Optimizing The Electron Transport Layer Of Zinc Oxide To Improve The Photovoltaic Performance Of Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells(PSCs)have attracted great attention for their high power conversion efficiency(PCE),simple preparation process,and low processing cost.Zinc oxide(ZnO)was a traditional cathode material for solar cells due to its excellent electrical and optical properties.However,when ZnO is used as the electron transport layer in PSCs,the deprotonation reaction at the interface and some specific groups will decompose the perovskite layer.The extremely poor chemical compatibility between ZnO and organic-inorganic hybrid perovskite materials restricts the further improvement of the PCE of ZnO-based planar PSCs,and limits the further application of ZnO electron transport layers in PSCs.In order to solve this problem,the ZnO electron transport layer was optimized by UV-O₃ and high temperature annealing,inserting non-stoichiometric Sn O_x layer,and PbS QDs-TBAI co-modification.The effects of different treatment methods on the photoelectric properties were studied,including the ZnO electron transport layer,the structure and photoelectric properties of perovskite absorption layer,and the photogenerated carriers behavior in PSCs.The mechanism of optimizing the electron transport layer to improve the photovoltaic performance of perovskite solar cells is revealed,and finally efficient and stable ZnO-based planar PSCs are obtained.The innovative results achieved are as follows:The functional groups on the surface of the ZnO electron transport layer are the key factors that limit the PCE of the ZnO-based planar structure PSCs.Firstly,UV-O₃ozone and annealing technology are used to treat ZnO surface,effectively remove hydroxyl and other surface functional groups,induce the formation of high-quality perovskite absorption layer,reduce the number of pinholes and gaps at ZnO/CH₃NH₃Pb I₃ interface,and realize effective interface passivation.Under the condition of similar Pb I₂ content,when treated with ultraviolet ozone for 20 min,the PCE of PSCs is not only 3 times that of untreated devices,but also significantly higher than that of devices annealed at 300°C.This is because in addition to passivating ZnO surface functional groups,UV-O₃ treatment can also regulate the number of deep level defect states(V_O⁺)and interstitial oxygen(O_i)in ZnO,promote the rapid separation of carriers at the interface,accelerate the transport of carriers in ZnO electron transport layer,and effectively improve the performance of perovskite solar cell devices.When the UV-O₃ treatment time reached 20 min,the device PCE reached 17.65%,which is the highest PCE obtained by ZnO-based PSC without any other material interface modification.Although UV-O₃ treatment can effectively improve the PCE of ZnO-based planar PSCs,the decomposition phenomenon of perovskite films still exists.In order to further solve the instability of perovskite film interface caused by ZnO,we further introduce a non-stoichiometric Sn O_x layer between ZnO electron transport layer and perovskite absorption layer as conformal insertion layer,and optimize the device performance by adjusting the thickness and annealing temperature of Sn O_x layer.When the thickness of Sn O_x layer is 10 nm and the annealing temperature is 350°C,the maximum PCE of PSCs based on ZnO/Sn O_x electron transport layer reaches 20.78%,and shows excellent UV stability.After continuous UV irradiation for 60 h,the PCE of the device still retains90%of the initial PCE.These improvements in photovoltaic performance can be attributed to the low photocatalytic activity of Sn O_x layer,the improvement of crystal quality of perovskite film and the formation of built-in electric field with a cascade structure in PSCs.This work not only provides an effective technology for PSCs to build a compatible electron transport layer and improve photovoltaic performance,but also promotes the further application of non-stoichiometric semiconductors in optoelectronic devices.In order to further reduce the overall preparation temperature of the device.We have studied a simple and controllable low-temperature strategy to overcome the above-mentioned problems and aimed to obtain high-efficiency and stable ZnO-based planar PSCs that can be processed at lower temperatures.Colloidal PbS QD and Tetrabutylammonium iodide(TBAI)were spin coated on the ZnO electron transport layer,and then the ZnO/PbS TBAI dense layer was obtained by mild annealing process.Due to the I ligand of TBAI,PbS-TBAI film acts as both surface wettability control layer and electric dipole layer,and regulates the crystal growth of perovskite film and the charge behavior in the device.The stronger non-wetting surface of ZnO/PbS-TBAI electron transport layer improves the crystallization quality of perovskite films by inhibiting the heterogeneous nucleation.By adjusting the number of I ligand coupling,the surface dipole moment of PbS TBAI is changed,the energy level arrangement between ZnO/PbS-TBAI/perovskite is optimized,the heterogeneous interface charge extraction is enhanced,and the interface charge accumulation of PSC is reduced.The PCE of optimized ZnO/PbS-TBAI PSCs device reaches 20.53%,which is the best PCE of ZnO-based PSCs device with MAPb I₃ as absorption layer.The stability test also shows that the PCE of ZnO/PbS-TBAI PSCs device remains above 86%of initial PCE after 30 days of exposure at 40%humidity,which is much better than that of the reference device.In this work,we used the I ligand to modify the PbS QDs coupling film for the first time,and obtained ZnO-based PSCs devices with excellent performance,which opened up new ideas for the application of QDs materials in optoelectronic devices.The above research results provide a variety of effective schemes for the optimization of ZnO electron transport layer,and effectively improve the photovoltaic performance and stability of the ZnO-based perovskite solar cells.