Improving Small-Molecules Based OSCs Performance Through Molecular Optimization: A Computational DFT Analysis

Organic solar cells (OSCs) have gained significant attention due to their potential for low-cost production and flexible applications. This study focuses on the computational design and optimization of donor molecules to enhance OSC efficiency. We have strategically designed benzodithiophene (BDT) and thieno[3,2-b]thiophene (TT) derivatives (M1 to M7) by varying the number of the central core and modifying the side chains with different substituents, allowing for fine-tuning of their electronic properties and enhancing their photovoltaic performance. This substitution promotes more ordered π-π stacking improving hole transport properties. Introducing a twisted backbone structure in the designed molecules M1-M7, with alteration of different side chains and substitutions, helps suppress molecular aggregation. The use of a selenophenyl (M6) side chain enhances intermolecular interactions compared to thiophenyl side chains (M1-M5). Thiophenyl substituted molecule exhibited higher HOMO energy levels. This higher energy level provides a larger HOMO offset, facilitating efficient exciton dissociation without increasing energy losses, which is favorable for achieving high V_oc (1.10-1.41 V). Among the derivatives, M6 exhibits the highest absorption maximum at 625 nm, attributed to the selenophenyl side chain. The order of maximum absorbed wavelengths M6 > M1 > M7 > M3 > M4 > M5 > M2 > R. Higher fill factor (84-91%) and short-circuit current density (24.85 mA cm^-2) contributed to the enhanced PCEs due to the optimized morphology and improved charge transport characteristics. Molecular engineering strategies such as incorporating twisted backbones, optimizing intermolecular interactions, and selecting appropriate side chains are important in designing small moleculer donors for ASM-OSCs. These approaches collectively enable the achievement of suitable absorption, higher efficiency and ultimately, superior device performance metrics.