Vinylene incorporated copolymers containing benzothiadiazole (BTZ) as an electron acceptor and fluorene (FL) or cyclophentadithiophene (CPDT) as a donor unit were synthesized for organic photovoltaic (OPV) applications. The alternating and random copolymers were prepared via Suzuki and Stille polycondensation, and the absorption spectral, electrochemical, and photovoltaic properties of the copolymers were investigated. The random polymer with higher molecular weight showed enhanced absorption and hole mobility, compared with the alternating polymer consisting of same donor-acceptor moieties, resulting in increased short circuit current (Jsc) of solar cell devices. We also found out that the optical absorption was improved at longer wave length by the displacement of adjacent thiophene groups around the acceptor core in copolymers due to low S1 transition energies. Whereas the vinylene linked copolymers incorporating CPDT donor units showed red shifted absorption with lower band gaps, HOMO levels were higher than those of copolymers incorporating FL donor units limiting open circuit voltage (Voc) in solar cell devices. The best performance of 1.42% was obtained in solar cells based on the vinylene linked polymer, which showed the higher hole mobility, the stronger absorption, and the deep-lying HOMO energy level. Our results demonstrate that designing donor materials in balance with hole mobility, absorption strength, and HOMO/LUMO energy levels is a plausible way towards highly efficient polymer solar cells.

Effect of absorption coefficient on the performance of organic photovoltaics based on vinylene linked copolymers

239th National Meeting (2010)

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