Acceptor engineering of BTBT derived donor materials for high performance organic solar cells via density functional and time dependent density functional theory

Creating donor-π-acceptor (D-π-A) molecular systems is an important strategy to further increase the efficiency of organic solar cells (OSCs). In this work, we have explored the optoelectronic effects of structural modifications on the reported small-molecule donor, BTBT-Ph-OC₈H₁₇ (Comp0). The original Comp0 structure was transformed into a D-π-A configuration of the form BTBT-CH = CH-C₆H₄-acceptor to achieve a more π-conjugated system and facilitate stabilization of charge transfer. Following this design strategy, we developed a series of new small-molecule donors (Comp1-Comp5) by incorporating different acceptor units. The density functional theory (DFT) in conjunction with time-dependent DFT (TD-DFT) at the HSEH1PBE/DGDZVP level of theory was employed to investigate their structure, electronic properties, and optical properties. All optimized geometries demonstrated that the designed molecules were planar and stable. TD-DFT results indicate that absorption spectra of the designed molecules were red shifted, with reduced HOMO-LUMO gap values (2.14–2.68 eV versus 3.56 eV for Comp0). The calculated dipole moments showed that Comp1 (9.84 D) and Comp5 (9.28 D) exceeded 9 D, whereas the other derivatives presented lower values. Nevertheless, all Comp1−5/ICBA heterojunctions exhibited high open-circuit voltages (VOC > 1.49 V), demonstrating their potential for photovoltaic devices. The charge transfer evaluation revealed enhanced carrier mobility and longer excited-state lifetimes (up to 4 ns), with Comp5 achieving the best performance due to its low reorganization energy and high dipole moment. The BTBT-based D-π-A systems that we developed in this work, particularly Comp5, represent strong candidates for efficient and stable organic solar cell applications.