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Flexible electronic devices, such as plastic displays, wearable radio frequency identification (RFID) tags, and roll up solar panels, based on pi-bonded (π-bonded) organic semiconductors have become highly desirable due to their light weight and high level of durability. These devices benefit from relatively low manufacturing costs and ease of scaling. However, poor charge mobility, and thus, poor device performance can occur unless the molecules of the organic thin film are properly oriented with respect to one another. In organic semiconductors, charge is transported preferentially along the direction of π-stacking of the aromatic rings of the constituent molecules. Thus, a reliable method for controlling molecular orientation is a highly desirable.

Recently a group of researchers from Princeton University, led by Professor Yueh-Lin Loo, developed post-deposition processing treatments that induce crystallization in the organic thin-film semiconductor hexabenzocoronene (HBC) [1]. Using Grazing Incidence X-ray Diffraction at the G1 beam-line of CHESS, Loo’s group was able to study the distribution of molecular orientation in post-deposition processed thin-films of HBC. Post-deposition processing treatments including hexane-vapor-annealing, thermal annealing, and physical contact with a substrate made of poly(dimethyl siloxane) (PDMS), resulted in a progressive increase in preferential edge-on (see bottom left, Figure 1) molecular orientation, which corresponds to a greater degree of in-plane π-stacking.

Figure 1

Figure 1. a) PDMS contact processing and b) hexane vapor annealing of hexabenzocoronene thin films helps align the molecules, as seen by x-ray scattering (upper figures). Alignment of the conjugated π bonds results in higher charge mobility.

Although diffraction images from all three of the post-deposition treated samples arise from the same underlying crystal structure, there is a dramatic variation in the distribution of scattered x-ray intensity as a function of azimuthal angle (Φ), which is caused by a distribution of molecular orientations. For HBC thin-films treated with hexane-vapor annealing (Fig. 1b) and thermal annealing (not shown), the diffracted intensities of the individual reflections are distributed across a broad range of azimuthal angles, whereas HBC thin-films treated with PDMS-contact exhibit sharp diffraction spots (Fig. 1a).

By tracking the intensity of crystal planes perpendicular to the central coronene core of the HBC, Loo’s group was able to quantify the extent of preferential molecular orientation. Hexane-vapor-annealed HBC thin-films show a slight preference for a face-on molecular orientation, whereas, thermally annealed HBC thin-films show a slight preference for an edge-on molecular orientation. HBC thin-films treated with PDMS contact exhibit a strong preference for edge-on orientation. An edge-on molecular orientation is particularly desirable for lateral charge transport in thin-film-transistors (TFT’s), since π-stacking occurs parallel to the substrate surface.

This research demonstrated the ability to tune the molecular orientation of organic thin-films through post-deposition treatments and independent of surface treatments, resulting in organic semiconductor thin-films with desirable properties for organic electronics applications. In the case of HBC, Loo’s group was able to increase the charge mobility by two orders of magnitude in TFT devices fabricated using PDMS treated thin-films, as compared to as-deposited thin films.

References:

[1] A. M. Hispanski, S. S. Lee, H. Wang, A. R. Woll, C. Nuckolls and Y.-L. Loo, "Post Deposition Processing Methods To Induce Preferential Orientation in Contorted Hexabenzocoronene Thin Films," ACS Nano, vol. 7, pp. 294-300, 2013.

 

 

Submitted by: Sol Gruner, CHESS, Cornell University
4/25/2013