Small Molecules Based on Thieno[3,4-c]pyrrole-4,6-dione for High Open-Circuit Voltage (V_OC) Organic Photovoltaics: Effect of Different Positions of Alkyl Substitution on Molecular Packing and Photovoltaic Performance

Two different thienopyrroledione (TPD)-based small molecules (SMs) with different alkyl substitution positions were synthesized, and their photovoltaic properties are measured and compared to examine the effect of the alkyl substitution position on their optical, electrochemical, and photovoltaic properties. The use of TPD as an electron-accepting unit in conjugated SMs effectively lowers the highest occupied molecular orbital (HOMO) energy levels of the conjugated SMs and leads to high open-circuit voltage (VOC). The two SMs with n-hexyl group substituted at different positions exhibit almost identical optical and electrochemical properties in the pristine state. However, the crystallographic and morphological characteristics of the two SMs are significantly different, because they are blended with PC71BM. The SM in which n-alkyl groups are substituted at the central accepting unit exhibits a power conversion efficiency (PCE) of 6.0% with VOC=0.94 V, which is among the highest PCE values of TPD-based SM devices, whereas the SM with n-alkyl groups being substituted at the chain ends shows a moderate PCE value of 3.1%.