Modification Of CH₃NH₃PbI₃ Thin Films And Performance Of Perovskite Solar Cells

The organic-inorganic metal halide perovskite materials possess some properties such as high light absorption coefficient,high carrier mobility,low exciton binding energy and balanced bipolar carrier transport,which satisfy many requirements of being an efficient optical absorption material for photoelectric conversion devices.Perovskite solar cells?PSCs?based on organic-inorganic metal halide materials have become a hot topic in the field of photovoltaic conversion due to their high efficiency and easy solution preparation process.PSCs have been developed rapidly with a power conversion efficiency?PCE?of 25.2%in just a single decade.Although many breakthroughs have been made,the further improvement of performance is also hindered by the defects in light absorption layer and interfaces.Therefore,from the perspective of optimizing film qualities and reducing defect states,this thesis improves PCE and stability of n-i-p type PSCs by modifying CH₃NH₃Pb I₃^-?MAPbI₃^-?thin films.The specific research works are as follows:?1?MAPbI₃ thin films are prepared by two-step spin coating method and applied in n-i-p type mesoporous structured PSCs.In order to improve the performance of PSCs,isopropanol?IPA?solvent vapor is introduced into thermal annealing process.MAI possess a good solubility in IPA solvent vapor,thus the IPA solvent vapor selectively dissolve MAI,leaving PbI₂ layer as a dense thin film to ensure the formation of pinhole free MAPbI₃ thin films,and the slight dissolution and recrystallization of Pb I₂-MAI precursor thin films in IPA solvent vapor is beneficial to the formation of MAPbI₃^- thin films.In addition,IPA solvent vapor annealing can provide a moist environment for the reaction of MAI and PbI₂,which would effectively promote the diffusion of MAI into PbI₂ for fully reaction.The results show that the dense and pinhole free MAPbI₃^- thin films with larger grain size are obtained by IPA solvent vapor annealing strategy.The J-V measurements exhibits an efficiency of 15.61%when IPA solvent vapor annealing for 60 min,which is higher than that of the device prepared by pristine annealing?14.16%?.The experiment results confirm that the IPA solvent vapor annealing is an effective strategy to improve the quality of perovskite films,and it also lays a foundation for further inhibiting surface defect states and realizing high performance PSCs device.?2?Here,an ionic compensation strategy based on tri-iodide ions[I₃^-]is presented to improve the performance of n-i-p type mesoporous structured PSCs through healing iodine vacancy defects during the solvent vapor annealing process and suppressing the defect induced non-radiative recombination.It involves thatiodine?I₂?can be ionized to[I₃^-]by oxidation of isopropanol?IPA?,and the IPAsolution which contain[I₃^-]ions is used as the annealing solvent.By this strategy,the iodine vacancy defects can be compensated effectively by the[I₃^-]ions in IPA vapor,and the quality of MAPbI₃^- thin films are improved simultaneously.Theinvestigation of carrier recombination dynamics show that the IPA/[I₃^-]solvent vapor annealing can suppress the non-radiative recombination and prolong thecarrier life.The concentration of[I₃^-]in IPA solvent have a great influence on performance of perovskite thin films and PSCs,when the concentration is 0.15%,the optimal MAPbI₃^- thin film is obtained,corresponding to the best performance PSCs device.For the best performance device,a PCE of 18.53%is achieved which is improved by 20%compared to solvent vapor annealing device.Meanwhile,the notorious hysteresis phenomenon is almost disappeared,and the long-term stabilityin air environment is greatly improved.In addition,by comparing IPA/[I₃^-]treated device and I₂ treated device,we find that the efficiency of PSCs device is reduced byI₂ vapor annealing,which indicate the negatively charged nature of[I₃^-]ions is important for defect_<sub>healing.Finally,the Kr?ger–Vink symbols are used to describe the mechanism of[I₃^-]induced defect reduction.?3?Here,a simple Ni ions incorporation strategy is presented to improve the performance of n-i-p type planar structured PSCs.The strategy makes use of the characteristic that Ni ions have a strong preference for octahedral coordination with halide ions which reduces the defects in perovskite lattice.The characterization results show that the Ni ions incorporation strategy can reduce the intrinsic iodine vacancy defects and improve the quality of MAPbI₃ thin films.And the investigation of carrier recombination dynamics show that the incorporation of Ni ions can improve the carrier transport efficiency and suppress non-radiative recombination.The J-V measurements show that the efficiency of PSCs increases first and then decreases with the incorporation of Ni²⁺.Under the optimal condition,the device formed by incorporating 1.5%Ni²⁺exhibits an efficiency of 18.82%which is improved by 26%compared to pristine device.Simultaneously,the hysteresis phenomenon is significantly reduced and the long-term stability is greatly improved.