Dramatically promoted crystallization control of organolead triiodide perovskite film by a homogeneous cap for high efficiency planar-heterojunction solar cells

A homogeneous cap-mediated crystallization concept with face-to-face configuration is proposed here to modulate the crystallization kinetics of organolead triiodide perovskite (OTP) films in a controlled way. The introduced OTP caps, especially those with low surface roughness, play a dramatically positive role in retarding the nucleation rate, promoting the growth, and preventing the composition loss of OTP grains, fully facilitating the formation of pinhole-free OTP films with numerous desirable characteristics, such as greatly enlarged grains, vertically aligned grain boundaries, preferred (110) orientation, significantly improved crystallinity, and proper stoichiometry. As a consequence, planar-heterojunction solar cells incorporating these high-quality films deliver a promising average efficiency of 17.87%, showing a remarkable enhancement of approximately 30% compared with control samples. In particular, large fill factors can be routinely achieved in these high efficiency cells. This excellent performance mainly originates from greatly suppressed non-radiative recombination as well as from the greatly enhanced diffusion and transfer properties of charge carriers in the cells as a result of the improved quality of the OTP films. Our work presents an effective and useful strategy to fabricate high-performance planar heterojunction solar cells based on OTP materials.