Basic electronic properties and optimization of TCO/a-Si:H(n)/c-Si(p) hetero solar cells

We report on a detailed analysis of the basic electronic properties and the optimization of amorphous/crystalline silicon heterojunction solar cells (a-Si:H(n)/c-Si(p)). The gap states density of the ultrathin a-Si:H emitter on c-Si was determined by photoelectron yield spectroscopy. By varying the a-Si:H film thickness the valence band offset was determined to about 0.45 eV. The density of states at the a-Si:H/c-Si interface amounts to about 2×10¹¹cm^-2eV^-1 at midgap. This result was obtained by field dependent surface photovoltage measurements. In addition, photoluminescence measurements were performed to investigate the recombination at the a-Si:H/c-Si interface. To gain an optimized solar cell performance the deposition temperature of the a-Si:H and the gas phase doping concentration was varied. These optimizations lead to a maximum efficiency of 17.2% for a TCO/a-Si:H(n)/c-Si(p)/a-Si:H(p) solar cell fabricated using low temperature processes only.