We quantify recombination losses in the bulk and interfaces for different perovskite compositions and popular charge transport layers.

Perovskite solar cells with all-organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high-temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron-transporting layer of inverted perovskite cells affects the open-circuit voltage (V ). It is shown that nonradiative recombination mediated by the electron-transporting layer is the limiting factor for the V in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH NH PbI perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a V as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge-blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high V and efficiency.

devices. Due to substantial research efforts, the power conversion efficiency of perovskite-based PV cells has been pushed beyond 20%. [3] The most widely studied materials are the triiodide CH 3 NH 3 PbI 3 and mixed halide CH 3 NH 3 PbI 3−x Cl x perovskites, which can be either solution processed [4] or thermally evaporated. [5] In particular, the mixed halide perovskites have been widely investigated due to their long carrier high diffusion length (over 1 µm [6] ) and low density of trap states, [7] making them especially efficient for charge generation and collection. Though substantial achievements for solar cells using solution processed CH 3 NH 3 PbI 3−x Cl x thin films have been reported, large variations in efficiency still persist among the devices even for nominally identical device configurations, [8,9] indicating significant differences in perovskite film properties. This is certainly to be related to the different preparation methods, e.g., one-step or twostep, [10] solvent treatment, [4] as well as the use of chlorine containing precursors, [11] which can greatly influence the film morphology and charge transport properties.Despite the rapid improvement in device performance, most of the fundamental questions regarding the physico-chemical properties of these materials remain to be unequivocally answered. In this context, the determination of the electronic structure and interface energetics with electron and hole transport layers, as well as contacts are crucial prerequisites to further device optimization.Photoelectron spectroscopy (PES) is the experimental technique of choice to investigate the core level and valence electronic structure of semiconductors. Particularly relevant parameters are the work function (φ), the ionization energy (IE), and the energy difference between the valence band maximum (VBM) and the Fermi level (E VBM − E f , where E f = 0 eV). To date, ultraviolet photoelectron spectroscopy (UPS) studies reporting the valence electronic properties of CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3−x Cl x thin films have shown a wide range of VBM binding energies. [12][13][14][15][16] In most UPS studies, E f was reported very close to (or even within) the conduction band edge, thus suggesting n-type character. The transport gap of these two perovskites was determined by Schulz et al. via combining UPS This study investigates the effect of white light illumination on the electronic and chemical properties of mixed halide perovskite (CH 3 NH 3 PbI 3−x Cl x ) thin films and CH 3 NH 3 PbI 3 single crystals using photoelectron and absorption spectroscopy. The pristine materials' surfaces are found to be n-type because of surface band bending due to the presence of donor levels, likely consisting of reduced lead (Pb 0 ) that acts as surface traps. When illuminating the sample with white light (up to 1 sun), the valence features shifted to lower binding energy due to surface photovoltage, i.e., the bulk of the materials is much less n-type. However, the surface photovoltage is only partially revers...