Research On The Interface Modification And Mechanism Of The Electron Transport Layer Of Perovskite Solar Cells

With the rapid economic development and social progress,the demand for energy will increase continuously.However,it threatens the living environment of human beings which the’ non-renewable feature of the traditional fossil energy and environmental pollution problems caused by the traditional fossil energy.The development and utilization of new energy is an effective way to solve the energy crisis and environmental pollution in the future.Solar energy is an inexhaustible clean energy,in which solar cells are the most important and effective way to make use of the solar energy.Since 2009,perovskite solar cells(PSCs)have attracted many researchers for its advantages of high power conversion efficiency(PCE),simple manufacturing process,low cost and easy preparation for large area device.The electron transport layer(ETL)is an important part of the perovskite solar cell,which plays the role of electron extraction and transportion.TiO2 is a wide bandgap semiconductor material with good light transmittance,chemical stability and strong charge separation.It is the most commonly used electron transport material(ETM)for the perovskite solar cell.However,the disadvantages of TiO2 electron transport materials affect the efficiency of cells:(1)charge recombination occurs due to the oxygen defects and vacancies in TiO2 nanomaterials,(2)the large band offset between CH3NH3PbI3 perovskite materials and TiO2 affects the quick injection of the electron and open circuit voltage of the cells,(3)the combination of electron and hole is easy to occur because of the low mobility of the electron in the TiO2.In this paper,we use the interface modification method to improve the performance of the perovskite.The corresponding work is shown as follows:(1)The dopping of TiO2 electron transport layer by the rare earth elements.The perovskite solar cell is prepared as follows:the compact TiO2 electron transport layer,CH3NH3PbI3 absorption layer and spiro-OMeTAD hole transport layer are deposited in sequence on the transparent conductive substrate FTO by solution spin-coating method.Then Ag electrode is prepared by magnetron sputtering.The dopping of TiO2 with elements,especially the rare earth elements,can tune the band-gap of TiO2 and enhance the electron affinity and electron mobility.Here,compact TiO2 electron transport layer is doped with Sm element,which widens the band-gap,improves the open circuit voltage,and restrains the recombination of photogenerated holes and electrons.When the concentration of Sm3+ ion is 0.3%,the cell has achieved the max power conversion efficiency(14.10%).At the same time,the performance of perovskite soalr cells is compared with the dye sensitized solar cells.(2)Preparation of high performance mixed crystal mesoporous TiO2 electron transport layer.The composition and structure of TiO2 electron transport layer affect the deposition of perovskite absorber and electron extraction and transportion property from perovskite layer to TiO2 electron transport layer.The mixed crystal mesoporous TiO2 films are prepared by one-step hydrothermal method on compact TiO2 seed layer.The phase composition and grain size of porous TiO2 film,film thickness and porosity are controlled through changing hydrothermal reaction time and annealing time.The results show that the perovskite solar cell based on H1-A0.5 mesoporous TiO2 film which prepared by hydrothermal reaction 1 h and annealing 0.5 h has achieved the max power conversion efficiency(19.87%).(3)Modification of the mesoporous TiO2 electron transport layer by Graphene quantum dots(GQDs).Graphene quantum dots exhibit many different properties from other graphene,such as unique photoluminescence,up conversion and down conversion luminescence due to their unique structure and quantum confinement and edge effects.The functioned graphene quantum dots banding with-OH,-COOH and-NH2 are prepared by hydrothermal method and then are used to modify the TiO2 mesoporous thin film and applied to the perovskite solar cell.The assembly of the GQDs between TiO2 and CH3NH3PbI3 surface can be realized by the formation of covalent bonds and hydrogen bond.Hydroxyl and carboxyl groups in GQDs bind very well onto the surface of TiO2,while amino groups coordinate with the Pb ions and form hydrogen bonds with iodide ions on the edge of CH3NH3PbI3 layer.Thus,it effectively improves carrier transport rate,reduces electron recombination and enhances performance of perovskite.When 0.001 mg ml-1 GQDs was used to modify the TiO2 film,the cell efficiency increased from 17.86%to 19.21%.