Fabrications and Device Characterizations of Inverted Polymer Solar Cells Using PTB7:PC71BM as Active Layer

In this thesis, the inverted polymer solar cells using PTB7:PC71BM as an active layer is studied. The potential advantages of polymer solar cells are numerous, including the low-cost of the process, mechanical flexibility, the ability for large area roll-to-roll processes, easy production, and light weight. Dependent clause researches have made great efforts on studying polymer solar cells. The improvements of polymer solar cells could result in both material innovation and state-of-the-art device architecture. Recently, a novel high efficiency active layer material was invented, which is PTB7:PC71BM. This promising material gives polymer solar cells a record high efficiency. Various research has studied this material. The state-of-the-art architecture is focused on this paper to further push the efficiency to another level, also the mechanics are discussed. The basic principle and mechanics are introduced in chapter 2. Some of the concepts are the same with inorganic solar cells. The mechanics of inorganic solar cells have been fully developed. However, some of the mechanics in polymer solar cells are still dubious. Hence, more details have been discussed in this chapter. In chapter 3, using ZnO as an electron transport layer and its further applications are discussed. The electron transport layer is relatively important in inverted architecture polymer solar cells, compared to conventional architecture polymer solar cells. Due to the high work function of ITO, the electron transport layer could lower the work function of the cathode electrode, preventing space charge effect; and hence, reducing the carriers recombination. Moreover, ZnO is an outstanding candidate to serve as the electron transport layer due to its high conductivity and transmittance. Some common active layers are compared using ZnO as an electron transport layer. In addition, multi-layers ZnO thin film could provide a smooth contact with the active layer; raising the power conversion efficiency to 6.25% with two layers ZnO thin film. It is noted that ZnO could exhibit a variety of nanostructures. An excellent solution to solve the short diffusion length of the active layer is to insert ZnO nanorods into the active layer. ZnO nanorods could provide an alternative tunnel to transport carriers. As a result, the carriers could be efficiently collected. The power conversion efficiency comes to 7.56% due to the enhancement of current density. Nevertheless, the ZnO nanorods length is a critical issue. In chapter 4, PEIE and PFN are introduced and used to modify the work function of metal or metal oxide. PEIE is reported to have the ability to lower the work function of variety materials. PEIE is used in this chapter to lower the work function of ITO and ZnO; and hence, the interfacial energy alignment has a better improvement. The device with PEIE on the top of two layers ZnO thin film has a better power conversion efficiency of 6.56%. Since PFN has the same ability to modify the work function of ITO and ZnO, a great enhancement is presented in the device with PFN on the top of three layers ZnO thin film. The power conversion efficiency comes to a higher level of 6.78%. Finally, the hole transport layer and anode electrode have been optimized. The optimized device has power conversion efficiency of 6.84% and FF of 62.73%. In conclusion, various state-of-the-art architecture have been discussed. The state-of-the-art architecture further pushes the efficiency to another level. Besides, they could be used, when a novel high efficiency material is invented. Nevertheless, great effort but more needs to be done.