Panoramic View of Electrochemical Pseudocapacitor and Organic Solar Cell Research in Molecularly Engineered Energy Materials (MEEM)

Abstract

Our program on capacitive energy storage is a comprehensive one that combines experimental and computational components to achieve a fundamental understanding of charge storage processes in redox-based materials, specifically transition metal oxides. Some of the highlights of this program are the identification of intercalation pseudocapacitance in Nb 2 O 5 , which enables high energy density to be achieved at high rates, and the development of a new route for synthesizing mesoporous films in which preformed nanocrystal building blocks are used in combination with polymer templating. The resulting material architectures have large surface areas and enable electrolyte access to the redox active pore walls, while the interconnected mesoporous film provides good electronic conductivity. Select first-principles density-functional theory studies of prototypical pseudocapacitor materials are reviewed, providing insight into the key physical and chemical features involved in charge transfer and ion diffusion. Rigorous multiscale physical models and numerical tools have been developed and used to reproduce electrochemical properties of carbon-based electrochemical capacitors with the ultimate objective of facilitating the optimization of electrode design. For the organic photovoltaic (OPV) program, our focus has been ongoing beyond the trial-and-error Edisonian approaches that have been responsible for the increase in power conversion efficiency of blend-cast (BC) bulk heterojunction blends of polymers and fullerenes. Our first approach has been to use molecular self-assembly to create the ideal nanometer-scale architecture using thermodynamics rather than relying on the kinetics of spontaneous phase segregation. We have created fullerenes that self-assemble into one-dimensional stacks and have shown that use of these self-assembled fullerenes lead to dramatically enhanced OPV performance relative to fullerenes that do not assemble. We also have created self-assembling conjugated polymers that form gels based on electrically continuous cross-linked micelles in solution, opening the possibility for water-processable "green" production of OPVs based on these materials. Our second approach has been to avoid kinetic control over phase separation by using a sequential processing (SqP) technique to deposit the polymer and fullerene materials in separate deposition steps. The polymer layer is deposited first, using solvents and deposition conditions that optimize the polymer crystallinity for swelling and hole mobility. The fullerene layer is then deposited in a second step from a solvent that swells the polymer but does not dissolve it, allowing the fullerene to penetrate into the polymer underlayer to the desired degree. Careful comparison of composition-and thickness-matched BC and SqP devices shows that SqP not only produces more efficient devices but also leads to devices that behave more consistently.

Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

“…[ 17,20 ] Optimal fabrication of these P3HT:PCBM SqP active layers, however, is unique in several important ways. First, P3HT does not signifi cantly swell in the presence of solvents due to its unusually high crystallinity, [ 51,52 ] which signifi cantly inhibits PCBM interdiffusion relative to most other polymer systems.…”

“…Moreover, with SqP, the fullerene network is likely to be better connected throughout the active layer, improving the global electron mobility. [ 17 ] In short, morphology control-the bane of blend-cast processing-is a more tractable problem with SqP, and the design rules we have outlined here detail how high-performance polymer:fullerene networks can be created with the SqP approach.…”

Sequential Processing: A Rational Route for Bulk Heterojunction Formation via Polymer Swelling

“…If there is insufficient fullerene intercalation, this not surprisingly leads to poor electron mobility and thus lousy photovoltaic performance. 1,2,17,102 SqP not only has been demonstrated for an increasingly wide variety of material combinations, 3,25,68,93,126,152 but also provides a route toward potential commercial scale-up. This is because blendcasting relies on spontaneous de-mixing that occurs during film formation, and drying kinetics change with device area.…”

“…65,72 Fortunately, SqP, which is based on the thermodynamics of swelling, avoids these problems with scale-up. 2,3,6,57 Figure 4 compares the performance of thickness-and composition-matched P3HT:PCBM devices produced by BC (circles) and SqP (diamonds) at two different size scales. In the small-area devices (∼7 mm 2 ) shown in Fig.…”

“…Since mass action and swelling are thermodynamic rather than kinetic effects, this means that SqP provides significantly greater reproducibility and control over the BHJ morphology needed to produced highly efficient polymer solar cells. 2,3 It is worth noting that when we originally developed SqP, we were unaware of the extent to which the solvent in the second step could swell the underlying polymer layer, and indeed our initial goal with SqP was to prepare clean polymer-fullerene bilayers. In fact, the fullerene-casting solvent we initially chose, dichloromethane (DCM), is a relatively poor swelling solvent for our initially-studied conjugated polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT).…”