The Study of 3,6-Carbazole Incorporated into D-A Type Conducting Polymers: Synthesis, Characterization and their Photovoltaic Application
- The Study of 3,6-Carbazole Incorporated into D-A Type Conducting Polymers: Synthesis, Characterization and their Photovoltaic Application
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- Organic photovoltaic solar cells (OPVs) containing donor (D)-acceptor (A) type copolymers which act as the active materials exhibited remarkable performance after the development. Herein, a series of 3,6-carbazole incorporated D-A type conjugated copolymers with a DTBT electron-deficient unit were synthesized using Suzuki coupling reactions. The basic properties and their photo performance were studied in this thesis. 3,6-Carbazole-based D--A type copolymers always exhibited low molecular weight. Based on our knowledge, the molecular weight of semiconducting copolymers is strongly related with their performance in the electronic devices. Therefore, parameters influencing the molecular weight of the 3,6-carbazole-based polymer series and conjugation lengths were investigated. Three effects were proposed as potential factors that limited chain growth, yielding persistently low molecular weight. Finally, strong intramolecular charge transfer (ICT) was present in 3,6-carbazole-based copolymers and, furthermore, conjugation breaks were generated at the centers of the 3,6-carbazole units in P(3,6C-DTBT), which resulted in rigid planar structures without extended conjugation. These properties are expected to facilitate chain–chain interactions that prevent the molecular weight of the linear oligomers from increasing during polymerization. Next, we introduced the conjugation breaks which was caused by the incorporation of 3,6-carbazole unit into P(2,7F-DTBT). The polymer, named P(F45C5-DTBT), incorporating 5 mol% 3,6-carbazole into P(2,7F-DTBT), showed an almost two-fold improvement (5.1%) in power conversion efficiency relative to P(2,7F-DTBT) (2.6%). Appropriate conjugation break was good for stacking and could increase the hole mobility of the corresponding copolymers. In the P(FxCy-DTBT)s system, the improvement was mainly due to the hole mobility increase which gave good balance between holes and electrons and a nanoscale morphology formation in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM).
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