Alkylthiophenyl side chain modulation for enhanced photovoltaic properties of polymer donors in organic solar cells

Abstract

Two new polymer donors, PB2T-S and PB2T, were designed and synthesized by incorporating alkylthiophenyl and alkylphenyl groups into the benzo[1,2-b:4,5-b′]-dithiophene (BDT) side chain, respectively. Calculations and experimental results demonstrate that the alkylthiophenyl groups in PB2T-S confer electron-withdrawing characteristics, leading to a lower HOMO level compared to PB2T. Temperature-dependent light absorption spectra reveal that PB2T-S exhibits significantly stronger pre-aggregation behavior in solution. Grazing-incidence wide-angle X-ray scattering measurements further show its strong crystallinity and a predominantly face-on molecular packing orientation. When combined with BTP-eC9, the PB2T-based device shows a power conversion efficiency (PCE) of 11.22%, with a V_OC of 0.80 V, a J_SC of 22.26 mA cm⁻² and an FF of 0.63. In comparison, the PB2T-S-based device delivers a higher PCE of 14.84%, with a simultaneously enhanced V_OC of 0.87 V, J_SC of 25.46 mA cm⁻², and FF of 0.67. This work highlights that alkylthiophenyl groups have the potential to optimize polymer energy levels and aggregation behavior, providing a promising method for designing highly efficient polymer donors in OSCs.