first solid-state perovskite solar cell (PSC) was successfully prepared by introducing Sprio as a hole transport layer (HTL), tremendous efforts have been devoted to further promoting the photovoltaic performance, including surface passivation, bulk doping, processing engineering, and optimizing the structure of the device. [9][10][11][12][13][14][15][16][17][18][19][20][21] Consequently, the power conversion efficiency (PCE) of PSCs has dramatically increased from the initial 9% to an impressive 25.2% in several years. [22,23] To date, the coexistence of high efficiency and long-term stability has become a crucial requirement for successful PSC applications. 2D perovskites (consisting of pure 2D and quasi-2D perovskites) have emerged as a new family of photovoltaic materials that have been proven to resolve the problem of instability in perovskites. Owing to the insulating spacer layer, inherent outstanding long-term stability is obtained in 2D perovskites, including hydrophobicity, suppression of ion migration, and larger formation energy. [24][25][26][27][28] However, severe carrier recombination occurs due to the combined effect of quantum confinement and dielectric confinement, rooted in the natural multiple quantum wells structure (MQWs) (Figure 1), where the inorganic parts act as potential "wells" while the organic parts act as "walls", which shows a boost in the photovoltaic performance of 2D PSCs. [29][30][31] To realize the coexistence of high efficiency and ultrastability in PSCs, researchers began to construct 2D/3D heterostructures by surface passivation. [32][33][34][35][36][37][38][39][40] In fact, before the boosting of the 2D/3D stacking concept, many researchers have applied large ammonium cations (such as phenethylamine (PEA), butylamine (BA), and quaternary ammonium (QA)) as defect passivants to 3D perovskite solar cells. [41][42][43][44][45] During this period, the pure PEA-or BA-based 2D perovskites have been explored as light-absorbing materials for better stability. [25,46] In 2015, Yao et al. introduced in situ grown 2D perovskite (PEI) 2 PbI 4 (PEI: polymeric ammonium) on an MAPbI 3 film, leading to more stable and efficient PSCs. [47] In 2016, discussions and analyses of the surface passivation effect began to emerge, which involved exploring a series of 2D perovskite molecules (such as aniline, benzylamine, and PEA). [42,45,48] The concept of 2D/3D stacking really stood out in 2018 by unveiling the chemical reaction mechanism for heterojunction formation and the mechanism for enhancing stability. For example, Huang 3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier ...

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA2MA3Pb4I13 (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N2 environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.

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