Back contact and reach-through diode effects in thin-film photovoltaics

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Abstract
The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTephotovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTesolar cell.