Tuning the Polarizability in Donor Polymers with a Thiophenesaccharin Unit for Organic Photovoltaic Applications

This paper describes the synthesis of low bandgap copolymers incorporating an artificial sweetener derivative, N-alkyl, 3-oxothieno[3,4-d]isothiazole 1,1-dioxide (TID). This new TID unit is identical to the well-known thieno[3,4-c]pyrrole-4,6-dione (TPD) unit except that one carbonyl has been replaced by a sulfonyl group. Semi-empirical calculations on the local dipole moment change between ground and excited states (ge) in the repeating units of the new polymer indicate that the replacement of the carbonyl by a sulfonyl group leads to larger ge values. The resulting polymers exhibit a diminished power-conversion efficiency (PCE) compared to a bulk heterojunction (BHJ) solar cells with PC71BM as an acceptor, which extends the correlation between PCE and ge of single repeating units in p-type polymers to a new regime. Detailed studies show that the strongly electron-withdrawing sulfonyl group is detrimental to charge separation in alternating copolymers containing a TID unit.