Temperature effects on the vibronic spectra of BEH–PPV conjugated polymer films

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Abstract
Good quality thin films of poly(2,5-bis(2′-ethyl-hexyl)–1,4-phenylenevinylene) (BEH–PPV) were optically characterized by photoluminescence, absorption, and Raman scattering techniques. The temperature evolution of the vibronic structures in the photoluminescence and absorption spectra was analyzed. At low temperatures it was possible to identify the different phonon modes that contribute to the vibronic spectra. The correlation of the Raman and photoluminescence spectra enabled us to conclude that the main vibrational modes are the two most intense Raman bands at 1310 and 1579 cm−1. The emission efficiency highly increases and the absorption spectra become much more resolved with decreasing temperature. The temperature dependence of the zero-phonon line in the absorption and photoluminescence measurements is attributed to an increase of the effective conjugation length at low temperatures. The results from the polarization-resolved photoluminescence demonstrated the high degree of the in-plane structural order in the BEH–PPV films, corroborating to the relatively high conjugation length obtained from the analysis of the Huang–Rhys factor.

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