Trends in Chemistry
ReviewOrganic Photovoltaics: Relating Chemical Structure, Local Morphology, and Electronic Properties
Section snippets
Active-Layer Morphology Impacts Device Performance
Since 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
Correlating Chemical Structure with Local Morphology by AA-MD Simulations
The output of an AA-MD simulation is an atomistic trajectory from which the local morphology (i.e., intermolecular packing and mixing) can be analyzed, for instance, by examining radial distribution functions (RDFs) among donors, among acceptors, or between donors and acceptors. Note that due to the size limitation of AA-MD simulations, the various regions of an active layer have to be modeled separately (i.e., the pure phases via a neat film and mixed phases via a blend). Recent studies have
Correlating Local Morphology to Electronic Properties via DFT Calculations
From the AA-MD outputs, intramolecular conformations and intermolecular configurations can be extracted; the former relate to single donor or acceptor macromolecules, while the latter comprise donor/donor, donor/acceptor, or acceptor/acceptor pairs. On extracting a large set (hundreds or thousands) of single donor (acceptor) molecules, their site energies and disorder relevant for hole (electron) transport can be evaluated via DFT calculations; here, a small disorder means more efficient charge
From Local to Global Morphology
As mentioned earlier, global morphology still largely remains to be experimentally explored in the case of efficient non-fullerene-based OSCs, although it is known to influence device performance [15,49]. This is thus an area where CG-MD simulations can play a useful role since they significantly reduce the degrees of freedom involved in the modeling and allow one to address greater system sizes and timescales and to connect local morphology to global morphology. Ultimately, the objective is to
Opportunities for ML
In recent years, artificial intelligence (AI) has started to play an increasing role in organic photovoltaics (OPVs). In particular, studies have focused on the use of ML to screen OPV materials based on molecular descriptors such as molecular level (highest occupied molecular orbital or lowest unoccupied molecular orbital) energies, band gaps, and chemical features (e.g., atom and bond types, connectivity) [90., 91., 92., 93., 94.]. These large-scale screenings aim ultimately to make a
Concluding Remarks and Future Perspectives
Recent computational studies based on a tight combination of AA-MD simulations and DFT calculations have enhanced our understanding of the connections among chemical structure, local morphology, and electronic properties in BHJ OSCs. However, since the investigations conducted to date have been limited to just a few systems, a comprehensive picture is still lacking. This is especially true since the effects from, for instance, solvents, additives, molecular weights, or fabrication processing
Acknowledgments
This work has been supported by the Office of Naval Research in the framework of Awards No. N00014-17-1-2208 and No. N00014-20-1-2110, as well as by the Georgia Institute of Technology and the University of Arizona. We acknowledge many stimulating discussions with Dr Veaceslav Coropceanu.
Glossary
- Atomistic trajectory
- trajectory that includes the coordinates of all of the atoms in a system as a function of simulation time.
- Charge-transfer (CT) electronic states
- occur at the donor–acceptor interfaces and correspond to a full or partial electron transfer from an electron donor to an acceptor on photoexcitation.
- Donor/acceptor pair
- comprises a donor and an acceptor directly interacting within a given distance of each other; the cutoff distance along the interaction direction is based on the
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