Research On Non-radiative Defects Healing Of Perovskite Films And Its Application In Large-Area Perovskite Solar Cells

Due to excellent photoelectronic properties of organolead trihalide perovskites materials,perovskite solar cells?PSCs?have attracted tremendous attentions and made a significant stride over the past few years with an inspiring power-conversion-efficiency?PCE?up to 23.7%,rapidly approaching its theoretical limit of 31%,while the performance of large area perovskite solar cells and modules are still far behind.For the sake of realizing commercialization of perovskite solar cells early,many researchers pay more and more attention to the challenge of fabricating large area perovskite solar cells and modules.There are three parts to cover what I have performed to solve the challenges,which are as follows:?1?In the first part,a facile and effective healing method has been proposed to elaborately diminish defects of perovskite polycrystalline layers by treating the surface of two-step deposited perovskite films using formanidinium iodide/isopropyl alcohol?FAI/IPA?solution.The synergistic effect of FAI and IPA has been carefully investigated,which is crucial for healing non-radiative defects of perovskite films and improving the performance of devices.From the characterization of scanning electron microscopy?SEM?and X-ray diffraction?XRD?,there is scarcely any effect observed on the growth of grain size for perovskite films treated by IPA or FAI solution.However,pure IPA rinsing effectively sweeps unbonded FAI and dangling bonds away from the surface of perovskite layers,resulting in significantly increased amount of PbI2 on the surface of perovskite films.As the concentration of FAI continuously increasing,the amount of PbI2 gradually decreases and the stoichiometric ratio of perovskite layers is further finely tailor for better crystallinity.The time-resolved photoluminescence?TRPL?indicates the significantly improved carrier lifetime by FAI/IPA healing process,resulting in the longest carrier lifetime at a concentration of 1 mg/ml,which is1.8 times longer carrier lifetime in comparison to the control.Furthermore,the steady-state photoluminescence?PL?,and the confocal laser scanning microscopy?CLSM?proves the high quality of perovskite films with significantly reduced defects healed by FAI treatment.Meanwhile,the results indicate that defects mainly locate along grain boundaries and the optimal concentration of FAI/IPA is determined at 1 mg/ml.Furthermorne,the space-charge-limited-current?SCLC?characterizations confirms that the lowest defect density of perovskite films treated by FAI solution of 1 mg/ml is1.72×10¹⁶ cm^-3.Finally,the optimized solar cell based on FAI-healed perovskite films achieve a PCE of 20.66%due to an improvement in short-circuit current density?Jsc?and fill factor?FF?.Moreover,FAI-healed devices show an obvious enhancement in long-term stability as well,maintaining nearly 95%of its initial PCE.We also discuss the healing mechanism of IPA and FAI and the effect of reductant PbI2 residuals on long-term stability and PCE of PSCs.?2?In the second part,the methylamine acetate?MAAc?has been introduced to prepare perovskite precursor solution,which can completely avoid the usage of toxic anti-solvent and simplify the fabrication processes of PSCs.Based on the MAAc contianed perovskite precursor solution,we propose a newly developed grooved roller coating?GRC?technique to fabricate perovskite films and devices with high quality.After a systematic investigation,the critical groove depth of 6?m and the optimal concentration of 1.1 M have been determined to generate uniform perovskite films.From the characterization of SEM and XRD,the perovskite films exhibit relatively smaller average grain sizes?300-400 nm?from diluted solutions,while the grain size is relatively larger but the distribution is relatively wider from highly concentrated solutions.Finally,we found the proper concentration of 1.1 M producing perovskite films with relatively uniform and average 594nm sized grains and shifted dominant plane from?110?to?224?.TRPL confirms the balanced interfacial charge transfer?60.8ns?and bulk transport lifetime?140.6 ns?for efficient PSCs.GRC-prepared perovskite films demonstrate a competitive quality in comparation to that of spin-coated one,based on both of which PSC shows similar PCE of 15.26%for the GRC and 15.76%for the SSC,respectively.Finally,the PSCs based on fully roller-coated SnO2,perovskite and Spiro-OMeTAD achieve a PCE of 12.34%with slightly inferior FF,which is the highest PCE value obtainable by the fully roller coating technique.Therefore,the results predict the GRC could be facile and effective method for large area and continuous mass production of PSCs for future commercialization.?3?In the third part,the utilization ratio of material of different technologies for large area PSCs is firstly obtained by the theoretical calculation.Compared with other coating techniques,the utilization ratio of material by the GRC is 49.3%,which is one of the highest value reported by now and about 23 times higher than that of the traditional spin coating for small-scale samples.Meanwhile,it takes only 2³ seconds to prepare 100 cm² sized perovskite film by the GRC,which is considered as the effective way.Furthermore,the production capability of perovskite films by the GRC could be at least 72 m²/h based on the calculation assuming the length?400 mm?of the commercially available roller and the optimized coating speed of 10-50 mm/s in this experiment.The uniformity of large area perovskite films and the performance of devices prepared by the GRC is verified based on the experiments results.Finally,PSCs with varioius active area,including 0.1 cm²,0.24 cm²,1.0 cm²,4.0 cm² and 10.24 cm²,are fabricated by the two-step spin coating and the GRC,respectively.Comparing the performance of devices fabricated by two methods,taking the GRC as an example,the0.1 cm² solar cells achieves an efficiency of 15.23%,while the efficiency of large area perovskite solar cell is only 2.13%when the active area of the devices increases to10.24 cm².Therefore,the preliminary conclusion is that the rapid growth of series resistance greatly affects the lateral transport and collection of carriers in large area PSCs,resulting in the rapid deterioration of FF and PCE.