Acetal- and Aldehyde-Substituted Thiophene-Benzodithiophene Copolymers for Organic Solar Cells

This study introduces two wide bandgap polymers, PBDTAT and PBDTFT, designed for cost-effective organic solar cells (OSCs). PBDTAT, synthesized through Stille coupling polymerization, features a 3-acetal-substituted thiophene building block, while PBDTFT, with the 3-formyl-substituted thiophene building block, is derived through a simple postpolymerization acetal-to-formyl conversion. The acetal substituents induce significant twisting in the polymer backbone, reducing the highest occupied molecular orbital energy (E_HOMO) of PBDTAT to −5.55 eV. Conversely, formyl groups have less steric impact, resulting in a more coplanar polymer backbone, and a strong electron-withdrawing effect, significantly lowering the E_HOMO of PBDTFT to −5.67 eV. These lowered E_HOMO levels contribute to achieving higher open-circuit voltages (V_OC) of 0.77 and 0.84 V for OSC devices with active layers of PBDTAT:Y6 and PBDTFT:Y6, respectively. Surprisingly, space-charge-limited current hole mobilities of PBDTAT, in neat and blend films with Y6, demonstrate similar or higher mobilities than those of PBDTFT, challenging assumptions about the impact of a significantly twisted backbone in PBDTAT on hole transport. This suggests that introducing controlled backbone twisting could strategically broaden the bandgap and reduce the HOMO energy level without compromising charge transport. Consequently, the OSC devices based on PBDTAT:Y6 can achieve a short-circuit current density (J_SC) of 24.00 mA/cm². Furthermore, photoluminescence quenching experiments confirm highly efficient hole transfer from the PBDTFT/Y6 interface, despite the small E_HOMO offset of only 0.07 eV. This leads to a high J_SC of up to 24.20 mA/cm² for the PBDTFT:Y6-based devices.