Improving the Performance of Carbon-Based Perovskite Solar Cells by the Incorporation of a Screen-Printed NiCo2O4 Interlayer

Hybrid lead halide perovskite solar cells (PSCs) stand out in terms of their high efficiency, yet the limited stability and process scalability pose challenges to their commercialization. Fully printable carbon-based perovskite solar cells (C-PSCs), consisting of a triple stack of mesoporous titania, zirconia, and carbon layers impregnated with the perovskite material, have been introduced as an attractive architecture; however, they generally exhibit lower efficiency. This study proposes a viable and scalable approach to increase the efficiency of C-PSCs by incorporation of an intermediate layer of mesoporous, nanostructured NiCo₂O₄ between the zirconia and carbon layers. The devices show an average increase in power conversion efficiency from 7.9 to 11%, with a champion device efficiency of 12.4%, associated with an enhanced average open-circuit voltage (V_OC) from 0.869 to 0.962 V. Electrochemical impedance spectroscopy reveals that the high-frequency recombination resistance (R_HF) decreases exponentially with V_OC with the same slope as for the reference triple-stack system, indicating that the mechanism is unchanged; however, a substantial increase in R_HF is observed. These results indicate that the hole extraction efficiency improves upon incorporation of the NiCo₂O₄ film thus decreasing surface recombination at the nonselective carbon contact. On the other hand, we postulate a possible contribution of the high capacitance of the interlayer, which may result in a shift of the Fermi energy of the carbon electrode and play a role in the hysteresis in the current - voltage curve.