Photoinduced Enhancement of Photoluminescence Intensity in PPV Films
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Photoinduced Enhancement of Photoluminescence Intensity in PPV Films Yara Galvão Gobato1, Alexandre Marletta2, Jucimar M. Souza3, Ernesto Pereira3, Roberto M. Faria2, Francisco E.G. Guimarães2 1 Departamento de Física-Universidade Federal de São Carlos, CP 676, 13560-970, São CarlosSP Brazil. 2 Instituto de Física de São Carlos,Universidade de São Carlos, CP 369, 1350-970, São CarlosSP, Brazil 3 CMDMC-LIEC-Universidade Federal de São Carlos, CP676, 13560-970, São Carlos, Brazil ABSTRACT Poly(p-phenylene vinylene) (PPV) films are usually photodegraded in air. We report on the enhancement of photoluminescence (PL) intensity of PPV films induced by Ar ion laser in the presence of air. The PL spectra were measured as function of laser power, time exposure and sample thickness. The initial PL intensity increases up to 300% without significant changes in peak positions by optimizing such parameters. This effect is accompanied by a blue-shift in the absorption spectrum resulting in shortened effective conjugation length and by a formation of defects such as carbonyl groups. Mechanisms for the observed photoluminescence intensity enhancements are suggested.
INTRODUCTION Conjugated polymers have attracted interest due to their potential applications in optoeletronic devices such as light-emitting devices [1] and photocells [2]. However, the lifetime of the polymer devices are shortened when compared to conventional devices based on inorganic semiconductors because PPV films are naturally prone to oxidative degradation. Several studies have been made on the effects of photo-oxidation on the physical properties of PPV both in solution [3] and in thin film [3-9]. It was demonstrated that the main chain of the polymer is modified by chemical reactions with oxygen during photo-excitation through formation of defects, such as carbonyl groups, which promote the shortening of the effective conjugation length and act as quenching sites. There are few recent descriptions of PL enhancement via photoexcitation in conjugated polymers [8, 9]. A small PL intensity increase followed by a very gradual decay was recently reported for poly (m-phenylene-co-2,5-dioctoxy-pphenylenevinylene(PmPV) [8] in Ar atmosphere, but is not yet understood. A reversibility of PL intensity quenching was reported for PPV films [9] and attributed to a conformational change of the chains that might be caused by the incorporation of oxygen and/or water into the polymer film. Recently, a significative enhancement of PL intensity induced by laser irradiation in poly(vinyl alcohol)-poly(3-thiopheneacetic acid) blends (PVA-PTAA) in air was also reported [11]. In the present study, we report a strong enhancement of photoluminescence (PL) intensity in PPV cast films upon laser irradiation in air which competes with the quenching process. This increase of PL is dependent on laser intensity and film thickness. FTIR and absorption studies reveal that this process is oxidative and causes shortening of the conjugation length. Based on our results, the PL enhancement can be e
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