Effect Of Optimized Metal Oxide Electron Transporting Layers In CsPbIBr₂ Perovskite Solar Cells

Hybrid organic-inorganic PSCs have aroused plenty of attention in photovoltaic community,since the first hybrid halide PSCs emerged in 2009.Attributed to their large absorption coefficients,long carrier diffusion length,low exciton binding energy and high carrier mobility,the PSCs have a rapid development in power conversion efficiency（PCE）in the past ten years.Despite the most exciting develpments for PSCs in photovoltaic community,the thermal instability arising from the organic moieties,such as methylammonium（MA⁺）and formamidinium（FA⁺）cations,has become an Achilles heel hindering the commercialized production and the application for flexible devices.Fortunately,a more stable perovskite based on cesium cation（Cs Pb X₃,X=I or Br）attracted great attention due to a range of significant advantages,including preeminent thermal stability（>300°C）and high electron mobility.Among the all-inorganic perovskite,CsPbIBr₂ demonstrates a balanceable feature both in its reasonable band gap（≈2.08 e V）and a high thermal stability.To date,huge discrepancies still exist between the theretical and experimental PCE values in CsPbIBr₂ PSCs,which is ascribed to deficient optical management and carrier dynamics.The latter severely corrode the photovoltaic parameters of PSCs,especially the open-circuit voltage.Therefore,we focus on the interface between electron tansporting layer（ETL）and perovskite layer,demonstrating an effective modified ETL and an interface contact modulation method to improve the perovskite quality and carrier dynamics in CsPbIBr₂ perovskite.Finally,we fabricate the CsPbIBr₂ PSCs via a low temperature process.In this thesis,our research was carried out as follows:（1）Gnerally,the typical sintering temperature for the widely-used electron transport material,i.e.TiO₂,is more than 400℃,elevating the cost and hindering the application.Owing to the high electron mobility and low fabrication temperature,ZnO becomes a desirable alternative for TiO₂ as the ETL in CsPbIBr₂ PSCs albeit with low V_oc.We introduce a trace of NH₄Cl to the sol-gel derived ZnO precursor to decrease the work function of ZnO film,tune the surface morphology of perovskite film and thus suppress the density of trap states in the CsPbIBr₂ films.Consequently,the full-coverage and pure-phase CsPbIBr₂ films consisting of micron-size and high-crystallinity grains are obtained.More importantly,for the optimal NH₄Cl modified,a shining improvement of V_oc from1.08 V to 1.27 V boosts the champion CsPbIBr₂ PSCs to obtain a power conversion efficiency of 10.16%.Besides,the NH₄Cl modified ZnO ETL reduces the severe hysteresis and increases the device stability significantly.（2）The interface defects and imperfect band alignment between ETL and perovskite layer is one of the main reasons for hindering further efficiency improvement.Herein,we modulate the band alignment and perovskite crystallization of the ETL/perovskite interface by employing ZnO and SnO₂ as ETL,which obtains high electron mobility and can be fabricated at low temperature.As a result,the SnO₂-based devices exhibit a better interface contact,leading to accelerated carrier injection and suppressed interface recombination,compared with ZnO-based devices.SnO₂-based PSCs show a higher PCE of 10.81%as a consequence of improved V_oc and fill factor（FF）as compared to 9.70%of ZnO counterpart.Remarkably,SnO₂ ETL can also reduce hysteresis and improve device stability as compared to ZnO ETL.This study unveils the critical role of interface contact modulation of CsPbIBr₂ PSCs and provides an insightful strategy for preparing efficient and stable low-temperature inorganic PSCs.In conclusion,we provide a facile method for ETL modulation in CsPbIBr₂ PSCs to realize energy level alignment between ETL and perovskite layer.Subsequently,we achieve a better interface contact in ETL/perovskite interface and reveal the critical role of the interface contact.Finally,we fabricate the whole device and systematically investigate the effect of ZnO and SnO₂ on the performance of CsPbIBr₂ PSCs.