Abstract
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells.
Highlights
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices
In order to better visualize the impact of reflection on an H-aggregate analysis, the exciton bandwidth W is determined with Equation (1)
Absorbance spectra evoke the impression that the exciton bandwidth is dependent on film thickness, W is a function of the electronic coupling between polymer chromophores only
Summary
Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. Intrachain transport along the polymer backbone demonstrates a higher mobility compared to interchain transport in π-πstacking direction, with a difference of about two orders of magnitude in case of poly(3-hexylthiophene)[4,5,6] For this reason it is necessary to characterize polymer systems with regard to their complex morphology behavior. We discuss the potential and challenges to experimentally obtain a quantitative analysis of UV/Vis and PL spectra within the H-aggregate framework For this purpose, it is necessary to investigate the influence of the dispersive refractive index by means of various film thicknesses
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