Microstructure And Composition Modifications Of CH₃NH₃PbI₃ Polycrvstalline Layers For Perovskite Solar Cells

Perovskite solar cells stemmed from traditional dye sensitized solar cells stand a new type of photovoltaic devices,which are named after the crystal structure of light absorption materials,namely metal organic halides such as CH₃NH₃PbI₃,which possess typical perovskite structure.Over the past few years,it has become a rapidly rising star due to low cost of raw materials,simple solution processability,and swiftly increased power conversion efficiency?PCE?.The PCEs so far certified have gone beyond 22%for perovskite solar cells and 23.6%for tandem devices with single crystalline silicon solar cells,which offer a promising photovoltaic technology for practical applications.Even so,there are still some urgent challenges in the commercialization of perovskite solar cells,especially the further improvement of cell efficiency,alleviation of current density-voltage?J-V?hysteresis,and realization of long-term cell stability.Previous works suggest that it is a feasible strategy to achieve the above goals by modifying optoelectronic properties and stability of absorption layer.Originally,such properties of absorption layer are largely dominated by its microstructure features and composition.Therefore,taking CH₃NH₃PbI₃ polycrystalline film as an example we propose a series of feasible routes to realize the modifications of its microstructure features including surface coverage,texture,and grain size as well as its composition.It should be noted that such routes stem from the achievements of previous studies on microstructure and composition modifications of functional thin films.This thesis provides a reference and research guide for realizing efficient and stable perovskite solar cells.Detailed research contents and main conclusions are as follows:?1?A facile solvent fumigation route that is frequently used to improve the coverage and crystallinity of organic thin films is introduced to modify surface coverage of the CH₃NH₃PbI₃ polycrystalline films prepared by traditional one-step spin-coating method.It is found that the surface coverage as well as crystallinity and adsorption intensity increase with the increase in solvent fumigation cycle.Under the optimized solvent fumigation cycle,the pristine films with dendritic structures can transform spontaneously to the uniform ones with full coverage and high crystallinity.Mesostructured perovskite solar cells with these high-quality CH3NH3Pbl3 polycrystalline films show reproducible optimal power conversion efficiency?PCE?of 11.15%and average PCE of 10.25±0.90%,which are much better than that of devices with the pristine ones.In addition,the hysteresis phenomenon in J-V curve test can also be effectively alleviated.Substantially enhanced photovoltaic performances can be mainly attributed to enhanced light absorption,reduced shunting paths,and increased charge transfer and separation efficiencies in the optimized devices.?2?Referring the strategy that using post-synthesis treatment to coarsen the grains of absorption layers in CdTe and CuInSe2 solar cells,a facile and effective post-synthesis high-temperature heating treatment assisted with spin-coated CH3NH3I is proposed to controllably induce the grain coarsening of one-step deposited CH₃NH₃PbI₃ polycrystalline films.The modified film shows significantly increased grain size and reduced defect states corresponding to the decrease of recombination centers,and then enhanced carrier transport and injection properties.Such desired properties lead to dramatically boosted efficiency from 14.54%to 16.88%for planar-heterojunction solar cells.More importantly,the improvement in film quality provides the possibility of thickening the absorber layer of cells to realize more sufficient absorption without serious aggravation of charge recombination.By further optimizing the film thickness,highly efficient cells with relatively excellent reproducibility and an optimal efficiency of 19.24%are achieved.?3?The homogeneous cap-mediated crystallization concept that has been developed well for inorganic functional films is introduced to modulate crystallization kinetics of CH₃NH₃PbI₃ polycrystalline films,thus realizing the regulation in grain size of the films.It is found that increase in surface roughness of cap films is companied by increase in average grain size of CH₃NH₃PbI₃ films.The optimized samples possess 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 with 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.?4?By improving the homogeneous cap-mediated crystallization method,we propose a facile face-down annealing route that can enable the formation of?110?textured CH₃NH₃PbI₃ polycrystalline films.And the texture degree can be controlled by adjusting the temperature in preheating precursor films.When such temperature is set to 40 ?,the obtained films possess the most desired features.They are consisted of high-crystallinity well-ordered micrometer-sized grains that span vertically the entire film thickness.As a result,the planar-heterojunction solar cells with these textured CH₃NH₃PbI₃ films show remarkably improved PCEs with the average of 17.11%and the highest up to 18.64%.The notorious J-V hysteresis is also alleviated substantially in the champion cell.More importantly,the champion cell can maintain 86%of its initial PCE after storage of 20 days in ambient conditions with relative humidity of 10-20%.Such desired performance features can be mainly attributed to the much reduced nonradiative recombination sites as well as greatly improved transport property of charge carriers in the textured CH₃NH₃PbI₃ films.?5?Referring ion exchange strategy proposed in inorganic functional films and nanomaterials,a halide exchange route based on CH3NH3Br solution post-treatment has been successfully employed to transform the two-step spin-coating deposited CH₃NH₃PbI₃ polycrystalline films into CH₃NH₃PbI₃-xBrx films.The physical properties as well as the performances of resultant devices are systemically investigated.With increase inCH3NH3Br solution concentration,CH₃NH₃PbI₃-xBrx films exhibited increased grain size,prolonged charge carrier lifetime,and enlarged bandgap with slightly reduced light absorption compared with parent CH₃NH₃PbI₃ film.Mesostructured perovskite solar cells with the CH₃NH₃PbI₃-xBrx films yield the optimal PCE of 14.25%and stabilized one of 13.29%.In particular,the champion can retained up to 93%of its efficiency after exposed to air for 14 days without any encapsulation,presenting a favorable stability.And then,the halide exchange route is further extended to prepare high-quality wide-bandgap CH₃NH₃PbI₃-xBrx polycrystalline films.By varying the halide exchange time,the content of Br ions and thus the bandgap of CH₃NH₃PbI₃-xBrx films can be controllably tuned.Taking the dense and homogenous flat wide-bandgap CH3NH3PbI2.1Br0.9 films as example,we realize planar-heterojunction solar cells with an optimal PCE of 12.67%and negligible J-V hysteresis.Such cells are compatible with Si or bandgap-tunable III-V semiconductors solar cells for tandem cell applications.