There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affected by photoinduced phase segregation, which triggers subbandgap defects that are detrimental to the open-circuit voltage. While this effect may be reversed, e.g., when leaving the cells in the dark, new perovskite compositions that exhibit enhanced stability may be required. In this Perspective, the compositional space beyond the conventional methylammonium-and formamidinium-based mixed halide compounds is reviewed in light of multijunction applications. These alternative absorber compositions include: (1) layered or quasi-2D perovskites, where larger organic cations are incorporated into the structure; (2) inorganic perovskites (i.e., when the organic components are removed altogether); and (3) lead-free structures, where the toxic lead is substituted by one or more elements. The development perspectives of highefficiency and stable perovskite materials based on these compositions are discussed in view of an integration in multijunction solar cells.

The mechanisms controlling the degradation of p–i–n perovskite solar cells in reverse bias are identified using various optoelectronic and microstructural characterization techniques.

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single‐junction devices and even perovskite‐silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long‐term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next‐generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up‐to‐date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO2, SnO2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and long‐term stability with an outlook on current challenges and further development are discussed.

accounts for only ≈3% of global electricity generation. [1] However, PV is experiencing an accelerated growth globally with >130 GW installed in 2020, an acceleration that should continue in the future to provide 20-30% of the global electricity on the 2050 horizon. [2] The key to materializing this ambitious goal is to reduce the cost of PV-generated electricity to make solar energy significantly cheaper than that produced by fossil fuels, and to promote the implementation of storage technologies. [3] Currently, the major cost component of a PV system stems from the balance-ofsystems (BOS). [4] The BOS refers to all the components of a PV system other than the solar module, including wiring, inverters, land, installation, labor, etc. With cell costs typically accounting for less than 20% of the total module cost (and module costs typically account for around 40% at the system level), [4,5] increasing power conversion efficiency at the cell and module level is the most efficient way to reduce the levelized cost of electricity (LCOE), provided this efficiency gain comes at affordable manufacturing costs. [6] Increasing the solar module efficiency is even more important for residential rooftops, facades, or other applications where This review focuses on monolithic 2-terminal perovskite-silicon tandem solar cells and discusses key scientific and technological challenges to address in view of an industrial implementation of this technology. The authors start by examining the different crystalline silicon (c-Si) technologies suitable for pairing with perovskites, followed by reviewing recent developments in the field of monolithic 2-terminal perovskite-silicon tandems. Factors limiting the power conversion efficiency of these tandem devices are then evaluated, before discussing pathways to achieve an efficiency of >32%, a value that small-scale devices will likely need to achieve to make tandems competitive. Aspects related to the upscaling of these device active areas to industryrelevant ones are reviewed, followed by a short discussion on module integration aspects. The review then focuses on stability issues, likely the most challenging task that will eventually determine the economic viability of this technology. The final part of this review discusses alternative monolithic perovskite-silicon tandem designs. Finally, key areas of research that should be addressed to bring this technology from the lab to the fab are highlighted.