Influence of morphological disorder on in- and out-of-plane charge transport in conjugated polymer films
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esearch Letters
Influence of morphological disorder on in- and out-of-plane charge transport in conjugated polymer films Anton Li, Ban Xuan Dong, and Peter F. Green*, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA; Biointerfaces Institute, University of Michigan, Ann Arbor, MI 48109, USA *Address all correspondence to Peter F. Green at [email protected] (Received 16 October 2015; accepted 5 November 2015)
Abstract Thin films of the conjugated polymer poly(3-hexylthiophene) (P3HT) of different morphological structures were fabricated using both conventional spin-casting and the matrix-assisted pulsed laser evaporation (MAPLE). Films deposited by MAPLE exhibit inhomogeneous morphologies comprised globular subfeatures with dimensions of the order of 100 nm. We show that whereas the in-plane carrier mobilities of MAPLE-deposited films (8.3 × 10−3 cm2/V/s) are comparable with those of spin-cast analogs (5.5 × 10−3 cm2/V/s), the out-of-plane mobilities are an order of magnitude lower (4.1 × 10−4 cm2/V/s versus 2.7 × 10−3 cm2/V/s). Both in- and out-of-plane carrier transport characteristics of MAPLE-deposited films indicate a broad density of states and high carrier trap concentration. Optical absorbance spectroscopy not only corroborates a high degree of energetic disorder in MAPLE-deposited films, but also suggests that the P3HT chains possess average conjugation lengths comparable with spin-cast counterparts. Our findings, rationalized in terms of the Gaussian Disorder Model, describing carrier transport in an environment characterized by both positional and energetic disorder, provide important perspectives on the extent to which disorder impacts mechanisms of charge transport in conjugated polymers.
Conjugated polymers are a widely studied class of semicrystalline organic semiconductors, exhibiting very different morphologies and physical properties, depending on their method of fabrication. Typically, thin polymer films are deposited from solution, which is the simplest strategy and most amenable for high-throughput, and low-cost fabrication. However, there are a number of situations wherein vacuum-based deposition provides advantages, such as creating patterned or multilayered architectures, or in cases where the desired substrate or surface has poor wettability.[1,2] Conventional physical vapor deposition techniques are unsuitable, as they involve large energy input which generally degrades polymeric materials. In recent years, however, an increasing number of groups have successfully fabricated polymer films using an analog of pulsed laser deposition known as matrix-assisted pulsed-laser evaporation (MAPLE).[3–5] Rather than directly ablating solid polymer, the MAPLE process utilizes a frozen dilute solution of the polymer and volatile solvent. Under ideal circumstances, laser energy is almost entirely absorbed by the host solvent “matrix”, thereby preventing photochemical degradation of the guest polymer. The MAPLE technique affords access to films with unique morphologi
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