Imaging the domain structure of organic semiconductor films

Abstract

State-of-the-art solution-processed organic fieldeffect transistors typically use polycrystalline organic semiconductor thin films as the active layer. Although it is widely regarded that boundaries between polycrystalline domains are a likely source of charge trapping limiting charge carrier mobility, little is known about the detailed domain structure of such films. Furthermore, variations in local order particularly in conjugated polymer films are likely to further impede charge transport. In recent years a number of techniques have been exploited that are able to provide information regarding local domain orientation and molecular order in polycrystalline organic thin films. These techniques have provided new information regarding the nature of domain structure providing an opportunity to directly evaluate the influence of domain structure on device operation. This article aims to provide a timely review of the experimental approaches used to date and provide a perspective for future work.

Imaging Nanoscale Morphology of Semiconducting Polymer Films with Photoemission Electron Microscopy

“…One of the key challenges is the polycrystalline nature of most organic semiconductor films. Recognition of this important relationship has led to extensive structural characterization of films, [1,4,[6][7][8][9][10][11][12] with the attempt to correlate morphology and device performance. [3] Films of organic semiconducting polymers are, if at all, only semicrystalline and therefore exhibit an additional degree of complexity.…”

“…While the first absorbed photon excites electrons within the material, a second one generates photoelectrons from the excited states. [7] This means that absorption of light is most likely if the polarization direction of the light is parallel to the polymer backbone. For P3HT, the transition dipole moment is oriented along the polymer backbone, i.e., along the polymer chain.…”

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Investigation of this relationship requires an experimental approach which is capable of providing information about the key features of the film morphology: the spatial distribution of ordered (crystalline) and unordered (amorphous) domains, the sizes and shapes of these domains, as well as the orientation of crystallites. [7] A commonly used technique is atomic force microscopy (AFM), [5] which allows for high-resolution imaging of the material surface and primarily delivers information about surface topography. A comprehensive overview is given by Huang et al [6] as well as McNeill.

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“…Scanning near-field optical microscopy (SNOM) overcomes this drawback exploiting the near-field in the vicinity of a sharp tip, which is scanned over the sample. Recognition of this important relationship has led to extensive structural characterization of films, [1,4,[6][7][8][9][10][11][12] with the attempt to correlate morphology and device performance. [26][27][28] While SNOM is a powerful technique, data analysis can be challenging due to possible artifacts, e.g., by sample topography and scattering.…”

Impact of Thermal Annealing on the Semicrystalline Nanomorphology of Spin-Coated Thin Films of Regioregular Poly(3-alkylthiophene)s as Observed by High-Resolution Transmission Electron Microscopy and Grazing Incidence X-ray Diffraction

“…21−23 This is essential as charge transport is limited by the interconnectivity between crystalline domains through the amorphous interlamellar zones. 23 Accordingly, mapping the nanomorphology (interconnectivity of domains, dimensions, preferred orientation) as a function of preparation conditions is an essential step toward a better modeling and understanding of charge transport in these semicrystalline systems. Moreover, P3ATs are reported to fold, similar to the more flexible chains like PE, 24−27 which is known to impact the growth features of semicrystalline polymers; e.g., lamellar thickening in the direction of the chains is expected upon thermal annealing.…”