Marked Consequences of Systematic Oligothiophene Catenation in Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione and Bithiopheneimide Photovoltaic Copolymers

As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor–acceptor copolymers, <b>PTPDnT</b> and <b>PBTInT</b>, based on thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione (<b>TPD</b>) or bithiopheneimide (<b>BTI</b>) as electron acceptor units, respectively, and oligothiophenes (<b>nT</b>s) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar <b>TPD</b> polymer backbones, however backbone torsion yields greater open-circuit voltages for <b>BTI</b> polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that <b>PTPDnT</b> and <b>PBTInT</b> conjugation lengths scale with <b>nT</b> catenation up to <i>n</i> = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The <b>3T</b> with “outward” alkylation in <b>PTPD3T</b> and <b>PBTI3T</b> affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm²/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for <b>PTPD3T</b>-based PSCs with PCE = 7.7%. The results demonstrate that <b>3T</b> is the optimal donor unit among <b>nT</b>s (<i>n</i> = 1–4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based <b>PTPD3T</b> blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance