Preparation And Properties Of Perovskite Solar Cells Based On Efficient Electron Transport Layer

In recent years,perovskite solar cell has attracted enormous attention due to its high efficiency,low cost and ease-of-fabrication.At present,the researchers focus on the fabrication of perovskite solar cells with high efficiency,low cost and excellent stability.Interfacial transport layer is one of the crutial factors to affect the efficiency and stability of devices,and the problems of electron transport layer and its heterojunction interface are widely researched because they are closely related to the transport and recombination process of charge carriers.Besides,the performance of perovskite solar cells depends significantly on the choice and preparation of perovskite materials,and the key is to obtain the perovskite absorbers with excellent photoelectric properties and high-quality film.In this paper,we mainly focused on the electron transport layer?ETL?of perovskite solar cell,and aimed to improve the performance of perovskite solar cells by developing efficient,low-cost and stable electron transport materials and controlling the crystallization and morphology of perovskite films.So four parts work were carried out as follows.?1?We demonstrated Sn O₂ films prepared by sinter-less spin-coating processes as an electron selective contact layer?ESL?for CH₃NH₃PbI₃-based planar-heterojunction perovskite solar cells?PSCs?,and the low-temperature sintering process will benefit the future fabrication of flexible devices.Besides,modified sequential deposition method,in which the grain size of Pb I₂ precursors was controlled by an equivalent solvent vapor annealing?SVA?process,was used to prepare the perovskite layer on SnO₂.Intriguingly,we observed the variation in morphology of PbI₂ and the resulting perovskite films with different solvent vapor annealing times for PbI₂,and hence the photovoltaic performance of PSCs was affected.With this SVA process,the optimized PSC device achieves the best power conversion efficiency?PCE?of up to 13%,which is highly durable over 30 days of storage time with exposure to the ambient air environment.?2?ZnO thin films prepared by spin-coating of nanoparticles at low temperature were utilized as electron collection layer in CH₃NH₃PbI₃-based perovskite solar cells having a planar heterojunction structure.The effect of ZnO layer thickness on device performance was mainly probed,and a highest PCE of 13.4%with an optimal ZnO layer thickness of 20 nm was achieved.Besides,the ZnO-based device degraded negligibly over 20 days,while the control device without the ZnO layer degraded significantly to 20%of the initial value.Semiconductor nanocomposites are a large group of nanomaterials that their chemical,physical and opto-electrical properties are tuned by varying the composition of each semiconductor to fulfill some particular requirements of semiconductor devices.On the basis of previous work,we prepared ZnO-SnO₂ nanocomposites with different Zn/Sn ratios at low temperature as ETLs for CH₃NH₃PbI₃-based PSCs.The PSCs exhibited relatively higher PCE with an optimal weight ratio of 2:1 for ZnO/SnO₂ in solution,and the higher PCE was attributed to joint contributions of the high charge extraction efficiency and low charge recombination process on the interface.Besides,the thermal stability of CH₃NH₃Pb I₃ absorber and the device stability of the corresponding PSC were improved due to the incorporation of SnO₂into ZnO.Finally,the PCE of the optimized device was further improved to 15.2%by introducing the low-temperature processable Al₂O₃ as a capping layer to the ZnO-SnO₂ composite.?3?Low-content doping/modification of metal oxides has been considered as a way of improving the selectivity of ESLs.We developed Mg-doped ZnO[Zn_1-xMg_xO?ZMO?]by a facile sol-gel method as a high optical transmittance ESL for CH₃NH₃PbI₃ perovskite absorber,and a maximum PCE of 16.5%was achieved with the optimal ZMO?0.4M?containing 10 mol%Mg.The optical and electrical properties of the ESL films with Mg contents of 0-30 mol%were further studied to investigate the origin of the difference in photovoltaic performance,the effect of ZMO ESLs on the interfacial dynamics of the CH₃NH₃PbI₃-based PSCs was also investigated by conducting the electrochemical impedance spectroscopy?EIS?measurements.More importantly,CH₃NH₃Pb I₃ deposited on 10%ZMO thin films exhibited better thermal stability and better durability as well as photo-stability of the resultant PSC device,as compared to pure ZnO.?4?The CH₃NH₃PbI₃-based perovskite rapidly decomposes on ZnO at elevated temperature through a deprotonation process.To solve this thermal instability issue and to further enhance the photovoltaic performance,we employed a?FA?-based perovskite,i.e.,FAPb I₃?HC?NH₂?₂Pb I₃?as the light absorber in ZnO-based PSCs,which was prepared by a sequential deposition technique.We observed the preheating of the Pb I₂ layers prior to immersion can effectively facilitate the reaction between Pb I₂ and FAI.The photovoltaic performance of the investigated FAPb I₃ solar cells was clearly dependent on both the pre-heating of the PbI₂ layer and post-annealing of the FAPb I₃ film in the fabrication procedure.The highest PCE of up to 16.1%was achieved,in which the pre-heating and post-annealing temperatures were 100°C and145°C,respectively.Importantly,the FAPbI₃ on ZnO exhibited better thermostability owing to basically the robust nature of FA compared with MAPbI₃?CH₃NH₃PbI₃?.Moreover,FAPb I₃-based PSCs exhibited excellent photostability and small J-V hysteresis.As well,the fact that MAPbI₃ easily undergoes thermal decomposition on a low-temperature processed Zn O surface limits the use of one-step deposition of perovskite and hence the resulting photovoltaic performance.We further demonstrated triple cation perovskite(Cs_x(MA_0.17FA_0.83)_?100-x?Pb(I_0.83Br_0.17)₃)prepared with a one-step deposition method as a stable light absorber in highly efficient PSCs with low-temperature processed ZnO as the ETL.The photovoltaic performance of the investigated PSCs was dependent on both the annealing temperature of the perovskite film and the composition of the Cs element in the perovskite structure.A remnant PbI₂ passivation phase in the perovskite layer,in which the composition is Cs₆(MA_0.17FA_0.83)₉₄Pb(I_0.83Br_0.17)₃ and the annealing temperature is 95°C,leads to the highest PCE of 18.9%,which is a record-high then for low-temperature processed ZnO-based PSCs.Importantly,this PSC based on the triple-cation perovskite exhibited excellent environmental durability and photostability.