Conjugated Polymer Based on Polycyclic Aromatics for Bulk Heterojunction Organic Solar Cells: A Case Study of Quadrathienonaphthalene Polymers with 2% Efficiency

Polycyclic aromatics offer great flexibility in tuning the energy levels and bandgaps of resulting conjugated polymers. These features have been exploited in the recent examples of benzo[2,1-b:3,4-b']dithiophene (BDT)based polymers for bulk heterojunction (BHJ) photovoltaics (ACS Appl. Mater. Interfaces 2009, 1, 1613). Taking one step further, a simple oxidative photocyclization is used here to convert the BDT with two pendent thiophene units into an enlarged planar polycyclic aromatic ring-quadrathienonaphthalene (QTN). The reduced steric hindrance and more planar structure promotes the intermolecular interaction of QTN-based polymers, leading to increased hole mobility in related polymers. As-synthesized homopolymer (HMPQTN) and donor-acceptor polymer (PQTN-BT) maintain a low highest occupied molecular orbital (HOMO) energy level, ascribable to the polycyclic aromatic (QTN) moiety, which leads to a good open-circuit voltage in BHJ devices of these polymers blended with PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester; HMPQTN: 0.76V, PQTN-BT: 0.72V). The donor-acceptor polymer (PQTN-BT) has a smaller optical bandgap (1.6eV) than that of HMPQTN (2.0 eV), which explains its current (5.69 mA cm(-2)) being slightly higher than that of HMPQTN (5.02 mA cm(-2)). Overall efficiencies over 2% are achieved for BHJ devices fabricated from either polymer with PCBM as the acceptor.