Dielectric Bragg Reflector as Back Electrode for Semi-Transparent Organic Solar Cells with an Average Visible Transparency of 52%

Abstract

A crucial challenge in the development of semi-transparent solar cells is to maintain a reasonable power conversion efficiency (PCE) while reaching a high average visible transparency (AVT). Typically, organic semiconductors are favorable for this application since they can selectively absorb infrared light while transmitting visible light. This ability stems from limited electronic states at high(er) energies in contrast to inorganic semiconductors with their typical rise of the absorption coefficient toward higher photon energies. To increase PCE at high AVTs, a series of infrared dielectric Bragg reflectors is developed for semi-transparent organic solar cells. Using the multi-layered back electrode (TiO₂|SiN|TiO₂|AZO|Ag|AZO) with PV-X Plus as photoactive layer and a metal-free PEDOT:PSS top electrode, a light utilization efficiency (LUE = AVT × PCE) of up to 4.32% is achieved, together with an AVT of 47.9%. Although the short circuit current and AVT agree well with optical simulations, a low fill factor (FF) and partial shunting limit the overall device performance. Using ZnO and PFN-Br as additional electron transport layers and modifying the back electrode stack (TiO₂|SiO₂|TiO₂|AZO|Ag|AZO) accordingly leads to an LUE of up to 4.6% with a remarkable AVT of 51.9% and a maximum PCE of 8.79%.