Study of Interfacial Charge Transfer Loss in Hybrid Solar Cells by Impedance Spectroscopy

Organic solar cells have been much studied because of the simplicity in their fabrication process in comparison to solar cells based on crystalline inorganic semiconductors. However, the actual power conversion efficiency is still much lower than the theoretical prediction, which may indicate the effect of inefficient charge transfer or charge recombination loss inside the cell. We have studied the photovoltaic characteristics of hybrid solar cells with transition metal doped ZnO layer as electron transport layers. Under dark condition, their J-V curves show a deviation from an ideal diode characteristic, which is indicated by current flow below the threshold voltage. Further investigations to clarify the photovoltaic characteristics of those cells were then carried out by the measurements of impedance spectroscopy. The impedance characteristics are dependent on the transport layer used, where the Nyquist plots of cells with TiZO exhibit larger impedance than those of cells with ZnO. However, those cells with larger impedance show better working performances. This fact was then interpreted as the result from smaller interfacial charge transfer loss, which may take place via charge carrier recombination at the interface of the transport layer and active layer, in the cells with TiZO.