Continuous Illumination of a Conjugated Polymer Causes Strong Enhancement of Photoluminescence

Abstract

We present measurements of absorbance and photoluminescence (PL) for films of poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) as a function of temperature (T) and time (t) of illumination. While having no detectable influence on absorbance of this conjugated polymer, our experiments clearly revealed that illumination of PDOPT caused a significant increase in the PL intensity (I PL (T,t)), that is, the emission probability of PDOPT. Without illumination, we always observed a decrease in I PL with time. An increase in I PL was only detectable when the sample was illuminated. Interestingly, while absorption and emission of photons occur on a time scale of nanoseconds, the herereported changes in the emission probability were slow and occurred on a time scale of minutes to hours. The influence of illumination on changes in I PL (T,t) was qualitatively similar for slowly and rapidly crystallized PDOPT, that is, the degree of crystallinity was not decisive for the observation. The rate of the increase in I PL depended clearly on the power of the illumination light source. As a function of the illumination time, the change in I PL (T,t) was nonmonotonic and depended on sample temperature. We speculate that changes in polymer interactions caused by excited electronic states might have induced slow changes in polymer conformations.

Illumination of Conjugated Polymers Reduces the Nucleation Probability and Slows Down the Crystal Growth Rate

“…32 Based on the results derived from a dewetting study of various conjugated polymers, absorption of photons was proposed to induce a change in conformation, which slowed down the dewetting process. 33 In other studies, an enhancement of photoluminescence (PL) induced by illumination was observed, 29 sometimes attributed to conformational changes at a segmental level. 34,35 The PL of poly[2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] films was boosted substantially by illumination with light.…”

“…3,54,55 In previous experiments on PDOPT films, we demonstrated that, compared to non-illuminated films, the intensity of PL increased significantly upon illumination. 29 These previous studies have suggested that slow changes in PDOPT conformations and possibly changes in interactions between polymers may be induced by illumination. 29 Here, we shed light on how crystallization in PDOPT and P3HT films was affected by simultaneous illumination with white light under various conditions.…”

“…29 These previous studies have suggested that slow changes in PDOPT conformations and possibly changes in interactions between polymers may be induced by illumination. 29 Here, we shed light on how crystallization in PDOPT and P3HT films was affected by simultaneous illumination with white light under various conditions.…”

“…For various organic optoelectronic devices containing a mixture of fullerene derivatives and conjugated polymers, researchers reported an influence of illumination by visible light on photovoltaic behavior. − A number of previous studies investigated how crystallization of photoisomerizable polymers containing compounds such as azobenzene can be induced by the absorption of light of certain wavelengths. − In other studies, crystallization was induced by locally heating the surface of the polymer through flashlight annealing at high light intensities . Furthermore, absorption of photons may potentially introduce changes at various time scales in the conformations of conjugated polymers. − However, it is unclear if such conformational changes induced by the absorption of light can have an impact on ordering processes such as crystallization. There are only very few investigations on the crystallization of conjugated polymers which explore the possible impact of illumination, where absorption of photons may induce changes in diffusion properties and polymer conformation .…”

“…Based on the results derived from a dewetting study of various conjugated polymers, absorption of photons was proposed to induce a change in conformation, which slowed down the dewetting process . In other studies, an enhancement of photoluminescence (PL) induced by illumination was observed, sometimes attributed to conformational changes at a segmental level. , The PL of poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] films was boosted substantially by illumination with light. , The absorption rate of blue light in previously illuminated films was higher than that in non-illuminated films, ascribed to illumination-induced changes in polymer conformations, resulting in a decrease in interchain energy and an increase in chain separation …”