Impact of Nonfullerene Molecular Architecture on Charge Generation, Transport, and Morphology in PTB7-Th-Based Organic Solar Cells

Document Type

Article

Publication Date

8-8-2018

Abstract

Despite the rapid development of nonfullerene acceptors (NFAs), the fundamental understanding on the relationship between NFA molecular architecture, morphology, and device performance is still lacking. Herein, poly[[4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophenediyl]] (PTB7-Th) is used as the donor polymer to compare an NFA with a 3D architecture (SF-PDI4) to a well-studied NFA with a linear acceptor–donor–acceptor (A–D–A) architecture (ITIC). The data suggest that the NFA ITIC with a linear molecular structure shows a better device performance due to an increase in short-circuit current (Jsc) and fill factor (FF) compared to the 3D SF-PDI4. The charge generation dynamics measured by femtosecond transient absorption spectroscopy (TAS) reveals that the exciton dissociation process in the PTB7-Th:ITIC films is highly efficient. In addition, the PTB7-Th:ITIC blend shows a higher electron mobility and lower energetic disorder compared to the PTB7-Th:SF-PDI4 blend, leading to higher values of Jsc and FF. The compositional sensitive resonant soft X-ray scattering (R-SoXS) results indicate that ITIC molecules form more pure domains with reduced domain spacing, resulting in more efficient charge transport compared with the SF-PDI4 blend. It is proposed that both the molecular structure and the corresponding morphology of ITIC play a vital role for the good solar cell device performance.

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