The Influence Of Colloid In Perovskite Precursor Solution On Film Morphology

In a short span of few years,tremendous development has been made in the perovskite solar cells field due to the strong light absorption,high carrier mobility and long carrier-diffusion length of the lead halide perovskite materials.As the core of the perovskite solar cell,the optical absorption layer has a crucial influence on the final performance of the device.During the crystal formation,the precursor solution undergoes first a nucleation and then a crystal growth process.More precisely,perovskite follows a heterogeneous nucleation process,thus various sizes of nuclei are formed at the same time.Generally,nucleation and growth govern the film morphology.Few nuclei on the surface generate large crystal grain size with the disadvantage that pin-holes are left between the grains.Rapid nucleation is critical in obtaining pin-hole free perovskite films.However,it remains a significant challenge to finely tune the nucleation speed during film crystallizing process.Although uniform connected film is good for the cell performance,the original energy decreases due to non-radiative recombination at the undesirable grain boundaries.As is well-known,the density of grain boundaries in the perovskite layers with large grains is significantly minimized.Large-grained perovskite films are beneficial for prolonging carrier diffusion length and reducing trap density.Thus contol the nulei is important for preparing solar cells.The introduction of dimethylsulfoxide(DMSO)has recognized as an effective way to prepare high-quality pervoskite films.During the developed DMSO process,the formation of an intermediate phase has proven to uniform crystal growth rates between CH3NH3I(MAI)and PbI2 in common solvents.In this work,controllable intermediates were obtained in dimethylformamide(DMF)by additional solvent of DMSO,which tend to be closely packed by means of intermolecular self-assembly In addition,the bond strength and chemical composition of intermediate phase were investigated with FTIR and TGA.The use of different ratios of DMSO leads to four different complexes of MAI PbI2·DMF and MAI·PbI2·(DMSO)y(y=0.6,1.5,1.9),thus dramatically influencing the crystallization of perovskite films.Of these complexes,optimized MAI·PbI2·(DMSO)1.5 intermediate adduct enables highly dense perovskite thin films with large grain size and enhances efficient perovskite solar cells with high charge carrier lifetime.The anti-polar solvent technique is an effective way to improve the film quality in a perovskite solar cell.In this work,we revealed the reason why the chlorobenzene(CBZ)plays an important role in controlling the crystallization process.By investigating the formation of intermediate phases in precursor solution,we observe that the MAI,PbI2-DMF or MAI-PbI2-DMSO adducts have not yet formed until washing with non-polar solvent.The accelerated formation of intermediate phase yields high crystalline perovskite layers.Rapid solvent evaporation and retarded perovskite crystallization in one-step method were efficient to obtain high-quality perovskite films.Consequently,MAI-PbI2-DMSO intermediate showed neat rod-like structure with high crystallinity,which eventually transformed extremely dense and uniform perovskite films.The nucleation stage has important influence on the lead halide perovskite film morphology,and therefore the solar cell performance.Here,we introduce a facile temperature-assisted rapid nucleation(TRN)method to improve the film crystallizing process and the film morphology.Upon employing low temperature anti-solvent(diethyl ether,chlorobenzene and toluene),we stimulate homogeneous nuclei growth,resulting in a highly dense perovskite thin layer.The TRN method prepared(FAPbI3)0.85(MAPbBr3)0.15 devices exhibits the power conversion efficiency up to 19.2%and maintain over 85%of the original efficiency after 40 days at room temperature under a relative humidity of 10-40%.Large-grained perovskite layers with less defects are beneficial to minimize carrier recombination at grain boundaries,hence boosting cell performance.Here,we present a facile solvent exchange strategy(SES)to yield large-grained perovskite films with high reproducibility.Upon employing a stoichiometric PbI2:MAI:DMSO(3:2:2)precursor solution,we reduced the number of colloidal intermediates(Pbl42-)which is correlated with the nucleation of seeds,resulting in a perovskite thin layer with large grain sizes up to 2μm.The SES prepared CH3NH3PbI3 devices shows significantly reduced grain boundaries corresponding to decrease of recombination centers,which dramatically boosted the efficiency up to 17.2%.