THE RESEARCH PROGRAMME CONCERNS A FEASIBILITY STUDY ON THE REALISATION OF P-I-N AMORPHOUS SILICON SOLAR CELLS BY LOW PRESSURE CHEMICAL VAPOUR DEPOSITION (LPCVD) FROM DISILANE SI2H6.

SUMMARY :

FURTHER PROGRESS HAS BEEN RECORDED CONCERNING THE QUALITY OF A-SI:H FILMS DEPOSITED AT 420 C BY LOW PRESSURE CHEMICAL VAPOUR DEPOSITION OF SI2H6. OXYGEN AND CARBON CONTENT IN THE SAMPLES WAS LARGELY BELOW 0,1 % AND THE HYDROGEN CONCENTRATION WAS UNIFORM ACCORDING TO N-15 TECHNIQUE RESULTS. HYDROGEN EVOLUTION CURVES INDICATE THAT H OUTDIFFUSION STARTS WELL ABOVE 400 C AND HAS A PEAK AT 630 C. THEREFORE, ONLY TIGHTLY BOND H IS PRESENT IN OUR SAMPLES.
OPTICAL DATA INDICATE CONSISTENTLY THAT OPTICAL ENERGY GAPS ARE ABOVE 1.7 EV AND REACH IN SOME CASES EVEN 1.8 EV. THE VALUES OF B ARE IN 700 CM-1/2 EV-1/2 RANGE. IT HAS TO BE NOTICED THAT VALUES OF EG LARGER THAN 1.7 EV HAVE NEVER BEEN REACHED BY LPCVD. THERE ARE TWO INDICATIVE PDS ABSORPTION SPECTRA FROM THESE ONE CAN EVALUATE THE URBACH'S TAIL LOGARITHMIC SLOPE E AND A TOTAL DEFECT DENSITY. THE ND VALUES OBTAINED ARE GENERALLY OF THE ORDER OF 10 /16 CM-3 AND THEREFORE, MUCH BETTER THAN THOSE ONES PREVIOUSLY REPORTED FOR SAMPLES OBTAINED BY THE SAME TECHNIQUE. ALSO THE VALUE OF E WHICH CAN BE DERIVED EITHER BY PDS AND BY CPM ARE NOW BETTER THAN BEFORE AND REACH OFTEN 60 MEV. THIS VALUE IS PROBABLY THE ONE WHICH MUST BE PARTICULARLY IMPROVED, AND IT SEEMS REALLY THAT BY A CAREFUL CONTROL OF DEPOSITION CONDITION A DENOINDENTEASE OF 10-20 MEV HAS BEEN ACHIEVED IN THE LAST SIX MONTHS. CPM DATA MAY BE USED BOTH TO GET THE BEHAVIOUR OF OPTICAL ABSORPTION COEFFICIENT AND TO CALCULATE THE N"MU"V PRODUCT FOR ELECTRONS. WHILE THE FORMER ONES ARE IN GOOD AGREEMENT WITH OPTICAL, PDS AND PAS DATA, THE LATTER ONES OVERCOME IN SOME CASES 10-6 CM2 V-1, FOR INCIDENT FLUXES AROUND 10 /18 PH. CM-2 S-1 AT 1.8 EV. AT 10 /14 PH. CM-2 S-1, A MORE CONSERVATIVE VALUE FOR N"MU" V IS AROUND 10-7 CM2V-1, WHICH IS STILL AMONG THE BEST ONES EVER OBTAINED FOR CVD A-SI:H SAMPLES. LPCVD A-SI:H SEEMS TO BE NOW AT SUCH A STAGE , THAT IT IS REALLY NOT UNREALISTIC TO FORESCAST APPLICATIONS TO THE OPTOELECTRONICS FIELD IN GENERAL AND TO THE PHOTOVOLTAIC FIELD IN PARTICULAR. ITS PROPERTIES ARE SIMILAR TO THOSE OF GD FILMS DEPOSITED FEW YEARS AGO, BUT IT IS CERTAINLY MORE STABLE AND REPRODUCIBLE. MOREOVER, STABLER-WRONSKI EFFECT IS PRACTICALLY ABSENT, AND THE CLASSICAL INSTABILITIES DUE TO WEAKLY BOUND HYDROGEN ARE NOT PRESENT. WE ARE PRESENTLY PLANNING TO REALIZE P-I-N SOLAR CELLS WITH FILMS ENTIRELY DEPOSITED BY LPCVD IN THE NEAR FUTURE: THE EXPECTATION IS CLEARLY TO OVERCOME A 5% CONVERSION EFFICIENCY, WHICH REPRESENTS THE BEST OBTAINED BY LPCVD SO FAR.