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

Abstract

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 (Eg 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 Jsc 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 (Eg > 1.85 eV) small molecular donor, demonstrating the feasibility of branched alkylated‐indacenodithieno[3,2‐b]thiophene core in enhancing overall photovoltaic performance.