2D Ruddlesden−Popper (2DRP) tin (Sn) perovskite solar cells (PSCs) play an irreplaceable role in advancing the commercialization of perovskite-based photovoltaic devices due to their low toxicity and improved stability. However, the efficiency of 2DRP Sn PSCs has not made a breakthrough owing to incompletely oriented crystal growth and poor film morphology, which is limited by a complex and uncontrollable crystallization process. Here, we first introduce the mixed spacer organic cations [n-butylamine (BA) and phenylethylamine (PEA)] in 2DRP Sn perovskite to control the crystallization process. We find that when the BA + and PEA + cowork to form [(BA 0.5 PEA 0.5 ) 2 FA 3 Sn 4 I 13 ] 2DRP perovskites, the intermediate phase impeding the homogeneous and ordered nucleation of the crystal is suppressed effectively, thus enabling a high-quality film morphology and improved crystal orientation. Benefitting from it, the power conversion efficiency (PCE) is improved to 8.82%, which is the highest one among the 2DRP Sn PSCs as far as we known.

Two‐dimensional (2D) AA′n−1MnX3n+1 type halide perovskites incorporating straight‐chain symmetric diammonium cations define a new type of structure, but their optoelectronic properties are largely unexplored. Reported here is the synthesis of a centimeter‐sized AA′n−1MnX3n+1 type perovskite, BDAPbI4 (BDA=NH3C4H8NH3), single crystal and its charge‐transport properties under X‐ray excitation. The crystal shows a staggered configuration of the [PbI6]4− layers, a band gap of 2.37 eV, and a low trap density of 3.1×109 cm−3. The single‐crystal X‐ray detector exhibits an excellent sensitivity of 242 μC Gyair−1 cm−2 under the 10 V bias (0.31 V μm−1), a detection limit as low as 430 nGyair s−1, ultrastable response current, a stable baseline with the lowest dark current drift of 6.06×10−9 nA cm−1 s−1 V−1, and rapid response time of τrise=7.3 ms and τfall=22.5 ms. These crystals are promising candidates for the next generation of optoelectronic devices.

Dion–Jacobson (DJ) phase halide perovskites have attracted extensive attention in photovoltaic devices due to their significantly enhanced stability when compared with conventional 3D analogs. However, fundamental questions concerning the quantum well (QW) barrier thicknesses, which are critical to design efficient DJ phase perovskite photovoltaics, remain unknown. Herein, it is unambiguously demonstrated that QW barrier thickness, depending on bulky organic ammonium spacers with different chain lengths, such as 1.3‐propanediamine (PDA), 1.4‐butanediamine (BDA), 1.5‐pentamethylenediamine (PeDA), and 1.6‐hexamethylenediamine (HDA), allows the control of orientation and QW distribution. The DJ phase perovskites based on PDA and BDA have suitable QW barrier thicknesses, which exhibit excellent orientation and more uniform QW distribution, allowing a smooth bandgap transition that leads to longer carrier diffusion length, higher charge mobility, and lower defect density. Conversely, PeDA and HDA, with thicker QW barriers, result in lower orientation and multiple DJ perovskite phases. DJ phase perovskite photovoltaic devices based on PDA and BDA show significantly improved power conversion efficiencies (PCEs) of 14.16% and 16.38% compared with PCEs of 12.95% and 10.55% for PeDA and HDA analogs, respectively.

Due to the additional introduction of bulky organic ammonium and the competition between bulky organic ammonium and methyl ammonium in 2D Ruddlesden-Popper (2DRP) perovskite, the crystallization process becomes complicated. Here, it is demonstrated that the rapid crystallization controlled by processing solvents plays an important role in achieving highquality 2DRP perovskite films. It is found that the processing solvents, e.g., dimethylacetamide (DMAC), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), with a different polarity and boiling point, have almost no effect on crystal structure and phase distribution but have a remarkable effect on crystallization kinetics, crystal growth orientation, and crystallinity of 2DRP perovskite. Compared to polar aprotic solvent DMF and DMSO with a high boiling point, DMAC with low polarity and a suitable boiling point has a weak coordination to lead and ammonium salts and is easy to escape during solution processing, which is able to accelerate the crystallization rate of 2DRP perovskite. Benefitting from the rapid crystallization enabled high-quality 2DRP perovskite films, the best-performing device with improved stability and a power conversion efficiency of 12.15% is obtained using DMAC solvent. These findings may give guidance for solvent engineering for highly efficient 2DRP perovskite solar cells in the future.