photovoltaic properties of MAPbI 3 perovskites across the tetragonal to cubic transition, due to structural fluctuations on a sub-picosecond timescale that make the instantaneous electronic energy levels and band-gap of the formally cubic, high temperature structure, to differ only slightly from those of the room temperature stable tetragonal phase. disordered position of the Cl anions 45 and the presence of a Raman signal at 66 cm -1 in the high temperature phase of MAPbCl 3 . 47 From a technological perspective, these results help to explain the lack of an observable abrupt change in photovoltaic device performance above room temperature 23 as would be expected to be observable if the light-harvester undergoes a phase transition. This is also further evidence indicating that ferroelectricity contributions to the optoelectronic properties, as in traditional inorganic materials, 54-57 are negligible, since the ferroelectric polarizability is expected to change dramatically across the transition between two different crystalline structures.On the contrary, this view supports other proposed mechanisms, as the spatial charge localization 50, 58 and/or stable band bending effects at the interfaces and grain boundaries, 59 as the basis of the impressive inherent performance of hybrid lead halide perovskites. ASSOCIATED CONTENT Supplementary InformationExperimental methods; theoretical methods and models; EQE measurements on different MAPbI3 devices; corresponding JV measurements of the devices; EQE of a cell with MAPbI 3-xClx ; comparison between theoretical and experimental radial distribution function of MAPbI 3 perovskite; theoretical fluctuation of the band edges.

We present here a planar perovskite solar cell with a stabilized power conversion efficiency (PCE) of 17.6% at the maximum power point and a PCE of 17% extracted from quasi-static J–V with an open-circuit voltage of 1.11 V. Such excellent figures of merit can be achieved by engineering a solution-processed electron buffer layer that does not require high temperature steps. A compact thin film of perovskite absorber is grown onto a PCBM-based electron extraction layer by implementing a novel two-step procedure which preserves the soluble organic interlayer during the deposition of successive layers. We demonstrate that efficient charge extraction is the key for high steady state efficiency in perovskite solar cells with a highly integrable architecture