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— q uadra t hieno n aphthalene ( 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‐C₆₁‐butyric acid methyl ester; HMPQTN : 0.76 V, PQTN ‐ BT : 0.72 V). The donor–acceptor polymer ( PQTN ‐ BT ) has a smaller optical bandgap (1.6 eV) 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.