Investigation of photophysical, morphological and photovoltaic behavior of poly(p-phenylene vinylene) based polymer/oligomer blends

Two cyano oligo-phenylene-vinylenes [α-CNOPV and β-CNOPV] distinguished by the position of cyano (CN) group at the vinylic double bond were synthesized. The acceptor oligomer CNOPVs were blended with poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) MEH-PPV to achieve α and β blends. The pronounced influence of cyano position on the photophysical and morphological properties of blends was observed through UV-vis absorption, photoluminescence and atomic force microscopy. The optical characterization suggests wider spectral photon harvesting in α blends and more planar conformation of molecules in β blends. The steady-state and time-resolved photoluminescence study provides evidence for efficient energy transfer from α-CNOPV to MEH-PPV. On the other hand, the 3:1 β blend exhibits quenching of PL intensity indicative of charge transfer. In addition, the feasibility of MEH-PPV:CNOPV blends in organic photovoltaic devices have been investigated. The initial device parameters show that power conversion efficiency is as high as 0.05% in β 1:1 devices. The photovoltaic efficiencies were limited by weak exciton dissociation in α blends while poor morphology restricted the efficiency in β blends.