Objective THE PROGRAM CONSTITUTES THE ITALIAN CONTRIBUTION TO A LARGER JOINT PROGRAM SUBMITTED BY FRENCH, ITALIAN, DUTCH AND SPANISH RESEARCH LABORATORIES, AIMED AT DEVELOPING AMORPHOUS SILICON-BASED ALLOYS EXHIBITING NARROW BAND-GAP TO BE USED AS EFFICIENT ABSORBERS IN THE LONG WAVELENGHT SPECTRAL REGION. THE AVAILABILITY OF SUCH A MATERIAL IS EXPECTED TO IMPROVE THE RESPONSE OF SINGLE-JUNCTION SOLAR CELLS AND TO ALLOW THE PREPARATION OF TANDEM SOLAR CELLS. By using optimized deposition conditions, low gap (1.4-1.5 eV) silicon germanium alloys, showing a density of states as low as 1-2E16 per cubic centimetre, have been grown. These materials have been tested as absorbing materials for photovoltaic solar cells and preliminary devices show a conversion efficiency higher than 5%.In order to evaluate the effect on solar cell performances of decreasing the optical gap of the absorbing layer, a semiempirical model has been developed which takes into account experimental optical and transport properties. By this model, the behaviour of single and multiple junction pin devices can be evaluated as a function of the gap and the thickness of active layers. The model clearly shows that the best low gap alloys can be profitably used to prepare both single and tandem devices. In the first case, an efficiency of 10%, comparable to that of plain amorphous silicon devices, can be obtained while using a much thinner active layer, so improving the cell stability. In the second case, the use of 2 absorbing layers allows a significant increase of cell efficiency of about 14%.THE ACTIVITIES HERE DESCRIBED ARE THE ITALIAN CONTRIBUTION TO A LARGER PROGRAM JOINTLY CARRIED OUT BY ITALIAN, FRENCH, SPANISH AND DUTCH LABORATORIES.ON THE ITALIAN SIDE, ENEA (PORTICI) LABORATORY) WAS THE PROJECT LEADER AND THE DEPARTMENT OF PHYSICS AND ENIRICERCHE (ENI GROUP) WERE THE OTHER PARTENRS. ABOUT TWENTY RESEARCHERS HAVE BEEN ENGAGED IN THE ACTIVITIES PERFORMED BY THE THREE TEAMS. EACH LABORATORY HAS LARGELY INCREASED ITS OWN EXPERIMENTAL FACILITIES AND HAS CONTRIBUTED BOTH TO THE PREPARATION OF SILICON ALLOYS AND TO THE SAMPLE CHARACTERIZATION. THE PROGRAM WAS AIMED TO PREPARE LOW GAP SILICON GERMANIUM ALLOYS TO BE USED AS ABSORBING MATERIALS IN SOLAR CELLS. SILICON - GERMANIUM ALLOYS WERE PREPARED BY STANDARD RF GLOW DISCHARGE TECHNIQUE, USING BOTH SINGLE AND MULTICHAMBER REACTORS, FROM MIXTURES OF SILANE, GERMANE AND HYDROGEN. IN ORDER TO PREPARE DEVICE QUALITY LOW GAP-GERMANIUM RICH ALLOYS, SEVERAL DEPOSITION PARAMETERS HAVE BEEN SYSTEMATICALLY VARIED, INCLUDING HYDROGEN DILUTION, SUBSTRATE TEMPERATURE, GAS PRESSURE AND RF POWER. HYDROGEN DILUTION WAS FOUND TO BE PARTICULARLY EFFECTIVE IN IMPROVING MORPHOLOGY AND PHOTOSENSITIVITY OF GERMANIUM RICH MATERIALS. CHEMICAL COMPOSITION OF ALLOYS WAS DETERMINED BY ELECTRON MICROPROBE AND BY AUGER SPECTROSCOPY, USING A SCANNING SPECTROMETER ALSO EQUIPPED FOR IN-DEPHT PROFILING. MATERIALS WERE CHARACTERIZED BY IR AND OPTICAL ABSORPTION, PHOTOTHERMAL DEFLECTION SPECTROSCOPY (PDS), CONDUCTIVITY, AND SURFACE PHOTOVOLTAGE TECHNIQUE. CONDUCTIVITY AND PHOTOCONDUCTIVITY MEASUREMENTS WERE PERFORMED IN A PLANAR CONFIGURATION. BY USING OPTIMIZED DEPOSITION CONDITIONS, LOW GAP (1.4 - 1.5 EV) SIGE ALLOYS, SHOWING A DENSITY OF STATES AS LOW AS 1 - 2X10 16 CM3 HAVE BEEN GROWN. THIS MATERIALS HAS BEEN TESTED AS ACTIVE LAYER FOR PHOTOVOLTAIC SOLAR CELLS: PRELIMINARY SMALL AREA DEVICES(<1 CM2) SHOW A CONVERSION EFFICIENCY OF ABOUT 6%. IN ORDER TO EVALUATE THE ADVANTAGES PROVIDED BY THE AVAILABILITY OF A LOW GAP MATERIAL, A SEMI-EMPIRICAL MODEL HAS BEEN DEVELOPED, TAKING INTO ACCOUNT EXPERIMENTAL OPTICAL AND TRANSPORT PROPERTIES. BY THIS MODEL, THE BEHAVIOUR OF SINGLE AND MULTIPLE JUNCTION P-I-N DEVICES CAN BE EVALUATED AS A FUNCTION OF THE GAP AND THE THICKNESS OF ACTIVE LAYERS. THE MODEL CLEARLY SHOWS THAT OUR BERST LOW GAP ALLOYS (EG = 1.4 - 1.5 EV) CAN BE PROFITABLY USED TO PREPARE BOTH SINGLE AND TANDEM DEVICES. IN THE FIRST CASE, AN EFFICIENCY OF 10 %, COMPARABLE TO THAT OF PLAIN AMORPHOUS SILICON DEVICES, CAN BE OBTAINED WHILE USING A MUCH THINNER ACTIVE LAYER, SO IMPROVING THE CELL STABILITY. IN THE SECOND CASE, THE USE OF TWO ABSORBING LAYERS (1.75 AND 1.4 EV) ALLOWS A SIGNIFICANT INCREASE OF CELL EFFICIENCY UP TO ABOUT 14%. Fields of science natural scienceschemical sciencesinorganic chemistrymetalloidsnatural sciencesphysical sciencesopticsspectroscopy Programme(s) FP1-ENNONUC 3C - Research and development programme (EEC) in the field of Non-Nuclear Energy, 1985-1988 Topic(s) Data not available Call for proposal Data not available Funding Scheme CSC - Cost-sharing contracts Coordinator Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA) EU contribution No data Address Via Vecchio Macello 80055 Portici Napoli Italy See on map Total cost No data
