Large-grain crystallized metal chalcogenide film, colloidal solution of amorphous particles, and preparation methods

Abstract

The present invention relates to a method for preparing an aqueous or hydro-alcoholic colloidal solution of metal chalcogenide amorphous nanoparticles notably of the Cu.sub.2ZnSnS.sub.4 (CZTS) type and to the obtained colloidal solution. The present invention also relates to a method for manufacturing a film of large-grain crystallized semi-conducting metal chalcogenide film notably of CZTS obtained from an aqueous or hydro-alcoholic colloidal solution according to the invention, said film being useful as an absorption layer deposited on a substrate applied in a solid photovoltaic device.

Claims

1. A method for preparing a colloidal aqueous, or hydro-alcoholic solution of amorphous nanoparticles of metal chalcogenides of formula M-C wherein: M represents first metals consisting of Cu, Zn, and Sn, and C represents at least one chalcogenide element selected from S and Se, the method comprising the following successive steps being performed at a room temperature from 0° C. to 50° C., wherein: a) a first solution of precursors of said first metals M is prepared in a solvent consisting of pure acetonitrile, or a mixture of acetonitrile with water and/or an alcohol other than methanol, and b) a second aqueous or hydro-alcoholic solution of precursors of the at least one chalcogenides C consisting of one or two chalcogenide salts of second metals, other than said first metals M, is prepared, the alcohol of said second solution, if any, being an alcohol other than methanol, and c) both of said first and second solutions of precursors are mixed at atmospheric pressure and at room temperature until a crude colloidal solution is obtained, including a solid portion comprising primary amorphous nanoparticles containing all of the first metals Cu, Zn, and Sn, with sizes of less than 30 nm, and a liquid supernatant, and d) the solid portion is separated from said colloidal solution of step c), in order to obtain a solid residue after removal of the liquid supernatant, and e) the solid residue obtained in step d) is rinsed by pouring onto it an aqueous or hydro-alcoholic solution in order to form a subsequent colloidal solution including a solid portion and a liquid supernatant, the alcohol of said hydro-alcoholic colloidal solution being other than methanol, and f) again, the solid portion is separated from said subsequent colloidal solution of step e), in order to obtain, after removal of the liquid supernatant, a rinsed solid residue as a humid paste, and g) said humid paste of step f) is re-dispersed in a dispersion solvent consisting of an aqueous or hydro-alcoholic solution, the alcohol of said hydro-alcoholic solution, if any, being a non-toxic alcohol, so as to obtain a dispersion of nanoparticles of chalcogenides of Cu, Zn, and Sn.

2. The method according to claim 1, wherein, in step a), the solvent is acetonitrile alone, in step b), the solvent is water alone, and in steps e) and g) said solvent consists of a hydro-alcoholic solution, the alcohol of said hydro-alcoholic solution being a non-toxic alcohol having a boiling temperature below the boiling temperature of water.

3. The preparation method according to claim 1, wherein: in step a), said precursors of said first metals are halides of said first metals M, and in step b), said one or two chalcogenide salts of said second metals are alkaline, or earth-alkaline metal salts.

4. The preparation method according to claim 1, wherein M is a ternary mixture of Cu, Zn, and Sn, and C is S, and, in step c), amorphous nanoparticles of Cu.sub.2ZnSnS.sub.4 are obtained.

5. The preparation method according to claim 3, wherein: in step a), said first solution contains CuCl.sub.2, ZnCl.sub.2, and SnCl.sub.4, and in step b), said one or two chalcogenide salts consist of NaSH, and in step c), amorphous nanoparticles of Cu.sub.2ZnSnS.sub.4 are obtained.

6. The method according to claim 1, wherein in step g), said nanoparticles are primary amorphous nanoparticles with sizes of less than 30 nm.

7. The method according to claim 6, wherein said nanoparticles are primary amorphous nanoparticles of sizes of 3 to 20 nm.

8. The method according to claim 6, wherein the dispersion solvent in step g) is a hydro-alcoholic solution, and wherein said alcohol of said solution has a boiling temperature below that of water.

Description

EXAMPLE 1

Preparation of a CZTS Colloid

(1) A colloid of nanoparticles Cu—Zn—Sn—S was made by reacting a mixture of metal salts, CuCl, ZnCl.sub.2, SnCl.sub.4.5H.sub.2O in water/acetonitrile with an aqueous solution of NaSH, at room temperature and under an inert nitrogen atmosphere, according to the global reaction:
2CuCl+ZnCl.sub.2+SnCl.sub.4+4NaSH<>Cu.sub.2ZnSnS.sub.4+4NaCl+4HCl

(2) This reactive system is suitable in the sense that the byproducts of the reaction, for example NaCl or HCl are soluble in water while the nanoparticles are solid and dispersed as a colloid.

(3) The aqueous solution (0.12 M) of NaSH is prepared in a 50 ml bottle, by weighing 0.56 grams of hydrated NaSH powder (provider Aldrich, product 16,152,7) and adding 50 ml of deionized water, deoxygenated beforehand by bubbling for 30 minutes with nitrogen. This aqueous solution of the sulfur precursor NaSH is then sealed with a plug, and then stored.

(4) The solution of copper-zinc-tin (CZT) metal chlorides in water/acetonitrile is prepared in a nitrogen glove box by:

(5) 1. weighing 469 mg of copper precursor powder: CuCl (provider Aldrich 224332), 415 mg of zinc precursor powder: ZnCl.sub.2 (provider Aldrich 208086) and 893 mg of tin precursor powder: hydrated SnCl.sub.4 (Aldrich 244678); and then

(6) 2. by adding 10 ml of anhydrous acetonitrile (Aldrich 271004);

(7) 3. after dissolution and mixing with ultrasound for a few minutes, a yellow-greenish solution is obtained with a concentration of 1 mol/L (Cu+Zn+Sn) which is then diluted 5 times in acetonitrile (volumes in a ratio from 1 to 4) and then itself diluted twice with water (volumes 1 and 1) and thereby obtaining a concentration of 0.1 mol/L.

(8) The colloidal synthesis reaction is conducted by pouring 10 ml of the NaSH solution (0.12 M) in 10 ml of a solution of metal precursors CZT (0.1 M). This synthesis carried out at ambient pressure and temperature is very rapid and gives rise to a colloidal CZTS solution, according to the global reaction indicated above.

(9) The pH of this crude colloid was measured to be equal to pH=0.3 which defines a very strongly pronounced acidity, favorable for avoiding hydrolysis of the metal elements or particles.

(10) This crude colloid was analysed by Transmission Electron Microscopy. A model 2100 FEG 200kV from JEOL (Japan), equipped with EDX (Energy-dispersive X-ray Spectroscopy) detectors, STEM BF (Scanning Transmission Electron Microscope Bright Field) and STEM DF (Surface Transmission Electron Microscope Dark Field) with a wide angle HAADF (High Angular Annular Dark Field) is used.

(11) For this, a sample holder consisting of a carbon membrane on a nickel grid was soaked in the non-diluted colloid and simply dried in ambient air before being introduced into the TEM vacuum chamber. According to FIG. 1A, the dried colloid forms aggregates of primary nanoparticles, the characteristic size of which is from 2 to 5 nm and with a rounded shape characteristic of amorphous particles. The average elementary analysis achieved by EDX measurement on many areas, indicates that these dried particles contain the majority elements Cu, Zn, Sn, S, and Cl as impurities. With larger TEM magnification (FIG. 1B), i.e. by concentrating the beam of incident electrons, certain crystalline planes seem to be observable, ascribable to probable crystallization under the beam, during the TEM observation. Electron diffraction analysis as illustrated with the example of FIG. 1C, shows the presence of a diffuse diffraction ring, corresponding to atomic diffraction planes characterized by interatomic distances compatible with the known crystalline structure of kesterite Cu.sub.2ZnSnS.sub.4 (CZTS). The crude colloid observed under TEM therefore appears to be amorphous or slightly crystallized in the probable kesterite structure (in particular with crystallization during TEM observation with strong magnification).

(12) The analysis of the rnmnncitinn (TFM FDX is the fnllnwing:

(13) TABLE-US-00001 Cu Zn Sn S Ni Cl at. % 24.2 15.7 10.4 47.4 0.0 2.3 100

(14) This crude colloid is then poured into a centrifugation tube and then centrifuged for 5 min at 6,000 rpm (Universal centrifuge 16 from Hettich Zentrifugen AG), i.e. an acceleration of 3,700 G expressed relatively to gravity. This allows separation of the solid and liquid portions. The transparent upper liquid portion (the supernatant) is removed by pouring it into a bottle of acid liquid waste. The lower solid portion is then rinsed by adding 20 ml of water. After introducing a magnetic bath covered with Teflon, this solution was placed on a magnetic plate and mixed with magnetic stirring at about 200 rpm for 5 minutes. New centrifugation for 10 min at 9,000 rpm (i.e. 8,400 G) is carried out, followed by removal of the supernatant. This rinsing procedure aims at removing the reaction products such as NaCl, HCl and other excess ionic species. We measured that the lower and humid solid residual portion forms a slurry which consists of about 100 mg of dry material (CZTS) and 500 mg of liquid, by weighing before and after drying in vacuo.

(15) This slurry was then re-dispersed in a water/ethanol mixture (5 ml/5 ml), and then mixed with magnetic stirring for 5 minutes at room temperature; the obtained colloid is then stable for several days and may be used for deposition by spraying/atomization.

(16) The particles suspended in this rinsed colloid were then analyzed by TEM microscopy (FIG. 2A) by using the same procedure with a carbon membrane on a nickel grid, as described before for the TEM measurements of crude colloid (FIG. 1A). The microscopy with a STEM DF detector of FIG. 2A shows primary nanoparticles of a rounded shape, agglomerated and with a similar size between 2 and 7 nm typically. EDX analysis (not shown) indicates the majority presence of elements Cu, Zn, Sn and S but the absence of the chlorine element, which illustrates the effect of the rinsing. As high resolution TEM analysis and electron diffraction is unreliable because of the crystallization under a beam of electrons, the crystallographic characteristics were measured by X ray diffraction on the slurry from the rinsed crude colloid (but not re-dispersed), coarsely spread out on a glass plate of 2.5×2.5 cm.sup.2 at room temperature.

(17) The X-ray diffraction measurements were conducted in a diffractometer of the Bruker AXS D8 series 2 type, by using an X-ray source corresponding to the copper emission line Kα, in a grazing mode (with an angle of incidence set to 1°) and a detector movable over a circular arc in order to obtain a 2θ diffraction angle spectrum scanned from 10° to 70° with a pitch of 0.04.

(18) Curve A of FIG. 3 (lower spectrum) shows the diffraction spectrum of X-rays of the humid slurry of FIG. 2A. This spectrum does not exhibit well-defined diffraction peaks but rather two very wide bumps, for which the positions in 2θ=28.4° and 2θ=47.3° may correspond to those of the two main peaks (112) and (220)/(204) respectively, of the kesterite crystalline structure from X-ray diffraction reference spectra measured on powder crystalline materials. Thus, the particles making up the rinsed colloid and then dried in ambient air, are in majority amorphous or slightly crystallized, in consistency with the TEM observations of FIGS. 1A and 1B (crude colloid) or TEM observations of FIG. 2A (rinsed and reconditioned or re-dispersed colloid).

(19) The solid nanoparticles of the crude colloid and of the rinsed and then re-dispersed colloid are both characterized by a nanometric size (2-7 nm), consisting of the elements Cu, Zn, Sn and S, of an amorphous crystalline or even very slightly crystallized structure.

EXAMPLE 2

Preparation of Crude Colloid of Concentrated CZTS

(20) The concentration of the CZTS colloid noted in moles per liter (or M), is defined as the number of molecules of the compound CZTS (Cu.sub.2ZnSnS.sub.4 or equivalent to the number of tin atoms) per unit volume. In Example 1 above, the concentration of the crude colloid is 0.0125 M. In the present example, the concentration of the CZTS colloid was brought to 0.25 M, which is equivalent to about 100 mg/ml. One skilled in the art will recognize there a value corresponding to the typical concentration of a slightly diluted slurry, which may be deposited by tape casting. This illustrates the versatility of the colloidal synthesis method.

(21) In this alternative, an aqueous solution (6 M) of NaSH is prepared, by weighing 2.24 g thereof for 4 ml of solution. A solution of copper-zinc-tin (CZT) metal chlorides in acetonitrile is prepared by weighing 188 mg of CuCl, 166 mg of ZnCl.sub.2 and 357 mg of SnCl.sub.4 hydrate for 5 ml of solution. The synthesis is achieved by pouring, in a first phase, 11 ml of deionized and deoxygenated water into the solution of metal precursors, and then in a second phase, the 4 ml of NaSH solution.

(22) The thereby made crude colloid of the compound CZTS is concentrated (0.25 M) and, further has strong stability after adding water in the rinsing step indicated in Example 1 above.

EXAMPLE 3

Preparation of a Crystallized CZTS Film on a Glass Substrate Covered with Molybdenum

(23) The composition of the crystallized film Cu.sub.2ZnSnS.sub.4 and the composition of the amorphous film deposited by spraying before annealing described in the form of Cu—Zn—Sn—S, are conventionally distinguished hereafter.

(24) Amorphous layers of Cu—Zn—Sn—S were deposited from suspended nanoparticles, by spraying on substrates of the Mo/glass type formed with sodium-lime glass with a thickness of 1 mm covered with a 700 nm molybdenum layer.

(25) In the present example of deposition by spraying, a colloid according to Example 1 above was prepared and the colloid was then re-dispersed in a water/ethanol mixture (5 ml/5 ml).

(26) The spraying step was carried out in a glove box (model GP concept type T3 in stainless steel, from Jacomex S.A.S., France) filled with nitrogen and equipped with a purification unit (<1 ppm O.sub.2, <10 ppm H.sub.2O) and with an airlock being used for introducing/extracting samples. Amorphous Cu—Zn—Sn-S films were deposited on Mo/glass substrates (2.5 cm x 2.5 cm) brought to a temperature of 250° C. by means of a heating plate (model 18×18 cm of standard ceramic, reference 444-0617 from VWR International SAS, France) with modified thermal regulation in a closed loop on a thermocouple of the K type placed under the substrate. An X-Y Cartesian robot was used (of the Yamaha type, FXYx 550×550 with an RCX222 controller, distributed in France by New-Mat France) for sweeping over a surface of more than 16 cm.sup.2 with the spray nozzle used (a sprayer flask on a test tube in borosilicate glass from Glasskeller Basel AG). For injecting the colloidal solution into the nozzle, application of nitrogen pressure was controlled intermittently: open for 0.3 seconds and then a waiting time of 1.7 seconds; this 2 second cycling being maintained during the spraying duration. Good films were obtained with a nozzle-substrate distance of about 15 cm with an average flow of nitrogen carrier gas of 14 L/min at a cylinder nitrogen pressure of 0.2 bars. Thicknesses of 6±1 μm were obtained by deposition by spraying for two minutes of the colloid of concentration 10 mg/ml (i.e. a volume of about 2 ml).

(27) The morphology of the thereby deposited amorphous layers Cu—Zn—Sn—S was determined by Scanning Electron Microscopy SEM (Hitachi Ltd, model S-4700 equipped with an EDX analyser and data processing by the software package NORAN). FIG. 2B is a sectional view of a Cu—Zn—Sn—S film deposited by spraying at 240° C. On this picture, the glass substrate may be seen, covered with a layer of 700 nm of polycrystalline molybdenum with a column structure, and the film Cu—Zn—Sn—S. It is possible to discern that this film is porous and consists of fine particles agglomerated together and separated by vacuum.

(28) The amorphous/slightly crystallized nature of the films obtained by spray deposition from CZTS colloids, was shown by X-ray diffraction measurement as indicated by the spectrum of FIG. 3 (middle curve b). There again, no diffraction peak is clearly detectable.

(29) The annealing step used for densifying and crystallizing the layer in order to form large crystalline grains was carried out in a nitrogen glass box (reference GT concept, from Jacomex SAS, France). The film deposited by spraying was then laid on a heated plate (model Titane plate with a Detlef control case, Harry Gestigkeit, GmbH) and heated gradually under nitrogen up to an annealing temperature of 525° C. maintained for 1 hour, and then cooled for 1 hour. FIG. 2C shows a sectional view of the film obtained after annealing: above the polycrystalline molybdenum layer, a polycrystalline film with a thickness of 1.8±0.2 μm expresses densification of the film which is accompanied by crystallization with formation of the desired large grains (from 1 to 2 μm) i.e. close to the thickness of the film.

(30) The surface condition of the film above was analyzed and its roughness Sa was measured according to the ISO 25178 standard. The average roughness Sa is defined as the arithmetic mean of the absolute values of the ordinates of the roughness profile. The following values were obtained for a film with a thickness of 1.8 μm: 313 nm for a surface of 50×50 μm.sup.2, 247 nm for a surface of 20×20 μm.sup.2.

EXAMPLE 4

Preparation of a Crystallized CZTS Film on a Glass Substrate Covered with Molybdenum from a CZTS Ink Formulated in Pure Dispersion Solvents

(31) In the present example, a CZTS colloid was prepared according to Example 1, except that the slurry rinsed with water and then centrifuged was then mixed in dispersion solvents different from the water-ethanol 50-50 mixture of Example 3. Among the latter, four were selected for the present example notably for their low vapor pressures at 20° C. for TEP (Tri-Ethyl-Phosphate) (40 Pa) or DMSO (80 Pa) or else high vapor pressures for water (2,330 Pa) and for ethanol (5,850 Pa). The concentration was adjusted to 10 g/L. After deposition by spraying in a glove box according to the invention, the CZTS/Mo/glass samples obtained were then subject to crystallization heat treatment at 525° C. under nitrogen. The surface images of the obtained samples are shown in FIGS. 4A to 4D.

(32) This CZTS film is formed with crystallized CZTS grains, as indicated by the X diffraction spectra, (not shown). However, in the cases, A) ethanol, B) Tri-Ethyl-Phosphate (TEP) or C) DiMethyl-SulfOxide (DMSO), the adhesion to the substrate is not sufficient, the covering level of the substrate is not complete, and the grain size is not homogeneous. Only the water solvent (FIG. 4—case D) gives the possibility of obtaining both a high covering level of the CZTS film with large crystalline grains and good adhesion on the molybdenum/glass substrate.

(33) The present example shows that water is the preferred pure dispersion solvent. Examples 3 and 4 show that the dispersion solvent is preferentially a water-ethanol mixture, which is an abundant solvent, easy to use and non-toxic and which gives the possibility of making, after deposition by spraying followed by a crystallization heat treatment, continuous and dense (without any cracks or holes) crystalline CZTS layers with large grains, and adherent on the molybdenum substrate.

EXAMPLE 5

Preparation of an Sb2S3 Colloid

(34) A sulfur precursor solution is first prepared by mixing 18 ml of acetonitrile and 2 ml of water at room temperature, and then by pouring therein 18 mg of NaSH powder (0.321 mmol) which spontaneously dissolves. As the NaSH is not or very little soluble in acetonitrile, dissolution occurs in the aqueous portion of the water/acetonitrile mixture.

(35) An antimony metal precursor solution with a concentration of 10.7 mol/L is then prepared by dissolving 4 mg (0.214 mmol) of SbCl.sub.3 powder (Aldrich) in a 20 ml solution of pure acetonitrile. No hydrolysis of the SbCl.sub.3 salt is noticed during this dissolution.

(36) By pouring the sulfur precursor solution into the metal solution at room temperature, orange coloration is immediately observed, characteristic of the amorphous solid phase Sb.sub.2S.sub.3, resulting from spontaneous formation and within a few seconds, of a stable colloid. This colloid is difficult to centrifuge, which shows the great stability of the colloid, which is related to the small size/mass of the primary suspended nanoparticles. Indeed, an analysis by transmission microscopy (TEM) is then conducted: a copper grid with a carbon membrane is soaked for a few seconds in the colloidal liquid so as to collect a small portion of it, and then it is left to dry in ambient air. Observation under TEM indicates agglomerated small primary particles, the individual size of which is of about 20 nanometers. Elementary analysis TEM+EDX shows a majority composition of Sb.sub.2S.sub.3 as well as the presence of chlorinated impurities (of the order of 1 atomic percent). No crystalline phase is identifiable by electron diffraction under a TEM electron beam, which indicates that the solid particles of the colloid consist of amorphous antimony sulfide Sb.sub.2S.sub.3. The colloidal synthesis by mixing both antimony/acetonitrile and sulfur/water solutions was then achieved according to the global reaction: 2SbCl.sub.3+3 NaSH<=>Sb.sub.2S.sub.3+3NaCl+3HCl.

(37) By using two sulfur and metal solutions prepared under identical conditions with those of the example above, the mixing order was reversed by pouring the metal solution into the sulfur-containing solution. There also, an orange coloration was immediately observed with spontaneous formation of an amorphous colloid Sb.sub.2S.sub.3 with a characteristic orange color.

EXAMPLE 6

Preparation of Colloid SnS

(38) A metal solution of tin 0.05 M is prepared in a glass bottle with a capacity of 50 ml, into which is first poured 348 mg of tin precursor powder (SnCl.sub.2, anhydrous, Fluka 96529) and then 36 ml of acetonitrile solvent. Dissolution is facilitated at room temperature by ultrasonication for a few minutes.

(39) As in the examples above, a sulfur-containing 0.2 M aqueous solution is prepared by weighing 1.12 g of NaSH powder, and then by adding 100 ml of deionized and deoxygenated pure water in order to achieve their spontaneous dissolution.

(40) The colloidal synthesis is then achieved by mixing at room temperature both solutions, for example by pouring 9 ml of the sulfur-containing 0.2M solution into 36 ml of 0.05 M metal tin solution. A black colloid is then formed spontaneously according to the global reaction: SnCl.sub.2+NaSH<=>SnS+NaCl+HCl. This colloid is stable under ambient conditions for several days. The TEM observation of this colloid thereby made and without any other treatment (without any rinsing, centrifugation, re-dispersion etc.) is shown hereafter. In particular, elementary analysis TEM-EDX indicates that the composition is in majority of tin sulfide, with presence of a chlorinated impurity, which is a residual impurity of the reaction according to the following composition (TEM-EDX):

(41) TABLE-US-00002 Sn S Cl C N O at. % 45.2 52.3 2.5 0.0 0.0 0.0 100

(42) The primary particles formed are relatively small, with a characteristic size of the order of 3 to 5 nm. This colloid may then be used in order to be rinsed and then re-formulated as a slurry or an ink which may be used for deposition of thin layers.

EXAMPLE 7

Photovoltaic Device of the Thin Layer Type Made with a CZTS Layer Annealed in a Nitrogen Atmosphere

(43) A thin CZTS layer was prepared on a glass substrate probably molybdenum, annealed under an N.sub.2 atmosphere, like in Example 3.

(44) On the crystallized continuous CZTS layer with large grains, a buffer layer of approximately 50 nm of CdS was deposited, by deposition in a chemical bath according to the customary procedure of the state of the art (see for example G. Hodes, Chemical Solution Deposition Of Semiconductor Films, ISBN 08247-0851-2, M. Dekker Inc.), by quenching in a mixture maintained at 60° C. of deionized water, of ammonia (NH.sub.3, 4M), cadmium nitrate (Cd(NO.sub.3).sub.2, 4mM) and of thio-urea (SC(NH.sub.2).sub.2, 0.2M). After 10 minutes, the samples were rinsed in deionized water and then dried under nitrogen flow.

(45) On this buffer layer, two optically transparent layers were deposited successively by magnetron sputtering with the use of a commercial apparatus H2 from Intercovamex: A first insulating layer of about 50 nm of ZnO [135 W RF, 0.5 Pa of argon] followed by a conductive transparent layer of about 250 nm of indium oxide doped with 10% by mass of tin (ITO) [70 W RF, 0.25 Pa of argon]. The square resistance of the obtained ITO layer is approximately 30 ohms per unit square.

(46) Next, the substrate was divided into 16 electrically insulated cells, each with square dimensions 0.5 cm x 0.5 cm. In order to collect the current and measure the photovoltaic performance, a front face contact was made with a small spot of 0.5 mm of silver deposited by drying a lacquer loaded with silver, on the conductive ITO layer. The rear contact was also directly taken on the molybdenum, at the edge of the substrate.

(47) The photovoltaic yield (or photovoltaic efficiency) was computed from the current-voltage electric characteristics of the photovoltaic diode measured under light irradiation. The conversion yield is the percentage of the electric power delivered by the device at the maximum power point, relatively to the power of the incident radiation: η=(electric power at the maximum power point)/(power of the incident radiation). This photovoltaic efficiency was measured with an electric test bench and which uses a solar simulator delivering an irradiation of 1000 W/m.sup.2 corresponding to the AM1.5G standard. The measurement bench was calibrated according to the standard procedure on the basis of the known photocurrent of reference cells, as provided by different recognised official institutes.

(48) As illustrated in FIG. 5, the preliminary yields as described in this example, were of the order of 1%, the short-circuit currents were around 8 mA/cm.sup.2 and the open circuit voltages around V.sub.oc=0.25V.