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Design and Synthesis of Donor-Acceptor Type Low-Bandgap Polymers for High Performance Polymer Solar Cells
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- Authors
- Advisor
- 조원호
- Major
- 재료공학부
- Issue Date
- 2012-02
- Publisher
- 서울대학교 대학원
- Abstract
- Currently, BHJ solar cells fabricated by simple blending of regioregular poly(3-hexylthiophene) (P3HT) as an electron-donating polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have reached the power conversion efficiency (PCE) as high as 5 %. Although possessing several advantageous properties, P3HT has a relatively high bandgap of 1.9 eV and thus harvests photons only up to 22 % of available photons.
To overcome the inherent weakness of the conducting polymers, we have designed and synthesized several kinds of donor-acceptor type alternating copolymers. In the design of low-bandgap polymers, some important aspects should be considered. If the bandgap of the donor polymer is too narrow, that is, the HOMO level of polymer is high-lying and LUMO level is low-lying, there exist opposite effects. Since the open circuit voltage (VOC) of polymer solar cell linearly depends on the difference between the LUMO level of acceptor (PCBM) and HOMO level of donor polymer, the high-lying HOMO level of polymer can result in reduced VOC. Secondly, since a LUMO-LUMO offset of 0.3 ~ 0.4 eV is necessary for efficient electron transfer from the donor polymer to PCBM, the small offset between the LUMO of the polymer and the LUMO of PCBM caused by low-lying LUMO level of polymer prevents electron transfer from polymer to PCBM. In this aspect, we have successfully controlled the molecular energy levels of D-A type copolymers by introducing different substituent on [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) unit which is used as an acceptor moiety in D-A type copolymer. By fine-tuning of the molecular energy level of copolymers, the photovoltaic performance was achieved up to 2.17% which is the highest value among the TQ-based solar cells.
Along with the low-bandgap nature, high charge mobility of conjugated polymers is also important for improving the performance of polymer solar cells. In order to exhibit good charge transport property, the polymer chains are required to pack closely each other facilitating charge transport through intermolecular hopping. For the purpose of inducing denser packing of polymer chains, we have synthesized low-bandgap polymers with fine-tuned number of solubilizing side groups and low-bandgap polymers incorporated with planar component. In both cases, the molecular packing characteristics was greatly improved which resulted in enhanced charge carrier mobility. The power conversion efficiency was largely increased up to 3.8% when planar quinoxaline derivative was introduced in D-A type alternating copolymer.
- Language
- eng
- URI
- https://hdl.handle.net/10371/156559
http://dcollection.snu.ac.kr:80/jsp/common/DcLoOrgPer.jsp?sItemId=000000000194
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