Revealing the Working Mechanisms of Planar Perovskite Solar Cells With Cross-Sectional Surface Potential Profiling

With the rapid improvement in the efficiencies of perovskite solar cells (PSCs), understanding their working mechanisms becomes more and more critical for their further optimization. In this work, Kelvin probe force microscopy (KPFM) is utilized to characterize the surface potential (SP) distribution on the cross-sectional surface of planar PSCs. Devices with regular structure Indium Tin Oxide (ITO)/ZnO/perovskite/2,2',7,-7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD)/Au and inverted structure Fluorine doped Tin Oxide (FTO)/nickel oxide (NiO )/perovskite/ZnO/Ag are studied. The analysis of KPFM measurement indicates that, while p-n junction forms at the perovskite/ZnO interface in regular planar PSCs, p-i-n junction exists in inverted planar PSCs with electric field peaks atperovskite/NiO and perovskite/ZnO interfaces. In addition, charge injection barriers in planar PSCs are revealed with relative SP profiles obtained under different biases. In regular PSCs, charge injection barriers are only observed at the perovskite/ZnO interface and the contact between perovskite and spiro-OMeTAD is an ideal ohmic contact, while in inverted PSCs, charge injection barriers are observed at both perovskite/ZnO and perovskite/NiO
_x
contacts. The appearance of additional charge injection barriers in inverted PSCs exacerbates the charge recombination in these devices and leads to their relatively low open-circuit voltages. Our result indicates that the inefficient charge extraction is a major problem affecting the performance of planar PSCs.