The Device Optimization And Photovoltaic Performance Of Bulk-heterojunction Polymer Solar Cells

Bulk-heterojunction（BHJ） polymer solar cells（PSCs） have gained intensive attention and the power conversion efficiencies（PCEs） have steadily increased over the past two decades. OPV researchers around the world constantly try to improve the performance of solar cells, and hope to eventually achieve a low-cost and large-scale commercial production. This thesis summarizes the development of bulk-heterojunction polymer solar cells first. And then the influence of side chain length of polymer donor on polymer-fullerene solar cells and the effect of molecular weight of polymer acceptor N2200 on the performance of all-polymer solar cells are studied.C₆₀, C₇₀, PC₆₁ BM and PC₇₁ BM are most widely used acceptors in polymer-fullerene solar cells. In this paper, we try to apply a series of PBD-x donors with different length of side chain as an effective tool to modify the different impact on acceptors in order to change the optical, electronic, structural as well as morphological properties, and finally affect the PCE. We blend the PBDs with PCBM to fabricate solar cell devices to analyze the relations between changes on side chain lenghth and device performance, which may help to guide the further device optimization work in the future.We adopt P（NDI2OD-T2）（also named N2200）, with high electron mobility, strong electron affinity, and light absorption ability at near-infrared wavelengths, as acceptor in our study for all-polymer solar cells（all-PSCs）. N2200 with different molecular weight were controlled by varying the polymerization time. We found that the rate of acceptor aggregates, charge carrier mobility valves, μh/μe ratios and the degree of phase separation are quite dependent on the molecular weight of N2200, and a maximum PCE of 4.81% is observed for the device based on medium-molecular-weight N2200. All the results demonstrate that molecular weight optimization is significantly crucial for devices improvement and should be considered as an effective strategy to enhance the PCE of all-polymer solar cells.