New Thieno[3,2- b]thiophene-Based Acceptor: Tuning Acceptor Strength of Ladder-Type N-Type Materials to Simultaneously Achieve Enhanced V_oc and J_sc of Nonfullerene Solar Cells

A new thieno[3,2-b]thiophene-incorporated acceptor TTC has been developed. The TTC acceptor was installed in a haptacyclic ladder-type core (BDCPDT) to furnish an n-type BDCPDT-TTC. The standard PBDB-T:BDCPDT-IC device showed a PCE of 9.33% with a V_oc of 0.86 V and a J_sc of 16.56 mA/cm². By molecular engineering of the acceptor unit, the BDCPDT-TTC:PBDB-T-based device delivered an enhanced efficiency of 10.29% with a simultaneously enhanced V_oc of 0.94 V and J_sc of 17.72 mA/cm². Incorporation of the electron-donating thieno[3,2-b]thiophene unit into the acceptor moiety decreases the electron-accepting strength, thereby upshifting the HOMO/LUMO energy levels to decrease the ΔE_HOMO and E_loss, achieving a larger V_oc. Second, the extended conjugated bicyclic thieno[3,2-b]thiophene ring beneficially induces an additional optical transition at short wavelengths, leading to improvement of J_sc. Alternatively, BDCPDT-FIC installed with the fluorinated acceptor shows more red-shifted absorption to achieve a high J_sc of 19.12 mA/cm².