Collaborative Research: Solution Processing of Organic Semiconductors: A Coupled Atomistic-Continuum Framework

The field of organic electronics has grown substantially over the last two decades. However, the widespread deployment of OSC requires further improvements in performance along with substantial advances in reliability, device-to-device uniformity, and tolerance of robust manufacturing processes. Many current issues related to device performance result from material disorder and defects in the thin-film - including (i) formation of crystalline grain mismatches / grain boundaries within the OSC film, (ii) uncontrolled interactions at interfaces with an electrode or dielectric, and (iii) presence of remnants from solution casting. Molecular- / nano-scales events lead to an array of material defects and disorder that hinder effective charge-carrier transport, amongst other electronic and optical properties. Furthermore, mesoscale processing conditions (shear stresses, pinned boundaries, Marongani effects) intimately interact with lower scale to affect the morphology and hence properties of interest. Current computational approaches are primarily focused on either the meso-scale (where very approximate, and usually erroneous phenomenological information is used about lower scales) or the molecular scale (where meso-scale processing effects are ignored). Both approaches, while useful, do not account for critical mechanisms occurring at the other scale. Motivated by this challenge, this proposal seeks to integrate the decade long efforts of the two PI in first principles/molecular methods (Risko) and meso-scale/continuum methods (Ganapathysubramanian) for OSC to create a cohesive, atomistic-continuum framework. We will specifically investigate the fundamental issues towards effectively and reproducibly solution cast molecular-based OSC of interest for organic field-effect transistors (OFET). The computational platform developed will delineate critical interrelationships between OSC molecular structure, processing conditions (e.g. variations in solvent, processing additives, and temperature), and interactions at material interfaces. Success in achieving this objective will result in a comprehensive set of design principles that will enable targeted, first-principles-based processing protocols for OSC applications. We have letters of collaboration with three experimental teams who are interested in validating and leveraging the insights gained from our three key proposed tasks. We will target a suite of OSC molecules that contain oligoacene cores, largely responsible for the material electronic and optical characteristics, appended with trialkylsilylethynyl side groups that direct solubility and packing in the solid state; importantly, this molecular class has been the focus of a number of experimental explorations of the relationships between OSC chemical structure, processing, and thin-film morphology. The intellectual merit of the proposal arises from these objectives Objective #1: Establish the interplay of OSC chemical and molecular structure and solution conditions on aggregation. The dynamic nature of molecular assemblies / aggregates in solution, and how the choice of solvent and external stimuli influence the dynamic nature of these aggregates, is not well understood in OSC materials. This is an inherently multiscale phenomena. We propose to utilize electronic-structure calculations within the context of symmetry-adapted perturbation theory (SAPT) to quantify how chemical structure influences the nature and strength of intermolecular interactions (exchange repulsion, dispersion, electrostatics, and induction). These electronic-structure calculations and associated MD simulations will then be used to determine Hildebrand and Hansen solubility parameters for the OSC materials. The (concentration and temperature dependent) solubility parameters are used to construct the phenomenological closures for the mesoscale model and will be used to model the device scale aggregation and morphology formation. Objective #2: Reveal the impact of chemisorbed surface modifiers on early stages of film growth. Chemisorbed surface modifiers can direct thin-film morphology formation by altering the surface energies of underlying electrodes or dielectrics and are often used to improve charge collection and injection in OSC-based devices. We propose to explore how changes in the chemical functionality of surface modifier directs aspects of the initial growth of the OSC thin film. Electronic-structure calculations will be used to understand the nature and strength of the intermolecular interactions that direct preferred molecular orientations at the material interface. These will serve as material specific inputs to both MD and meso-scale morphology modeling to investigate the effect of OSC and solvent effects on morphology. Objective #3: Uncover the complexity of OSC film formation in multicomponent polymer-molecule blends. Co-dissolving OSC molecules in a polymer matrix, combined with solvent drying and post-deposition processing, can invoke the formation of stratified layers of the small molecule and polymer, leading to well-ordered OSC layers that effectively transport charge carriers over large distances. However, the molecular-scale details of this process, and the impact that the chemical functionality of the OSC molecule and polymer have on the stratification process, is not understood. We will leverage the outcomes of Objective 1 and 2 (computing solubility parameters, surface energies, and mobilities) to perform large-scale meso-scale simulations under various solvent and thermal conditions to identify mechanisms of OSC film formation.