Universality of non-Ohmic shunt leakage in thin-film solar cells

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu ( In , Ga ) Se 2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias ( V < ∼ 0.4 ) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage ( I s h ) , across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V = 0 , (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited (SCL) current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism.