The interest in organometal trihalide Pb perovskite (CH3NH3PbI3)-based solid-state hybrid solar cells has increased in recent years due to the high efficiencies achieved, with a record of over 20%, and the simple low temperature preparation method [1]. The high efficiency was thought to mainly originate from the strong optical absorption over a broader range (up to 800 nm for Pb ), low Urbach energy due to low defect states, and longer lifetimes of photoexcited charge carriers of the organometal trihalide Pb perovskite absorbers. Further improvements in the photovoltaic performance can be obtained by increasing the light harvesting in the NIR region up to 1000 nm. Recently, successful energy harvesting up to a wavelength of 1060 nm using Sn/Pb cocktail halide based perovskite materials has been achieved [2]. However, the energy conversion efficiency with such solar cells (less than 10%) is much lower than that of CH3NH3PbI3 based solar cells, which is due to the lower open circuit voltage (Voc) and fill factor (FF). In order to improve this, we need to have a good understanding of the key factors governing the photovoltaic performance of these solar cells, i.e., the optical absorption, the charge separation and the recombination dynamics.

  We have systematically characterized the optical absorption properties including the bandgap and the Urbach energy, and clarified the photoexcited charge separation and recombination dynamics at each interface in both Pb and Sn/Pb cocktail halide perovskites, and lastly investigated the relationships between these and the photovoltaic performance [3,4]. First, larger Urbach energy (34 meV) and the faster recombination process (lifetime: 4 ps) were observed in the Sn/Pb cocktail perovskite, which indicate more defects existing. These results are significantly different from those of MAPbI3 perovskite, where the photocarrier lifetimes are larger than 100 ns and almost no defects are observed. Secondly, the low Voc and FF in TiO2/Sn/Pb cocktail perovskite/P3HT solar cells were found to result from the large Urbach energy and recombination occurring at the TiO2/P3HT interface. These findings indicate that the photovoltaic performance of Sn/Pb cocktail perovskite solar cells can be improved by reducing the defects in the material and the recombination occurring at the TiO2/HTM interface through some interfacial engineering.