Abstract
In this contribution, we studied the effect of fluorine substitution on photogenerated charge generation, transport, and recombination in polymer solar cells. Two conjugated polymer materials, PBDTTT-E (fluorine free) and PTB7 (one fluorine substitution), were compared thoroughly. Meanwhile, various characterization techniques, including atomic force microscopy, steady-state spectroscopy, transient absorption spectroscopy, spectroelectrochemistry, and electrical measurements, were employed to analyse the correlation between molecular structure and device performance. The results showed that the influence of fluorine substitution on both the exciton binding energy of the polymer and the carrier recombination dynamics in the ultrafast timescale on the polymer was weak. However, we found that the fluorine substitution could enhance the exciton lifetime in neat polymer film, and it also could increase the mobility of photogenerated charge. Moreover, it was found that the SOMO energy level distribution of the donor in a PTB7:PC71BM solar cell could facilitate hole transport from the donor/acceptor interface to the inner of the donor phase, showing a better advantage than the PBDTTT-E:PC71BM solar cell. Therefore, fluorine substitution played a critical role for high-efficiency polymer solar cells.
Highlights
In the field of solar energy, polymer solar cells (PSCs) have attracted much attention owing to their flexibility, low cost, light weight, material diversification, and large-area solution processing (Kim et al, 2007; Arias et al, 2010; Lu et al, 2018)
The results showed that fluorine substitution had an insignificant effect on polymer exciton binding energy and carrier recombination dynamics in the ultrafast timescale
It is well known that VOC of PSCs is related to the highest occupied molecular orbital (HOMO) energy level of the donor and the lowest unoccupied molecular orbital (LUMO) energy level of the acceptor
Summary
In the field of solar energy, polymer solar cells (PSCs) have attracted much attention owing to their flexibility, low cost, light weight, material diversification, and large-area solution processing (Kim et al, 2007; Arias et al, 2010; Lu et al, 2018). It is usually adopted as a substituent to substitute H atom on benzene (or thiophene) rings, so as to adjust the energy levels and optical absorption of a polymer in the active layer. Fluorine substitution would play an important role in the photophysical process and, the performance of polymer solar cells. The ultrafast photoelectric conversion processes in PSCs that are based on fluorine-substituted conjugated polymers (such as PTB7-Th, PffBT4T-2OD, and PBDBT-2F) have been studied extensively by using time-resolved spectroscopy and transient photoelectric measurements (Lu and Yu, 2014; Wu et al, 2018; Zhang et al, 2019; Wang et al, 2020). The targeted research about the influence of fluorine atoms on the photoelectric conversion process is still unclear yet for fluorine-substituted polymers
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