Toward Understanding the Built-in Field in Perovskite Solar Cells through Layer-by-Layer Surface Photovoltage Measurements

The built-in voltage (<i>V</i>_BI) is a key parameter for solar cell operation, yet in perovskite solar cells the distribution, magnitude, and origin of the <i>V</i>_BI remains poorly understood. In this work, we systematically studied the <i>V</i>_BI in <i>pin</i>-type perovskite solar cells based on different hole transport layers (TLs). To this end, we determine the surface photovoltage (SPV) of partial and complete device stacks layer-by-layer by measuring the work function (WF) under dark and light (equivalent AM1.5G) conditions with Kelvin probe (KP) and photoemission spectroscopy (UPS) measurements in 3 different laboratories. We demonstrate that the SPV increases upon the addition of each additional layer until it equals the open-circuit voltage (<i>V</i>_OC) of the full device. This suggests that both the electron and hole transport layer (HTL/ETL) enlarge the SPV, by improving the separation of photogenerated carriers. Yet, the contribution of both transport layers to the total SPV of the device is small (in the range of ≈100 to 200 meV) and the largest contribution to the SPV originates from the top metal electrode (≈500 meV). The results suggest that the <i>V</i>_BI of <i>pin</i>-type perovskite solar cells is largely a result of the work-function difference of the electrodes. With regard to films (or incomplete cell stacks), our simulations can reproduce the measured SPV, and measured quasi-Fermi level splitting (><i>V</i>_OC) in partial cell stacks without a significant internal field consistent with the experimental data. This work establishes layer-by-layer SPV measurements, which are easily accessible, as a key tool for understanding device performance and internal energetics, similar to layer-by-layer QFLS measurements.