and the measured TJ, of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alqj film 4 0 sandwiched between the immersion oil 42 and the glass 38. However, a considerable fraction of this waveguided radiation is also collected by the solar cell 44 due to total internal reflection at the oil-air interface, as shown in FIG. 4. Thus, the results obtained by both embodiments, as shown in examples 1 and 2, are in good agreement.
EXAMPLE 3
The dependence of the internal luminescence efficiency on
film thickness for samples grown on glass substrate is shown by
curve 64 in FIG.7. To obtain these data, the luminescence from
both the thin and previously calibrated thick samples is measured

with a low pass filter (passing X>42 0 nm) placed over the photodetector 44 to eliminate any non-absorbed excitation and scattered light from the monochromator 24. The amount of absorbed excitation light is directly measured for films with thickness >50oA, and is calculated for thinner films using the data in the inset of Fig. 5. Data shown in FIG. 7 correspond to freshly prepared samples, with air exposure times coating.
Although various embodiments of the invention are shown and
illustrated above, they are not meant to be limiting. Those
skilled in the art may recognize certain modifications to these
embodiments, which are meant to be covered by the spirit and
scope of the appended claims. For example, C.W. Tang et al.,
"Electroluminescence of Doped Organic Thin Films", J. Appl. Phys.
65, No.9 3610 (1989), teaches that Alq, may be doped with traces
I i of coumarin 540 and DCMl to' increase its efficiency and to shift
i I ■
its emission from the gre^n to anywhere between the blue-green to orange-red regions of the spectrum.

As a further example of another embodiment of the invention, for estimation of the internal quantum efficiency yj^ of a film, reference is made to FtG. 8.
In this embodiment, the material of interest is deposited directly at a Si-photodiode 70 surface as a film 74. The photodiode includes a p layer 71 and an n layer 72. In this case, the value of ij, c^n be estimated from simple equations and the measurement of the magnitude of current flowing through photodiode 70 via a current measuring device 7 6:
where J^ is the photodiode photocurrent (in electrons/sec.) measured by current measuring device 76, I^ is the intensity of the UV excitation radiation 78 (in photons/sec.) , and /3ph (in electrons/photon) is the photodiode quantum efficiency at a wavelength corresponding to the peak of film 74 photo-luminescence.
Also, D.Z Garbuzov et al., "Organic film deposited on Si p-n junctions: Accurate measurements of florescence internal efficiency, and application to luminescent antireflection coatings", Journal of Applied Phvsics, Vol. 80, No. 8, October 15, 1996, pages 4644-4648, shows and describes the results obtained by the present inventors for testing materials other than Alq,. More specifically, through use of the present invention, coatings of TPD {N,N'-diphenyl-N, N'-bis-(3-methylphenyl)-!, l'-biphenyl-4, 4' diamine}, and of GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallinm} were tested, resulting in the measurement of internal fluorescence efficiency 17, of 0.35 + 0.3, and 0.36 ± 0.3, respectively.

other efficient chromophores have also been j-ucjiiLj-riea oy the inventors for use as high efficiency florescent anti-reflection (AR) coatings. These materials include poly¬pheny lenevinylene, which can be prepared as a very uniform and flat film via wet chemical growth; and distyriarelene derivatives, which are vacuum depositable (see C. Hosokawa, et al. , ADPI. Phvs. Lett.. Vol. 67, pages 3853-3855 (1195)). Another material is Al (acetylacetonate) 3[A1 (acac) 3] , a compound that absorbs in the ultraviolet spectral region, and emits blue or purple spectral region.

We Claim;
1. A combination comprising:
a light detector selected from the group consisting of photocells, photodiodes and solar cells; and
an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector.
2. A combination comprising:
a light detector; and
an organic luminescent coating capable of converting UV to visible light in the range from blue-green to red at high quantum efficiencies as light absorbs in passing through said coating into said light detector.
3. The combination of Claim 2, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, GaqJCl, TPD, polyphenylenevinylene, and distyriarelene derivatives.
4. The combination of Claim 2, wherein the light aetector is selected from the group consisting of photocells, photodiodes and solar cells.
5. A combination comprising:
a light detector; and

an organic luminescent coating capable of conveptln^, UV to green light at high quantum efficiencies as light passes through said coating into said light detector.
6. The combination of claim 5, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq;ci, TPD, polyphenylenevinylene, and distyriarelene derivatives.
7. The combination of Claim 5, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells.
8. The combination of Claim 7, wherein the light detector is made from silicon.
9. The combination of Claim 7, wherein the light detector is made from gallium arsenide.
10. The combination of Claim 5, wherein the light detector
further includes a conventional anti-reflection coating-
11. The combination of claim 6, wherein the Aluminum-Tris-Quinolate coating is from about lOoA to about 1.35 fiv\ in thickness.
12. The combination of Claim 6, wherein the Aluminum-Tris-Quinolate coating is from about 30oA to about 40oA in thickness.

13. An apparatus for measuring film spectral properties',
comprising:
means for providing monochromatic light;
means for chopping the monochromatic light;
means for focusing the monochromatic light onto a sample, the sample being a substrate with a film to be analyzed on its surface;
coupling the film side of the sample to a first light detector with Immersion oil;
means for focusing light emitted by the sample onto a second light detector; and
means for analyzing the signals from the first and second light detectors to provide the desired film spectral properties.
14. An apparatus as in Claim 13, further comprising a UV
bandpass filter between the monochromatic light and the sample-
15. An apparatus as in Claim 13, further comprising a bandpass filter between the sample and the second light detector.
16. An apparatus as in Claiin 15, wherein the bandpass filter has light transmission between about 250 and 420 nm.
17. An apparatus as in Claim 13, further comprising a bandpass filter above the sample.
18. As in Claim 17, wherein the bandpass filter has peak transmission at about 370 nm.

19. An apparatus as in Claim 13, wherein the means for providing a monochromatic light comprises a Xenon lamp and a monochromator.
20. An apparatus as in Claim 13, wherein the means for focusing the light onto the sample is a lens.
21. An apparatus as in Claim 20, further comprising a mirror.
22. An apparatus as in Claim 13, wherein the substrate is transparent.
23. An apparatus as in Claim 13, wherein the substrate is selected from the group consisting of glass and sapphire.
24. An apparatus as in Claim 13, wherein the means for analyzing the signals from the first and second light detectors consists of a current and lock-in amplifier.
25. An apparatus as in Claim 13, wherein said first and second light detectors are selected from the group consisting of silicon solar cells, silicon photodetectors and UV enhanced photodiodes.
26. An apparatus as in Claim 13, further comprising an improved method of measuring the internal quantum efficiency of a luminescent film, comprising the steps of:

using the apparatus to obtain the calibration efficiency of the luminescent film on a transparent substrate;
determining the absolute value of the external quantum efficiency for the luminescent film on a semiconductor substrate from the obtained calibration efficiency on glass; and
dividing the absolute value of the external quantum efficiency for the luminescent film on a semiconductor substrate by (1-R,) (1+R,) (n-(n'-l)'^) (2n)' to obtain the internal efficiency of the luminescent film, substantially free of measurement losses.
27. The method as in Claim 26, wherein the semiconductor
substrate is silicon.
28. An apparatus for measuring film spectral properties
including a light source, a monochromator, a chopper, lenses,
filters, an optional mirror and at least one light detector,
wherein the improvement comprises the combination of the light
detector coupled by immersion oil to an organic luminescent film
sample deposited on a substrate-

semiconductors such as silicon, gallium arsenide, germanium, selenium, and the like. Typically, the light gathering ( efficiency of these devices is improved by providing them with an Anti-Reflection (AR) coating. If less light is reflected, then more is available for absorption and conversion to an electrical signal. These coatings can be a single layer thick, in which case they are typically a 1/4 wavelength of light thick, or they may be multilayer coatings for even greater efficiency. Such coatings are typically composed of glassy insulators such as aluminum oxide, titanium oxide, silicon dioxide, silicon nitride or magnesium fluoride. Furthermore, high sensitivity in the blue and ultraviolet (UV) region of the spectrum is difficult to achieve with conventional semiconductor detectors, since reflectance of semiconductor materials strongly increases in this spectral range. Also, a typical AR-coatings is typically optimized for the green spectral range, becoming less effective in the blue and UV spectral regions.
Summary Of The invention:
An object of the invention is to provide high efficiency solar cells and photodiodes for a broad range of wide spectral bandwidth applications.
Another object of the invention is to provide photocells and photodiodes with increased sensitivity, especially in the blue and UV regions of the spectrum.
Another object of the invention is to provide an apparatus for measuring film properties such as internal and external luminescence quantum efficiency with an accuracy heretofore

unobtained with traditional measurement methods such as the integrating spheres. This apparatus can be utilized for identifying candidate coatings for use in high efficiency solar cells, and high sensitivity photocells and photodiodes.
These and other objects of the invention are provided by an improved light detector, which consists of a coating of an organic luminescent material, such as Aluminum-Tris-Quinolate (Alq^ also known as 8-hydroxyquinoline aluminum, Alq), or TPD {N,N'-diphenyl-N, N'-bis(3-methylphenyl) 1, l'-biphenyl-4, 4' diamine}, or GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallium, on a light detector, preferably as a substitute for a conventional AR coating. When organic film is substituted for the AR coating, the film also acts as an AR coating, if it is thick enough, since it does not absorb appreciably in its own luminescent region of the spectrum. If the coating has a 100% internal luminescence efficiency (efficiency of conversion of the absorbed light into luminescent light), the power efficiency for an amorphous silicon solar cell with such a coating is improved by 11%. Conventional AR coatings can be retained for scratch resistance, at slightly less gain. Alternatively, the organic film can be intentionally designed as a final layer of a multi¬layer AR coating.

Accordingly, the present invention provides an improved light detectc comprising a light detector selected from the group consisting of photocells, photodiodes and solar cells; and an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector.
Various embodiments of the invention are illustrated and described below with reference to the accompanying drawings, in which like items are identified by the same reference designation, wherein

FIG. la shows a conventional photocell or solar cell of the prior art, the UV portion of the spectrum being partially -reflected by the AR coated device surface.
FIG. lb shows a preferred embodiment of the invention, in which a photocell or solar cell is coated with an organic luminescent coating for converting UV to green light, and for acting as an AR coating;
FIG. lc shows another embodiment, in which a luminescent organic material is coated over a conventional AR coating.
FIG. 2 shows curves illustrating the comparative performance of a conventional solar cell, and one coated with 100% quantum efficiency (Q.E.) organic material;
FIG. 3 shows curves illustrating the calculated quantum efficiency improvement of 10.9% when comparing a cell coated with 100% QE organic material to a conventional cell;
FIG. 4 shows an apparatus for measuring film spectral and luminescence properties;
FIG. 5 shows curves illustrating the absorption spectra of Alq3 films obtained from three different film thicknesses, while the inset shows light attenuation vs. film thicknesses at X=370 nm, the slope giving the absorption coefficient;
FIG. 6 shows curves illustrating the photoluminescence (PL) excitation spectra for film thicknesses of 150 A and 1.35 μ m, while the inset is a schematic of a Alq3-excited state relaxation process;
FIG. 7 shows a curve illustrating the dependence of the internal luminescence efficiency for the \=530 nm band vs. film

thickness for films grown on glass, the excitation wavelength for these data being 370 nm; and
FIG. 8 shows an end view of a simple apparatus set-up and' method for measuring film luminescence properties for another embodiment of the invention.

Detailed Description Of The Invention:
In FIG. la, a photocell 4 is shown with a conventional monolayer AR coating 3 optimized for X = 500 nm. Device with such coatings undesirably reflect away substantial fractions of UV light with \
^ convert the absorbed near-UV light into green light of 530 mo-wavelength. Therefore, wavelengths greater than 400 nm are transmitted through the film with low loss, while more than one-half of near-UV light is absorbed in a 300-400 A thick Alq3 film. An organic thin film 2 can thus act as a UV-to-green light converter and at the same time serve as AR coating 2 for X> 400 nm, as shown in FIG. lb.
It is well known that the efficiency of conventional photodiodes and solar cells decreases rapidly in the near-UV range. The dashed line in FIG. 2 (curve 16) illustrates the energy distribution of photons in the solar AM0 (atmospheric mass zero, the standard of intensity and wavelength distribution of sunlight in space) spectrum. Also plotted in Fig. 2 is the quantum efficiency of a typical conventional solar cell 4 with an AR coating 3, shown by curve 12 plotted between solid circles (R.A. Street, "Hydrogenated Amorphous Si1icon", Cambridge University Press, 1991). It is clear that for wavelengths shorter than 400 nm, conventional cells (see FIG. la) can not provide efficient conversion of solar radiation. Also, curve 14 plotted between square points in FIG. 2 shows the calculated quantum efficiency for conventional cells coated with a 100% Q.E. organic material. Here, it was assumed that all the solar radiation with wavelengths less than 420 nm is absorbed by Alq^ and converted into the 530 nm light (100% internal efficiency of luminescence). It is also assumed that all the photons emitted by the Alq3 coating 2 are coupled into the solar cell 4 (see FIG. lb), which is reasonable since the refraction index of Si (x.xx) is much higher than that of Alq, (1.73) or air (-1.00).

FIG. 3 illustrates photocurrent vs. wavelength curves fdr conventional solar cells (taken from M.A. Green," High Efficiency Silicon Solar Cells", Trans Tech Publishing, p.416, 1987) represented by curve 18 plotted between solid circles, and the calculated spectrum (curve 20, plotted between open squares) , for the same cell coated with a 100% Q.E. organic layer thick enough to absorb all the incident UV light (the exact film thickness should also satisfy the anti reflection condition for the solar radiation spectral maximum). The difference in area under these two curves, 18 and 20, corresponds to the expected solar cell power efficiency increase and is close to 11%. It should also be noted that design of the photocells 2,4 themselves may now be optimized for 530 nm, allowing for the use of more efficient, deep p-n junctions for solar cells.
It is apparent that a similar approach can be used to increase the UV sensitivity of photodiodes. Low reflectance in the UV range is provided by the low reflectance of the organic thin film. In this case, however, the film thickness should satisfy the antireflection condition for the UV light. For 250 nm wavelength UV, the quarter-wavelength AR coating thickness should be about 350 A. This thickness of film is sufficient to fully absorb incident UV light, as discussed above. It should also be noted that, according to the measurements made by the inventors, the quantum efficiency of luminescence is independent of the film thickness for layers thicker than 100 A, due to the relatively short diffusion length of excited states in amorphous organic compounds. There is, therefore, no obstacle to the

available. For example, the data on optical absorption are apparently limited to the work of P.E. Burrows et al. (See APDI. Phys. Lett. 64, pg. 2285, 1993), where only a narrow spectral range was studied. The inventors believe that no previous data has been published on the photoexcitation spectra of Alg3.
The absorption, photoexcitation, external and internal quantum efficiencies of Alq3 thin films grown by thermal evaporation in vacuum have been successfully measured by the inventors. The absorption measurements determined absorption coefficients (a ) in solid films higher than 104 cm*1 in the spectral range between \= 250 nm and 425 nm. Investigation of the photoluminescence excitation (PLE) spectra shows that over this spectral range, the external Quantum Efficiency IJC for the prominent green luminescence band (with a center wavelength at X=530 nm) does not depend on the excitation radiation wavelength. Direct measurements of TJ„ suggest that the efficiency of radiative transitions centered at 530 nm is much higher than previously assumed, exceeding 30% at room temperature. The discovery of this information, made possible via the inventors novel measurement methods, explains the lack of past interest in using Alg3 in the manufacture of light detectors.
Measurement of Alq3 optical characteristics was performed on Alq3 films grown on glass, sapphire and silicon substrates. Prepurified Alo^ films with thicknesses ranging from 50A to 1.35 μ m were deposited under vacuum by thermal evaporation as described by P.E. Burrows et al., in "Electroluminescence from Trap-Limited Current Transport in Vacuum deposited Organic Light Emitting Devices", Appl. Phys. Lett. 64, p. 2285 (1994). Prior

to growth, glass and sapphire substrates were cleaned by three^ successive rinses in boiling 1,1,1-trichloromethane, followed by three cycles in acetone, and a final rinse in boiling methanol. Silicon substrates were used as received from the manufacturer, with -20 A-thick native oxide layer on the surface. Alq3 films grown on Si substrates were used for ellipsometric determination of the layer thickness and refractive index (n) , which was determined to be n=1.73±0.05 at X=633 nm.
As shown in FIG. 4, the experimental apparatus for measuring the film spectral properties consists of a 0.25 m monochromator 24 used to select light with a full width at half maximum of AX = 15 nm from the broad spectral band of a Xe-arc lamp 22. After passing through a 165 Hz chopper 26, the excitation light is focused to a 1x2 mm spot using a CaF2 lens system 28, 34, and a 45° angle tilted mirror 32. Calibrated, UV-enhanced 1 cm diameter Si-photodiode 50, and 4x2 cm Si solar cell 44 were used to detect PL and excitation radiation in conjunction with a current and lock-in amplifier 51. All measurements were made at room temperature.
For absorption measurements, a bandpass filter 36 with transmission between X=250 nm and 420 nm was introduced between the sample and the Si-detector to eliminate the Alc^ photoluminescence signal, as well as scattered light at \>420 nm. Samples grown on sapphire substrates were used for the absorption experiments. The low intensity of the Xe-arc lamp 22 as well as the low sensitivity of the Si-detector 50 at \500oA) , and hence thinner samples (510A and 2400A)

were used for absorption measurements in the short wavelength range. The results of the absorption measurements for three films of different thickness are given in FIG. 5 (see curves 52, 54 and 56) . The absolute value of the absorption coefficient is calibrated at X=370 nm by measuring transmission through several samples of different thicknesses (see inset, FIG. 5, curve 58), and is a=(4.4± 0.1)xl04 cm"1. Figure 5 shows that the overlap of two broad absorption bands with maxima at X=385 nm and X=260 nm (indicated by arrows) results in a>104 cm"1 over the entire spectral range from X=425 nm to X=250 nm, which is the short wavelength detection limit of the apparatus.
In FIG. 6, the X=530 nm PLE (photoluminensce excitation) spectra for thin (150 A) Alq3 is shown by curve 60, and for thick (1.35 pm) Alq3 films is shown by curve 62. For thin samples, the PLE spectral maximum corresponds to the short wavelength absorption maximum at X=260 nm. The 1.35 pm thick sample, however, absorbs 95% of the incident light at all wavelengths shorter than X=4 2 5 nm. The plateau at X Determination of the absolute value of the external PL efficiency, ije, was accomplished using a bandpass filter 36 with peak transmission at X=370 nm placed immediately above the sample (see FIG. 4) to reduce scattered light from the monochromator. A 1.35 μ m thick organic film 48, such as Alq3 film, on a glass substrate 38 was set directly onto the surface of the silicon

solar cell 44. Using the values obtained for a, (FIG. 6), and taking into account the decreased UV photoresponse of the Si solar cell 44 as compared to its response at longer (~500 nm) wavelengths (see FIGS. 2 through 12), it was found that X=370 nm radiation transmitted through the 1.35 fan. thick film does not contribute appreciably to the diode photoresponse, and hence can be neglected. The PL efficiency for X=370 nm, integrated over a 2TT solid angle (estimated as discussed by Garbuzov in J. Luminescence 27, p.109, (1982)), gives rje from 2.8% to 10%. This measured external PL efficiency depends on the excitation area, the thickness of the organic film and area of the substrate, indicating that the measured value of ije is strongly affected by scattering losses and waveguiding of PL in the film and the glass substrate. Note that waveguiding effects, substrate losses and other experimental artifacts have also been observed to distort the results of ije measurements using an integrating sphere. In the present invention one embodiment includes an apparatus to measure film spectral properties, which provides improvements that overcome these limitations of the prior art.
The film 40 measured by the present apparatus is deposited on a glass substrate 38 and is thick enough to absorb all the excitation radiation. The substrate is placed film side down on the surface of a calibrated Si solar cell 44. The excitation radiation is focused at the film 40 through the glass 38. The film luminescence power is measured by the solar cell 44 (all the radiation (P) coupled out from the sample in 2TT solid angle) and by the second photodetector 50, located over the sample (which signal with amplitude A, is proportional to the intensity of

* 42, and the PL signals of the two samples were compared using a second photodetector 50 placed above the samples (see FIG. 4). The PL efficiency, as recorded by the second photodetector 50, of the immersed sample was 0.8 times as large as that of the non-immersed sample, whose efficiency was measured by the solar cell 44 to be -3.6%. Therefore, the total efficiency of the immersed samples was 28%+(0.8)x3.6%*31%. This number can be considered as a lower limit for rje since the refractive index of the immersion fluid is less than n for Alq3, and hence the waveguiding losses to the Alq3 film, although improved, are not completely eliminated.
EXAMPLE 2 In the second, preferred embodiment, the luminescence intensities of the 1.35 /un thick film grown on Si substrate is compared with that of the film grown on a glass substrate. Since the refractive index of Si (4.16 at X=530 nm) is much higher than that of Alqj, waveguide losses are eliminated and only photons emitted at angles smaller than the total internal reflection angle (35°w.r.t. normal) can contribute to ije. In this case, a good approximation to tjc is given by ijt;=iii(l-'Rl) (I+R3) (n-(n2-l)w)(2n)"1. Where n is the luminescent film refractive index (in this case the luminescent film is Alg3) . R, and R2 are the Fresnel reflection coefficients for the PL radiation at the luminescent film-air interface and semiconductor-luminescent film interfaces, respectively, averaged over angles
caused by the total internal reflection at the Alq,-air interface, perhaps explaining why past efforts that did not use the improvement of index matching fluid coupling failed to anticipate how high the efficiency of Alq3 actually is. Using the previous calibration efficiency of the film on glass (see above) , it is found that the value of 7}e for the film on the Si substrate equals to (3.2±0.2%). The inventors, by using the above equation, calculated that the internal efficiency for the green luminescent band centered at X= 530 nm is IJ~(32±2)%. Note that this is lOx the (3.2±0.2%) absolute value of rje for the film on the Si substrate, which requires using the previous calibration efficiency of the film on glass , which in turn requires the immersion oil improvement of example 1.
The difference between the calculated TJ-, and the measured ije of the immersed samples is unexpectedly small if one considers the large fraction of luminescence waveguided in the Alq3 film 40 sandwiched between the immersion oil 42 and the glass 38. However, a considerable fraction of this waveguided radiation is also collected by the solar cell 44 due to total internal reflection at the oil-air interface, as shown in FIG. 4. Thus, the results obtained by both embodiments, as shown in examples 1 and 2, are in good agreement.
EXAMPLE 3
The dependence of the internal luminescence efficiency on
film thickness for samples grown on glass substrate is shown by
curve 64 in FIG.7. To obtain these data, the luminescence from
both the thin and previously calibrated thick samples is measured

with a low pass filter (passing \>420 nm) placed over the photodetector 44 to eliminate any non-absorbed excitation and scattered light from the monochromator 24. The amount of absorbed excitation light is directly measured for films with thickness >500A, and is calculated for thinner films using the data in the inset of Fig. 5. Data shown in FIG. 7 correspond to freshly prepared samples, with air exposure times Although various embodiments of the invention are shown and i1lustrated above, they are not meant to be 1imiting. Those skilled in the art may recognize certain modifications to these embodiments, which are meant to be covered by the spirit and scope of the appended claims. For example, C.W. Tang et al., "Electroluminescence of Doped Organic Thin Films", J. Appl. Phys. 65, No.9 3610 (1989), teaches that Alg3 may be doped with traces of coumarin 540 and DCM1 to increase its efficiency and to shift its emission from the green to anywhere between the blue-green to orange-red regions of the spectrum.

As a further example of another embodiment of the invention, for estimation of the internal quantum efficiency r}t of a film, reference is made to FIG. 8.
In this embodiment, the material of interest is deposited directly at a Si-photodiode 70 surface as a film 74. The photodiode includes a p layer 71 and an n layer 72. In this case, the value of J/J can be estimated from simple equations and the measurement of the magnitude of current flowing through photodiode 70 via a current measuring device 76:
where J9 is the photodiode photocurrent (in electrons/sec.) measured by current measuring device 76, 1^ is the intensity of the UV excitation radiation 78 (in photons/sec.), and £„, (in electrons/photon) is the photodiode quantum efficiency at a wavelength corresponding to the peak of film 74 photo-luminescence .
Also, D.Z Garbuzov' et al., "Organic film deposited on Si p-n junctions: Accurate measurements of florescence internal efficiency, and application to luminescent antireflection coatings", Journal of Applied Phvsics. Vol. 80, No. 8, October 15, 1996, pages 4644-4648, shows and describes the results obtained by the present inventors for testing materials other than Alq3. More specifically, through use of the present invention, coatings of TPD {N,N'-diphenyl-N, N'-bis-(3-methylphenyl)-l, l'-biphenyl-4, 4' diamine}, and of GaqJCl {bis-(8-hydroxyquinaldine)-chlorogallinm} were tested, resulting in the measurement of internal fluorescence efficiency rj, of 0.35 + 0.3, and 0.36 ± 0.3, respectively.

■
Other efficient chromophores have also been identified by the inventors for use as high efficiency f lorescent anti-reflection (AR) coatings. These materials include poly¬pheny lenevinylene, which can be prepared as a very uniform and flat film via wet chemical growth; and distyriarelene derivatives, which are vacuum depositable (see C. Hosokawa, et al. , Appl, Phvs. Lett.. Vol. 67, pages 3853-3855 (1195)). Another material is Al (acetylacetonate)3[Al(acac)3], a compound that absorbs in the ultraviolet spectral region, and emits blue or purple spectral region -

WE CLAIM :
1. An improved light detector comprising a light detector selected from the group consisting of photocells, photodiodes and solar cells; and an organic luminescent coating on said light detector capable of converting UV to visible light at high quantum efficiencies as light absorbs in passing through said coating into said light detector.
2. A light detector comprising a light detector; and an organic luminescent coating capable of converting UV to visible light in the range from blue-green to red at high quantum efficiencies as light absorbs in passing through said coating into said light detector.
3. The light detector as claimed in claim 2, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq2Cl, TPD, polyphenylenevinylene, and distyriarelene derivatives.
4. The light detector as claimed in claim 2, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells.
5. A light detector comprising a light detector; and an organic luminescent coating capable of converting UV to green light at high quantum efficiencies as light passes through said coating into said light detector.
6. The light detector as claimed in claim 5, wherein said coating is selected from the group consisting of Aluminum-Tris-Quinolate, Gaq2Cl, TPD, polyphenylenevinylene, and distyriarelene derivatives.

7. The light detector as claimed in claim 5, wherein the light detector is selected from the group consisting of photocells, photodiodes and solar cells.
8. The light detector as claimed in claim 7, wherein the light detector is made from silicon.
9. The light detector as claimed in claim 7, wherein the light detector is made from gallium arsenide.
10. The light detector as claimed in claim 5, wherein the light detector has a conventional anti-reflection coating.
11. The light detector as claimed in claim 6, wherein the Aluminum-Tris-Quinolate coating is from 100A to 1.35 urn in thickness.
12. The light detector as claimed in claim 6, wherein the AIuminum-Tris-Quinolate coating is from 300A to 400A in thickness.
13. An apparatus for measuring film spectral properties, comprising means for providing monochromatic light; means for chopping the monochromatic light; means for focusing the monochromatic light onto a sample, the sample being a substrate with a film to be analyzed on its surface; coupling the film side of the sample to a first light detector with immersion oil; means for focusing light emitted by the sample onto a second light detector; and means for analyzing the signals from the first and second light detectors to provide the desired film spectral properties.

14. The apparatus as claimed in claim 13, wherein a UV bandpass filter provided between the monochromatic light and the sample.
15. The apparatus as claimed in claim 13, wherein a bandpass filter is provided between the sample and the second light detector.
16. The apparatus as claimed in claim 15, wherein the bandpass filter has light transmission between about 250 and 420 nm.
17. The apparatus as claimed in claim 13, comprises a bandpass filter above the sample.
18. The apparatus as claimed in claim 17, wherein the bandpass filter has peak transmission at about 370 nm.
19. The apparatus as claimed in claim 13, wherein the means for providing a monochromatic light comprises a Xenon lamp and a monochromator.
20. The apparatus as claimed in claim 13, wherein the means for focusing the light onto the sample is a lens.
21. The apparatus as claimed in claim 20, comprises a mirror.
22. The apparatus as claimed in claim 13, wherein the substrate is transparent.

23. The apparatus as claimed in claim 13, wherein the substrate is selected
from the group consisting of glass and sapphire.
24. The apparatus as claimed in claim 13, wherein the means for analyzing
the signals from the first and second light detectors consists of a current
and lock-in amplifier.
25. The apparatus as claimed in claim 13, wherein said first and second light
detectors are selected from the group consisting of silicon solar cells,
silicon photodetectors and UV enhanced photodiodes.
26. An improved light detector substantially as herein described with
reference to the accompanying drawings.

Documents:

020-mas-1997 abstract-granded.pdf

020-mas-1997 claims-granded.pdf

020-mas-1997 description (complete)-granded.pdf

020-mas-1997 drawings-granded.pdf

20-mas-1997 abstract.pdf

20-mas-1997 assignmnt.pdf

20-mas-1997 claims duplicate.pdf

20-mas-1997 claims.pdf

20-mas-1997 correspondence others.pdf

20-mas-1997 correspondence po.pdf

20-mas-1997 description (complete) duplicate.pdf

20-mas-1997 description (complete).pdf

20-mas-1997 drawings duplicate.pdf

20-mas-1997 drawings.pdf

20-mas-1997 form-2.pdf

20-mas-1997 form-4.pdf

20-mas-1997 form-6.pdf

20-mas-1997 petition.pdf

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Patent Number

199048

Indian Patent Application Number

20/MAS/1997

PG Journal Number

23/2006

Publication Date

09-Jun-2006

Grant Date

03-Mar-2006

Date of Filing

06-Jan-1997

Name of Patentee

THE TRUSTEES OF PRINCETON UNIVERSITY

Applicant Address

NEW SOUTH BUILDING, 5TH FLOOR, POST BOX 36 PRINCETON, NEW JERSEY 08544,

Inventors:

#	Inventor's Name	Inventor's Address
1	STEPHEN R FORREST	148 HUNT DRIVE, PRINCETON NEW JERSEY
2	DMITRI Z, GARBUZOV	7-T HIBBEN APTS., FACULTY ROAD, PRINCETON, NEW JERSEY
3	VLADMIR BULOVIC	41-B, MIDDLESEX AVRENUE, METUCHEN, NEW JERSEY
4	PAUL E. BURROWS	262 EAST STANWORTH DRIVE, PRINCETON, NEW JERSEY

PCT International Classification Number

G01J1/48

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	60/010,013	1996-01-11	U.S.A.