Strain-released method to enhance the photovoltaic performance in solution-processed organic solar cells

To release the steric-strain in largely fused molecular backbone and achieve the much planar molecules for efficient organic solar cells, a facile method was proposed by dividing the rigidly fused dibenzosexithiophene (DBST) units in FT3 with large molecular skeletons to two smaller building blocks to construct the electron donor-acceptor type compound FT4. As observed, much improved molecular co-planarity was achieved for FT4 in contrast to that of FT3, then leading to the higher charge carrier mobilities up to 6.7 × 10⁻⁴ cm²V⁻¹s⁻¹ by SCLC method. Meanwhile, FT4-based BHJ solar cells also exhibited much enhanced photovoltaic performance with the maximum PCE value of 3.07% and higher J_sc value of 8.23 mA/cm², which is only 2.13% and 5.67 mA/cm² for FT3-based solar cells. The results here indicated that the strain-released method, especially in the highly fused molecules, is beneficial to further improve their co-planarity, then leading to the high charge carrier mobilities and much enhanced photovoltaic performance in solar cells.