Abstract
We report surface and interface effects in dynamics and chain conformation in the thin film of conjugated polymer PCDTBT. To probe dynamic anomalies, we measure the glass transition temperature (T(g)) of PCDTBT films as a function of thickness, and find that there is a significant depression in T(g) for films less than 100 nm thick; a result qualitatively similar to that observed in many other polymer film systems. However, for films less than 40 nm, the T(g) converges to a constant value of 20 K below its bulk value. Grazing incidence X-ray diffraction shows depth-dependent molecular organization that is associated with the unusual thickness-dependent dynamics.
Highlights
We explore the changes in Tg and molecular organization of thin films of a conjugated polymer poly[N-9 -heptadecanyl-2,7-carbazole-alt-5,5-(4, 7 -di-2-thienyl-2, 1, 3 -benzothiadiazole)] (PCDTBT)
Our previous work has demonstrated that any residual CB casting solvent trapped in a PCDTBT film during spin-casting evaporates rapidly during the initial heating cycle once the film reaches a temperature of 135 ◦C [13]
We do not expect degradation of PCDTBT to occur in our measurements as all heating/cooling cycles were performed under a N2 atmosphere at temperatures well below the degradation temperature [17] of PCDTBT (> 450 ◦C)
Summary
We explore the changes in Tg and molecular organization of thin films of a conjugated polymer poly[N-9 -heptadecanyl-2,7-carbazole-alt-5,5-(4 , 7 -di-2-thienyl-2 , 1 , 3 -benzothiadiazole)] (PCDTBT). PCDTBT is one of a relatively new class of low-energy-gap polymers that have been used to fabricate photovoltaic devices and field effect transistors [12,13]. Our measurements suggest the existence of a series of distinct depthdependent morphological phases within the film, with a π-π stacked layer having increased coherence length existing close to the substrate and lamellar-type order towards the film surface We speculate that such structural heterogeneity may directly affect the electronic properties of conjugated polymer thin films; a conclusion that has relevance for the operation of organic-electronic devices [14,15]
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