Impedance investigation of the highly efficient polymer solar cells with composite CuBr₂/MoO₃ hole transport layer

Developing an air-stable, low-cost, non-toxic, and high-transparency charge buffer layer is a critical strategy to achieve the high photoelectric conversion efficiency of polymer photovoltaic cells. This paper reports the remarkable improvement of device performance by employing a combination of copper bromide (CuBr₂) and molybdenum trioxide (MoO₃) (CuBr₂/MoO₃) as the hole transport layer (HTL) of inverted-type polymer solar cells (PSCs). The bulk transport processes and resistive capacitance elements in the operating PTB7:PC₇₁BM bulk heterojunction PSCs were characterized using impedance spectroscopy. The impedance response was modeled using two equivalent circuital models, which are the general transmission line circuit (GTLC) model and the electrochemical polarization model. The effective carrier lifetime, conductivity, and mobility for both devices were extracted from the models. The improved hole transport at the anode and the efficient electron transport blocking decreased interface recombination and contact resistance, resulting in improved power conversion efficiency (PCE) values ranging from 7.30% to 9.56%. These results suggest that quantitative interpretation and modeling of the impedance spectroscopy results provide an effective way to unravel the operating mechanism of photovoltaic devices.