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HIGH EFFICIENCY THIN-FILM SOLAR CELLS ON UPGRADED METALLURGICAL GRADE (UMG) SILICON SUBSTRATES.

Objective

TOGETHER WITH THREE OTHER EUROPEAN TEAMS, WE AIM AT PROVING THE FEASIBILITY OF EFFICIENT (12%) SOLAR CELLS ON CHEAP (1$/WP) CRYSTALLINE SI SUBSTRATES.
The 3 main aspects of this work were the simulation and optimisation of epitaxial cells, the building and study of the cells, and the improvement of the initial upgraded metallurgical grade (UMG) silicon.

Using several kinds of UMG polycrystals as substrates, we succeeded in making epitaxial cells with an efficiency between 11.5 and 12 %. Including the expected improvement of the emitter, the efficiency should surpass 12 %. Apart from the intermediate steps involved in epilayer growth, which remains the weak stage of the process economically, this technology is a simple and cheap one, because impure raw materials are used to grow the ingots and because an integral screen printing technology is used to complete the cells.
THE TECHNICAL GOAL OF THE CONTRACT, WHICH WAS TO OBTAIN A CONVERSION EFFICIENCY OF 12% FOR EPITAXIAL CELLS GROWN ON UMG-SI SUBSTRATES, HAS BEEN REACHED. IN COOPERATION WITH M.CAYMAX (IMEC-LEUVEN), WE HAVE IN A FIRST STEP BUILT EPITAXIAL CELLS ON WAFERS CUT FROM DIFFERENT INGOTS FROM CNRS-VITRY, CGE, ITALSOLAR AND PHOTOWATT, AND OBTAINED THE FOLLOWING EFFICIENCIES: MAXIMUM EFFICIENCY WAS 11.7% AND 12.5% FOR A REFERENCE MONOCRYSTALLINE EPITAXIAL CELL. THESE RESULTS WERE OBTAINED USING THE SOLPRO CELL TECHNOLOGY, WHICH LEADS TO A RATHER DEEP P-N JUNCTION: AS A RESULT THE BLUE EFFICIENCY IS LOW AND MIGHT BE USING THE TECHNOLOGY DECELOPED LAST YEAR AT IMEC. THE COMPARISON WITH THE SIMULATED CURVES CALCULATED LAST YEAR SHOWS THAT THE EPILAYER DIFFUSION LENGTH LN IS LARGER THAN 100 MICRO M, A VERY GOOD VALUE WHICH EXPLAINS THE HIGH EFFICIENCY REACHED. THIS WORK CONFIRMS THAT FOR THIS TYPE OF CELLS THE SUBSTRATE SHOULD BE HIGHLY DOPED (0.02 - 0.06 OHMCM), AS EXPECTED THEORETICALLY. THE STUDY OF THE OPEN CIRCUIT VOLTAGE ALSO CONFIRMS THE BENEFICIAL INFLUENCE OF THE BACK SURFACE FIELD AT THE SUBSTRATE/EPILAYER INTERFACE. IN A SECOND STEP, THESE RESULTS HAVE BEEN STILL IMPROVED BY TEXTURIZING THE BACK AND FRONT SURFACE OF THE WAFER IN A NAOH-KOH MIXTURE: AN EFFICIENCY OF 12% WAS REACHED. THIS WORK WAS MAINLY THE TASK OF LE HOANG THI TO AND LE QUANG NAM. ANOTHER PIECE OF WORK HAS BEEN THE TASK OF B. KAYALI AND R. SURYANARAYANAN; IT CONSISTS OF PREPARING ALTERNATIVE SUBSTRATES BY A PLASMA TORCH TECHNIQUE. THIS TECHNIQUE HAS BEEN MODELLIZED. ALSO, FROM A WACKER POWDER, WE HAVE BEEN ABLE TO OBTAIN A NUMBER OF WAFERS REACHING 5X5CM2 IN DIMENSIONS. THESE WAFERS ARE FAGILE (MICROCRACKS, HOLES) BUT THEIR ELECTRICAL QUALITY IS GOOD AND MAY STILL BE INCREASED BY AN EPILAYER DEPOSITION (LN > 60 MICRO M). AT THIS STAGE, NO SOLAR CELL COULD BE MADE ON SUCH WAFERS, DUE TO THE MICRO-HOLES THAT BRING SHORT CIRCUITS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Centre National de la Recherche Scientifique (CNRS)
Address
1 Place Aristide-briand
92195 Meudon
France