Amorphous ZnO/SnO₂ Nanocomposite As The Electron Transport Layer In Perovskite Solar Cells And The Mechanism Of Interfacial Charge Separation

Organic halide metal perovskite materials have excellent photoelectric properties,and have made great progress in the field of perovskite solar cell(PSC).To date,25.2%power conversion efficiency has been achieved.From the perspective of future industrialization,planar PSCs are more attractive due to their advantage in low temperature processing.It is therefore desirable to develop suitable electron transport layer for improving photovoltaic performance of PSCs.In this thesis,ZnO/SnO₂ composite,serving as an electron transport layer,was prepared by varing annealing temperature.The SEM results show that the surface of the ZnO/SnO₂ film annealed at 250 ^oC(ZS-250)is relatively dense and smooth,while cracks are found to appear at the surface of the films annealed at lower and higher temperature.For instance,pinholes are observed at the film annealed at 400 ^oC(ZS-400),which results in strong surface recombination and serious current leakage.According to UPS and UV-vis measurements,ZnO/SnO₂ composite is demonstrated as amorphous semiconductor with wide bandgap.The band gap can be adjusted by varing annealing temperature.The energy level of the ZnO/SnO₂ film annealed at 180^oC(ZS-180)is 40 m V higher than the perovskite phase,which limits the injection of free electrons.By contrast,the ZS-250 has a lower minimum conduction band and proper energy level matching,which leads to higher photocurrent and open circuit voltage.The PSCs based on ZS-180 exhibits high impedance due to its high energy barrier for charge transfer.The well-matching of interface energy alignment in ZS-250based PSCs engenders the reduction the impedance arc and hence promotes charge transfer.With the depositon of PCBM onto ZnO/SnO₂ film,interfacial recombination was further suppressed,filling factor and open-circuit voltage are improved.Resultantly,a power conversion efficiency of 19.5%is achieved.According to the above studies,the band matching between electron transport layer(ETL)and perovskite layer(PS)has a profound influence on device efficiency.The drift diffusion equations are therefore employed for modeling the dynamics of charge transport in PSCs.It was found that the energy level mismatch between ETL and PS is an important cause to the reduction of cell efficiency and well as the occurance of J-V hysteresis.When energy levels match,cations accumulate at the ETL/PS interface,while mobile anions are at the PS/HTM interface.When the energy level mismatch,the aggregation of mobile ions is in reverse.By considering the migration of mobile ions in the simulations,we have confirmed the dependence of the hysteresis phenomenon on the voltage scan rate.It also results in different hysteresis curves,such as normal hysteresis,inverted hysteresis,and peak shaped hysteresis,which can be obtained by changing the voltage sweep speed.By incorperating surface recombination into simulations,the dependence of scanning direction on the open circuit voltage is demonstrated.It illuminates that the suppression of surface recombination is crucial to improve device performance.