PbI₂ Heterogeneous-cap-induced Crystallization For An Efficient CH₃NH₃-PbI₃ Layer In Perovskite Solar Cells

CH3NH3PbI3 perovskite materials were introduced for the first time as sensitizers in solar cells in 2009 and afterwards they have been applied in photodetectors,light-emitting diodes,lasers,and especially perovskite solar cells(PSC)due to their favorable direct bandgap,low exciton binding energy,long exciton lifetime and diffusion length,and,most importantly,high mobility for charge carriers of both polarities.The power conversion efficiency(PCE)of PSCs has been boosted from 3.8%to 22.1%,and PSCs have been seen as a new prospective generation of photovoltaic devices,considering their low cost,simple fabrication,good tolerance of defects and easy self-assembly ability.Generally,in PSC,it is essential to obtain high-quality uniform CH3NH3PbI3 perovskite films with large-size grains and few grain boundaries.Lots of works have been done towards this end,such as solvent engineering,laser-assisted annealing,subsequent deposition,room-temperature formation,and so on.Of all,one-step spin-coating is the most widely applied method.Herein,we propose an innovative solution to enhance the crystallinity of the CH3NH3PbI3 perovskite film and hence the PCE of PSC by successfully introducing PbI2 heterogeneous-cap-induced crystallization.Based on the traditional one-step spinning-coating method plus anti-solvent diethyl ether washing down,we cover CH3NH3PbI3 perovskite precursor film face-to-face with a PbI2 heterogeneous cap during thermal annealing for about 15 min.After that,the cap was removed.Through our study,we find that the grain sizes are much large and grain orientation(110)is more preferred.The transport of charge carriers is also more efficient.The as-fabricated PSC reach a PCE of 17.6%higher than that of 14.2%of the conventional PSC.Moreover,they also show good reproducibility.Therefore,we see PbI2 heterogeneous-cap-induced crystallization as an innovative and manageable method.We come up with an explanation that this shift is mainly because of the depressed volatilization of the residual solvent.We hope this paper can inspire the future work on boosting PCE and lead us to better understand of the dynamics and thermodynamics in the annealing process in PSC.