Highly crystalline Nb-doped TiO2 nanospindles as superior electron transporting materials for high-performance planar structured perovskite solar cells

Planar-structured perovskite solar cells (PSCs) have received tremendous attention due to their high power conversion efficiency (PCE), simple process and low-cost fabrication. A compact thin film of electron transport materials (ETMs) plays a key role in these PSCs. However, the traditional ETMs of PSCs, TiO2 nanoparticulate films, suffer from low conductivity and high trap state density. Herein, we exploited TiO2 nanospindles as a compact ETM in planar PSCs for the first time, and achieved an efficient device with a PCE of 19.1%. By optimization with Nb doping into the TiO2 nanospindles, the PCE of the PSC was further improved up to 20.8%. The carrier transfer and collection efficiency were significantly improved after Nb5+ doping, revealed by Mott-Schottky (MS) analysis, space charge limited current (SCLC), photoluminence (PL), time-resolved photoluminence (TRPL) spectra, electrochemical impedance spectra (EIS) and so forth. Moreover, the hysteresis behavior was effectively inhibited and the stability was significantly enhanced. This work may provide a new avenue towards the rational design of efficient ETMs for perovskite solar cells.