Quantifying Quasi‐Fermi Level Splitting and Open‐Circuit Voltage Losses in Highly Efficient Nonfullerene Organic Solar Cells

The power conversion efficiency (PCE) of state‐of‐the‐art organic solar cells is still limited by significant open‐circuit voltage (VOC) losses, partly due to the excitonic nature of organic materials and partly due to ill‐designed architectures. Thus, quantifying different contributions of the VOC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state‐of‐the‐art systems that demonstrate different VOC losses in their performance are presented. By evaluating the quasi‐Fermi level splitting (QFLS) and the VOC as a function of the excitation fluence in nonfullerene‐based PM6:Y6, PM6:Y11, and fullerene‐based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non‐radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high‐open‐circuit‐voltage organic solar cells. Quasi‐Fermi level splitting (QFLS), which sets the maximum value of the open‐circuit voltage (VOC) of a photovoltaic device, in state‐of‐the‐art organic solar cells is evaluated using spectroscopic and semiconductor device physics approaches.