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Toward Efficient All-Polymer Solar Cells via Halogenation on Polymer Acceptors
journal contribution
posted on 2020-07-07, 23:03 authored by Yuxiang Li, Zhiyan Jia, Qilin Zhang, Ziang Wu, Hongmei Qin, Jianye Yang, Shuguang Wen, Han Young Woo, Wanli Ma, Renqiang Yang, Jianyu YuanAlthough
halogenation has been widely regarded as an effective
approach to adjust the properties of organic semiconductors, systematic
investigation on the comparison of nonhalogenated and halogenated
polymer acceptors only received minor attention in all-polymer solar
cell (all-PSC) community. Herein, we report three IDIC-based narrow
band gap polymer acceptors, PIDIC2T, PIDIC2T2F, and PIDIC2T2Cl, which
are composed of IDIC-C16 building blocks as acceptor units, linking
pristine bithiophene, fluorinated bithiophene, or chlorinated bithiophene
as donor units. Although these three polymer acceptors exhibit nearly
identical lowest unoccupied molecular orbital (LUMO) levels of ca.
−3.87 eV with a similar optical band gap of ca. 1.54 eV, we
found that different halogen species significantly affect the electron
mobility and thin-film morphology of the polymer acceptors. All-PSCs
were fabricated by pairing three polymer acceptors with a PBDB-T polymer
donor, while PIDIC2T2Cl delivered a highest power conversion efficiency
(PCE) of 5.34% due to its favorable bulk morphology with smaller root-mean-square
(rms) roughness values, which induce the relatively more balanced
charge carrier mobilities. By blending the fluorinated analogue of
PBDB-T, PM6, further improved VOC, JSC, and fill factor (FF) of devices were achieved
(5.46% for PM6:PIDIC2T, 4.96% for PM6:PIDIC2T2F, 7.11% for PM6:PIDIC2T2Cl),
which can be due to the synergistic effect of the deeper highest occupied
molecular orbital (HOMO) energy level of PM6, enhanced crystallinity,
and more matched charge transport. This systematic study provides
an insight into the influence of halogenation (fluorination and chlorination)
on the optoelectrical properties of n-type organic semiconductors
and demonstrates an efficient strategy that the design guideline for
polymer acceptors can be enriched by backbone halogenation to further
develop high-performance all-PSCs.