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Polarized X-Rays Reveal Molecular Alignment in Printed Electronics Print

The printing of electronic devices using giant roll-to-roll presses or inkjet-style printers has recently been made possible by the development of solution-processable organic materials with optoelectronic properties. Organic light-emitting diodes (OLEDs) are already being produced commercially, and sensors, organic thin-film transistors (OTFTs), and organic photovoltaics (OPVs) are also well on their way to commercial viability. With low processing costs, scalability, and mechanical flexibility, they could revolutionize how we use technology and embody an abundant source of renewable energy. Yet the fundamental connection between how these materials organize on small length scales and how that affects their operation has so far lagged empirical knowledge. Recently, an international team of researchers shed light on that connection using polarized x-rays with resonant scattering at ALS Beamline 11.0.1.

Printable Electronics

The invention of the printing press had a revolutionary impact on society. The ability to mass-produce printed material in assembly-line fashion greatly enhanced the dissemination of information across borders and over time. Later, the invention of the transistor, the fundamental building block of modern electronic devices, led to a second "information revolution" driven by radios, television, and computers. Now, researchers are intrigued by a combination of these two technologies: namely, large-volume printing of electronic circuits onto thin sheets of plastic material. The "ink" in this case consists of organic polymer molecules that conduct electricity and are solution processable (they can be dissolved in a solution and deposited as a liquid). The process results in devices, such as LED displays or solar cells, that are light, flexible, and inexpensive.

The catch, however, is in the relatively low efficiency of such devices, with some materials performing better than others. To understand the reasons behind this, Collins et al. found a way to use polarized x-rays to reveal the molecular orientations of the organic polymers in the "ink." By correlating the results with device performance, they gained important insight into how to produce better-performing materials, reducing the need to rely on trial-and-error methods to achieve potentially revolutionary breakthroughs in electronic technology and renewable energy.

Concept of polarized scattering. Photons from a polarized x-ray beam are tuned to an orientation-sensitive resonance of the semiconducting polymer in an OTFT channel/dielectric bilayer. Passing through the film, the photons scatter onto a CCD detector. The pattern reveals structure due to correlations in molecular alignment in the channel.

Polymer microstructure is generally difficult to characterize due to low electron density and relatively low levels of crystallinity. Hard x-ray interactions with matter typically scale with electron density and therefore lack sufficient contrast to probe polymer domains. However, x-ray contrast increases dramatically if tuned to a material's molecular resonances. In polymers, these molecular resonances are highly directional, providing a marker for each molecule's orientation. Furthermore, polarized x-rays will interact strongly with molecules if their electric field is aligned to these markers.

Both resonance and polarization effects are often used in x-ray microscopy to identify material and orientational domains, respectively. While microscopy can image individual domains, scattering can most readily measure the statistics of domains (size distribution, average purity/level of ordering, etc.). In scattering, resonance enhancement methods are being increasingly utilized, but this is the first work that exploits polarization as well. The results revealed the impact of polymer-chain alignment on charge-carrier mobility in transistors and uncovered the existence of the preferential alignment of polymers at domain interfaces of blend films used for solar cells. These discoveries are valuable in guiding the molecular design of improved polymers for transistors and in gaining a better understanding of charge generation at complex 3D interfaces in blend-based OPVs and OLEDs.

Utilizing polarized resonant x-rays, the researchers probed semiconducting polymers and dielectric polymers that represent exactly the structures used in OTFTs. Tunable soft x-rays, a specialty of the ALS, allowed the scattering to target the layer of interest, the semiconducting polymer layer. These multilayers are formed from solution, and interactions with neighboring layers during processing can alter their structure. By adjusting the thermal processing steps, the length over which the semiconducting polymer chains aligned could be increased or decreased depending on the interfacial interactions with the underlying layer. The charge mobility in these devices was found to scale exponentially with this length, suggesting that increased molecular alignment—likely causing greater overlap in the electron orbitals responsible for conduction—is key to charge transport in polymer materials.

Molecular alignment promoting charge transport. Left: Resonant scattering profiles for devices under different processing conditions reveal the spacing between two regions of aligned molecules. This spacing defines the distance over which the molecules go out of and back into alignment locally, as shown in the inset. Right: Hole mobility of the OTFT devices compared with the average alignment length of the molecules. Inset shows the chemical structure of the polymer investigated (PBTTT).

In solar cells containing semiconducting polymer blends, charge is generated at interfaces between electron-donating and electron-accepting materials, requiring 3D bulk, nanoscale phase separation to maximize the interfaces. To date, there is very little knowledge of how molecules orient themselves at these interfaces due to their complexity. However, the results here revealed that, in this system, the molecular orientation of one material was being patterned via interactions at the interface. One of the polymers, P(NDI2OD-T2), was preferentially aligned along the interface with its conducting electronic orbitals pointed at the other material (P3HT). This arrangement is likely to have a major impact on electronic interactions and, therefore, on charge generation between these two materials. In OLEDs, charge recombination at this interface, a process that generates light, could also be influenced by this effect. Further study of these alignment effects—now possible with resonant scattering—will be key to understanding how fundamental processes like charge transport and generation take place and, ultimately, to improving the performance and commercial viability of this novel technology.

Patterning of molecular orientation. Left: Anisotropic scattering patterns that rotate with x-ray polarization, indicating alignment of the molecules around domains. Right: Schematic of the molecular alignment of P(NDI2OD-T2) molecules with their electron orbitals pointed across the interface.



Research conducted by: B.A. Collins,  H. Yan, E. Gann, and H. Ade (North Carolina State University); J.E. Cochran and M.L. Chabinyc (University of California, Santa Barbara); C. Hub and R. Fink (Universität Erlangen-Nürnberg, Germany); C. Wang (ALS); T. Schuettfort (University of Cambridge,UK); and C.R. McNeill (Monash University, Australia).

Research funding: National Science Foundation, the German Federal Ministry of Education and Research (BMBF), and the UK Engineering and Physical Sciences Research Council (EPSRC). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: B.A. Collins, J.E. Cochran, H. Yan, E. Gann, C. Hub, R. Fink, C. Wang, T. Schuettfort, C.R. McNeill, M.L. Chabinyc, and H. Ade, "Polarized x-ray scattering reveals non-crystalline orientational ordering in organic films," Nat. Mater. 11, 536 (2012).

ALS Science Highlight #258


ALSNews Vol. 336