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Organic-inorganic hybrid lead halide perovskites (e.g., CH3NH3PbX3, X=Cl, Br, I) possess a unique combination of excellent electronic and photoelectrochemical properties including suitable and tunable bandgap, low exciton binding energy in the range of 9–80 meV, high extinction coefficient, and long electron and hole diffusion lengths [1], which make them excellent photovoltaic materials. The absorption coefficient of CH3NH3PbI3 is 5.7×10 cm at 600 nm, which is more than one order of magnitude larger than that of silicon. With large absorption coefficient, efficient light harvesting can be obtained with a thin absorber that in turn enhances the open circuit voltage. Photovoltaic solar cells with hybrid lead halide perovskites as active layers have achieved high power conversion efficiencies (PCE) [2], comparable to that of crystalline silicon and thin film solar cells. The perovskite layer is at the core of perovskite solar cells (PSCs), whose quality would directly determine the device performance. According to the in-situ X-ray scattering studies, the nucleation and crystallization activation energy of the perovskite (56.6–97.3 kJ mol) is much lower than that of amorphous silicon (280–470 kJ mol) [3]. The low crystallization energy barrier allows the perovskite films being readily prepared by a variety of low-temperature large-scale fabrication processes including ink-jet printing, doctor-blading and roll-to-roll print. However, such low crystallization energy also permits the easy formation of defects in perovskite crystals, which in turn compromises the material properties and device performance. The deficiency of long-term stability of the hybrid perovskite material has been one of the greatest barriers to the commercialization of PSCs. The hybrid perovskites are ionic crystals, and the organic and inorganic components are bonded by hydrogen bonds or van der Waals forces. They readily and sensitively respond to even subtle changes in ambient environment, for example, humidity, temperature, oxygen content, and UV light [4]. Phase transition occurs easily and they are easy to decompose into inorganic components due to the loss of organic components with the external stimulus [4]. A great number of approaches have been explored to improve the long-term stability of hybrid perovskite materials. Tuning the perovskite component, constructing two-dimensional perovskite, post treatment, surface passivation, encapsulation and device architecture modification all are investigated as effective strategies to protect the perovskite structure from inside and outside [5]. One of the most important strategies to achieve stable solar cells is to improve the intrinsic stability of the materials. Considering the volatility and thermal instability of methylammonium (CH3NH3) cation, replacing the volatile CH3NH3 cation with larger and heavier formamidinium (FA, NHCHNH2) cation is an effective way [6]. The relatively narrow band gap of FAPbI3 allows broader absorption of solar spectrum, which helps to obtain a high device performance. However, FAPbI3 forms a non-perovskite yellow film consisting of hexagonal phase at room temperature due to the large size of FA cation. Combination of the larger FA cation with the smaller Cs cation on the A site to form FA1−yCsyPb(I1−xBrx)3 can tune the tolerance factor close to 1, allowing the formation of the desired black perovskite phase. FA1−yCsyPb(I1−xBrx)3 perovskites exhibit enhanced phase

science china. materials

Surface-defect passivation through complexation with organic molecules leads to enhanced power conversion efficiency and long term stability of perovskite photovoltaics