Perovskite synthesis out in the open: Although methods have been developed that create the photoactive black perovskite phase of formamidinium lead iodide (α-FAPbI ₃ ), these routes are temperature and humidity sensitive and less compatible with large-scale solar cell production. Hui et al. report an alternative route in which vertically aligned lead iodide thin films are grown from the ionic liquid methylamine formate. Nanoscale channels in the films lower the barrier to permeation of formamidinium iodide and enable transformation to α-FAPbI ₃ , even at high humidity and room temperature. Solar cells made with these films have power conversion efficiencies as high as 24.1% that display high stability. Science , this issue p. 1359

Metal halide perovskites (MHPs), emerging as innovative and promising semiconductor materials with prominent optoelectronic properties, has been pioneering a new era of light management (ranging from emission, absorption, modulation, to transmission) for next-generation optoelectronic technology. Notably, the exploration of fundamental characteristics of MHPs and their devices is the main research theme during the past decade, while in the next decade, it will be primarily critical to promote their implantation in the next-generation optoelectronics. In this review, we first retrospect the historical research milestones of MHPs and their optoelectronic devices. Thereafter, we introduce the origin of the unique optoelectronic features of MHPs, based on which we highlight the tunability of these features via regulating the phase, dimensionality, composition, and geometry of MHPs. Then, we show that owing to the convenient property control of MHPs, various optoelectronic devices with target performance can be designed. At last, we emphasize on the revolutionary applications of MHPs-based devices on the existing optoelectronic systems. This review demonstrates the key role of MHPs played in the development of modern optoelectronics, which is expected to inspire the novel research directions of MHPs and promote the widespread applications of MHPs in the next-generation optoelectronics.