Research On Interfacial Regulation For Low-Temperature Carbon-Based Inorganic CsPbI₂Br Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells have attracted considerable attentions in the recent years,and their power conversion efficiency（PCE）has achieved a great rising from initial 3.8%to certified 26%.However,the volatilization and thermal decomposition of organic cations are seriously harmful to device stability.In contrast,all-inorganic CsPbX₃（X=I,Br,or mixed halides）perovskites possess better thermal stability and thus exhibit a great development potential.Among them,the mixed halide CsPbI₂Br is considered as the most promising light absorber owing to its preferable trade-off between optical response range and phase stability.However,most of efficient CsPbI₂Br perovskite solar cells are composed of organic hole transport layer（HTL）and noble metal electrode,and usually adopt high-temperature preparation process that are energy-consuming and incompatible with flexible substrates.The additives in organic HTL will accelerate device degradation because of their hygroscopicity and deliquescence.Besides high costs,the noble metal electrode is sensitive to the halide ion migration from perovskites,which will cause phase segregation and irreversible degradation,thus damaging the stability of devices.As an alternative,carbon electrode is earth abundant,cheap,hydrophobic,and inert to halide ion migration.In this scenario,low-temperature carbon-based inorganic CsPbI₂Br perovskite solar cells without HTLs and noble metal electrodes are more suitable for commercial application,which hold great advantages of low costs,simple process,and excellent stability.However,the perovskite crystal defects and interfacial energy level mismatch seriously restrict the photovoltaic performance of low-temperature carbon-based inorganic CsPbI₂Br perovskite solar cells.These defects can act as charge recombination centers and shunt pathways,resulting in charge transport barrier and moisture invasion,which eventually leads to the open-circuit voltage(V_OC)loss,current density-voltage（J-V）hysteresis effect,and device instability.On the other hand,the interfacial energy level mismatch can also decrease charge extraction efficiency and induce interfacial charge accumulation.To address these problems,this dissertation chose a series of simple and universal interface regulation methods.Firstly,the upper interface of CsPbI₂Br films was post-treated,and then the photovoltaic performance of low-temperature（≤160 ^oC）carbon-based inorganic CsPbI₂Br perovskite solar cells was further optimized by fluorinated dual-interface design and buried interface strategy.The general research contents are as follows:（1）In order to solve the issues of poor quality for CsPbI₂Br films and insufficient hole transport at the perovskite/carbon interface,hydrophobic aromatic phenylethylammonium halides（PEAI and PEABr）and their fluorinated derivatives of4-fluorophenylethylammonium halides（P-F-PEAI and P-F-PEABr）were used as the upper interface passivation layer to post-treat low-temperature CsPbI₂Br films.It is found that the introduction of passivation layer effectively reduced the defects of perovskite films and adjusted interfacial energy level alignments,thus inhibiting charge non-radiative recombination,and promoting hole extraction and transport.Compared with the reference device,the optimized devices achieved a PCE of13.97%.Meanwhile,the optimized devices without encapsulation exhibited improved moisture and thermal stability in ambient air.（2）The above work focuses on the defect passivation and energy level alignment adjustment at the perovskite/carbon interface,but ignores the key role of electron transport layer（ETL）/perovskite buried interface.It is reported that the defects at the ETL/perovskite buried interface are about 100 times that of the perovskite films,which is considered as the main charge recombination center.In order to optimize the interfaces of ETL/perovskite and perovskite/carbon electrode at the same time,a fluorinated dual-interface design was proposed for low-temperature carbon-based inorganic CsPbI₂Br perovskite solar cells,in which the KTFA and CF₃PMABr were used to modify the buried interface and the upper interface of CsPbI₂Br films,respectively.The different role of K⁺,TFA^-,CF₃PMA⁺,and Br^-was investigated systematically.It is found that TFA^-was located at the Sn O₂/CsPbI₂Br buried interface,while a small amount of K⁺diffused into perovskite lattice to participate in the nucleation and crystallization of CsPbI₂Br,thus adjusting interfacial energy level alignments,improving the film quality,passivating interfacial defects,releasing interfacial residual strain,and inhibiting charge recombination and ion migration.On the other hand,CF₃PMABr passivated I/Br halide vacancies and formed 2D perovskite capping layer,which promoted the hole extraction and transport at the CsPbI₂Br/carbon interface.As a result,the optimized devices achieved the highest PCE of 14.05%and V_OC of 1.273 V.This PCE is one of the highest values reported in low-temperature carbon-based CsPbI₂Br perovskite solar cells.Meanwhile,the optimized devices without encapsulation demonstrated improved moisture,thermal,and illumination stability in ambient air.（3）Although inserting interfacial molecules can improve the photovoltaic performance of the device,this method is still complicated.In order to further simplifying the fabrication process and reducing the costs,the above functions can be realized by directly introducing interfacial molecules into ETL,that is,doping ZnO ETL with cesium salts（CsAc,CsF and CsTFA）containing Ac^-,F^-,and TFA^-.Theoretical and experimental results showed that these functional anions could passivate the defects of ETL and perovskite films by coordinating with both Zn²⁺in ZnO and Pb²⁺in CsPbI₂Br.Meanwhile,the Cs⁺could passivate the hydroxyl（-OH）defects of ZnO surface and form interfacial dipoles on the ZnO surface through Zn-O-Csbonds,thus achieving stable and efficient interfacial electron transport at the buried interface.Compared with Ac^-and F^-,TFA^-has the best effect.Therefore,CsTFA-modified devices achieved a best PCE of 14.25%with less J-V hysteresis.This PCE is the highest value of carbon-based inorganic perovskite solar cells using ZnO as ETL reported so far.Furthermore,the optimized devices without encapsulation exhibited improved moisture,thermal,and ultraviolet light stability in ambient air.