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 Pb²⁺/Pb⁰ and oxidation reactions of I^-/I₂ are observed on MAPbI₃ 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 Pb²⁺/Pb⁰ and X^-/X₂ (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.