MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: the Role of Chloride as Dopant on the Transport and Structural Properties

Finding viable alternatives to silicon-based photovoltaics, through low-cost solution processable materials, is crucial, facing as we are, a complex transition out of the fossil fuelled civilization. In this scenario, the utilization in nanostructured solid-state solar cells of an underexplored eclectic class of materials, the hybrid halide perovskites, has represented a field breakthrough, allowing novel device architectures leading to record device performances up to 15%,[2] thus holding the promise of cost effective solar energy production. Among the first results of the pioneer reports on perovskites-based solar cells, probably the most intriguing discover concerned the application of a iodide/chloride mixed-halide perovskite CH3NH3PbI3-xClx in a so called "meso-superstructured" Solid State Solar Cell, where the perovskite is concomitantly capable of both absorbing light and transporting charge within a mesopourus network.[3] This mixed system has recently been compared to the I-based perovskite CH3NH3PbI3 via spectroscopical investigation and interesting differences have been reported, some of them leading to hypothesizing a better charge transport within CH3NH3PbI3-xClx.[4] However, an investigation on the exact materials composition and structure is still missing. Here[1] we report a detailed investigation on Cl/I mixed halide self-assembling perovskites, and study the relation between the I:Cl ratio in the material and the solar cell characteristics, aiming at optimizing device performances through composition tuning. We found out that the exact composition of the final compound does not reproduce the stoichiometry of the precursor solution. We demonstrated, both experimentally and theoretically, that the formation of continuous solid phase MAPbI3-xClx is actually not allowed and that chloride incorporation into MAPbI3 is possible only at relatively low concentration (3-4%), so that it could be classified as a dopant agent. However, even if the material band-gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material. [1] Colella S., Mosconi E., Fedeli P., Listorti A., Gazza F., Orlandi F., Ferro P., Besagni T., Rizzo A., Calestani G., Gigli G., De Angelis F., Mosca R.; Chemistry of Materials, in press. [2] Liu M., Johnston M. B., Snaith H. J.; Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395. [3] Lee M.M., Teuscher J., Miyasaka T., Murakami T.N., Snaith H.J.; Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. [4] Stranks S.D., Eperon G.E., Grancini G., Menelaou C., Alcocer M.J.P, Leijtens T., Herz L.M., Petrozza A., Snaith H.J.; Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341.