High efficiency of the organo-lead-halide perovskite cells is backed by high open-circuit voltage, 1.0-1.1V, in addition to strong light absorption due to the hybrid crystalline structure. High voltage is realized by minimised thermal loss in the charge transfers across solid-solid hetero junctions. For the narrow band gap tri-iodide perovskite, voltage loss of the cells using spiro-MeOTAD hole transport material ranges from 0.4 to 0.55 eV with respect to the band gap of 1.55 eV (800 nm). Here, relatively high voltage (V_oc >1.05V) is obtained with use of Al₂O₃ mesoporous scaffold. Mesoporous scaffolds also help to minimize the hysteresis of I-V characteristic, especially in combination with a Cl-doped tri-iodide perovskite coated on a thin TiO₂ compact layer.
    Our group has been focusing the perovskite PV development on two subjects. One is how to achieve higher voltage for narrow gap perovskite cells, and the other is how to exclude the hysteresis in the I-V characteristics to ensure stable cell performance. We found that combination of uniform and very thin TiO₂ compact layers (thickness <10 nm) and mesoporous TiO₂ scaffolds for Cl-doped tri-iodide perovskite based cells gives I-V characteristics that has excellent responsivity under high-speed scanning and causes no hysteresis, realising perfect matching of efficiencies for forward and reverse scan. Use of Al₂O₃ scaffold in the same conditions however tends to cause hysteresis depending on the scaffold thickness. Cell stability was highly affected by the quality and density of the compact layer and interfacial structures of metal oxide and perovskite, in which Cl doping improves the quality of perovskite including crystalline orientation.
    For high voltage generation, one of the best cell structure we found is use of a well-oriented crystalline hole transport material in junction with an oriented high quality perovskite layer. A thin crystalline film of perylene is an efficient hole conductor for this purpose. Self-organised formation of perylene on the surface of perovskite was influenced its orientation by the orientation of underlying perovskite. The fully crystalline perovskite-perylene hybrid cell is capable of Voc exceeding 1.2V maintaining sufficiently high conversion efficiency of 4%. The voltage loss of this cell, <0.35 eV, is one of the smallest value ever achieved by solid state thin photovoltaic cells, and can be compared to GaAs photovoltaic cell which is capable of high V_oc up to 1.12V with respect to band gap of 1.42 eV.