IntroductionIn recent decades, the world has come to a consensus that new green energy sources are vital for sustainable development. Solar energy has been regarded as an alternative source of energy supply. Photovoltaic (PV) technology has progressed for decades with massive efforts devoted. Alternative technologies for photon to electron power conversion include, for instance, This review presents various hole transport layers (HTLs) employed in perovskite solar cells (PSCs) in pursuing high power conversion efficiency (PCE) and functional stability. The PSCs have achieved high PCE (over 23%, certified by NREL) and more efforts have been devoted into research for stability enhancement. Inorganic HTLs become a popular choice as selective contact materials because of their intrinsic chemical stability and low cost. HTLs and electron transport layers (ETLs) are critical components of PSCs due to the requirement to create charge collection selectivity. Herein the authors provide an overview on inorganic HTLs synthesis, properties, and their application in various PSCs for both mesoporous and planar architectures. InorganicHTLs with appropriate properties, such as proper energy level and high carrier mobility, can not only assist with charge transport, but also improve the stability of PSCs under ambient conditions. The importance of interfacial chemistry and interfacial charge transport is further addressed to understand the underlying mechanism of related degradation and carrier dynamic. It is expected that the success of the inorganic HTL in PSCs can stimulate further research and bring real impact for future photovoltaic technologies.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201800882. super-mesostructure Al 2 O 3 scaffold, achieving a PCE above 9% [6] and 10%, [7] respectively.

Perovskite solar cells (PSCs) have achieved a high power conversion efficiency (PCE) with a credible certified value over 25%. More efforts have been devoted to the development of stable and ecofriendly perovskite materials. Lead-free double perovskites (LFDPs) are a noteworthy choice as a photoactive layer because of their favorable photovoltaic (PV) properties, intrinsic chemical stability, and environmental friendliness. This Review presents various LFDP materials whose structural stability and optoelectronic properties are predicted by theoretical calculations. The synthesis and experimental properties of LFDPs and their applications in PSCs and optoelectronics in pursuing high performance, low toxicity, and functional stability are also reviewed. Perovskites active layers are critical for PSCs, and their appropriate properties are responsible for achieving a high PCE. On the other side, the stability of PSCs under working conditions is a critical requirement for their practical applications. Defect-ordered perovskites are also presented to provide another outlook on lead-free perovskite-based PVs. The introduction and interest toward LFDP in PSCs can represent a viable solution to the toxicity issue, stimulate further research, and bring a real impact to future PV technologies.

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