Manganese Dopant-Induced Isoelectric Point Tuning of ZnO Electron Selective Layer Enable Improved Interface Stability in Cesium-Formamidinium-Based Planar Perovskite Solar Cells

The higher basicity and uncontrolled defect states in the planar zinc oxide (ZnO) electron selective layer (ESL) cause rapid deprotonation of the perovskite absorber which results in higher interface charge recombination at the perovskite/ESL interface restricting the usage of ZnO as the ESL in perovskite solar cells. In this work, the isoelectric point (IEP) of ZnO was tuned by introducing a manganese (Mn4+) dopant in ZnO for the first time. The higher oxidation state of the Mn dopant reduces the basicity of the doped ZnO ESL and controls the perovskite deprotonation at the Mn:ZnO/perovskite interface. The doping of Mn4+ in ZnO results in the generation of two free electrons causing higher conductivity of Mn:ZnO films. The dual effect of a lower IEP and higher conductivity of the Mn:ZnO film along with its improved n-type behavior results in higher surface photovoltage and reduced trap-filled limited voltage (V-TFL), which resulted in a higher open-circuit voltage (V-o(c)) (0.92 to 0.99 V). The negligible PbI2 formation at the Mn:ZnO/perovskite interface, lower leakage current (1 order lower than that of ZnO), and a comparatively reduced diode ideality factor (n(id)) validate the improvement of perovskite/interface stability. The above-mentioned merits of the Mn-doped ZnO-based ESL improved the mixed-cation perovskite power conversion efficiency from 11.7 to 13.6%, which is similar to 15% higher than that of a bare ZnO-based ESL. Furthermore, a considerably improved device stability of over 100 h under high relative humidity condition (RH >70%) was observed for the Mn-doped ZnO ESL without any encapsulation.