REDUCTION OF SURFACE RECOMBINATION IN SILICON SOLAR CELLS

THE NEXT GENERATION OF HIGH EFFICIENCY SILICON SOLAR CELLS WILL REQUIRE VERY LOW SURFACE RECOMBINATION AT BOTH FRONT AND BACK SURFACES. ONE APPROACH TO REDUCING SURFACE RECOMBINATION IS BY GROWTH OF AN EPITAXIAL HETEROJUNCTION "WINDOW" OF A HIGHER BANDGAP MATERIAL WHICH IS TRANSPARENT AND OFFERS BOTH LOW INTERFACE RECOMBINATION AND LOW RESISTANCE CONTACT. TRANSPARENCY REQUIRES A BANDGAP OF AT LEAST 3 EV, WHILE A LOW DENSITY OF INTERFACE STATES MAY BE OBTAINED BY EPITAXIAL GROWTH OF THE WINDOW LAYER. THIS REQUIRES A SEMICONDUCTOR WITH FCC LATTICE AND A LATTICE CONSTANT NEAR 5.431 A. OF THE SEMICONDUCTOR MATERIALS AVAILABLE, ONLY ZNS, WITH A BANDGAP OF 3.6 EV AND A LATTICE CONSTANT 5.406 A, APPEARS TO FULLY MEET THE CRITERIA. IN COLLABORATION WITH BROWN UNIVERSITY, THIS PROGRAM WILL INVESTIGATE GROWTH OF EPITAXIAL LAYERS OF ZINC SULFIDE ON HIGH-EFFICIENCY SOLAR CELL SUBSTRATES BY CHEMICAL VAPOR TRANSPORT. MATERIAL QUALITY OF THE DEPOSITED LAYERS WILL BE EVALUATED BY X-RAY DIFFRACTION, SEM, EDAX, AND HALL MEASUREMENTS. ELECTRICAL CHARACTERIZATION WILL BE DONE ON SILICON WAFERS WITH ION IMPLANTED P-N JUNCTIONS. BY MEASURING THE SPECTRAL RESPONSE OF THE CELL WHEN ILLUMINATED BOTH FROM FRONT AND BACK, DIFFUSION LENGTH AND SURFACE RECOMBINATION CAN BE DETERMINED. ONCE LOW RECOMBINATION VELOCITY EPITAXIAL LAYERS CAN BE GROWN AND CHARACTERIZED, REPRESENTATIVE HIGH EFFICIENCY SOLAR CELLS WILL BE MADE. THE END RESULT OF PHASE I WILL BE THE DEMONSTRATION OF THE PASSIVATION OF SILICON SOLAR CELLS BY EPITAXIAL ZINC SULFIDE OR ZINC SELENIDE/SULFIDE LAYERS. THUS, THE PROPOSED RESEARCH WILL RESULT IN A MEANS OF LOWERING THE SURFACE RECOMBINATION OF SILICON SOLAR CELLS AND MAKE POSSIBLE THE PRODUCTION OF SOLAR CELLS OF GREATLY INCREASED EFFICIENCIES. THIS WILL BE FOLLOWED IN PHASE II BY THE APPLICATION OF THIS TECHNIQUE TO THE MANUFACTURE OF EXTREMELY HIGH EFFICIENCY SOLAR CELLS, AND IN PHASE III BY THE COMMERCIALIZATION OF THE TECHNOLOGY.