Synthesis of main-chain polyoxometalate-containing hybrid polymers and their applications in photovoltaic cells

Hexamolybdate clusters have been embedded through covalent bonds into the main chain of poly(phenylene acetylene)s. These hybrid polymers were synthesized by palladium-catalyzed coupling reactions of a diiodo functionalized cluster with a diethynylbenzene derivative or a diethynyl functionalized cluster with a diiodobenzene derivative. These polymers are soluble in organic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and free-standing films can be spin-coated or cast from solutions. While hybrid monomer 2a exhibits a sharp melting transition at 246 degreesC, polymers 5a and 5b show glass transitions at 125 and 102 degreesC, respectively. Cyclovoltammetry studies of the hybrid polymers revealed a reversible reduction wave at 1.19 V versus Ag/Ag+, comparable to those of bifunctionalized imido derivatives of hexamolybdates. These polymers show intense absorption in the visible range but with little fluorescence emissions, indicating efficient fluorescence quenching of the embedded polyoxometalate (POM) cluster on the organic phenylene acetylene units. Simple single-layer photovoltaic (PV) cells with a device configuration of indium-tin oxide (ITO)/polymer/Ca have been fabricated and a power conversion efficiency of 0.15% has been obtained, which is significantly higher than PV cells fabricated with other conjugated polymers in the same device configuration. These results convincingly demonstrate the potential applications of POM-based organic-inorganic hybrids in molecular electronics and photonics.