Complementary Semiconducting Polymer Blends: The Influence of Conjugation-Break Spacer Length in Matrix Polymers

The concept of complementary semiconducting polymer blends (c-SPBs) for efficient charge transport was recently proposed and established by our group. In this study, we aim to reveal the influence of the length of conjugation-break spacers (CBSs) on charge transport properties of the matrix polymers and their corresponding complementary polymer blends. A series of 11 DPP-based semiconducting polymers DPP-Cm (m = 2-12) that incorporate CBSs of 2-12 methylene units along the polymer backbones were prepared and characterized. The UV-vis spectra and the ultraviolet photoelectron spectroscopy (UPS) measurements show that the CBS length has marginal influence on the polymer absorption spectra, energy levels, and band gaps. It also has little impact on polymer decomposition temperatures. However, the CBS length has a profound influence on polymer phase transition and the heat of fusion. As for the melt transitions, an odd-even effect is observed from DPP-C2 to DPP-C7, in which polymers with even-numbered CBSs show higher melting points than their adjacent odd-numbered derivatives. The trend is opposite for heat of fusion. The polymers with odd-numbered CBSs exhibit larger heat of fusion, indicating higher ordering and crystallinity. The odd-even effect is also found in surface morphologies of the polymers by atomic force microscopy (AFM). The polymers with the even CBSs have a more interconnected feature that appear more fibrillar than the polymers with the odd linkages. As far as charge carrier mobility is concerned, the average number drops from 0.023 cm² V^-1 s^-1 to 7.9 × 10^-6 cm² V^-1 s^-1 as the CBS moves from C2 to C12. It is intriguing to observe that even-numbered polymers outperform the adjacent odd-numbered polymers, despite the fact that the latter show higher ordering and crystallinity in thin films. When these polymers are mixed with fully conjugated DPP-C0 (2 wt %, designated as tie chain polymer), the obtained c-SPBs witness a dramatic increase (2-4 orders of magnitude) in charge carrier mobility. Interestingly, the odd-even effect is not found for charge transport in the c-SPBs. This work reveals that the length of CBSs plays a significant role in charge transport properties of the matrix polymers and reconfirms that efficient charge transport properties of the c-SPB result from the interactions between matrix polymers and tie chain polymers. This begins to provide guidelines as to what spacer lengths may be utilized to offer the best balance between processing and charge transport properties.