Cavendish Laboratory, University of Cambridge, Cambridge, UK.
Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Japan.
Nature. 2020 Apr;580(7803):360-366. doi: 10.1038/s41586-020-2184-1. Epub 2020 Apr 15.
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 devices. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance, 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 performance. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions and with local strain, both of which make devices less stable. 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 process, 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.
卤化物钙钛矿材料在低成本光电应用方面具有有前景的性能特点。由钙钛矿吸收体制成的光伏器件在单结器件中达到了超过 25%的功率转换效率，在串联器件中达到了 28%。这种优异的性能（尽管低于约 30%和 35%的实际极限）在低温溶液处理的薄膜中令人惊讶，这种方法通常会产生丰富的晶体缺陷。尽管点缺陷通常只会在钙钛矿带隙中诱导浅的电子态，而不会影响性能，但钙钛矿器件仍然在带隙中有许多深的状态，这些状态会捕获电荷载流子并导致它们非辐射复合。这些深陷阱状态因此会引起光致发光的局部变化，并限制器件性能。这些陷阱状态的起源和分布尚不清楚，但它们与混合卤化物钙钛矿成分中的光诱导卤化物分凝以及局部应变有关，这两者都会使器件变得不稳定。在这里，我们使用光发射电子显微镜来成像先进的卤化物钙钛矿薄膜中的陷阱分布。我们观察到离散的纳米级陷阱簇，而不是在光致发光效率较差的区域内相对均匀的分布。通过将显微镜测量与扫描电子分析技术相关联，我们发现这些陷阱簇出现在晶体学和组成上不同的实体之间的界面处。最后，通过生成光致激发载流子俘获过程的时间分辨光发射序列，我们揭示了空穴俘获特性，其动力学受空穴扩散到局部陷阱簇的限制。我们的方法表明，在纳米尺度上管理结构和组成对于优化卤化物钙钛矿器件的性能至关重要。
