Nat. Commun. 9, 1336 (2018)

Two-dimensional (2D) Ruddlesden–Popper metal halide perovskites (MHPs) offer a promising solution to the notorious moisture sensitivity of bulk perovskites. However, in devices with vertical architecture, the insulating bulky ammonium cations that separate 2D perovskites slabs impede efficient out-of-plane charge transport. Now, Chen et al. have devised a strategy to enhance optoelectronic properties of 2D MHPs by controlling the alignment of vertically oriented layers.

The researchers used a two-step fabrication procedure involving solution processing and low-temperature annealing to achieve thin films with various degrees of molecular orientation. X-ray measurements reveal that nucleation and growth of 2D perovskites occur at the precursor liquid–air interface. The assumed orientation stems from the tendency of the aliphatic chain of butylammonium molecules to remain in the solution environment. By tuning the crystallization conditions, specifically by adding a pre-crystallization annealing step, Chen and co-workers were able to obtain thin films of vertically oriented 2D perovskites with near 100% degree of the out-of-plane preferential alignment. When made into a solar cell, such optimized 2D MHPs show a strong short-circuit current increase and an overall efficiency improvement in comparison with randomly oriented crystals.