Research On Enhancing Photovoltaic Cell Performance Through Optimizing Carrier Transport Properties Of Two-Dimensional Perovskite Films

Due to their notable advantages such as exceptional environmental stability,tunable optoelectronic properties,and the ability for solution-based fabrication,the emerging organic-inorganic hybrid two-dimensional（2D）perovskite materials exhibit significant potential in photovoltaic cell applications.However,random crystal orientation,disordered or uneven phase distribution,and surface defects of 2D perovskite films deteriorate charge transport and induce serious non-radiative recombination,leading to a generally low power conversion efficiency（PCE）in 2D perovskite photovoltaic cells（2D PPCs）.Furthermore,the severe carrier recombination losses caused by the poor quality of large-scale deposited 2D perovskite films limit the practical application of 2D perovskites in photovoltaic cell modules.To address these issues,this dissertation focused on optimizing the properties of 2D perovskite films through controlling crystal orientation and phase distribution,implementing rational surface modifications,and fine-tuning crystallization during the large-scale deposition process.These efforts resulted in the improved carrier transport and the suppressed non-radiative recombination in 2D perovskite films,ultimately enhancing the PCE of 2D PPCs and their modules.The specific research is as follows:1.A simple and highly effective method was employed by introducing dimethyl sulfoxide（DMSO）and thiourea-sulfur compound（TSC）additives into the precursor solution to gain the vertical crystal orientation of 2D perovskite films.DMSO and TSC additives exhibit a synergistic effect on regulating the quality of 2D perovskite films.DMSO plays a critical role in slowing solvent evaporation,and TSC induces the aggregation of perovskite precursors.Their synergistic effect is conducive to reducing nucleation density and slowing down the crystallization process,resulting in the formation of large vertically-oriented grains.The 2D perovskite films treated with both DMSO and TSC show a continuous surface without pinholes or cracks,characterized by enlarged grain sizes,vertical crystal orientation,and high crystallinity,thereby improving charge carrier transport performance.Ultimately,the PCE of 2D PPCs is significantly boosted from 1.05%for control device to an impressive 14.15%for device treated with DMSO and TSC.Furthermore,the environmental stability of both the 2D perovskite films and devices is greatly enhanced.2.A sulfobetaine zwitterion（ZW）-assisted crystallization process of 2D perovskite was proposed to effectively reduce the phase distribution non-uniformity in 2D perovskite films.ZW can form hydrogen bonding/electrostatic interactions with the organic cations and coordinate with Pb²⁺ions in the precursor solution.This property contributes to the balanced growth of 2D and 3D-like phases,and inhibits the asynchronous consumption of different organic cations.Compared to control films without ZW treatment,ZW-treated films exhibit a more uniform phase distribution and fewer undesirable perovskite phases,enabling the faster direct charge transfer from low n-value phases to high n-value phases.Moreover,ZW also plays a role in defect passivation and optimization of crystal orientation to a certain extent.Moreover,ZW also plays a role in defect passivation and optimization of crystal orientation to a certain extent.As a result,the PCE of 2D PPCs based on ZW treatment is improved from 13.85%for control devices to 17.04%,with an open-circuit voltage(V_oc)reaching as high as 1.19 V.It’s worth noting that these devices were prepared in the high-humidity environment,and the unencapsulated ZW-treated device showed remarkable environmental stability.3.The rational surface modification of 2D perovskite films was achieved through a multifunctional ionic liquid,1-butyl-3-methylimidazolium trifluoro-methane-sulfonate（BMIMOTF）.BMIMOTF effectively passivates surface defects of 2D perovskite films and reduces the hole transfer barrier,thereby significantly suppressing non-radiative carrier recombination.Theoretical calculations and experimental results indicate that the role of the OTF^-anion of BMIMOTF is more crucial than the BMIM⁺cation,as the anion exhibits higher binding affinity energy with the dominant iodine vacancy defects and stronger interaction with uncoordinated Pb²⁺cations on the film surface.Compared to BMIMOTF,its iodide counterpart（BMIMI）exhibits destructive and limited surface modification efficacy,further highlighting the significant defect passivation effect of the OTF^-anion.Therefore,2D PPCs based on BMIMOTF surface modification achieved a champion efficiency of 21.38%,surpassing the efficiency of control devices（19.02%）and BMIMI-modified devices（20.37%）.The V_oc and fill factor reached as high as 1.195V and 80.02%,respectively,making it one of the highest-efficiency 2D PPCs reported to date.Furthermore,BMIMOTF enhances the hydrophobicity and suppresses the ion migration in 2D perovskite films,significantly improving environmental stability and thermal stability of devices.4.High-quality large-area 2D perovskite films were successfully deposited through controlling solvent composition in perovskite precursor solution,with the fabrication method using blade coating and vacuum-assisted crystallization.It was found that replacing DMSO with DMPU,which has a lower saturated vapor pressure and higher Lewis-basicity,introduces more crystal nuclei and more stable solvate intermediate phases during the film formation process.This,in turn,promotes solute diffusion and Ostwald ripening.As a result,the 2D perovskite films prepared with the mixed solvent of DMF and DMPU（DMPU-2D）display a continuous and dense morphology,large grain sizes,high crystallinity,and superior crystal orientation.The enhanced film quality extends the carrier lifetime,mitigates non-radiative recombination,and enhances carrier transport performance.Consequently,single-junction 2D perovskite photovoltaic cell modules（2D PPMs）with effective areas of 14.7 cm² and 57.3 cm² achieved a PCE of16.21%and 14.86%,respectively,making them the most efficient 2D PPMs known to date.Furthermore,due to the improved film quality,unencapsulated DMPU-2D PPMs exhibited excellent operational stability.