Bias‐Dependent Quantum Efficiency Reveals Recombination Pathways in Thin Film Solar Cells

Abstract Identifying where recombination predominantly occurs—whether at the front interface, back interface, or throughout the bulk—is crucial for optimizing CdSeTe solar cells and many other photovoltaic device architectures. Here, a simple and effective diagnostic is demonstrated: measuring external quantum efficiency (QE) under varying forward biases. The drift–diffusion simulations reveal that each recombination pathway leaves a distinct bias‐induced signature in the normalized QE: a progressive drop at long wavelengths for back‐limited devices, a short‐wavelength decline for front‐limited devices, and a relatively uniform decrease across all wavelengths for bulk‐limited devices. These predictions are validated with experiments on As‐doped and Cu‐doped CdSeTe devices, with and without passivation layers or different front buffers. In each case, the observed bias‐dependent QE spectral changes align with the simulated recombination map. Because this method uses standard QE instrumentation, it offers a broadly accessible and cost‐efficient means to diagnose recombination pathways—extending beyond CdSeTe to other thin‐film and emerging solar cell technologies. By pinpointing recombination bottlenecks, researchers and manufacturers can strategically refine doping profiles, passivation schemes, and interface designs to further improve device performance.