A polymer acceptor with an optimal LUMO energy level for all-polymer solar cells. Issue 9 (22nd June 2016)

• Record Type: Journal Article
• Authors: Ding, Zicheng; Long, Xiaojing; Dou, Chuandong; Liu, Jun; Wang, Lixiang
• Is Part Of: Chemical science. Volume 7:Issue 9(2016:Sep.)
• Page Start: 6197
• Page End: 6202
• Publication Date: 2016-06-22
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c6sc01756h
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : A new polymer acceptor based on the BNBP unit with an optimal LUMO energy level has been developed. The resulting all-polymer solar cells show high PCEs, remarkably high V oc values and small photon energy losses. Abstract : A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level ( E LUMO ). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their E LUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage ( V oc ) of the resulting all-polymer solar cells (all-PSCs). We report that polymer electron acceptors based on the double B←N bridged bipyridine (BNBP) unit exhibit tunable E LUMO because of their delocalized LUMOs over polymer backbones. The E LUMO of the copolymer of the BNBP unit and selenophene unit (P-BNBP-Se ) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit (P-BNBP-T ). As a result, the energy levels ofP-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th ). The electron mobility ofP-BNBP-Se ( μ e = 2.07 × 10 −4 cm 2 V −1 s −1 ) is also higher than that ofP-BNBP-T ( μ e = 7.16 × 10 −5 cm 2 V −1 s −1 ). While the all-PSC device based on thePTB7-Th :P-BNBP-T blend shows a moderate power conversion efficiency (PCE) of 2.27%, the corresponding device withP-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, owing to the suitable E LUMO ofP-BNBP-Se, the all-PSC device ofP-BNBP-Se shows a V oc up to 1.03 V, which is higher by 0.22 V than that with the conventional NDI-based polymer acceptor. These results indicate that BNBP-based polymers can give all-PSCs with high PCEs, remarkably high V oc values and small photon energy losses.