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DEVELOPMENT OF THE SCIENTIFIC AND TECHNICAL BASIS FOR INTEGRATED AMORPHOUS SILICON MODULES

Objective

THE AIM OF THIS STUDY IS TO REALIZE A TEXTURIZED TCO FRONT ELECTRODE FOR A-SI:H SOLAR CELLS IN ORDER TO ENHANCE LIGHT TRAPPING. THE TCO WAS AN ITO FILM OBTAINED BY POWDER PYROLYSIS. TEXTURIZING WAS ACHIEVED BY INCLUDING AN ADDITIVE INTO THE USUAL POWDER. WE TESTED AEROSIL (SIO2) AND ALUMINA AS TEXTURIZING AGENTS. THE = 5 CM X 5 CM SAMPLES WERE OPTICALLY AND ELECTRICALLY CHARACTERIZED BY SPECTROPHOTOMETRY IN THE VISIBLE AND 4-PROBE SHEET RESISTANCE (R SH) MEASUREMENTS. TECHNOLOGICAL PROBLEMS IN PYROLYSIS APPEARED WHEN USING ALUMINA WHICH WAS THUS DISREGARDED FOR THIS APPLICATION. IN THE CASE OF AEROSIL, A BETTER REPRODUCEBILITY IN H (HAZE RATIO) VALUES WAS OBTAINED WHEN IMPROVING THE HOMOGENEITY OF TEXTURIZING AGENT/PYROLYSIS POWDER MIXING. AT 550 NM, AND FOR A 10 % - AEROSIL MIXTURE, WE OBTAINED H=9.1 % WHEN USING A PROGRESSIVE AND CONTINUOUS PROJECTION ONTO THE GLASS AND H = 24.5% FOR AN INSTANTANEOUS PROJECTION. BUT THE FORMER TECHNIQUE WAS CHOOSEN FOR FURTHER EXPERIMENTS AS IT GAVE BETTER HOMOGENEITY IN GLASS COATING AND LOWER DISPERSION IN RESULTS. THESE SELECTED SAMPLES (R SH = 14) WERE TESTED BY SOLEMS AS SUBSTRATES IN SOLAR CELLS WHICH SHOWED LOW JSC (= 10 MA/CM2). IMPROVEMENT OF THE PYROLYSIS TECHNIQUE IS BEING CARRIED OUT, PARTICULARLY WITH OTHER TEXTURIZING AGENTS.
The use of an indium-2O3 tin oxide (ITO) thin film deposited by powder pyrolysis as a front electrode in amorphous silicon hydrogen solar cells has been demonstrated. The photovoltaic performances are worse than those obtained with a tin oxide fluorine (tin oxide (TO)) substrate. Chemical reduction of ITO (magnetron or pyrolysis deposited) and TO has been observed during the deposition at 250 C, but does not appear at temperatures below 150 C. Zinc oxide (ZO) is not attacked by the plasma. A 6 nm ZO overlayer is thick enough to passivate the ITO film. The cell performances seem to be slightly worse than those obtained with TO. 2 types of texturised electrodes have been tested in order to enhance the cell performances. Chemically frosted glass followed by an ITO deposition led to a high pin hole density. Texturised ITO deposition on flat glass gave cells with inhomogeneous visual aspects and with slightly lower performances than those using scattering TO.

The use of an indium tin oxide (ITO) thin film as a front electrode in a-Si:H solar cells were shown to have a reduced performance in comparison to those obtained with an SnO2:F(TO) substrate. This was true if the film was deposited by powder pyrolysis or by magnatron, and also if the film was protected from plasma attack with zinc oxide. Texturised electrodes have also been evaluated with respect to cell performance.
OPTICAL ABSORPTION IN AN AMORPHOUS SILICON SOLAR CELL CAN BE ENHANCED BY TEXTURING A SUBSTRATE WHICH SCATTERS LIGHT. THIS RESULTS IN INCREASING THE PHOTOCURRENT OF THE CELL WHEN COMPARED TO A CELL CONTAINING A FLAT SUBSTRATE.

SEVERAL TECHNIQUES ARE USED TO OBTAIN SUCH TEXTURIZED SUBSTRATES, AS LITHOGRAPHY (I), GROWTH OF TRANSPARENT OXIDE FILMS WITH LARGE GRAINS (II) AND DEPOSITION OF TEXTURIZED SIO2 FILM ON FLAT GLASS (III). IN THE PRESENT STUDY, WE ATTEMPT TO OBTAIN THIS EFFECT BY ROUGHENING THE SURFACE OF THE INITIALLY FLAT GLASS SUBSTRATE. THE EXPECTED ROUGHNESS WAVELENGHT IS IN THE 500-5000 ANGSTROEM RANGE. THREE METHODS OF GLASS ETCHING WILL BE INVESTIGATED: (1) ABRASION BY USING HIGH HARDNESS POWDERS, (2) PROJECTION OF POWDERS ONTO THE GLASS SUBSTRATE (IV,V) AND (3) WET CHEMICAL ETCHING (IV,VI,VII) BY USING HYDROFLUORIC ACID-BASED SOLUTIONS OR READY-TO-USE SOLUTIONS. INFLUENCE OF ETCHING PARAMETERS (TIME, TEMPERATURE, ETC.) ON THE OPTICAL PROPERTIES OF THE SUBSTRATES WILL BE TESTED. SCATTERING EFFICIENCY AND TEXTURING HOMOGENEITY WILL BE CHECKED BY OPTICAL MEASUREMENTS. DIRECT OBSERVATION OF THE MICROSTRUCTURE OF THE SURFACE WILL BE ALSO PERFORMED. A TRANSPARENT AND CONDUCTIVE OXIDE WILL BE DEPOSITED ON THE SUBSTRATES WHICH SHOW BEST SCATTERING RESULTS. THESE SUBSTRATES WILL BE INTEGRATED BY SOLEMS S.A. IN COMPLETE P-I-N SOLAR CELLS WHICH WILL BE TESTED.

REFERENCES:
(I) H.W. DECKMAN AND J.H. DUNSMUIR. APPL.PHYS.LETT. 41 (4), 377 (1982).
(II) H. IDA, N. SHIBA, T. MISHUKU, H. KARASAWA, A. ITO, M. YAMANAKA AND Y. HAYASHI. IEEE ELECTRON DEVICE LETT. 4, 157 (1983).
(III) M. MISONOU, M. HYODO, H. NAGAYAMA AND H. KAWAHARA. PROC. 18TH IEE PHOTOV. SPEC. CONF. LAS VEGAS, OCT. 21-25 (1985)P. 925.
(IV) F. ORGAZ AND I. JIMENEZ. BOL. SOC. ESP. CERAM. VIDR. 21(3), 163 (1982)
(V) GUYSON INTERNATIONAL LTD. GLASS. P. 194, MAY 1980.
(VI) W.C. NIXON. GLASS, P. 221, JUNE 1982.
(VII) P.N. HOMER AND B.J. CRAWFORD. GLASS TECHNOLOGY. 11(1), 10 (1970).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

SAINT-GOBAIN RECHERCHE S.A.
Address
Quai Lucien Lefranc 39
Aubervilliers
France