Asymmetrical Single Crystals Containing Tilted Ruddlesden–Popper Phases for Efficient Perovskite Solar Cells

2D perovskites are receiving increasing amounts of attention because of their superior device stability. The framework is “cut” into corner‐sharing PbX 6 layers, known as Ruddlesden–Popper (RP) phases, by employing bulky cations that cannot fit into the previous A‐site interspaces. Unfortunately, the insulating moieties of the cations can significantly affect charge transport in 2D perovskites, thereby limiting the power conversion efficiencies (PCEs) of perovskite solar cells. Herein, a vapor venting space‐limited crystallization method is developed for growing asymmetric 2D perovskite single crystals (SCs) featuring nearly vertically oriented RP phases. The high crystallinity and preferred orientation of the 2D layered structures minimize the impact of the bulky side chains of the ammonium cations. By using grazing incidence small‐angle X‐Ray scattering, it is found that the RP phases in the 2D SCs are aligned with a tilt angle with respect to the substrate normal. It is suspected that the strong π–π interaction between the benzene rings of the ammonium cations and the surface of the hole‐transport layer plays important roles in determining the 2D crystal structure. The PCE is improved to greater than 16% after surface passivation to lower the degree of surface defects.