Equivalent-circuit Modeling of Microcrystalline Silicon pin Solar Cells prepared over a Wide Range of Absorber-layer Com
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1245-A07-17
Equivalent-circuit Modeling of Microcrystalline Silicon pin Solar Cells prepared over a Wide Range of Absorber-layer Compositions Steve Reynolds 1 and Vladimir Smirnov 2 1
Carnegie Laboratory of Physics, University of Dundee, Dundee DD1 4HN, UK.
2
IEF-5 Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany.
ABSTRACT An equivalent-circuit electrical model is used to simulate the photovoltaic properties of mixed-phase thin-film silicon solar cells. Microcrystalline and amorphous phases are represented as separate parallel-connected photodiode equivalent circuits, scaled by assuming that the photodiode area is directly proportional to the volume fraction of each phase. A reasonable correspondence between experiment and simulation is obtained for short-circuit current and open-circuit voltage vs. volume fraction. However the large dip in fill-factor and reduced PV efficiency measured for cells prepared in the low-crystalline region is inadequately reproduced. It is concluded that poor PV performance in this region is not due solely to shunting by more highly-crystalline filaments, which supports the view that the low-crystalline material has transport properties inferior to either microcrystalline or amorphous silicon.
INTRODUCTION The optoelectronic properties of thin-film silicon solar cells may be controlled and enhanced by the use of a process gas consisting of silane diluted in hydrogen. At low silane concentrations SC = [SiH4]/([H2] + [SiH4]), of the order of 5% under typical VHF PECVD deposition conditions, mixed-phase microcrystalline silicon consisting of roughly equal volumes of crystalline and amorphous material is obtained. When this material forms the absorber layer, optimized PV conversion efficiencies in excess of 8% can be achieved, with open-circuit voltages VOC in the region of 500-550 mV [1]. As SC is increased towards 10%, both the optical band-gap of the absorber and VOC increase as the crystalline fraction continues to fall, but ultimately a point is reached where both the fill-factor FF and short-circuit current ISC decrease rapidly and PV efficiency drops to as low as 2%. Further increase in SC yields material containing no detectable crystalline volume fraction when analyzed by Raman spectroscopy and the efficiency begins gradually to improve. By re-optimization of the deposition conditions in this regime good-quality solar cells may again be obtained, with VOC now in the region 850-1000 mV [2]. Thus there are two regimes, that when approached from the microcrystalline and amorphous ‘ends’ of the dilution spectrum, may yield good quality solar cells. Why, then, do solar cells with what will be termed here ‘low-crystallinity’ absorbers lying between these two regimes, exhibit such poor PV performance? ESR studies [3] have shown that paramagnetic defects remain at a low level in this region and thus there is no reason to suspect an increase in defect-mediated recombination. However, time-of-flight transport measurements [4] indicate that both electron and hole mobilitie
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