Novel Cerium-Based Electron Transport Layer For Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells（PSCs）have attracted significant attention from the scientific community since 2009,because perovskite light-absorbing materials have many advantages,such as high absorbance coefficient,high carrier mobility,tunable energy bandgap,solution processibility,and low cost.Due to these merits,the power conversion efficiencies（PCEs）of PSCs have rapidly surged from 3.8%to 22.1%over the past few years.Although PCEs exceeding 20%have been achieved in perovskite solar cells,commercialization of these cells will require the development of high-efficiency solar cells with long-term stability and no I-V hysteresis.Conventional high-performance perovskite solar cells are typically fabricated using titanium oxide（TiO₂）as the electron transport layer（ETL）.However,the formation of TiO₂ ETL usually requires high-temperature（up to 500 ℃）sintering process,which is incompatible with plastic substrates.Besides,perovskite solar cells using TiO₂ ETL often suffer from serious photocurrent hysteresis.Moreover,TiO₂-based solar cells often suffer from reduced stability under UV light,which has been attributed to changes occurring in the compact TiO₂ and/or mesoporous TiO₂ layers.To overcome these problems,we focused on designing new ETL and developing new type of perovskite light-absorbing layer in this dissertation,and the main results and conclusions are carried out as follows:（1）High quality CH₃NH₃PbI₃ film was prepared by vacuum chemical vapor deposition method.PbI₂ film was converted into a uniform and full surface coverage CH₃NH₃PbI₃ film on TiO₂ substrate through a gas-solid reaction with CH₃NH₃I under vacuum conditions.This method is highly reproducible,and suitable for large-scale fabrication of PSCs.We fabricated a planar perovskite solar cells using TiO₂ as the ETL with PCE of 16.44%.To further enhance the efficiency of PSCs,a thin layer of[6,6]-phenyl-C₆₁-butyric acid methyl ester(PC₆₁BM)was introduced to modify the surface of the TiO₂.The PC₆₁BM film could fill up the pinholes or cracks along TiO₂ grain boundaries to passivate the defects and make the ETL extremely compact and uniform.As a result,the efficiency of the TiO₂-based PSC was improved to 17.31%.（2）For the first time,we reported cerium oxide（CeO_x,x=1.87）,that was prepared facilely through a simple sol-gel method at low temperature（150 ℃）,as an alternative to high-temperature sintering processed TiO₂ in the regular architecture of PSCs with high performance and enhanced stability.The best-performing planar PSC using a CeO_x ETL was achieved a PCE of 14.78%,through optimizing the concentration of the precursor solution.A further improvement in the PCE,up to 17.30%,had been demonstrated through,introducing a PC₆₁BM interfacial layer between the CeO_x ETL and the perovskite layer.Besides,CeO_x-based devices exhibited superior stability under light soaking when compared to TiO₂-based PSCs.CeO_x-based devices retained 85%of their initial PCE after light soaking through continuously illuminating the devices under simulated full spectrum AM 1.5 illumination under ambient atmosphere at room temperature for about 300 min,whereas the TiO₂-based control device decreased to 35%of its initial value under the same aging conditions.（3）We applied perovskite with triple cation mixtures（FA⁺,MA⁺,Cs⁺）as light-absorbing layer in perovskite solar cells based on mesoporous-TiO₂ and TiO₂ nanowire array.High-quality perovskite films were prepared by one-step method using chlorobenzene as anti-solvent.High PCE of 18.36%,and 18.91%was obtained in perovskite solar cells based on mesoporous-TiO₂,and TiO₂ nanowire array,respectively.The higher PCE of perovskite solar cells based on TiO₂ nanorod array demonstrated better electron transporting properties of TiO₂ nanowire array compared to mesoporous-TiO₂.