Absorption and luminescence characteristics of optically thin conjugated polymer films

Abstract

Poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] (MO-PPV) optically thin films have been prepared from their chloroform solutions of different concentrations. Then their UV–Vis absorption (Abs), photoluminescence (PL), and selective-excitation photoluminescence (SEPL) spectra have been measured at room temperature. The position of S0 → S1 absorption peak shows red shift and broadened relative to that in MO-PPV solution form due to solidation effect, while no relative shifts between the absorption spectra of these film samples are observed. A “long-wavelength tail” appears in Abs spectra which should attribute to the disorder morphology of these thin films. A long-wavelength emission component near 630 nm has been identified as S2 → S0 vibronic band through Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This absorption tail and the second vibronic band cannot be observed in thick films. The PL efficiency of MEH-PPV thin film and that of MO-PPV thin film are also compared and explained by the side substituent effect, which can be proved both by theoretical simulation of interchain interactions and by comparing their morphology in film forms. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)