Improved charge transport of Nb-doped TiO2 nanorods in methylammonium lead iodide bromide perovskite solar cells

Abstract

Nb-doped rutile nanorod-based methylammonium lead iodide bromide (MAPbI_3−xBr_x) perovskite solar cells have been developed by integrating an excellent photon-active perovskite sensitizer with the superior electron transporting rutile nanorods. It is found that there are two distinct stages in the formation of the perovskite materials prepared using non-stoichiometric mixed halide precursors, namely the orange colored bromine-rich transient state formed at 105 °C and the dark brown colored iodine-rich crystallized state formed at 155 °C. Optical, compositional, and crystalline properties of the perovskite samples at the two stages are studied by using UV-vis spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The mixed halide materials undergo a transition from an intermediate cubic phase to a well-crystallized tetragonal perovskite phase through complicated diffusion, evaporation, and intercalation processes. Furthermore, a well-crystallized mixed halide perovskite is integrated with Nb-doped rutile nanorods and undoped rutile nanorods to fabricate perovskite solid state solar cells. Perovskite solar cells with Nb-doped rutile nanorods have significantly improved performance including the increased short circuit current and open circuit voltage compared to perovskite solar cells with undoped rutile nanorods. The overall power conversion efficiency enhancement of the device with Nb-doped rutile nanorods is over 50% compared with an undoped nanorod-based device, which is attributed to the superior charge collection efficiency of the Nb-doped rutile nanorods as evidenced by the electrochemical impedance measurement.