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

Abstract

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. Interfaces2009, 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-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.