Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
One of the main problems of organic photovoltaics, which have to be solved in order to realize its potential for the market of alternative energy sources, is increasing the power conversion efficiency (PCE) of organic photovoltaic devices made by solution processing, which is considered as significantly cheaper as commonly used vacuum processing. Solution of this problem relies upon design of novel soluble organic semiconducting materials with strong light harvesting properties in the maximal area of the solar light spectrum. In this work we consider a series of novel low bang gap copolymers and star-shaped small molecules based on thiophene chemistry (Figure 1).
The donor-acceptor copolymers synthesized are based on 4,4-difluoro-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) as an acceptor unit and 4,4-dialkyl derivatives of CPDT or its silicon analogue, dithieno[3,2-b:2′,3′-d]-silol, as donor units. These copolymers have an almost identical bandgap of 1.7 eV, but their HOMO energy level varies from −5.34 to −5.73 eV. Bulk heterojunction organic solar cells (BHJ OSC) based on these copolymers as donor and PCBM[60] as acceptor components yield a PCE up to 2.4% [1].
The star-shaped small molecules synthesized have a triphenylamine (TPA) central donor unit connected through oligothiophene conjugated spacer to strong dicyanovinyl acceptor units with solubilizing alkyl end groups. Varying the number of oligothiophene rings as well as the length of aliphatic end groups allowed to increase the PCE of BHJ OSC prepared from these molecules as donor and PCBM[70] as acceptor from 1.4% to 3.6% [2]. Recent improvements for TPA-based molecules with the shortest methyl end group led to PCE approaching 5% without any post-treatment, which is among the highest reported for solution-processed OSCs based on star-shaped molecules [3]. This work was supported by RFBR (grant 13-03-01315a) and Program P-8 of the Presidium of Russian Academy of Sciences.
Figure 1. Chemical structured of low bang gap copolymers and star-shaped small molecules developed in this work.
[1] Drozdov, F. V.; Myshkovskaya, E. N.; Susarova, D. K.; Troshin, P. A.; Fominykh, O. D.; Balakina, M. Y.; Bakirov, A. V.; Shcherbina, M. A.; Choi, J.; Tondelier, D.; Buzin, M. I.; Chvalun, S. N.; Yassar, A.; Ponomarenko, S. A. Novel Cyclopentadithiophene-Based D–A Copolymers for Organic Photovoltaic Cell Applications. Macromol. Chem. Phys. 2013, 214, 2144-2156. [2] Min, J.; Luponosov, Y. N.; Ameri, T.; Elschner, A.; Peregudova, S. M.; Baran, D.; Heumьller, T.; Li, N.; Machui, F.; Ponomarenko, S.; Brabec, C. J. A solution-processable star-shaped molecule for high-performance organic solar cells via alkyl chain engineering and solvent additive. Organic Electronics 2013, 14, 219-229. [3] Min, J.; Luponosov, Y. N.; Gerl, A.; Polinskaya, M. S.; Peregudova, S. M.; Dmitryakov, P. V.; Bakirov, A. V.; Shcherbina, M. A.; Chvalun, S. N.; Grigorian, S.; Kausch-Busies, N.; Ponomarenko, S. A.; Ameri, T.; Brabec, C. J. Alkyl Chain Engineering of Solution-Processable Star-Shaped Molecules for High-Performance Organic Solar Cells. Adv. Energy Mater. 2014, DOI: 10.1002/aenm.20130123.
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