Branched 2-Ethylhexyl Substituted Indacenodithieno[3,2-b]Thiophene Core Enabling Wide-Bandgap Small Molecule for Fullerene-Based Organic Solar Cells with 9.15% Efficiency: Effect of Length and Position of Fused Polycyclic Aromatic Units

A pair of 2-ethylhexyl-substituted indacenodithieno[3,2-b]thiophene (EH-IDTT) based tetrafluorinated substituted wide-bandgap small molecular donor materials with different electron-rich terminal units, DT4FIDTT and TT4FIDTT, has been synthesized for high efficiency fullerene-based organic solar cells. The utilization of the EH-IDTT center core increased the optical bandgap of DT4FIDTT (E_g of 1.86 eV) and decreased HOMO energy level in thin films in comparison with those of BIT-4F-T with indacenodithiophene core and thiophene π bridge. Compared to DT4FIDTT with 2,2′-bithiophene terminal unit, TT4FIDTT with further fused thieno[3,2-b]thiophene terminal units exhibited a larger optical bandgap and more straight conformation due to less effective conjugated length of thieno[3,2-b]thiophene. Devices based on DT4FIDTT/PC₇₁BM exhibited high power conversion efficiency (PCE) up to 9.15%, significantly higher than that of the device based on TT4FIDTT/PC₇₁BM (PCE of 7.35%). The higher J_sc of the device based on DT4FIDTT/PC₇₁BM can be attributed to its better charge transport properties and favorable phase separation features. Such high efficiency of 9.15% is the highest value reported for the fullerene-based organic solar cells based on wide-bandgap (E_g > 1.85 eV) small molecular donor, demonstrating the feasibility of branched alkylated-indacenodithieno[3,2-b]thiophene core in enhancing overall photovoltaic performance.