| The excellent photoelectric properties of organic-inorganic perovskite materials have attracted widespread attention.The photoelectric conversion efficiency of solar cells prepared based on them has reached the certified 25.5%,which is comparable to largescale industrialized crystalline silicon cells.However,there is still a lot of room for improvement in performance from its theoretical limit efficiency.Moreover,its stability is poor,especially thermal stability,because organic components are prone to degradation and decomposition under heating,which is the bottleneck of its industrialization.The allinorganic CsPbI3 material introduces the largest monovalent metal cation Cs instead of organic components,and has attain a good thermal stability.However,it still faces the poor phase stability problem.Under high humidity condition,CsPbI3 is prone to phase transition and damage the photovoltaic performance of the device.Therefore,how to prepare highly efficient and stable perovskite solar cell is an urgent problem to be solved.In addition,defects in the film are an important factor affecting device stability and photovoltaic performance and understanding the information of defects in the perovskite film is very necessary for the improvement of device performance.In response to the above problems,we adopt different methods to optimize perovskite devices:1.Use quartz glass to cover the surface of the perovskite film to form a semi-closed space,manipulating the amount of dimethyl sulfoxide(DMSO)remaining in the perovskite film during the annealing treatment,and form the MA2Pb3I8(DMSO)2 intermediate to retards the crystallization process.Moreover,the compensation effect of DMSO vapor atmosphere will cause secondary crystallization process,produce largeaspect-ratio grains with fewer defect states,and then improve the morphology of film,thereby nonradiative processes are greatly suppressed.Besides,combined with X-ray photoelectron spectroscopy measurement and the surface energy of MAI-and PbI-terminated surface model calculated by density functional theory,the defect states are identified and the causes of Pb0 defect states are explained.Using this strategy,a high power conversion efficiency of 20.09%is achieved based on MAPbI3 photovoltaic solar cell.The stability test showed that after 1320 hours under 10%and 40%humidity conditions,the efficiency of the devices prepared under the optimized and reference conditions decreased 28%and 48%,respectively.The long-term ambient stability is obviously improved.Research shows that influencing crystallization kinetics is an effective way to achieve high-performance perovskite solar cells.2.CsPbI3 system has the same excellent photovoltaic performance as organic and inorganic perovskites,and possess better thermal stability,we turned to study inorganic CsPbI3 system.There are many defects in a pure CsPbI3 material and its cubic phase(?-phase)is thermodynamically unstable,which will prevent a high device performance.Here,we incorporated the pseudohalide ion SCN-in a CsPbl3 thin film to achieve X-site doping and improve the crystallinity with smooth perovskite films.The results of XRD and PL showed that the crystallization property and thermal stability of CsPbI3 thin films with the addition of Pb(SCN)2 exhibited a huge improvement compared to the reference.The champion power conversion efficiency(PCE)of CsPbI3 PSCs with the optimized Pb(SCN)2 additive(2%)could reach as high as 17.04%,which is close to the world's highest efficiency(17.06%)of this system at that time.X-site doping can significantly improve the performance of CsPbI3 perovskite,then we explored the effect of B-site doping on its crystal structure and performance.3.Mn2+is used to substitute Pb2+ in the CsPbI3 film to improve the phase stability of the material.The crystalline quality of perovskite materials with Mn2+ doping is significantly improved,and the defect density is reduced.The power conversion efficiency(PCE)of an inorgllic perovskite solar cell with optimized Mn2+ doping(2%)reached 16.52%,which is higher than the 15.05%of the reference,with an enhancement of?10%.Simultaneously,the humidity and thermal stability were boosted by Mn doping,which is attributed to the introduction of Mn,shrinking the lattice of the perovskite material and enhancing the formation energy of the CsPbI3 film.The doping of Mn2+at B site of CsPbI3 significantly improves the structural stability and reduces the defect state density of the film.Because the above work has a significant impact on the defects,the next step is to study the defect information in the perovskite film.4.Deep level transient spectroscopy(DLTS)is used to measure deep defect concentrations and to determine defect energy levels in all-inorganic perovskite solar cells.Combining that data with the density functional theory(DFT)calculation,it is determined that the dominant deep defect states to antisite defect pairs(Pbi and IPb)and interstitial defects(Pbi)in freshly prepared CsPbI3 films.After resting the films in the dark for 12 hours,the concentration of these defects is reduced by approximately two orders of magnitude.Fluorescence spectra,transient photovoltage,and space charge limited current(SCLC)measurements also confirmed this conclusion.The reduced defect concentration is because they can be self-regulated during the storage.To assess the thermodynamics possibilities,two reaction procedures were designed to calculate their formation enthalpies and a negative Gibbs energy change indicated that the self-healing process is spontaneous.Then,lattice strain is revealed as the direct driving force for ion migration and self-healing by tracing the XRD patterns.The power conversion efficiency(PCE)was improved to 18.43%by allowing the defect "self-digestion" before device assembly,which is 6.6%the reference device efficiency of 17.29%. |
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