Study On Anode Buffer Layers By The Solution-immersion Method For Polymer Solar Cells

Polymer solar cells （PSCs） have attracted many attentions due to their various materials,simple production processes, and ability to fabricate lager area and flexible devices.However, the energy conversion efficiency （PCE） of PSCs is still lower than that of theother kinds of solar cells. Therefore, several measures have been enforced to improve thePCE of PSCs such as optimizing the structure of devices, improving the production process,using new materials （donor and acceptor materials）, reforming the interface buffer layerand so on. There are two methods which can be used to reform the interface buffer layer.One is the application of novel materials as interface buffer layers, and the other issimplifying the production process to cut down the production cost. Currently, the solutionprocess has been applied to the fabrication of interface buffer layers as well as the vacuumvapor deposition, pulsed laser deposition, and chemical vapor deposition. Comparing withthe vacuum deposition, the solution process consumes less power during the fabrication,which contributes to cost reduction. Nevertheless, the substrate must be relatively smoothin order to ensure the quality of films, and numerous solutions are wasted during the spinprocess.In this thesis, the solution immersion method was proposed as a replacement for anodebuffer layer fabrication. Pre-treated indium tin oxide （ITO） coated glass substrates were immersed inMoO3solutions with different concentrations, ultrasonic cleaned for10min and dried with a pressurenitrogen gun. This method did not need a smooth substrate and the solution can be usedefficiently. Moreover, the interface buffer layer could be prepared during the process ofcleaning substrates, simplifying the production process. As a result, this method wassuitable for large-area, flexible and non-flat substrates and compatible with the roll to rollprocess.Firstly, the MoO3thin layer as an anode buffer layer was fabricated via the solutionspin coating method and the solution immersion method on ITO glasses. The morphologyof the films was observed by an atomic force microscope and the absorption spectrum ofthe films was observed a UV spectrophotometer. Then solar cells with the structure ofITO/MoO3/P3HT:PC₆₀BM（150nm）/LiF（1nm）/Al（100nm） were manufactured with the different MoO3concentration of solutions and the soaking time and the PCE wasdetermined by the J-V curve of the devices. The result showed that the PCE of devices was2.84%when the MoO3concentration was2g/L, which was higher than that of devicesusing the solution spin coating method. The transmission of the film decayed when theconcentration was higher than2g/L, while the substrate could not be covered completelywhen the concentration was lower than2g/L. It was also demonstrated that the soakingtime of ITO substrates in MoO3solutions had little influence on the performance ofdevices, and the MoO3molecular could adhere on the substrate immediately.Secondly, the WO3solution was prepared via a new method. Then, the WO3thin layeras an anode buffer layer was fabricated via the solution spin coating method and thesolution immersion method, which was observed by an atomic force microscope and a UVspectrophotometer. Then solar cells with the structure of ITO/WO3/P3HT:PC₆₀BM/LiF/Alwere manufactured with the different WO3concentration of solutions and the conditions ofannealing and the PCE was determined by the J-V curve of the devices. When the WO3film was fabricated via the solution spin coating method, the device using a0.2g/L WO3solution, the optimal device, showed the highest PCE3.06%. When the WO3film wasfabricated via the solution immersion method, the device using a0.02g/L WO3solution,the optimal device, showed the highest PCE2.93%. When the annealing temperature wasabove100°C, the efficiency of devices did not increase significantly with the annealingtemperature increasing. The device with the WO3film annealed at25°C showed thepoorest performance. It was also demonstrated that the annealing time of WO3films hadlittle influence on the performance of devices.