Organolead halide perovskites for all-solid-state sensitised solar cells

Recent reports of high conversion efficiencies (up to 15%) for solid-state solar cell devices based on hybrid organic-inorganic perovskites (CH3NH3)PbI3-xXx (X=I, Br, Cl) make these materials very promising as light-harvesting materials for mesoscopic solar cells. Although hybrid lead halide perovskites were discovered many decades ago [1] they have been employed as active components of solar converting devices only in 2009, when TiO2 sensitization by the MAPbBr3 and MAPbI3 perovskites (MA=CH3NH3) was demonstrated in photovoltaic cells using liquid redox electrolyte [2] achieving an efficiency of 6.5% [3], but the liquid electrolyte was observed to dissolve the hybrid perovskite in a few minutes. In 2012 CH3NH3PbI3-sensitized all-solid-state solar cells (SS-SC) with efficiency ? larger than 9% were reported[4]. Shortly after a "meso-superstructured" SS-SCs based on a iodide/chloride mixed-halide perovskite MAPbI3-xClx with efficiencies up to 12.3% was demonstrated [5,6], thus pointing out that mixed halide perovskites may be suitably used to improve the performances of existing solar cells as well as conceive novel photovoltaic devices. Besides, the chemical management of MAPbI1-xBrx allows the material bandgap to be controllably tuned so that almost the entire visible spectrum can be covered by this mixed halide perovskite, which enables the realization of colorful solar cells [7]. In all these works the perovskite pigment has been deposited in a single step onto mesoporous metal oxide films using mixtures based on lead and methylammonium iodide in common solvents. Very recently a power conversion efficiency of 15% was obtained by using the CH3NH3PbI3 perovskite prepared through a two step dipping procedure [8]. The above results confirm the potential of these materials in photovoltaic applications, but at the same time points out that enhancing the photovoltaic performances requires a proper choice of the material combined with the optimization of its functional properties which strongly depend on the synthesis procedures. Thus clarifying the relation between material properties and the procedures used for their preparation is of fundamental importance for light harvesting optimization and cell efficiency increase. Indeed, despite the widening literature on the photovoltaic use of hybrid lead perovskites, many questions remain to be addressed regarding the diverse structural chemistry of these materials and the nature of charge transfer, before it will be possible to rationally modify and tailor chemistries and nanostructures in solar cells to improve solar cell performances. In this communication we report about the influence of the preparation procedures on the properties of lead-based hybrid halide perovskites, in particular methylammonium triiodoplumbate (MAPbI3) films and mixed lead halide perovskites having different I:Br(Cl) ratios. We investigate the role of solvents and self-assembling process parameters (temperature, time, ...) in determining the morphological, crystallographic, optical and electrical properties of the obtained materials. The aim is identifying the procedures most suited to reproducibly prepare hybrid lead perovskites having the capability of both light absorption and charge transport within a mesoporous network combined with enhanced dye loading for the most efficient sunlight harvesting. Issues related with dye loading are also discussed considering the infiltration of the obtained perovskites into photoanodes made of either nanostrucutred TiO2 or porous nanosheets of ZnO, which is considered an environmental friendly and low-cost potential alternative to TiO2. [1] Weber, D. Z. Naturforsch 1978, 33b, 1443. [2] Kojima A., Teshima K., et al., J. Am. Chem. Soc. 2009, 131, 6050. [3] Im J.H., Lee C.R., et al., Nanoscale 2011, 3, 4088. [4] Kim H.-S., Lee C.R, et al., Sci. Rep. 2012, 2, 591. [5] Lee M. M., Teuscher J, et al., Science 2012, 338, 643. [6] Ball M., Lee M. M., et al., Energy & Environmental Science 2013, 6, 1739. [7] Noh J. H., Im S. H., et al., Nano letters 2013, 13, 1764. [8] Burschka J., Pellet N., et al., Nature 2013, 316, 499.