Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

The certified power conversion efficiency of state-of-the-art organic–inorganic hybrid perovskite solar cells has surpassed 25%, showing promising potential for commercialization. Compared with volatile methylammonium-based perovskites, formamidinium- and cesium-based halide perovskites have attracted attention due to their resistance to thermal degradation. However, the photoactive perovskite phases of these materials suffer from limited phase stability at room temperature and are prone to spontaneous transformation into the photoinactive perovskite phases. This Review sheds light onto the fundamental understanding of the origin of phase instability for both the intrinsic structure and the extrinsic factors. We highlight the methodologies used to suppress the undesired phase transitions of formamidinium- and cesium-based halide perovskites with an emphasis on structure–property relationships.

Halide perovskite semiconductors have risen to prominence in photovoltaics and light‐emitting diodes (LEDs), but traditional oxide perovskites, which overcome the stability limitations of their halide counterparts, have also recently witnessed a rise in potential as solar absorbers. One of the many important factors underpinning these developments is an understanding of the role of dimensionality on the optoelectronic properties and, consequently, on the performance of the materials in photovoltaics and LEDs. This review article examines the role of structural and electronic dimensionality, as well as form factor, in oxide and halide perovskites, and in lead‐free alternatives to halide perovskites. Insights into how dimensionality influences the band gap, stability, charge‐carrier transport, recombination processes and defect tolerance of the materials, and the impact these parameters have on device performance are brought forward. Particular emphasis is placed on carrier/exciton‐phonon coupling, which plays a significant role in the materials considered, owing to their soft lattices and composition of heavy elements, and becomes more prominent as dimensionality is reduced. It is finished with a discussion of the implications on the classes of materials future efforts should focus on, as well as the key questions that need to be addressed.

Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 1018 cm−3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.