Cubic: Column composite structure (NH2CH=NH2)x(CH3NH3)1-xPbI3 for efficient hole-transport material-free and insulation layer free perovskite solar cells with high stability

We have designed a novel hole-conductor-free and insulation layer free mesoscopic perovskite solar cells (PSCs) with a low cost carbon counter electrode (CE). Significantly, to enhance solar light harvesting capability, a mixture of methylammonium (CH3NH3+, MA) and formamidinium (HNCHNH3+, FA) cations were explored to synthesize superior performance perovskite materials. With the introduction of FAI, Cubic: Column composite structure of (FA)x(MA)1-xPbI3 was formed on the top of mesoscopic TiO2. This nanostructure not only leads to fully coverage of perovskite on TiO2 but also provides salutary electron transport channels to effectively reduce the electron recombination probability. As a result, optimal solar cell efficiency of 13.03% of (FA)x(MA)1-xPbI3 composite structure based PSC (when x is fixed at 0.25) was achieved, while PSC based typical MAPbI3 cuboids only obtained an efficiency of 12.49%. Despite of the improved efficiency, the Cubic: Column composite structure caused by the addition of FAI also contributes to higher long-term and thermal stability.