In the development of perovskite solar cells, a new version of Don Quixote is needed if scientists are to keep on seeking the most celebrated works of literature, according to the evaluation criterion of ‘THE FIRST’ and ‘THE BEST’.

The electron transport layer plays an essential part for high-performing perovskite solar cells, on which successful extraction of charges from light harvester and the transfer to the electrode are critically dependent.

Organic–inorganic lead halide perovskites solar cells (PSCs) show great potential in the photovoltaic system, reaching overall power conversion efficiencies (PCE) up to 25.2%. The rapid increase of PCE in PSCs has made them the rising star of the photovoltaics world, with great interest to the academic community. Long‐term stability under working environments remains a significant challenge for the commercialization of PSCs, particularly those using inorganic–organic halide lead perovskite absorbers. In this regard, only the devices that can maintain long‐term stability under conditions of temperature, humidity, and UV irradiation can be called stable solar cells. This Review highlights the sources for the chemical instability problems in conventional CH3NH3PbI3‐based perovskite solar cells from humidity instability, phase instability, thermal instability, and ion migration. In pursuit of highly stable PSCs, this article also discusses the strategies to stabilize PSCs through both external and internal aspects without sacrificing the PCE, specifically including additive engineering with surface passivation and composition engineering.

Low‐temperature solution‐processed perovskite solar cells (PSCs) based on organic–inorganic hybrid perovskites have emerged as a low‐cost and high‐efficiency thin‐film photovoltaic technology. The reported power conversion efficiency (PCE) of laboratory produced PSCs with an active area of less than 0.1 cm2 has already exceeded 25%, which, however, decreases significantly to about 16% for a large device area of about 100 cm2. Therefore, the scalability has become one of the most significant limits on successful commercialization of perovskite photovoltaics. This includes realizing a homogenous and compact electron transport layer (ETL), facing with issues of defects, energy level mismatch, and high‐temperature annealing requirements. Therefore, an exploration of effective and low‐cost charge transport materials is crucial for scalable fabrication of highly efficient perovskite devices. The 2D materials have drawn wide attention in the PSC community with tunable bandgap and high carrier mobility. So far, the search for a wide range of novel 2D materials for use in PSCs has documented considerable progress; however, a lot remains to be done in this field. This review summarizes recent advancements in the application of emerging 2D materials as effective ETL, thus providing direction for future development toward efficient and large‐scale perovskite devices.

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