Tailoring side chains of dithienophthalimide-based non-halogenated polymer donors to enhance the efficiency of polymer solar cells

Abstract

Non-halogenated low-toxicity polymers demonstrate significant commercialization potential for organic solar cells (OSCs), owing to their advantageous processing characteristics. However, high-performance non-halogenated polymer donors are still limited. Herein, dithienophthalimide (DPI)-based monomers bearing various side chains have been synthesized at low cost. And then, a series of BDT-DPI-based non-halogenated polymer donors (PBDT-DPIs) were designed and obtained by synergistically tuning the alkyl side chains on both DPI and BDT units, in which thiophene served as a π-bridge, BDT with various side chains (R₁) served as the D unit, and DPI with various side chains (R₂) served as the A unit. The effects of alkyl side chains on the polymers’ optical absorption spectra, electronic and morphological properties, and photovoltaic performance were investigated to understand the relationship between the structure and photovoltaic (PV) performance. It is found that alkyl side chain lengths have negligible impact on optical absorption spectra due to their same backbones, but have profound effects on their aggregation and crystallinity in the solid state. The length of alkyl side chains has a significant influence on the active layer morphology, charge transport properties, and device performance when blended with the non-fullerene acceptor BTP-eC9. As a result, through fine-tuning the alkyl side chains, a PBDT_(8,10)-DPI_(4,6)-based device exhibited the highest PCE of 14.56%, with an open-circuit voltage of 0.85 V, a short-circuit current density of 24.76 mA cm⁻², and a fill factor of 69.05%. Our study demonstrated that tailoring the side chain on both BDT and DPI units could optimize crystallinity, aggregation and miscibility for developing high-performance non-halogenated polymer donors.