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Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7and with local strain8, both of which make devices less stable9. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process10,11, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices.

Operational stability of perovskite solar cells (PSCs) is rapidly becoming one of the pressing bottlenecks for their upscaling and integration of such promising photovoltaic technology. Instability of the hole transport layer (HTL) has been considered as one of the potential origins of short life-time of the PSCs. In this work, by varying the molecular weight (MW) of doped poly(triarylamine)(PTAA) HTL, we improved by one order of magnitude the charge mobility inside the HTL and the charge transfer at the perovskite/HTL interface. We demonstrate that this occurs via the enhancement of polaron delocalization on the polymeric chains through the combined effect of doping strategy and MW tuning. By using high MW PTAA doped combining three different dopant, we demonstrate stable PSCs with typical power conversion efficiencies above 20%, retain more than 90% of the initial efficiency after 1080 h thermal stress at 85 °C and 87% of initial efficiency after 160 h exposure against 1 sun light soaking. By using this doping-MW strategy, we realized perovskite solar modules with an efficiency of 17% on an active area of 43 cm2, keeping above 90% of the initial efficiency after 800 h thermal stress at 85 °C. These results, obtained in ambient conditions, pave the way toward the industrialization of PSC-based photovoltaic technology.