Modeling morphology evolution during solvent-based fabrication of organic solar cells

Abstract

Solvent-based thin-film deposition constitutes a popular class of fabrication strategies for manufacturing organic electronic devices like organic solar cells. All such solvent-based techniques usually involve preparing dilute blends of electron-donor and electron-acceptor materials dissolved in a volatile solvent. After some form of coating onto a substrate to form a thin film, the solvent evaporates. An initially homogeneous mixture separates into electron-acceptor rich and electron-donor rich regions as the solvent evaporates. Depending on the specifics of the blend, processing conditions, and substrate characteristics different morphologies are typically formed. Experimental evidence consistently confirms that the resultant morphology critically affects device performance. A computational framework that can predict morphology evolution can significantly augment experimental analysis. Such a framework will also allow high throughput analysis of the large phase space of processing parameters, thus yielding considerable insight into the process-structure-property relationships governing organic solar cell behavior.In this paper, we formulate a computational framework to predict evolution of morphology during solvent-based fabrication of organic thin films. This is accomplished by developing a phase field-based model of evaporation-induced and substrate-induced phase-separation in ternary systems. This formulation allows most of the important physical phenomena affecting morphology evolution during fabrication to be naturally incorporated. We discuss the various numerical and computational challenges associated with a three dimensional, finite-element based, massively parallel implementation of this framework. This formulation allows, for the first time, to model three-dimensional nanomorphology evolution over large time spans on device scale domains. We illustrate this framework by investigating and quantifying the effect of various process and system variables on morphology evolution. We explore ways to control the morphology evolution by investigating different evaporation rates, blend ratios and interaction parameters between components.

A Novel Framework for Visual Detection and Exploration of Performance Bottlenecks in Organic Photovoltaic Solar Cell Materials

“…As case studies, we employ two different simulated data sets that represent two main BHJ lab synthesis techniques, which are called Solvent-Based Fabrication [24], and Thermal Annealing [25], respectively. The former is used to create morphologies with different patterns and connectivity by manipulating physical parameters such as pressure, donor/acceptor percentage, etc.…”

“…Lab experiments cannot enable this type of characterizations since they provide no access to this level of detail. A successful step towards obtaining this characterization is through studying charge paths, as shown in [24]. However, this earlier work only used standard statistical methods, which limits it with respect to two major considerations: (1) It is unable to detect bottlenecks, and (2) it can only study fractions as independent parameters without the critical interplay between them.…”

“…An OPV is a device composed of three main parts: the anode, the cathode, and the so-called Bulk Heterojunction (BHJ) that is sandwiched in between the electrodes (anode and cathode) [24], as shown in Fig. 1(a).…”

How do evaporating thin films evolve? Unravelling phase-separation mechanisms during solvent-based fabrication of polymer blends

“…A detailed description of the physical and computational models was stated by Wodo and Ganapathysubramanian. 18,19 We briefly describe the model for completeness. We consider a ternary system consisting of two components, p 1 and p 2 , along with a volatile solvent, s. We denote the volume fractions of the three components as / p 1 ; / p 2 , and / s , respectively.…”

“…To ensure generality of our analysis, we non-dimensionalize the evaporation rate via Biot number. 19 The Biot number naturally accounts for the interplay between external solvent evaporation from the top layer and the internal diffusion of solvent within the film during evaporation, D s : Bi ¼ k e L/D s , where L is the characteristic length scale.…”