Origin of VOC in Perovskite Solar Cells by Faradaic Junction Model

Perovskite solar cells have attracted much interest due to the very fast increasement of power conversion efficiency (PCE) as well as the low-cost solution processing, excellent absorption coefficient, long charge carrier diffusion length. To date, further improvement of PCE in perovskite solar cells is mainly limited by the open-circuit voltage (V _OC ). In previous studies, the origin of V _OC is usually explained by the energy band alignment theory. However, in some experiments, the V _OC does not change apparently when the band positions of electron or hole transport layers are changed. Therefore, the energy band alignment theory is not suitable to explain the origin of V _OC in perovskite solar cells, and it’s desirable to reveal the origin of V _OC to further improve the PCE of perovskite solar cells. Here, we develop a new method to simulate the interface charge transfer process in the all solid-state devices in working conditions by connecting perovskite/ETL half device with perovskite/HTL half device under simultaneous illumination. Reversible reduction reactions of Pb ²⁺ /Pb ⁰ and oxidation reactions of I ^- /I ₂ are observed on MAPbI ₃ surfaces under illumination, respectively. Moreover, the oxidation processes of Spiro-OMeTAD are also investigated by in situ FTIR and NMR. We find that the V _OC comes from the difference between the electrode potentials of Pb ²⁺ /Pb ⁰ and X ^- /X ₂ (X=I, Br) on perovskite surfaces. The results suggest that the origin of V _OC does not come from the energy band alignment but from the electrode potential alignment, which can offer new perspectives to improve the V _OC of perovskite solar cells.