Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 112, Issue 43, Pages 10764-10773Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp805817c
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Funding
- TOP
- Netherlands Organization for Scientific Research
- Joint Solar Programme
- Foundation for Fundamental Research on Matter
- NWO
- Foundation Shell Research
- European Commission
- Human Potential Program Marie-Curie Research Training Network NANOMATCH [MRTNCT-2006-035884]
- Belgian National Fund for Scientific Research
- Ramon y Cajal
- Spanish Ministry for Education and Science
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Small band gap polymers may increase the energy conversion efficiency of polymer solar cells by increased absorption of sunlight. Here we present a combined experimental and theoretical study on the optical and electrochemical properties of a series of well-defined, lengthy, small band gap oligo(5.7-bis(thiophen-2yl)thieno[3,4-b]pyrazine)s (E-g = 1.50 eV) having alternating donor and acceptor units. The optical absorptions of the ground state, triplet excited state, radical cation, and dication are identified and found to shift to lower energy with increasing chain length. The reduction of the band gap in these alternating small band gap oligomers mainly results from an increase of the highest occupied molecular orbital (HOMO) level. The S-1-T-1 singlet-triplet splitting is reduced from similar to 0.9 eV from the trimeric monomer to -0.5 eV for the pentamer. This significant exchange energy is consistent with the fact that both the HOMO and the lowest unoccupied molecular orbital (LUMO) remain distributed over virtually all units, rather than being localized on the D and A units.
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