Interfacial Energy Level Alignment Constructed Efficient Electron Transport-layer-free Perovskite Solar Cells

Perovskite solar cells（PSCs）are considered one of the most promising next-generation high-tech photovoltaic energy converters due to their excellent energy conversion efficiency and low cost.The power conversion efficiency（PCE）has reached 25.7%.The traditional perovskite solar cell structure is that the perovskite layer is sandwiched between the hole transport layer（HTL）and the electron transport layer（ETL）.Because the perovskite material itself has the advantage of ambipolar carrier transport,the free electrons and holes can be efficiently and spontaneously transported without HTL and ETL.However,some common electron transport layers,such as Ti O₂,Sn O₂,often require high temperature annealing to ensure the constant energy of charge transport;the UV photocatalytic degradation induced by oxygen vacancies will lead to instability at the interface between the electron transport layer and the perovskite layer.Therefore,in order to reduce the cost,simplify the device structure and better fabricate flexible devices,researchers have turned more attention to ETL-free PSCs structure.At present,the efficiency of ETL-free PSCs is always lower than that of PSCs with traditional structure,mainly due to the energy level mismatch at the interface when the perovskite material is in direct contact with the conductive substrate.In this paper,a synergistic strategy is developed to optimize the perovskite material and interface energy level arrangement to simplify the structure of PSCs without sacrificing performance as much as possible.The work is mainly carried out in the following two aspects:（1）Perovskite materials with bandgap gradient distribution design:Due to the energy level mismatch between perovskite and conductive substrate,a large energy transfer potential barrier is created.Therefore,designing perovskite materials with adjustable bandgap to act as ETL provides multiple possibilities for simplifying device structure and avoiding the use of ETL materials.In this thesis,a special perovskite material with cascaded bandgap,called gradient homojunction perovskite（GHJP）,has been designed and synthesized by a large cation-assisted method.The inherent nature of GHJP was the type-II cascaded energy level alignment,which could block holes during the electrons collection.It facilitated the dissociation of the excitons in the GHJP.Due to the excellent properties,ETL-free PSCs based on GHJP obtained20.55%PCE,which was over 90%higher than that of ETL-free PSCs based on control perovskite material.（2）In situ growth of bifunctional modification interface materials:the optimized interface energy level arrangement can be improved by depositing ultra-thin interfacial layers on conductive substrates.However,most of the deposited ultrathin layers are single-function and prepared by the spin-coating method,which results in uneven distribution of thin layers and is not conducive to large-scale preparation.In this thesis,a bifunctional interfacial modification material,3-aminopropyl trimethoxy silane（APS）,has been selected and in situ self-assembled on the transparent conducting oxide（TCO）.On the one hand,the lower surface energy of APS could improve the crystal growth of perovskite,resulting in larger grain sizes and effectively passivate the surface defects.On the other hand,the APS modification could improve the energy level alignment of Indium Tin Oxide（ITO）and perovskite,leading to enhance electron extraction and hole blocking.Therefore,the PCE of the ETL-free PSCs based on APS modified ITO reached 19.09%.