Electron Transport Layer Doping And Absorbing Layer Optimization In ZnO/PbS Quantum Dot Solar Cells

Quantum dots(QDs)have advantages of adjustable bandgap,wide absorption range and strong chemical stability,due to their size effect and multi-exciton generation(MEG)effect.Quantum dot solar cells(QDSCs),using PbS as the main material,have attracted much attention owing to their simple preparation,low cost and large room for improvement of power conversion efficiency and have achieved great development in the past decade.At present,the highest certified efficiency of this type of cells has exceeded 12%.Zinc oxide(ZnO)nanoparticles have been widely used as the electron transport layer material of QDSCs because of their advantages of low synthesis temperature,high optical transparency and suitable energy level for charge conduction.ZnO/PbS QDSCs,with a basic constructure of FTO/ZnO/PbS-EMII/PbS-EDT/Au,were taken as the object for the study on optimization of ZnO electron transport layer and PbS QD layer in this paper.The effect of working temperature on the cell performance and the air storage stability of the cell were studied.Experimental study was performed step by step,and finally a more systematic law was obtained.The main content of this paper is as follows:(1)Surface modification of ZnO electron transport layer.Surface modification is an effective way to reduce the recombination of charge carriers which results from the internal and external defects.The surface modification of ZnO was carried out by Mg doping before synthesis and Na Cl treatment after synthesis.Optimized ZnO made by the two methods were respectively used as electron transport layer to prepare solar cell devices.The results show that the performance of the modified ZnO electron transport layer is optimized,and the performance of the solar devices prepared based on that are also greatly improved.The efficiency of solar cells with ZMO(Mg doped ZnO)and ZnO-Na Cl(Na Cl treated ZnO)is9.46% and 9.15%,respectively.The changes of the electron transport layer and the cell performance before and after ZnO surface modification were analyzed,which confirmed that surface modification could effectively reduce the defects of ZnO material and adjust its energy level structure.The short-circuit current density and fill factor of modified devices are dramatically enhanced.the optimization effect of surface modification on cells,providing a basis for improving the power conversion efficiency of QDSCs.(2)Optimization study of PbS QD layer thickness.Surface morphology and thickness of the light absorbing layer are related to the ability of devices to light absorption and charge transfer path.Based on the first section,the QDSCs(using ZMO as the electron transport layer)with different QD layer thickness were prepared by spin-coating different layers of PbS.The results show that the thicker the QD layer,the better the light absorption,but also the greater the defects introduction.Among the devices with different thickness(6-10 layers of PbS QD layer)tested in this paper,the cells with 7 layers were the most efficient.We confirmed that QD layer thickness has a significant effect on the cell performance,resulting from a trade-off between the favorable light absorption and undesired defeat number.(3)The influence of working temperature on the cell performance and the air storage stability of cells.With ZMO as electron transport layer and 7 PbS QD layers,the performance of the QDSCs was tested under different working temperatures.The results show that the cell efficiency first increases with the increase of temperature,and then decreases with the further increase of temperature.According to the analysis,the increase of temperature will increase the ability of charge transfer between QDs and increase the chance of electron-hole recombination.Among different working temperatures(253-343 K)studied in this paper,the best cell efficiency(12%)was obtained at the working temperature of 273 K.When the QDSC is stored in the air at room temperature,its efficiency rises to the highest value(10.48%)after 10 days,then decreases slightly.But the value can still reach 9.64% after 30 days,indicating that the cell has good stability in the air storage.