|Title of Invention
"A PORTABLE EQUIPMENT FOR QUICK MEASUREMENT OF DIFFUSED REFLECTANCE OF TEXTURED SILICON WAFERS FOR SOLAR CELLS"
|This invention relates to a portable equipment and method for quick measurement of diffused reflectance of textured silicon wafers for solar cell comprising: a 12 volt power supply (1) to power a halogen lamp (2) which produces a collimated light beam through a lens (2) to fall on a textured silicon wafer (3), a solar detector cell (4) to generate short circuit current indicated in a digital mA meter (5) which measure the intensity of the reflected light being directly proportional to the said short circuit current.
FIELD OF THE INVENTION
The present invention relates to a portable non-destructive equipment for quick measurement of average diffused reflectance of large area textured surface of silicon wafers used for solar cells manufacturing industry.
The novel equipment can be used in photovoltaic solar cell manufacturing industry to measure the relative diffused reflectance of the textured silicon wafers for sorting good and bad quality of texturisation. As the testing method in solar cell manufacturing industry is to be very quick taking less than 15 seconds per wafer, it can be used in a solar cell industry to test the quality of texturisation from batch to batch. As the testing method is non-destructive, wafers of sizes from 5 to 6 sq. inches can be tested with great ease.
BACKGROUND OF THE INVENTION
Majority of solar cell manufacturers use either mono crystalline or multi crystalline Si wafers as basic input material for the production of solar cells. Typically, the as cut mono or multi crystalline silicon wafers received from wafer manufacturers reflect about 30-40 % of incident light. The amount of light lost due to this reflection from the silicon surface is a big loss and affects the solar cell efficiency. Hence, in solar cell manufacturing process, various surface preparation procedures are used to reduce the reflection to a minimum. In this process step, the surface of silicon wafer is treated with chemicals or etched with reactive ions to make the surface rough / textured so that the diffused reflection is reduced to a minimum leading to maximum light absorption. Typically, diffused reflectance from the textured Si wafer is in the range: 10-15 %. To maintain a very high yield of solar cells, it is required to maintain a consistent texturisation process. For this purpose the quality of texturisation needs to be monitored on a regular basis on the shop floor. In order to ascertain the quality of texturisation, a simple non-destructive tool is required for the operator on the shop floor to measure the diffused reflectance of textured wafers.
Nearest prior art references of non-destructive measurements of solar cell wafers are found as follows.
US Patent 4, 563, 642 has disclosed an apparatus for non-destructively measuring characteristics of a semi conductor wafer with a junction comprising means for radiating a frequency chargeable pulsated photo beam to the wafer, means for taking out the resulting photovoltage by capacitance complying and means for calculating the carrier lifetime and cutoff frequency from the chopping frequency dependence characteristics of its photovoltage.
EP 20030808673 20031009 (Publication No. EP 1556900) has disclosed a nondestructive analysis method for determining the quality of a solar cell based on Raman spectroscopy by which significant structural parameters (FWHM) relating to the structural properties of a processed absorber layer is determined in a solar cell by establishing a correlation between structural parameters with electric parameters (Voc) which are relevant in terms of the expected electrical properties of the finished processed solar cell.
None of the said prior documents are based on measuring diffused reflectance of silicon wafer to which the present invention is aimed for and at the same time those process equipments are also not cost effective.
Standard laboratory equipment, like spectrophotometer fitted with an integrating sphere, is available to measure the diffused reflectance. This method is very accurate but has the following disadvantages:
• Destructive in nature (125 mm x 125 mm wafer has to be cut to small
• Time consuming, as it has to measure reflectance at all wavelengths in
the visible region. Approximately time taken to run one sample is 10-15
• It is not portable.
• Needs to be operated in air-conditioned and clean atmosphere.
• Equipment is very expensive (More than Rs. 30-35 lakhs).
To circumvent some of the above disadvantages, a non-destructive and non-contact technique for diffused reflectance measurement on large area crystalline silicon wafers / solid surface has been invented.
DESCRIPTION OF THE INVENTION
One object of the invention is to develop a portable, non-destructive, non-contact equipment and method for measuring average diffused reflectance of a textured crystalline silicon wafer / solid surface.
Another objective of the invention is to provide a simple cost effective and time saving equipment for measuring diffused reflectance of textured silicon wafer.
In this equipment, solar cells are used as the detector to detect light reflected from the surface of the textured silicon wafer. The electrical signal is detected in the form of short circuit current and displayed on an ampere meter. As the current generated by the detector cells is directly proportional to the reflectivity of the silicon wafer, the quality of texturisation can be compared by comparing
the value of current on the ampere meter reading. Less the value of the current, less is the reflected light from the wafer and better is the quality of texturisation of the wafer.
A photovoltaic solar cell generates electricity by absorbing solar radiation / light and also can act as a photo detector. The short circuit current generated by the cell is directly proportional to the light intensity falling on it. The response of the solar cell is good even in very low light intensity and it generates photocurrent. In the present invention solar cells are used as photo detectors. A halogen lamp has been selected as the light source as the spectrum of the halogen lamp matches well with the solar spectrum. The photocurrent generated by the detector cells is very sensitive to the intensity of light from the source. Therefore a regulated supply has been used to power the lamp to keep the intensity of light constant to enhance the measurement accuracy. All the control electronics and the optics have been housed in a suitable box and care has been taken to make the box light proof to reduce stray light interference with the actual signal.
In the proposed equipment a collimated light beam from the halogen lamp is allowed to fall on the textured silicon wafer mounted on a sample holder and the intensity of the reflected light from silicon wafer is measured in terms of short circuit current generated by the detector cells. As the light is reflected uniformly, over a sufficiently large area, from the textured wafer, the current measured by the detector cells is observed as directly proportional to the reflectance of the textured surface. Hence the current generated by the detector can be used as the qualification test for the quality of texturisation.
The proposed invention will be better understood from the following description with reference to the accompanying drawings in which
Figure 1 represents a schematic of the diffused reflectance measurement equipment.
A 12 volt regulated power supply (1) is connected to supply line (S). The 12 V power supply (1) is used to power the Halogen Lamp (2). A collimated light beam through a lens (L) from the halogen lamp falls on a mounted textured silicon wafer sample (3), the intensity of the reflected light from the wafer is
measured in terms of short circuit current in mA by a Digital mA Meter (5). The short circuit current is generated by the solar cells used as detectors (4). From the measured current the reflectance is measured easily as the reflectance is directly proportional to the measured current produced by the detector cells (4) and the quality of texturisation of the silicon wafer is sorted out by this simple equipment and procedure.
The present invention is illustrated with the following test datas and example. Table 1: Detector Current ID of Different types of wafers
Wafer No. As cut wafer (Av. reflectance ~ 35 %) (ID in milli Amp) Textured wafer (Av. reflectance ~ 15-16 %) (ID in milli Amp) ARC* coated wafer (Av. reflectance ~ 5-6 %) (ID in milli Amp)
1 1.60 0.60 0.17
2 1.69 0.65 0.18
3 1.67 0.62 0.16
4 1.66 0.68 0.17
5 1.69 0.68 0.18
Anti Reflection Coating
Table 1 clearly indicates that the current generated by the detector cells is directly proportional to the reflectance of the silicon wafer. Lower the ID better will be solar cell efficiency.
10 numbers of samples from a mixed batch of good and bad texturisation were studied using this set-up. The comparison of diffused reflectance in terms of detector current is given in the table below:
ID (ref.) = 0.7 mA (Reference wafer) corresponding to average diffused reflectance = 15-16 %
Table 2: Acceptance criteria of Texturisation
Wafer No. ID (Sample) (mA) ID (ref.) - ID (Sample) Rejected * /Accepted
1 0.50 0.2 Accepted
2 0.86 -0.16 Rejected
3 0.56 0.14 Accepted
4 0.65 0.05 Accepted
5 0.55 0.15 Accepted
6 0.89 -0.19 Rejected
7 0.54 0.16 Accepted
8 0.96 -0.26 Rejected
9 1.16 -0.46 Rejected
10 0.53 0.17 Accepted
The texturisation will be accepted if ID (ref.) -ID (sample) > 0, and will be rejected if ID (ref.) -ID (sample) Table 2 shows that the samples with different quality of texturisation can be segregated. It is obvious that wafers with bad quality of texturisation give higher value of ID. Thus wafers have been compared with a reference sample (ID (ref.) = 0.7 mA), as determined by the user. The rejected silicon wafers need not go for next process steps and can be retextured to achieve proper quality of texturisation. By using this non-destructive testing method, it is possible to reduce the wastage of costly silicon wafers and save the add-on cost of further processes resulting in huge savings.
The invention as described and illustrated herein should not be read and construed in a restrictive manner as various adaptations, alterations and modifications are possible within the scope and limit of the defined claims as appended.
1. A portable equipment for quick measurement of diffused reflectance of
textured silicon wafers used for solar cell production comprising:
- a 12 volt power supply (1) to supply power to a halogen lamp (2) which produce a collimated light beam through a lens (2) to fall on a textured silicon wafer (3), a solar detector cell (4) generates short circuit photo current which is displayed in a digital mA meter (5) which measures the intensity of the reflected light being directly proportional to the said short circuit current.
2. The portable equipment as claimed in claim 1, wherein the photovoltaic
solar cell (4) generates electricity by absorbing solar radiation and also
act as a photodetector of the reflected light from the sample surface.
3. The portable equipment as claimed in claim 1, wherein the 12 volt power
supply is a regulated power supply to enable accurate measurement of
diffused reflectance of textured silicon wafer by ensuring a constant light
intensity independent of any mains power supply voltage fluctuations.
4. The portable equipment as claimed in claim 1, wherein samples of the
silicon wafers coated with anti reflection coating (ARC) of silicon nitride
with low reflectance (5-6 %) also can be compared and segregated.
5. The portable equipment as claimed in the preceding claims wherein the
equipment enables to compare the quality of texturisation of silicon wafer
size of 125 x 125 mm giving result within 15 seconds non-destructively.
6. The equipment as claimed in the preceding claims wherein comparison of
diffused reflectance of textured silicon wafer is carried out in terms of
reference detector current of wafer ID (ref) = 0.7 mA corresponding to
average diffused reflectance = 15-16 %.
7. The equipment as claimed in the preceding claims wherein textured
silicon wafer is accepted when ID (ref) - ID (sample) is >0 and is rejected
when ID (ref) - ID (sample) is
8. The equipment as claimed in claim 6, wherein the rejection of silicon
wafers are avoided by not carrying through rest of process steps and can
be retextured to achieve proper quality of texturisation, thus saving
wastage of costly silicon wafers and add on cost from further processing.
9. A method of measuring diffused reflectance of textured silicon wafers for
solar cell by the equipment according to claim 1 comprising the steps of
supplying a 12 volt regulated power to a halogen lamp to produce a
collimated light beam through a lens to fall on a textured silicon wafer to
generate short circuit photo current, displayed in a digital mA meter and
measuring the intensity of the reflected light being directly proportional
to the said short circuit current.
This invention relates to a portable equipment and method for quick measurement of diffused reflectance of textured silicon wafers for solar cell comprising: a 12 volt power supply (1) to power a halogen lamp (2) which produces a collimated light beam through a lens (2) to fall on a textured silicon wafer (3), a solar detector cell (4) to generate short circuit current indicated in a digital mA meter (5) which measure the intensity of the reflected light being directly proportional to the said short circuit current.
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|BHARAT HEAVY ELECTRICALS LIMITED,
|REGIONAL OFFICE: REGIONAL OPERATIONS DIVISION(ROD), PLOT NO:9/1, DJ BLOCK, 3RD FLOOR, KARUNAMOYEE, SALTLAKE CITY, KOLKATA-700091 REGISTERED OFFICE: BHEL HOUSE,SIRI FORT,NEW DELHI-110049,INDIA,
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date