Design, Synthesis And Properties Of Polymers For Organic Solar Cells

Over the last years, there has been an increasing interest in polymeric solar cells (PSCs).Conjugated polymers are now recognized as the emerging semiconductors due to their low cost,ease of processing and flexibility.However,compared to inorganic materials,conjugated polymers have some serious limitations for photovoltaic materials because of their relatively low mobility of charge carriers and their narrow absorption bands that have poor coverage with solar spectrum. In this thesis, a series of conjugated polymers, containing of PPV, 4,7 - diphenyl-benzothiadiazole and 4,7 - thienyl- benzothiadiazole in the main chain, were synthesized and applied to the PVCs. The thermal, optical, electrochemical and photovoltaic properties of the polymers were studied.A novel poly(p-phenylenevinylene) PPV-based copolymer (3C-OXD-PPV) with electron-deficient oxadiazole segments as the side chain has been successfully synthesized through the Gilch polymerization. The obtained copolymer is soluble in common organic solvents such as chloroform, tetrahydronfuran, and 1,1,2,2-tetrachloroethane. The copolymer was characterized by 1H NMR, elemental analysis and GPC. TGA measurement of the copolymers shows it has good thermal stability with decomposition temperature higher than 350°C. The absorption, electrochemical properties of the 3C-OXD-PPV were investigated and also compared with the properties of MEH-PPV. The HOMO and LUMO levels of 3C-OXD-PPV were estimated from the electrochemical cyclic voltammograms. Bulk-heterojunction PVCs were fabricated by using 3C-OXD-PPV blended PCBM as an active layer. The PCE of the PVC is 1.60% under 100 mW/cm2 AM 1.5 illumination, which indicates that 3C-OXD-PPV is a potential candidate for the application of polymer PVCs.Three novel conjugated copolymers containing alkoxylated 4,7-Diphenyl-2,1,3-Benzothiadiazole and dialkylfluorene or dialkyloxyphenylene or dialkylthiophene units were prepared by Horner polycondensation reactions. They are all soluble in common organic solvents such as chloroform, tetrahydrofuran, and chlorobenzene. The novel copolymers were characterized by NMR, GPC, and elemental analysis. Thermogravimetric analysis of the copolymers showed they have good thermal stability with the decomposition temperature higher than 350°C. Cyclic voltammetric study shows that the HOMO energy levels of the three copolymers are deep-lying which implies that these copolymers have good stability in the air and the relative low HOMO energy level assures a higher open circuit potential when they are used in photovoltaic cells. Bulk-heterojunction photovoltaic cells were fabricated with the copolymers as the donors and PCBM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.65, 1.25, 1.62% with high open circuit potential of 0.76, 0.96 and 1.04 V under AM 1.5 solar simulator illumination (100 mW/cm2)We report the synthesis and photovoltaic properties of three new 4,7-dithienyl-2,1,3-benzothiadiazole-based copolymers: PFV-HBT, PPV-HBT and PTV-HBT by Horner polycondensation which is fit for the synthesis of well-defined strictly alternating copolymers. These narrow-band-gap conjugated polymers show a wide absorption spectrum (350nm-850nm), can effectively cover the solar spectrum to improve the collection rate of the sun. Bulk-heterojunction photovoltaic cells were fabricated with the copolymers as the donors and PCBM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.38, 0.33, 0.76%. The efficiency of photovoltaic devices is still relatively low, presumably due to the small molecular weight of polymers which lead to poor film forming ability.