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 / PC 71 BM exhibited high power conversion efficiency (PCE) up to 9.15%, significantly higher than that of the device based on TT4FIDTT / PC 71 BM (PCE of 7.35%). The higher J sc of the device based on DT4FIDTT / PC 71 BM 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.