Perovskite solar cells (PSCs) based on 2D/3D heterostructures show great potential to combine the advantages of the high efficiency of 3D perovskites and the high stability of 2D perovskites. However, an in‐depth understanding of the organic‐spacer effects on the 2D quantum well (QW) structures and electronic properties at the 2D/3D interfaces is yet to be fully achieved, especially in the case of 2D perovskites based on diammonium spacers/ligands. Here, a series of diammonium spacers is considered for the construct ion 2D/3D perovskite heterostructures. It is found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers, including their phase purity and orientation. Density functional theory calculations indicate that the spacer modifications can induce shifts in the energy‐level alignments at the 2D/3D interfaces and therefore influence the charge‐transfer characteristics. The strong intermolecular interactions between the 2,2‐(ethylenedioxy)bis(ethylammonium) (EDBE) cations and inorganic [PbI6]4− slabs facilitate a controlled deposition of a phase‐pure QW structure (n = 1) with a horizontal orientation, which leads to better surface passivation and carrier extraction. These benefits endow the EDBE‐based 2D/3D devices with a high power conversion efficiency of 22.6% and remarkable environmental stability, highlighting the promise of spacer‐chemistry design for high‐performance 2D/3D PSCs.

2D/3D Perovskites In article number 2102973, Qifan Xue, Hong Li, Jean‐Luc Brédas, Hin‐Lap Yip and co‐workers report a spacer tailoring strategy to regulate the interfacial properties of diammonium‐based 2D/3D perovskite heterostructures, which allows the well‐controlled phase composition and crystalline orientation and enhanced surface passivation of a 2D capping layer. This work provides an important insight into the further development of high‐performance perovskite solar cells through interfacial reconstruction.

Metal oxides are commonly employed as electron transport layers (ETLs) for n-i-p perovskite solar cells (PSCs), but the presence of surface traps and their mismatched energy alignment with perovskites limits the corresponding device performance. Therefore, the interfacial modification of ETLs by functional molecules becomes an important strategy for tailoring the interfacial properties and facilitating an efficient charge extraction and transport in PSCs. However, an in-depth understanding of the influences of their molecular structures on the surface chemistry and electronic properties of ETLs is rarely discussed. Herein, three carboxylic acid-based molecules with different chemical structures were employed to modify the SnO2 ETL and their effects on the performance of PSCs were systematically investigated. We found that the alkyl-chain length and carboxyl number in molecular structures can dramatically alter their binding strength to SnO2, providing a good strategy to fine-tune their film quality, electron mobility, and energy offset at the cathode interface. Benefiting from the optimal coordination ability of citric acid (CA) to SnO2, the corresponding PSCs show better charge transport properties and suppressed nonradiative recombination, leading to a champion efficiency of 23.1% with much improved environmental stability, highlighting the potential of rational design of molecular modifiers for high-performance ETLs applied in PSCs.