End group engineering enabling organic solar cells with high open-circuit voltage

To investigate the influence of thiophene-fused indanone or thieno[3,2-b]thiophene-fused indanone on acceptors, we designed and synthesized three molecules BTP-CC, BTP-TTC and BTP-TC. Impressively, a high open-circuit voltage (V oc) of 0.98 V was achieved in BTP-CC based device, which is one of the highest values based on A-DA′D-A type acceptors to date. The small ΔE HOMO (0.05 eV) between PBDB-T and BTP-TTC resulted in the inefficient exciton dissociation and charge transfer. Despite obtaining a relatively high V oc of 0.95 V, BTP-TTC based organic solar cell (OSC) displayed poor short-circuit current density (J sc) and efficiency. Compared with BTP-CC and BTP-TTC, BTP-TC exhibited red-shifted absorption and down-shifted energy levels. Benefiting from the increased crystallinity and the stronger intermolecular interaction, the device of PBDB-T:BTP-TC achieved improved charge dissociation, enhanced charge transport and suppressed charge recombination. Eventually, BTP-TC based OSC realized a higher efficiency of 13.57% with V oc of 0.86 V, J sc of 22.64 mA cm–2 and fill factor of 0.70. This systematic work provides an approach to improving V oc and establishes a guideline for developing A-DA′D-A type acceptors with thiophene- or thieno[3,2-b]thiophene-containing end group.