A Tale of Current and Voltage: Interplay of Band Gap and Energy Levels of Conjugated Polymers in Bulk Heterojunction Solar Cells
We recently proposed a design strategy of “weak donor−strong acceptor” copolymer to approach ideal polymers with both a low HOMO energy level and a small band gap for bulk heterojunction (BHJ) polymer solar cells in order to achieve both a high open circuit voltage (V oc) and a high short circuit cu...
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Published in | Macromolecules Vol. 43; no. 24; pp. 10390 - 10396 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Washington, DC
American Chemical Society
28.12.2010
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Subjects | |
Online Access | Get full text |
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Summary: | We recently proposed a design strategy of “weak donor−strong acceptor” copolymer to approach ideal polymers with both a low HOMO energy level and a small band gap for bulk heterojunction (BHJ) polymer solar cells in order to achieve both a high open circuit voltage (V oc) and a high short circuit current (J sc) [ACS Appl. Mater. Interfaces 2010, 2, 1377]. To further demonstrate the delicate interplay of the V oc and J sc through molecular design of conjugated polymers, two such “weak donors”, naphtho[2,1-b:3,4-b′]dithiophene (NDT) and dithieno[3,2-f:2′,3′-h]quinoxaline (QDT), which differ only by two atoms, were copolymerized with a common acceptor, 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT). The BHJ devices based on PNDT−4DTBT and PQDT−4DTBT as the donor polymer (with PC61BM as the acceptor) exhibit overall efficiencies of 5.1% and 4.3%, respectively. Through these two structurally related polymers, we demonstrate that incorporating electron-withdrawing atoms in the donor unit would lead to a lower HOMO energy level. This lower HOMO energy level translates into a higher open circuit voltage (V oc) of 0.83 V in PQDT−4DTBT-based BHJ devices. In contrast, the slightly higher HOMO energy level (− 5.34 eV) of PNDT−4DTBT limits the V oc to 0.67 V. Since the LUMO levels of both polymers are similar, the higher HOMO energy level of PNDT−4DTBT leads to a smaller band gap of 1.61 eV (vs 1.70 eV of PQDT−4DTBT), which contributes to a noticeably higher J sc of 14.20 mA/cm2 (vs 11.38 mA/cm2 of PQDT−4DTBT-based BHJ devices). Future research needs to focus on the employment of even weaker donor and stronger acceptors via innovative structural modification in order to concurrently achieve a deeper HOMO and a lower band gap, thereby enhancing both V oc and J sc. |
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ISSN: | 0024-9297 1520-5835 |
DOI: | 10.1021/ma101646r |