Title of Invention	A PROCESS FOR THE PREPARATION OF INTRINSICALLY CONDUCTING PRE-VULCANIZED NATURAL RUBBER LATEX FILMS FOR ELECTRONIC AND OPTOELECTRONIC APPLICATIONS
Abstract	There is provided a process for producing natural rubber based intrinsically conducting poly^ier from prevulcanized rubber latex, by solution doping reaction wherein the doping utilizes the doing agent Iodine and a medium of cyclohexane or toluene for solution doping and a sterric stabilizer prevents the latex from coagulation. The flexible thin films produced by the method of invention are useful for preparing polymer PV device.

Title of Invention

A PROCESS FOR THE PREPARATION OF INTRINSICALLY CONDUCTING PRE-VULCANIZED NATURAL RUBBER LATEX FILMS FOR ELECTRONIC AND OPTOELECTRONIC APPLICATIONS

Abstract

There is provided a process for producing natural rubber based intrinsically conducting poly^ier from prevulcanized rubber latex, by solution doping reaction wherein the doping utilizes the doing agent Iodine and a medium of cyclohexane or toluene for solution doping and a sterric stabilizer prevents the latex from coagulation. The flexible thin films produced by the method of invention are useful for preparing polymer PV device.

Full Text	BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a process for preparing intrinsically conducting 1, 4- polyisoprene (natural rubber) films from pre-vulcanised latex and fabrication of a photovoltaic device using the same. [0003] 2, Description of the Prior Art [0004] Electrically conducting rubbers from fresh latex of natural rubber as well as unvulcanised natural rubber are generally prepared by conducting a doping reaction using different doping agents such as iodine & and SbCb using the two doping methods, viz gas phase doping and solution doping. [0005] Amongst the conventional processes for doping with iodine utilize the technique of adding specific amounts of iodine into a solution of fresh latex or into a solution of uncrosslinked NR solution in toluene, cyclohexane or the like. After the completion of the reactions, end solution is evaporated on suitable substrates to get conducting films. For gas phase reaction fresh NR latex or an uncrosslinked dry rubber is first made into films by evaporating the solvent followed by vacuum drying. In the second step the resulting films are kept in an evacuated chamber containing iodine vapour for specific time periods. However all such known in the art about the development of intrinsic electric conductivity in the 1, 4-polyisoprene are about unvulcanised rubber. A major disadvantage of fresh latex is its lack of enough mechanical strength and this limits its potential especially in thin film form. Another disadvantage of applying the known methods of preparing unvulcanised conducting film to pre-vulcanised latex is that on adding iodine solution in toluene for doping, the pre-vulcanised latex gets coagulated and the product will be lumps of swelled rubber pieces, which makes film preparation process impossible. Hence, with conventional solution process for producing electrically conducting NR films, it has proved difficult to obtain a NR based conducting film having good mechanical characteristics with high electrical condjuctivity. [0006] It is known that the photovoltaic materials based on organic semiconductors differ from, inorganic semiconductors in various aspects such as photo generated excitations (excitons) are strongly bound and do not spontaneously dissociate into separate charges; charge transport proceeds by hopping between localized states, rather than transport within a band. [0007] Recently it has been reported that in polymeric planar heterojunction or bilayer device the p-type conjugated polymer (donor) and n type conjugated polymer (acceptor) interface separates excitons much more efficiently than the organic metal interface in a single layer device. [0008] Polymer PVs are advantageous for the formation of a film having large area and reduction in cost since an organic layer can be easily formed by coating using a polymeric electro luminescent substance. [0009] Conventionally MEH-PPV is the most used semi conducting polymer for PV devices since they are soluble in common solvents and can be processed from solution at room temperature to uniform larger area, however the synthesis procedure is rather found to be tedious and not cost effective. [0010] The object of the present invention is to develop iodine doped pre-vulcanized natuj'al rubber as a novel p- type conjugated polymer and use it to fabricate polymer photovoltaic cell (Py) with n-type conjugated polymer. SUMMARY OF T^E UNVf NTIpN [0011] The objept of the present invention is to develop mechanically stronger and intrinsically conducting flexible films of cis 1,4-polyisoprene by solution processing and fabricate a photovoltaic (PV) device using the same. [0012] According to this invention there is provided a process in which, the fresh latex taken in a rotating vessel system is first prevulcanized by gamma (^^C, 7.38 x 10^"^ Bq) radiation source at a rotating speed of lO-rev min'^ at a temperature of 25^^C. The dose rate is 0.565-kpy h' (total dose = 12). 2-ethy]hexylacrylate and CCU are used as the sensitizers. The pre-vulpanized latex having various DRC of 50, 53.8 and 55 % are stabilized by adding sterric stabilizers such as starch or Do-decylbenzene sulfonic acid (DBSA) ranging frpm 2 % weight/weight (w/w) to 15 % weight/weight (w/w) of latex that sterically prevents the latex particles from coagulation followed by the addition of iodine solution in toluene or cyclqhexane at two different [C= C]/[l2] molar ratios of 8 and 12 at different reaction temperatures of 20^ C or 50^ C under nitrogen atmosphere. Thin film of the 4oped solution are casted on the ITO coated glass substrate by spin coating and dried under vacuum. A thin film of C 6o is spin coated on it. A metallic electrode of Indiurq (Ip) or Aluminium (Al) is then deposited on the top of C6o film through vacuum eyapofation technique. DETAILED DESCRIPTION OF THE PREFERI^ED EM^pIjJ^ENTS [0013] As follows is ^ more detailed description of the present invention [0014] In this descriptipn, the term "pre-vulcanization" refers to the partial cross linking of the latex prior to any chemical treatment and this is achieved by exposing the fresh latex to gamma irradiation under the conditions described above. This latex can ensure eno^gh mechanical strength to the fabricated latex fi^ms. [0015] A number of methods such as sulfur vujcanizatjon, pej-pxide vulcanization, chemical grafting, gajttima-irradiation etc are well known for cfiefpical modification of NR latex. In the pif^Sgnt invention the process of gamma irra^i^tjon is selected for pre- vulcanization since it 4oes not irjvolve any chemical ingredients, which may adyprsely affect the doping re^ption. [0016] Sterric S^iU?:ers [0017] The sterric stabilisers starch or DBSA is used to stabihze the latex from coagulation on addition of any foreign materials to the latex solution and here iodine-toluene or iodine - cyclohexane solution are the foreign materials. The adsorption of the stabiliser molecule to the rubber moieties is in fact the probable mechanism for the stabilization of the latex. [0018] In the present invention the doping reaction was carried out for each different quantity of stabilizer (between 2-15 % weight/weight of latex) [0019] Starch and DBSA are particularly selected as stabilizer for the reason that the stabilizer should be capable of providing latex the required stability and also it should not cause any coloration for the latex. [0020] There are no restrictions on the methods used to add the stabilizer, and typically used methods include addition of the stabilizer directly while stirring or adding the stabilizer as a water based solution. [0021] In the present invention adding sterric stabilizer enables the initial stabilization of the latex and the electrical stability of the product polymer. The stabilizer has a retarding function in the doping rate and the decreased doping rate enables the effective control of the electrical conductivity of the product polymer. [0022] Methods for measuring the molar ratio of C=C double bonds of Polyisoprene in the reaction medium are well known. The molar ratio is calculated using the basic equation [€<: weight of latex taken in g x> 68 [0023] With the mixture undergoing constant stirring, a pre-determined quantity of a predetermined doping agent (iodine at 0.00623 g/ml) in a pre-determined solvent (toluene or cyclohexane) was added and the doping reaction was allowed to proceed. The molar ratio of iodine is calculated from the equation [I2] == weight of iodine taken in ^ml x 1000 258 [0024] Here the [C=C] /[I2] is calculated to be 8. After 24 h, part of the solution was casted on glass substrates and dried under dynamic vacuum (10"^ torr). Rest of the doped solution was coated at subsequent time intervals of 24 h each up to 240 h. [0025] Using the same method, the doping reaction was carried out for different concentrations of iodine ( 0.0046 g/ml) [0026] The decrease in iodine concentration results in an increase in the ratio [C=C] /[I2] to 12 resuhing in lower doping rate as inferred from the color changes. [0027] There are no particular restrictions on the doping time of the system and typical time values are from 24 h to 240 h. [002 §] Polymerization Conditions , [0029] In a production process of the present invention, the reaction temperature is varied in the range of 20" 50 V. [0030] Other Conditions [0031] In a production process of the present invention, the reaction was carried out with latex of ditferent dry rubber content (DRC) of 50, 53.8 and 55%. [0032] Next, the polymer fabrication of PV cell of the present invention v^ll be illustrated. The polymer PV of the present invention is a polymer PV comprising of a pair of electrodes composed of an anode and a cathode and at least one of which is transparent or semitransparent with the p-type and n-type polymers sandwiched between the electrodes. [0033] As the film is formed from a solution, coating methods such as a spin coating method, solution casting method and dip coating method can be used. [OO34] The method for forming the electron-transporting layer is not particularly restricted, and in this case, a method of film-forming from a solution is employed. [0035] The thickness of the electron-transporting layer is, for example, from 1 nm to 1 )i.m, preferably from 2 nm to 500 nm, further preferably from 5 nm to 200 nm. [0036] The substrate forming the polymer PV of the present invention may preferably be that does not change in forming an electrode and layers of organic materials, and there are exemplified glass, plastics, polymer film, silicon substrates and the like. In the present invention, it is preferable that an anode is transparent or semitransparent, and as the materia of this anode, electron conductive metal oxide films, semitransparent metal thin films and the like are used. Specifically, there are used indium oxide, zinc oxide, tin oxide, and films (NESA and the like) fabricated by using an electron conductive glass composed of indium tin oxide (ITO), indium' zinc oxide and the like, which are metal oxide complexes, and gold, platinum, silver, copper and the like are used, and among them, ITO, indium- zinc -oxide, tin oxide are preferable. [0037] The thickness of the anode can be appropriately selected while considering transmission of a light and electrical conductivity, and for example, from 10 nm to 10 \|i.m, preferably from 20 nm to 1 \|a.m, further preferably from 50 nm to 500 nm. [0038] As the material of a cathode used in the polymer PV of the present invention, that having lower work ftmction is preferable. For example, there are used metals such as lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium, aluminum, scandium, vanadium, zinc, yttrium, indium, cerium, samarium, europium, terbium, ytterbium and the like, or alloys comprising two of more of them, or alloys comprising one or more of them with one or more of gold, silver, platinum, copper, manganese, titanium, cobalt, nickel, tungsten and tin, graphite or grapl^ite intercalation compounds and the like. Examples of alloys include a magnesium-silver alloy, magnesium-indium alloy, magnesium-aluminum alloy, indium-silver alloy, lithium-aluminum alloy, lithium-magnesium alloy, lithium-indium alloy, calcium-aluminum alloy and the like. The cathode may be formed into a laminated structure of two or more layers. The thickness of the cathode can be appropriately selected while considering transmission of a light and electric conductivity, and for example, from 10 nm to 10 ji.m, preferably from 20 nm to 1 ^i.m, further preferably from 50 nm to 500 nm. [0039] As the method for fabricating a cathode, there are used a vacuum vapor deposition methpd is used. EXAMPLES [0040] The following examples further illustrate the present invention in detail. [0041] [0042] : 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2% w/w) of starch. The pre-vulcanized latex solution remains as a colloidal solution. Then the h dopirjg reaction is carried out by adding iodine- cyclohexane solution (0,00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction can be inferred by the colour changes from purple to dark brown and from conductivity measurements. Thin films spin coated on glass substrates at diffe^'ent time intervals of 24, 48, 72, 9'6 and 240h and dried under dynamic vacuum are used for conductivity studies. The conductivity was found to increase from giga ohms to kilo ohms with the increase in doping time. [0043] [0044]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.0125 g (5% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. Flexible conducting films were obtained and the resistivity values were comparable to films obtained for the above example. [0045]: 0.025 g(10 %w/w) of starch is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The colour changes and the conductivity values were comparable to the above reaction. [0046]: 0.036 g (15% w/w) of starch is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The doping reaction was continued for about 240 h and films were spin coated on glass substrates at definite intervals of 24, 48, 72, 96 & 240h. Tke colour of th? solution was found to change from purple - yellow - dark brovm with the increase in the doping time. [0047]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2 % w/w) of DBSA. The latex solution remains as a colloidal solution. Then the I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction can be inferred by the colour change from purple- dark brown and from conductivity measurements. Thin films spin coated on glass substrates at different time interyals of 24, 48, 72, 96 & 240h and dried under dynamic vacuum are used for conductivity studies. [0048]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.0125 g (5% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. Flexible conducting films were obtained and the resistivity values and colour changes were comparable to films obtained for the above example. [0049] ; 0.025 g (10% w/w) of DBSA is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the alpove examples. The doping time was varied and films were spin coated at definite interyals of 24, 48, 72, 96 & 240h. The conductivity was measured and the results were same as for [0045]. [0050] : 0.036 g (15% w/w) of DBSA is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The results were compared with [0046]. [0051]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2% w/w) of starch. The pre-vulcanized latex solution remains as a colloidal solution. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction can be inferred by the colour changes from purple to dark brown and from conductivity measurements. Thin films are spin coated on glass substrates at different time intervals of 24, 48, 72, 96 & 240h and dried under dynamic vacuum are used for conductivity studies. The conductivity was found to increase from giga phms to kilo ohms with the increase in doping time. However the rate of doping was less compared with samples of same doping periods from [0042]. [0052]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.036 g (15% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- toluene solution (0.0046 g/ml) at a [C=C] /[h] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. [0053]: 0.005 g (2% w/w) of DBSA is added to 0.25 g (DRC - 53.8%) of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The results were compared with [0047]. [0054] : 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.036 g (15% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[l2i molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. Comparable results to that of [0052] are obtained. [0055]: 0.25 g of latex (DRC = 50 %) is stabilized by adding 0.036 g (15% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. The results were compared with [0052]. [0056]: 0.25 g of latex (DRC = 55 %) is stabilized by adding 0.036 g (15% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[h] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic coloqr changes from purple to dark brown. Sartie results as that of the above example. [0057]: 0.025 g (10% w/w) of starch is added to 0.25 g (DRC = 53.8%) of latex. The I. dopii^g reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere at a higher temperature of 50*^ C. The procedure is same as for the above examples. The doping time was varied and films were spin coated at definite intervals of 24, 48, 72, 96 & 240h hrs and Qonductivity was measured and the results were compared with samples at room temperature. The iodine induced conjugation of 1,4-polyisoprene was enhanced by heating the polymer/iodine solutioti to 50 C. [0058]: 0.025 g (10% w/w) of DBSA is added to 0.25 g (DRC -^ 53.8%) of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere at a higher temperature of 50^ C. The procedure is same as for the above examples. The doping time was varied and films were spin coated at definite intervals of 24, 48, 72, 96 & 240h and conductivity was measured. The results were compared with [0049] and [0057]. [0059]: 0.036 g (15% w/w) of DBSA is added to 0.25 g (DRC - 55%) of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0623 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere at a temperature of 50^ C. The procedure is same as for the above examples. [0060] [0061]: The conjugation reaction in solution is carried by dissolving a 0.25 g (DRC = 53.8%) of latex stabilized by 0.005 g (2% w/w) of DBSA dissolved in cyclohexane (0.015 g/ml) to iodine solution at a definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature. The reaction is characterized by the respective color changes with increase in doping time. Thin film of the doped solution is casted on the ITO poated glass substrate by spin coating and are dried under vacuum. A thin film of C 60 is spin coated on it. The metallic electrode Indium (In) is then deposited on the top of C(,o f\|lm through vacuum evaporation technique. The active area of the device is about 0.5 cm . The electrical conductivity measurements and the J-V characteristics are measured using Keithley electrometer model 6514 and a stabilized power supply. [0062] [0063] :The same procedure is repeated by using Al electrode. The solution is prepared using 0.25 g (DRC = 53.8 %) of latex stabilized by 0.005 g (2% w/w) of DBSA dissolved in cyplohexane (0.015 g/ml) to iodine solution at a definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature. Thin film of the doped solution was casted on the ITO coated glass substrate by spin coating technique and were dried under vacuum. The metallic electrode Indium (Al) is then deposited on the top of C6o filni through vacuum evaporation technique. The results are comparable to [0061]. [0064] : The solution is prepared using 0.25 g (DRC = 53,8 %) of latex stabilized by 0.005 g (2% w/w) of starch dissolved in cyclohexane (0.015 g/ml) to iodine solution at a definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature. Thin film of the doped solution is casted on the ITO coated glass substrate by spin coatipg technique and are dried under vacuum. The metallic electrode Indium (In) is then deposited on the top of Ceo film through vacuum evaporation technique. We Claim 1. A process of producing intrinsically conducting 1,4 polyisoprene (natural rubber) films by solution doping process, from natural rubber latex prevulcanized by gamma irradiation, said process comprising prevulcnized cis 1,4 polyisoprene latex stabilized by adding sterric satabilizers, wherein the said sterric stabilizers being starch or Do- decylbenzene sulfonic acid (DBSA), doping the said stabilized latex by adding dopants, said dopant being iodine, said solvents for dopants being toluene or cyclohexane, stirring the solution under nitrogen atmosphere at a temperatures 20^C or 50^^C, keeping the solution occasionally stirred and spin coating the said solution on to glass substrate at time intervals of 24, to 240 hrs, and drying under dynamic vacuum of 10'^ Torr, to obtain thin films. 2. A process as claimed in claim 1 wherein the said step of iodine doping reaction consists of adding iodine - cyclohexane or iodine- toluene solution at a molar ratio, [C = C]/[l2]-8&12. 3. A process as claimed in claim 1 wherein the DRC of prevulcanized rubber latex is from 53 to 55 %. 4. A process as claimed in claim 1 wherein the steps of stabilization consists in adding to the prevulcanized latex, 2, 5, 10, and 15 weight % of starch or 2, 5, 10 and 15 weight % of DBSA. 5. A process as claimed in claim 1 wherein the step of doping reaction consists in stirring the reaction mixture of stabilized, prevulcanized 1,4 polyisoprene latex at temperatures 20^C or 50^C for a period of 24 to 240 hours.. 6. A process for the fabrication of a polymer photovoltaic device, wherein a thin film of intrinsically conducting pre-vulcanised natural rubber latex solution is spin coated on to an Indium Tin Oxide (ITO) coated glass substrate, followed by spin coating a thin film of Ceo on the top of it and finally a third layer of Aluminium (Al) or Indium (In) deposited above it by vacuum evaporation method.

Full Text

BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for preparing intrinsically conducting 1, 4- polyisoprene (natural rubber) films from pre-vulcanised latex and fabrication of a photovoltaic device using the same.
[0003] 2, Description of the Prior Art
[0004] Electrically conducting rubbers from fresh latex of natural rubber as well as unvulcanised natural rubber are generally prepared by conducting a doping reaction using different doping agents such as iodine & and SbCb using the two doping methods, viz gas phase doping and solution doping.
[0005] Amongst the conventional processes for doping with iodine utilize the technique of adding specific amounts of iodine into a solution of fresh latex or into a solution of uncrosslinked NR solution in toluene, cyclohexane or the like. After the completion of the reactions, end solution is evaporated on suitable substrates to get conducting films. For gas phase reaction fresh NR latex or an uncrosslinked dry rubber is first made into films by evaporating the solvent followed by vacuum drying. In the second step the resulting films are kept in an evacuated chamber containing iodine vapour for specific time periods. However all such known in the art about the development of intrinsic electric conductivity in the 1, 4-polyisoprene are about unvulcanised rubber. A major disadvantage of fresh latex is its lack of enough mechanical strength and this limits its potential especially in thin film form. Another disadvantage of applying the known methods of preparing unvulcanised conducting film to pre-vulcanised latex is that on adding iodine solution in toluene for doping, the pre-vulcanised latex gets coagulated and the product will be lumps of swelled rubber pieces, which makes film preparation process impossible. Hence, with conventional solution process for producing electrically conducting NR films, it has proved difficult to obtain a NR based conducting film having good mechanical characteristics with high electrical condjuctivity.

[0006] It is known that the photovoltaic materials based on organic semiconductors differ
from, inorganic semiconductors in various aspects such as photo generated excitations
(excitons) are strongly bound and do not spontaneously dissociate into separate charges;
charge transport proceeds by hopping between localized states, rather than transport
within a band.
[0007] Recently it has been reported that in polymeric planar heterojunction or bilayer
device the p-type conjugated polymer (donor) and n type conjugated polymer (acceptor)
interface separates excitons much more efficiently than the organic metal interface in a
single layer device.
[0008] Polymer PVs are advantageous for the formation of a film having large area and
reduction in cost since an organic layer can be easily formed by coating using a
polymeric electro luminescent substance.
[0009] Conventionally MEH-PPV is the most used semi conducting polymer for PV
devices since they are soluble in common solvents and can be processed from solution at
room temperature to uniform larger area, however the synthesis procedure is rather found
to be tedious and not cost effective.
[0010] The object of the present invention is to develop iodine doped pre-vulcanized
natuj'al rubber as a novel p- type conjugated polymer and use it to fabricate polymer
photovoltaic cell (Py) with n-type conjugated polymer.

SUMMARY OF T^E UNVf NTIpN
[0011] The objept of the present invention is to develop mechanically stronger and intrinsically conducting flexible films of cis 1,4-polyisoprene by solution processing and fabricate a photovoltaic (PV) device using the same.
[0012] According to this invention there is provided a process in which, the fresh latex taken in a rotating vessel system is first prevulcanized by gamma (^^C, 7.38 x 10^"^ Bq) radiation source at a rotating speed of lO-rev min'^ at a temperature of 25^^C. The dose rate is 0.565-kpy h' (total dose = 12). 2-ethy]hexylacrylate and CCU are used as the sensitizers. The pre-vulpanized latex having various DRC of 50, 53.8 and 55 % are stabilized by adding sterric stabilizers such as starch or Do-decylbenzene sulfonic acid (DBSA) ranging frpm 2 % weight/weight (w/w) to 15 % weight/weight (w/w) of latex that sterically prevents the latex particles from coagulation followed by the addition of iodine solution in toluene or cyclqhexane at two different [C= C]/[l2] molar ratios of 8 and 12 at different reaction temperatures of 20^ C or 50^ C under nitrogen atmosphere. Thin film of the 4oped solution are casted on the ITO coated glass substrate by spin coating and dried under vacuum. A thin film of C 6o is spin coated on it. A metallic electrode of Indiurq (Ip) or Aluminium (Al) is then deposited on the top of C6o film through vacuum eyapofation technique.
DETAILED DESCRIPTION OF THE PREFERI^ED EM^pIjJ^ENTS
[0013] As follows is ^ more detailed description of the present invention
[0014] In this descriptipn, the term "pre-vulcanization" refers to the partial cross linking
of the latex prior to any chemical treatment and this is achieved by exposing the fresh
latex to gamma irradiation under the conditions described above. This latex can ensure
eno^gh mechanical strength to the fabricated latex fi^ms.
[0015] A number of methods such as sulfur vujcanizatjon, pej-pxide vulcanization,
chemical grafting, gajttima-irradiation etc are well known for cfiefpical modification of
NR latex. In the pif^Sgnt invention the process of gamma irra^i^tjon is selected for pre-
vulcanization since it 4oes not irjvolve any chemical ingredients, which may adyprsely
affect the doping re^ption.
[0016] Sterric S^iU?:ers

[0017] The sterric stabilisers starch or DBSA is used to stabihze the latex from coagulation on addition of any foreign materials to the latex solution and here iodine-toluene or iodine - cyclohexane solution are the foreign materials. The adsorption of the stabiliser molecule to the rubber moieties is in fact the probable mechanism for the stabilization of the latex.
[0018] In the present invention the doping reaction was carried out for each different quantity of stabilizer (between 2-15 % weight/weight of latex)
[0019] Starch and DBSA are particularly selected as stabilizer for the reason that the stabilizer should be capable of providing latex the required stability and also it should not cause any coloration for the latex.
[0020] There are no restrictions on the methods used to add the stabilizer, and typically used methods include addition of the stabilizer directly while stirring or adding the stabilizer as a water based solution.
[0021] In the present invention adding sterric stabilizer enables the initial stabilization of the latex and the electrical stability of the product polymer. The stabilizer has a retarding function in the doping rate and the decreased doping rate enables the effective control of the electrical conductivity of the product polymer.
[0022] Methods for measuring the molar ratio of C=C double bonds of Polyisoprene in the reaction medium are well known. The molar ratio is calculated using the basic equation [€<: weight of latex taken in g x> 68 [0023] With the mixture undergoing constant stirring, a pre-determined quantity of a predetermined doping agent (iodine at 0.00623 g/ml) in a pre-determined solvent (toluene or cyclohexane) was added and the doping reaction was allowed to proceed. The molar ratio of iodine is calculated from the equation [I2] == weight of iodine taken in ^ml x 1000
258 [0024] Here the [C=C] /[I2] is calculated to be 8. After 24 h, part of the solution was casted on glass substrates and dried under dynamic vacuum (10"^ torr). Rest of the doped solution was coated at subsequent time intervals of 24 h each up to 240 h.

[0025] Using the same method, the doping reaction was carried out for different concentrations of iodine ( 0.0046 g/ml)
[0026] The decrease in iodine concentration results in an increase in the ratio [C=C] /[I2] to 12 resuhing in lower doping rate as inferred from the color changes. [0027] There are no particular restrictions on the doping time of the system and typical time values are from 24 h to 240 h. [002 §] Polymerization Conditions , [0029] In a production process of the present invention, the reaction temperature is varied in the range of 20" 50 V. [0030] Other Conditions
[0031] In a production process of the present invention, the reaction was carried out with latex of ditferent dry rubber content (DRC) of 50, 53.8 and 55%.
[0032] Next, the polymer fabrication of PV cell of the present invention v^ll be illustrated. The polymer PV of the present invention is a polymer PV comprising of a pair of electrodes composed of an anode and a cathode and at least one of which is transparent or semitransparent with the p-type and n-type polymers sandwiched between the electrodes.
[0033] As the film is formed from a solution, coating methods such as a spin coating method, solution casting method and dip coating method can be used. [OO34] The method for forming the electron-transporting layer is not particularly restricted, and in this case, a method of film-forming from a solution is employed. [0035] The thickness of the electron-transporting layer is, for example, from 1 nm to 1 )i.m, preferably from 2 nm to 500 nm, further preferably from 5 nm to 200 nm. [0036] The substrate forming the polymer PV of the present invention may preferably be that does not change in forming an electrode and layers of organic materials, and there are exemplified glass, plastics, polymer film, silicon substrates and the like. In the present invention, it is preferable that an anode is transparent or semitransparent, and as the materia of this anode, electron conductive metal oxide films, semitransparent metal thin films and the like are used. Specifically, there are used indium oxide, zinc oxide, tin

oxide, and films (NESA and the like) fabricated by using an electron conductive glass composed of indium tin oxide (ITO), indium' zinc oxide and the like, which are metal oxide complexes, and gold, platinum, silver, copper and the like are used, and among them, ITO, indium- zinc -oxide, tin oxide are preferable.
[0037] The thickness of the anode can be appropriately selected while considering transmission of a light and electrical conductivity, and for example, from 10 nm to 10 |i.m, preferably from 20 nm to 1 |a.m, further preferably from 50 nm to 500 nm. [0038] As the material of a cathode used in the polymer PV of the present invention, that having lower work ftmction is preferable. For example, there are used metals such as lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium, aluminum, scandium, vanadium, zinc, yttrium, indium, cerium, samarium, europium, terbium, ytterbium and the like, or alloys comprising two of more of them, or alloys comprising one or more of them with one or more of gold, silver, platinum, copper, manganese, titanium, cobalt, nickel, tungsten and tin, graphite or grapl^ite intercalation compounds and the like. Examples of alloys include a magnesium-silver alloy, magnesium-indium alloy, magnesium-aluminum alloy, indium-silver alloy, lithium-aluminum alloy, lithium-magnesium alloy, lithium-indium alloy, calcium-aluminum alloy and the like. The cathode may be formed into a laminated structure of two or more layers. The thickness of the cathode can be appropriately selected while considering transmission of a light and electric conductivity, and for example, from 10 nm to 10 ji.m, preferably from 20 nm to 1 ^i.m, further preferably from 50 nm to 500 nm. [0039] As the method for fabricating a cathode, there are used a vacuum vapor deposition methpd is used.
EXAMPLES
[0040] The following examples further illustrate the present invention in detail. [0041]
[0042] : 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2% w/w) of starch. The pre-vulcanized latex solution remains as a colloidal solution. Then the h dopirjg reaction is carried out by adding iodine- cyclohexane solution (0,00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The

progress of the doping reaction can be inferred by the colour changes from purple to dark brown and from conductivity measurements. Thin films spin coated on glass substrates at diffe^'ent time intervals of 24, 48, 72, 9'6 and 240h and dried under dynamic vacuum are used for conductivity studies. The conductivity was found to increase from giga ohms to kilo ohms with the increase in doping time.
[0043]
[0044]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.0125 g (5% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. Flexible conducting films were obtained and the resistivity values were comparable to films obtained for the above example.
[0045]: 0.025 g(10 %w/w) of starch is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The colour changes and the conductivity values were comparable to the above reaction.
[0046]: 0.036 g (15% w/w) of starch is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The doping reaction was continued for about 240 h and films were spin coated on glass substrates at definite intervals of 24, 48, 72, 96 & 240h. Tke colour of th? solution was found to change from purple - yellow - dark brovm with the increase in the doping time.
[0047]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2 % w/w) of DBSA. The latex solution remains as a colloidal solution. Then the I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The progress of the doping

reaction can be inferred by the colour change from purple- dark brown and from conductivity measurements. Thin films spin coated on glass substrates at different time interyals of 24, 48, 72, 96 & 240h and dried under dynamic vacuum are used for conductivity studies.
[0048]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.0125 g (5% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. Flexible conducting films were obtained and the resistivity values and colour changes were comparable to films obtained for the above example.
[0049] ; 0.025 g (10% w/w) of DBSA is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the alpove examples. The doping time was varied and films were spin coated at definite interyals of 24, 48, 72, 96 & 240h. The conductivity was measured and the results were same as for [0045].
[0050] : 0.036 g (15% w/w) of DBSA is added to 0.25 g of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The results were compared with [0046].
[0051]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.005 g (2% w/w) of starch. The pre-vulcanized latex solution remains as a colloidal solution. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction can be inferred by the colour changes from purple to dark brown and from conductivity measurements. Thin films are spin coated on glass substrates at different time intervals of 24, 48, 72, 96 & 240h and dried under dynamic vacuum are used for conductivity studies. The conductivity was found to increase from

giga phms to kilo ohms with the increase in doping time. However the rate of doping was less compared with samples of same doping periods from [0042].
[0052]: 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.036 g (15% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- toluene solution (0.0046 g/ml) at a [C=C] /[h] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown.
[0053]: 0.005 g (2% w/w) of DBSA is added to 0.25 g (DRC - 53.8%) of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The procedure is same as for the above examples. The results were compared with [0047].
[0054] : 0.25 g of latex (DRC = 53.8%) is stabilized by adding 0.036 g (15% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[l2i molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. Comparable results to that of [0052] are obtained.
[0055]: 0.25 g of latex (DRC = 50 %) is stabilized by adding 0.036 g (15% w/w) of starch. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere. The progress of the doping reaction was inferred from the characteristic colour changes from purple to dark brown. The results were compared with [0052].
[0056]: 0.25 g of latex (DRC = 55 %) is stabilized by adding 0.036 g (15% w/w) of DBSA. Then the I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0046 g/ml) at a [C=C] /[h] molar ratio of 12. The solution is stirred under nitrogen

atmosphere. The progress of the doping reaction was inferred from the characteristic coloqr changes from purple to dark brown. Sartie results as that of the above example.
[0057]: 0.025 g (10% w/w) of starch is added to 0.25 g (DRC = 53.8%) of latex. The I. dopii^g reaction is carried out by adding iodine- cyclohexane solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere at a higher temperature of 50*^ C. The procedure is same as for the above examples. The doping time was varied and films were spin coated at definite intervals of 24, 48, 72, 96 & 240h hrs and Qonductivity was measured and the results were compared with samples at room temperature. The iodine induced conjugation of 1,4-polyisoprene was enhanced by heating the polymer/iodine solutioti to 50 C.
[0058]: 0.025 g (10% w/w) of DBSA is added to 0.25 g (DRC -^ 53.8%) of latex. The I2 doping reaction is carried out by adding iodine- toluene solution (0.00623 g/ml) at a [C=C] /[I2] molar ratio of 8. The solution is stirred under nitrogen atmosphere at a higher temperature of 50^ C. The procedure is same as for the above examples. The doping time was varied and films were spin coated at definite intervals of 24, 48, 72, 96 & 240h and conductivity was measured. The results were compared with [0049] and [0057].
[0059]: 0.036 g (15% w/w) of DBSA is added to 0.25 g (DRC - 55%) of latex. The I2 doping reaction is carried out by adding iodine- cyclohexane solution (0.0623 g/ml) at a [C=C] /[I2] molar ratio of 12. The solution is stirred under nitrogen atmosphere at a temperature of 50^ C. The procedure is same as for the above examples.
[0060]
[0061]: The conjugation reaction in solution is carried by dissolving a 0.25 g (DRC = 53.8%) of latex stabilized by 0.005 g (2% w/w) of DBSA dissolved in cyclohexane (0.015 g/ml) to iodine solution at a definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature. The reaction is characterized by the respective color changes with increase in doping time. Thin film of the doped solution is casted on the ITO poated glass substrate by spin coating and are dried under vacuum. A thin film of

C 60 is spin coated on it. The metallic electrode Indium (In) is then deposited on the top of C(,o f|lm through vacuum evaporation technique. The active area of the device is about 0.5 cm . The electrical conductivity measurements and the J-V characteristics are measured using Keithley electrometer model 6514 and a stabilized power supply. [0062]
[0063] :The same procedure is repeated by using Al electrode. The solution is prepared using 0.25 g (DRC = 53.8 %) of latex stabilized by 0.005 g (2% w/w) of DBSA dissolved in cyplohexane (0.015 g/ml) to iodine solution at a definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature. Thin film of the doped solution was casted on the ITO coated glass substrate by spin coating technique and were dried under vacuum. The metallic electrode Indium (Al) is then deposited on the top of C6o filni through vacuum evaporation technique. The results are comparable to [0061]. [0064] : The solution is prepared using 0.25 g (DRC = 53,8 %) of latex stabilized by 0.005 g (2% w/w) of starch dissolved in cyclohexane (0.015 g/ml) to iodine solution at a
definite molar ratio of [C=C]/[l2] = 8 under nitrogen atmosphere at room temperature.
Thin film of the doped solution is casted on the ITO coated glass substrate by spin coatipg technique and are dried under vacuum. The metallic electrode Indium (In) is then deposited on the top of Ceo film through vacuum evaporation technique.

We Claim
1. A process of producing intrinsically conducting 1,4 polyisoprene (natural rubber) films
by solution doping process, from natural rubber latex prevulcanized by gamma
irradiation, said process comprising prevulcnized cis 1,4 polyisoprene latex stabilized by
adding sterric satabilizers, wherein the said sterric stabilizers being starch or Do-
decylbenzene sulfonic acid (DBSA), doping the said stabilized latex by adding dopants,
said dopant being iodine, said solvents for dopants being toluene or cyclohexane, stirring
the solution under nitrogen atmosphere at a temperatures 20^C or 50^^C, keeping the
solution occasionally stirred and spin coating the said solution on to glass substrate at
time intervals of 24, to 240 hrs, and drying under dynamic vacuum of 10'^ Torr, to obtain
thin films.
2. A process as claimed in claim 1 wherein the said step of iodine doping reaction consists of adding iodine - cyclohexane or iodine- toluene solution at a molar ratio, [C = C]/[l2]-8&12.
3. A process as claimed in claim 1 wherein the DRC of prevulcanized rubber latex is from 53 to 55 %.
4. A process as claimed in claim 1 wherein the steps of stabilization consists in adding to
the prevulcanized latex, 2, 5, 10, and 15 weight % of starch or 2, 5, 10 and 15 weight %
of DBSA.
5. A process as claimed in claim 1 wherein the step of doping reaction consists in stirring the reaction mixture of stabilized, prevulcanized 1,4 polyisoprene latex at temperatures 20^C or 50^C for a period of 24 to 240 hours..
6. A process for the fabrication of a polymer photovoltaic device, wherein a thin film of intrinsically conducting pre-vulcanised natural rubber latex solution is spin coated on to an Indium Tin Oxide (ITO) coated glass substrate, followed by spin coating a thin film of Ceo on the top of it and finally a third layer of Aluminium (Al) or Indium (In) deposited above it by vacuum evaporation method.

Documents:

710-che-2003-abstract.pdf

710-che-2003-claims duplicate.pdf

710-che-2003-claims original.pdf

710-che-2003-correspondnece-others.pdf

710-che-2003-correspondnece-po.pdf

710-che-2003-description(complete) duplicate.pdf

710-che-2003-description(complete) original.pdf

710-che-2003-form 1.pdf

710-che-2003-form 19.pdf

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Patent Number

200711

Indian Patent Application Number

710/CHE/2003

PG Journal Number

8/2007

Publication Date

23-Feb-2007

Grant Date

30-May-2006

Date of Filing

05-Sep-2003

Name of Patentee

DR. PADMANABHAN PREDEEP

Applicant Address

PADMA VILAS, KALLUMALA P.O KERALA 690 110

Inventors:

#	Inventor's Name	Inventor's Address
1	DR. PADMANABHAN PREDEEP	PADMA VILAS, KALLUMALA P.O KERALA 690 110

PCT International Classification Number

H01B001/04

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA