The Research Of Efficient Electron Transporting Layers For Perovskite Solar Cells

In recent years, perovskite solar cells have attracted enormous attention due to their high efficiency, simple fabrication process, and low cost. Perovskite solar cells typically contain electron transporting layers (ETLs), perovskite absorbers, hole transporting layers (HTLs), and electrodes. ETLs transport electrons and also block holes, HTLs transport holes and also block electrons, both of which are critical to separate and collect photogenerated charge carriers. High-efficiency perovskite solar cells usually use excellent ETLs and HTLs. In this doctoral thesis, we systematically study on efficient perovskite solar cells with simple structure and the mechanism of devices, mainly focus on some new ETL materials to replace the high-temperature sintered TiO2 ETL. In addition, excellent HTL and perovskite absorber have also been studied.Firstly, a new one-step sintering method instead of the conventional two-step sintering method was used to prepare the TiO2 compact and mesoporous layers. The TiO2 compact layer is denser and the recombination between the compact layer and mesoporous layer is lower by using the one-step sintering method. Therefore, the performance of the perovskite solar cells using the one-step sintered TiO2 compact and mesoporous layers can be improved. However, the TiO2 still needs to be sintered at high temperature.Secondly, we demonstrate that efficient hole blocking layer-free planar perovskite solar cells, simplifying the fabrication process and avoiding the use of the high-temperature sintered TiO2. Ultraviolet-ozone treatment on substrate and the interface passivation by Cl are the keys for realizing high open-circuit voltage and high efficiecny for the perovskite solar cells without hole blocking layers. The hole blocking layer-free solar cells without the high-temperature sintered TiO2 not only have a simpler device structure but also reduce the fabrication cost. The results also demonstrate that the interface passivation is very important for the cell performance, suggesting a method for achieving high-efficiecny perovskite solar cells.In order to improve the cell performance, we used the low-temperature solution-processed nanocrystalline SnO2 film as ETL to replace the conventional high-temperatue sintered TiO2. SnO2 can be prepared by a simple low-temperature solution process and also exhibits higher electron mobility and more suitable band edge positions than TiO2. Furthermore, nanocrystalline SnO2 film can not only act as ETL but also as antireflection layer and improve the transmittance of the substrate. Therefore, the best-performaing solar cell using the SnO2 ETL achieved an efficiency of above 17% with a higher Jsc, Voc, and FF than the reference cell using the TiO2 ETL. The low-temperature solution-processed SnO2 ETL is compatible with the roll-to-roll manufacturing of low-cost perovskite solar cells on flexible substrates.We also used the evaporated ultra-thin fullerene as ETL to replace the conventional high-temperatue sintered TiO2. The ultra-thin fullerene can not only act as ETL but also as passivation layer for the perovskite. It can significantly reduce the hysteresis and the interface recombination of the devices. Furthermore, efficient and flexible fully-vacuum-processed perovskite solar cells with a better stability have been achieved by using the evaporated fullerene ETLs and co-evaporated perovskite absorbers and evaporated copper phthalocyanine HTLs.To combine the advantages of SnO2 and fullerene, we used fullerene passivated SnO2 as ETLs, which have a better ability of transporting electrons and blocking holes. The SnO2 layer blocks holes effectively; whereas, the ultra-thin fullerene promotes electron transfer and passivates traps and defects at the SnO2/perovskite interface. Therefore, the perovskite solar cells using the SnO2/fullerene ETLs achieved better performance. In order to realize the perovskite solar cells with higher efficiency, we also fabricated better perovskite absorbers by using Pb(SCN)2 additive in perovskite precursor. A small amount of Pb(SCN)2 in the perovskite precursor can significantly increase the grain size and crystalline quality of the perovskite thin film. It can also result in excess PbI2 at the grain boundaries, which passivates grain boundaries and reduces dark current through the grain boundaries. Therefore, the solar cells using Pb(SCN)2 additives in perovskite precursors have lower hysteresis and better performance. Finally, combined with the use of SnO2/fullerene ESLs and Pb(SCN)2 additives, our best-performing planar perovskite solar cell achieved an efficiency of 19.45% measured under reverse voltage scanning.