STFTYT: A simple and broadly absorbing small molecule for efficient organic solar cells with a very low energy loss

Abstract A simple and new A-π-D-π-A small molecule denoted as STFTYT was designed and synthesized employing spiro[cyclopenta[1,2- b :5,4- b ′]dithiophene-4,9′-fluorene] (STF) as the central donor (D) unit, ethylhexylthiophene as the π-conjugated bridge, indenedione as the acceptor (A) unit, for application as an electron donor material in solution-processable organic solar cells (OSCs). Results demonstrate that the target material exhibits an incredibly broad absorption extended to 813 nm, a small bandgap of 1.53 eV, and a low-lying HOMO energy level of −5.26 eV. Theory calculations reveal that the shortened π-bridge is beneficial to improving molecular planarity and enhancing the intramolecular charge transfer (ICT) effect between the donor and acceptor groups, and ultimately contributing to a broad absorption and small bandgap. Bulk-heterojunction OSCs based on STFTYT as donor and PC 71 BM as acceptor were prepared and the photovoltaic properties were optimized. A considerable power conversion efficiency (PCE) of 6.17% was achieved along with an extremely high open-circuit voltage ( V oc ) of 0.97 V and a very low energy loss ( E loss ) of 0.56 eV.