Solution-processed Organic Semiconductor Thin Films Research Articles

Motion-Programmed Bar-Coating Method with Controlled Gap for High-Speed Scalable Preparation of Highly Crystalline Organic Semiconductor Thin Films

Solution-processed organic semiconductor thin films with high charge carrier mobility are necessary for development of next-generation electronic applications, but the rapid processing speed demanded for the industrial-scale production of these thin films poses a challenge to control of their thin-film properties, such as crystallinity, morphology, and film-to-film uniformity. Here, we show a new solution coating method that is compatible with a roll-to-roll printing process at a rate of 2 mm s-1 by using a gap-controllable wire bar, motion-programming strategy, and blended active inks. We demonstrate that the coating bar, the horizontal motion of which is repeatedly brought to an intermittent standstill, results in an improved vertically self-stratified structure and a high crystallinity for organic active inks comprising a semiconducting small molecule and a semiconducting polymer. Furthermore, organic transistors prepared by the developed method show superior hole mobility with high operational stability (hysteresis and kink-free transfer characteristics), high uniformity over a large area of a 4 in. wafer, good reproducibility, and superior electromechanical stabilities on a flexible plastic substrate. The bar-coating approach demonstrated here will be a step toward developing industrial-scale practical organic electronics applications.

Improved Stability and Performance of Visible Photoelectrochemical Water Splitting on Solution-Processed Organic Semiconductor Thin Films by Ultrathin Metal Oxide Passivation

Solution-processable organic semiconductors have potentials as visible photoelectrochemical (PEC) water splitting photoelectrodes due to their tunable small band gap and electronic energy levels, but they are typically limited by poor stability and photocatalytic activity. Here, we demonstrate the direct visible PEC water oxidation on solution-processed organic semiconductor thin films with improved stability and performance by ultrathin metal oxide passivation layers. N-type fullerene-derivative thin films passivated by sub-2 nm ZnO via atomic layer deposition enabled the visible PEC water oxidation at wavelengths longer than 600 nm in harsh alkaline electrolyte environments with up to 30 μA/cm2 photocurrents at the thermodynamic water-oxidation equilibrium potential and the photoanode half-lifetime extended to ∼1000 s. The systematic investigation reveals the enhanced water oxidation catalytic activity afforded by ZnO passivation and the charge tunneling governing the hole transfer through passivation l...

Spin-Coated Thin Films of Polycyclic Aromatic Hydrocarbons Exhibiting High SCLC Hole Mobilities

High charge carrier mobilities have been greatly sought after in the development of cutting-edge organic electronic and optoelectronic devices. Although high field-effect transistor hole mobilities have been reported for solution-processed organic semiconductor thin films, their space-charge-limited current (SCLC) mobilities are still substantially lower. Herein, we report the synthesis and thin film SCLC hole mobilities of four polycyclic aromatic hydrocarbons, specifically, 2,5,8,17-tetra-tert-butyldiacenaphtho[1,2-j:1′,2′-l]fluoranthene (1), 2,5,8,11,14,17-hexa-tert-butyldiacenaphtho[1,2-j:1′,2′-l]fluoranthene (2), 2,9,12,15-tetra-tert-butylacenaphtho[1,2-j]benzo[l]fluoranthene (3), and 2,9,12,15-tetra-tert-butyl-4,5,6,7-tetraphenylacenaphtho[1,2-j]benzo[l]fluoranthene (4). Spin-coated pristine thin films of compounds 1–4 exhibit SCLC hole mobilities of 1.18 ± 0.18 × 10–3, 3.8 ± 0.9 × 10–5, 2.00 ± 0.87 × 10–3, and 2.27 ± 0.67 × 10–4 cm2 V–1 s–1, respectively. Upon thermal annealing at 120 °C for 10 min...

Exciton dynamics reveal aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films

Large-scale organic electronics manufacturing requires solution processing. For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. Here we use transient absorption microscopy to isolate a unique signature of a hidden interface in a TIPS-pentacene thin film, exposing its exciton dynamics and intermolecular structure. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystallites interleaved by a web of interfaces that compound decreases in charge mobility. Our novel approach provides critical missing information on interface morphology necessary to correlate solution-processing methods to optimal device performance.

Morphology control strategies for solution-processed organic semiconductor thin films

Solution-based deposition techniques and strategies to control the morphology of organic semiconductor thin films are reviewed and discussed.

Quantifying Resistances across Nanoscale Low- and High-Angle Interspherulite Boundaries in Solution-Processed Organic Semiconductor Thin Films

The nanoscale boundaries formed when neighboring spherulites impinge in polycrystalline, solution-processed organic semiconductor thin films act as bottlenecks to charge transport, significantly reducing organic thin-film transistor mobility in devices comprising spherulitic thin films as the active layers. These interspherulite boundaries (ISBs) are structurally complex, with varying angles of molecular orientation mismatch along their lengths. We have successfully engineered exclusively low- and exclusively high-angle ISBs to elucidate how the angle of molecular orientation mismatch at ISBs affects their resistivities in triethylsilylethynyl anthradithiophene thin films. Conductive AFM and four-probe measurements reveal that current flow is unaffected by the presence of low-angle ISBs, whereas current flow is significantly disrupted across high-angle ISBs. In the latter case, we estimate the resistivity to be 22 MΩμm(2)/width of the ISB, only less than a quarter of the resistivity measured across low-angle grain boundaries in thermally evaporated sexithiophene thin films. This discrepancy in resistivities across ISBs in solution-processed organic semiconductor thin films and grain boundaries in thermally evaporated organic semiconductor thin films likely arises from inherent differences in the nature of film formation in the respective systems.

Understanding Heterogeneous Nucleation in Binary, Solution-Processed, Organic Semiconductor Thin Films

Heterogeneous nucleation is often the precursor to crystallization in solution-processed organic semiconductor thin films. Here, we study the efficacy of a series of nine small-molecule organic semiconductor additives in seeding the crystallization of solution-processable triethylsilylethynyl anthradithiophene (TES ADT). By systematically varying the concentrations of the additives in TES ADT thin films, we found the tendency of the additives to crystallize, their solubility in the casting solvent, and their similarity in chemical structure to TES ADT, to determine the nucleation and resulting density of nuclei. Tracking the crystallization process further yields information about the mechanism of nucleation. While pure TES ADT nucleates instantaneously at the onset of crystallization, nucleation transitions to a distributed process occurring throughout crystallization with the incorporation of increasing amounts of additives.

Orientation-Independent Charge Transport in Single Spherulites from Solution-Processed Organic Semiconductors

Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction.

Controlling Nucleation and Crystallization in Solution‐Processed Organic Semiconductors for Thin‐Film Transistors

Three orders of magnitude is the range over which the grain size (see figure) can be tuned in solution-processed organic semiconductor thin films for TFTs. Fluorinated triethylsilyl anthradithiophene (FTES-ADT) is added in fractional amounts to seed crystallization of TES-ADT. Correlation between device mobility and grain size in the active layer is described by a composite mobility model that assumes charge-carrier traps are located at grain boundaries.