Organometallic Donor−Acceptor Conjugated Polymer Semiconductors: Tunable Optical, Electrochemical, Charge Transport, and Photovoltaic Properties

A series of 11 organometallic conjugated polymer semiconductors bearing a platinum (Pt) center have been synthesized and their electronic band structures, field-effect charge transport, and use in bulk heterojunction solar cells were evaluated. The Pt-bridged donor−acceptor conjugated poly(aryleneethynylene)s were synthesized by Sonogashira-type polycondensation and are exemplified by poly[4-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-7-(5′′-ethynyl-thiophen-2′′-yl)-benzo[1,2,5]thiadiazole] (BT-BTPt), poly[5-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-8-(5′′-ethynyl-thiophen-2′′-yl)-2,3-diheptyl-pyrido[3,4-b]pyrazine] (HPP-BTPt), and poly[5-(5′-trans-bis(tributylphosphine)platinum ethynyl-thiophen-2′-yl)-7-(5′′-ethynyl-thiophen-2′′-yl)-thieno[3,4-b]pyrazine] (TP-BTPt). The Pt-bridged polymers had reversible electrochemical reduction waves from which electron affinities were found to be 2.95 to 3.28 eV. From the onset oxidation potentials of the polymers, we similarly determined ionization potentials to be 4.82 to 5.23 eV. Optical bandgaps of the donor−acceptor polymers were 1.49 to 1.97 eV. The spin coated polymer thin films showed p-channel field-effect charge transport with average hole mobilities of 3.87 × 10−7 to 3.32 × 10−5 cm2/(V s). Bulk heterojunction solar cells based on blends of the polymers with [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) gave power conversion efficiencies as high as 0.68% for HPP-BTPt and 2.41% for BT-BTPt. These results demonstrate the molecular engineering of the electronic band structures and the optical, charge transport, and photovoltaic properties of organometallic donor−acceptor conjugated polymer semiconductors.