Organic Photovoltaics: Relating Chemical Structure, Local Morphology, and Electronic Properties

Abstract

Substantial enhancements in the efficiencies of bulk-heterojunction (BHJ) organic solar cells (OSCs) have come from largely trial-and-error-based optimizations of the morphology of the active layers. Further improvements, however, require a detailed understanding of the relationships among chemical structure, morphology, electronic properties, and device performance. On the experimental side, characterization of the local (i.e., nanoscale) morphology remains challenging, which has called for the development of robust computational methodologies that can reliably address those aspects. In this review, we describe how a methodology that combines all-atom molecular dynamics (AA-MD) simulations with density functional theory (DFT) calculations allows the establishment of chemical structure-local morphology-electronic properties relationships. We also provide a brief overview of coarse-graining methods in an effort to bridge local to global (i.e., mesoscale to microscale) morphology. Finally, we give a few examples of machine learning (ML) applications that can assist in the discovery of these relationships. Active-Layer Morphology Impacts Device PerformanceSince the BHJ architecture was introduced in the mid-1990s [1,2], OSCs have witnessed substantial improvements in their power conversion efficiencies (PCEs). The active layer of a BHJ OSC is formed by a blend of an electron-donor and an electron-acceptor material, with the two components mixing down to the~10 nm scale; Figure 1A illustrates a typical polymer: fullerene BHJ architecture with the basic electronic processes described in the legend [3] (the chemical structures of the donor and acceptor materials mentioned in this review are provided in Figures 1B and 3A). In conjunction with molecular design, efforts to optimize the active-layer morphology have led to record PCEs of 11.0%, 11.7%, and 18.2% for all-polymer, polymer:fullerene, and polymer:non-fullerene small-molecule acceptor (NF-SMA) single-junction BHJ OSCs, respectively [4][5][6][7][8]. A key element of recent, impressive PCE enhancements has come from switching from fullerenes to NF-SMAs as electron-acceptor materials. Efficient NF-SMAs began with the synthesis of ITIC by Zhan and coworkers in 2015 and currently mainly revolve around the Y6 acceptor reported by Zhou and coworkers [9,10].One of the positive characteristics of efficient polymer:NF-SMA OSCs is their reduced nonradiative voltage losses [6,11,12], which comes at least partly from the molecular design of pairs of polymer donors and NF-SMAs where the energetic offset between either the ionization energies or the electron affinities of the donor and acceptor components is minimized [12]. When the donor and acceptor components possess appropriate energy levels and optical absorption profiles, a meta-analysis by Jackson and colleagues on some 150 BHJ OSCs has suggested that the PCEs are then morphology limited [13]. Thus, a critical task is to be able to

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