Morphological Disorder Affects Charge Carrier Movement in Conjugated Polymer Films

Morphological Disorder Affects Charge Carrier Movement in Conjugated Polymer Films

Films produced by MAPLE technique show a higher degree of structural disorder, forming localized trap sites that inhibit charge transport across the bulk. MAPLE-deposited P3HT thin-film transistors exhibit comparable mobilities to spin-coated analogues.

Conjugated polymers are organic macromolecules with a backbone chain which consists of alternating double and single bonds. Along the polymer chain, the p-orbitals of adjacent double bonds overlap and form a π-conjugated system, in which the electrons are able to move. This structure gives rise to electrical conductivity and optoelectronic properties that make this class of materials very interesting for photovoltaic and other organic electronic applications.

The designing of new materials and processes requires a very good understanding not only of the way that the charge carriers move inside a molecule, but also of the way they move between different molecules. This has proved challenging in these materials due to the structural complexity and anisotropy, where amorphous and ordered phases coexist, disturbing the transport of charge carriers. Researchers in the University of Michigan have recently taken a step forward in comprehending the relationship between the carriers’ movement and the structural disorder in conjugated polymers.

Peter F. Green, Professor of Materials Science and Engineering, Chemical Engineering, Macromolecular Science and Engineering at the University of Michigan, and his team fabricated thin films of the conjugated polymer poly(3-hexylthiophene) (P3HT), using resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), as well as conventional spin-casting. The different deposition techniques resulted in considerably different film morphologies, which enabled the comparison of the in- and out-of-plane charge transport in the deposited thin films. To their surprise, the scientists found that the increased disorder of the MAPLE-films did not have an impact on the in-plane transistor mobilities in comparison to the spin-cast analogues. The out-of-plane mobilities were, on the other hand, considerably different.

The team started by employing MAPLE, which is “a vacuum-based deposition technique that offers advantages in cases of film deposition on surfaces where there are wettability concerns,” according to Green. Atomic force microscopy (AFM) was used to characterize the morphologies of the MAPLE-films. The surface of the film was found to be rough because of the presence of globular features with diameters ranging from 50 nm to 200 nm. The optical properties of spin-cast and MAPLE-deposited films (both with average thicknesses of c.a. 55 nm) were studied with UV-visible absorbance spectroscopy and despite the very different film morphologies, the absorption spectrum obtained in each case was qualitatively similar.

The in-plane charge transport characteristics of P3HT films were quantified by fabricating thin-film transistors (TFTs) in top-contact, bottom-gate configuration on substrates of highly doped Si with 300 nm of thermally grown SiO2 . The effects of morphology on the out-of-plane carrier mobility were examined using the technique of charge extraction by linearly increasing voltage (CELIV). In-plane carrier mobilities of MAPLE-deposited films were measured at 8.3 × 10-3 cm2 V-1 s-1 and were comparable to those of spin-cast analogues at 5.5 × 10-3 cm2 V-1 s-1 . The out-of-plane mobilities were found to be an order of magnitude lower (4.1 × 10-4 cm2 V-1 s-1 versus 2.7 × 10-3 cm2 V-1 s-1 ). The smaller out-of-plane mobilities show that the structural disorder becomes, in this case, quite significant.

For Lin X. Chen, a professor of chemistry at Northwestern University, who was not involved in the study, “The modified MAPLE method represents a new approach for conjugated polymer film morphology control. This is a new way in deepening our understanding of correlations of electronic properties and morphology of polymer films which has been elusive in details.” Chen adds that “apart from achieving similar UV-visible spectra, intrinsic exciton bandwidth and in-plane carrier mobility for P3HT films and spin-casted films, the new method enables the control of the out-of-plane carrier mobility in P3HT films while maintaining in-plane carrier mobility.”

Alberto Salleo, associate professor at the Department of Materials Science and Engineering at Stanford University, who was also not involved in the study, says, “These results demonstrate that when transport relies more heavily on interchain hopping (e.g., out-of-plane currents when polymer chains lie in the substrate plane), carrier mobility is very sensitive to structural disorder. Conversely, when mobility is controlled by intrachain transport (e.g., in-plane currents), structural disorder does not have major effects.” According to Salleo the report is important because “understanding the impact of disorder on electronic properties is an essential part of the puzzle one has to solve when attempting to rationally design new high-performance conjugated polymers.”

Green considers this work as part of the effort of his team to understand the connection between the structural properties of polymers in nanoscale confinement. “We want to investigate a number of other polymers with higher carrier mobilities,” Green says. “By using a combination of processing techniques, like MAPLE, supercritical carbon dioxide processing of films, and grazing incidence x-ray diffraction, we hope to get deeper insights into the morphological structure and the connection between the morphology disorder and carrier transport.”

Read the abstract in MRS Communications.


This article originally appeared on Materials360online.