Bipolar Circuit Modeling of p+n a-Si:H/c-Si Heterojunction Solar Cells With TCO Schottky Collector Including Early Effect in a-Si:H

A bipolar equivalent circuit model is developed for silicon hetero-junction silicon solar cells (SHJ) with a Schottky contact between the p-type amorphous silicon and the transparent conducting oxide (TCO). The Schottky barrier is treated as a bipolar collector junction through which holes are transported by thermionic emission, while electrons reaching the barrier are swiftly collected. The electron current is shown to suffer an Early effect especially in cells with low a-Si:H(p) doping level. The results of the equivalent circuit analysis in the dark and under illumination perfectly match both the ${J}$ – ${V}$ characteristics resulting from AFORS-HET device simulations and the measured ${J}$ – ${V}$ characteristics of real cells, with low and with high doping levels in a-Si:H(p). The model provides a clear interpretation for the role of the Schottky diode in the degradation of the cell open circuit voltage and in the ${S}$ -shape distortion of the ${J}$ – ${V}$ characteristics under illumination. Such degradations disappear if hole tunneling through the Schottky contact prevails.