Study On Air-processed TiO₂ Electron Transfer Layer Modification And Performance Of Perovskite Solar Cells

Perovskite solar cell?PSC?owing to the low cost and high power conversion efficiency?PCE?has attracted wide attention and becomes hotspot of photovoltaic field in a short time,which is considered as the most promising and valuable next-generation photovoltaic device.Despite ten years of development,PSCs continue to make breakthrough in conversion efficiency,there are still many challenges in the process of moving from laboratory to industrialization.Organic-inorganic hybrid perovskite material very sensitive to water and oxygen has intrinsic instability,the device degenerates severely and shows poor stability,which can not meet the needs of practical application.Moreover,to gain high-quality perovskite films,the PSC is usually fabricated under inert gas shielding,this harsh process is not beneficial to device large-scale production and commercial application.In this work,air-processed fabrication of PSCs has investigated and the device performance has been improved by regulating defect density and chemical state at TiO₂/CH₃NH₃ PbI₃ interface.The effects of interfacial modification on the transport and recombination of photo-induced carriers has also been studied.By optimizing the CH₃NH₃ I concentration in two-step solution method,the perovskite films and efficient solar cells have been successfully prepared,optimum conditions of air-processed PSC fabrication based on two-step method have been investigated.Although the quality of perovskite film has been improved by regulating the CH₃NH₃ I concentration,the perovskite films still have pinholes.To address this issue,based on optimized two-step method the isopropanol is introduced to further control the crystallization process of perovskite films and the pinholes are successfully eliminated.By the isopropanol assisted two-step s method,the planar device with a FTO/TiO₂/MAPbI₃/Spiro-OMe TAD/Au architecture has been constructed.The best PCE of optimized device increases from 12.18 % to 15.10 %,the PCE maintains over 80% PCE after 20-day storing under 50 % to 70 % relative humidity.The TiO₂ ETL has been doped by the rare earth Er,the doping Er suppress the agglomeration of TiO₂ nanoparticles during hydrothermal preparation,generating homogeneous nanoparticles.It is indicating that the doping Er can suppress the phase transformation from anatase to rutile,generating mixed TiO₂ phase of anatase and rutile,which can accelerate the photo-induced electron extraction.Eliminateing the oxygen defect and neutralizing the local charge,and effective passivation has been realized.The mesoporous PSCs with a FTO/c-TiO₂/Er-TiO₂/CH₃NH₃ PbI₃/Spiro-OMe TAD/Au architecture are fabricated under ambient atmosphere,which yields a best PCE of 14.06%,maintaining over 80% even after 28-day storaging in air without any encapsulation.By the NH₄ F solution soaking treatment,the adsorbed organic solvent and surface hydroxyl of the TiO₂ electron transfer layer?ETL?have been rreduced.The surface oxygen vacancies have been passivated,which suppressed the interface recombination in PSC.The decreased work function of TiO₂ by NH₄ F solution soaking treatment means that the built-in potential can be enhanced,accelerating the extraction of pthoto-induced electron and reducing electron accumulation and recombination at TiO₂/CH₃NH₃ PbI₃ interface.Larger perovskite crystalline grains and pinhole-free perovskite films have been prepared onto the NH₄F-treated TiO₂ ETLs.The modified device yields a highest PCE of 15.61%,which can still maintain over 90% original PCE even after 28-day storing in air without any encapsulation.To further reduce fabrication cost,the TiO₂ ETL has been prepared at low temperature and surface modification has been carried out by UV-ozone technique.High-intensity UV radiation and ozone can remove residual organic species on low-temperature TiO₂ and improve the surface hydrophilicity,giving higher quality perovskite films.UV-ozone treatment can dramatically decrease the work function,suggesting that bulit-in potential can be reinforced and easier extraction of photo-induced electrons can be achieved.Meanwhile,the lattice strain of perovskite films crystallized on the modified TiO₂ can be released in some degree,delaying the degradation of perovskite film.The champion device with UV-ozone modified TiO₂ achieves a high efficiency up to 18.16 %,which can maintain over 90% of original value even after 40-day storage in air without any encapsulation.