Synthesis and characterization of benzodithiophene containing conjugated polymers for organic bulk heterojunction solar cells

Taşkaya Aslan, Sultan
Bulk heterojunction organic solar cells (BHJ-OSCs) have drawn attention as a renewable energy source due to their flexibility, lightweight and low cost in large-area applications. Besides the design of novel polymeric materials with different building blocks, the device architectures of the solar cells have been improved significantly to increase the power conversion efficiency (PCE) of solution-processed OSCs over the last years. Up to now, PCE for single layer BHJ-OSCs has reached 18%, and high performance of 18.6% was recorded in the tandem OSCs. Some requirements for designing ideal polymeric donor-acceptor (D-A) type materials should be considered to obtain high-efficiency BHJ-OSCs: a) wide and strong absorption spectra in the visible and near-infrared range to get high short circuit current (JSC); b) high molecular weight and good solubility in common organic solvents; c) suitable energy levels between polymeric donor and acceptor materials to obtain high open-circuit voltage (VOC); d) good morphology and high charge carrier mobility to facilitate fill factor (FF). With these aim, the D-A type conjugated polymers as the donor materials in the active layer of the OSCs were designed to obtain a narrower band gap and improve the intramolecular charge transfer (ICT) between donor and acceptor building blocks. This thesis investigated a wide range of polymeric materials containing benzodithiophene moiety to construct organic bulk heterojunction solar cells to obtain high power conversion efficiency. The novel conjugated polymers with benzothiadiazole, quinoxaline, thienopyroledione, benzotriazole and isoindigo acceptor moieties were studied in detail. The electrochemical, spectroelectrochemical and photovoltaic behaviors are examined for the conjugated polymers. The effect of the number of the F atom substitution on the benzothiodiazole based polymers was investigated. With the incorporation of fluorine atom, HOMO energy level of the PBDTSe-FBT and PBDTSe-FFBT were decreased compared to the polymer PBDTSe-BT. For the optimized active layer of benzothiodiazole based polymers, PBDTSe-FFBT showed the best PCE of 2.63%. Quinoxaline derivatives were synthesized with thiophene and selenophene π-bridge. The effect of the π-bridge and side groups on the benzodithiophene were investigated. PCE of 4.47% were obtained for PBDT-TQx with an increase in all photovoltaic parameters. Benzothiodiazole and thienopyrroledione based random polymers were obtained to investigate the effect of the different acceptor moieties and F atom substitution. The best device performance was found with the addition of one F atom substitution, RP2, giving 3.38% of PCE. Different side groups on benzodithiophene were used with benzotriazole acceptor to synthesize PBDTPh-BTz, PBDTT-BTz and PBDTTT-BTz. The optimized studies showed the best performance as 3.49% of PCE for PBDTT-BTz. Isoindigo containing polymers resulted broad absorption in UV-Vis spectra. Preliminary results showed a PCE of 2.03% for PBDTTT-IID.


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Citation Formats
S. Taşkaya Aslan, “Synthesis and characterization of benzodithiophene containing conjugated polymers for organic bulk heterojunction solar cells,” Ph.D. - Doctoral Program, Middle East Technical University, 2022.