Organic bulk-heterojunction (BHJ) solar cells made of polymeric donors and small molecule non-fullerene acceptors (NFA) have been the subject of interest due to their record-high efficiencies. Tremendous effort is being directed at the development of novel donor and acceptor building blocks, and the optimization of energetics and morphology of donor/acceptor systems. In this work, we demonstrate how computational insight from both ab initio ad molecular dynamics simulations can be used to guide the optimization of both donor and acceptors.<br/>The polymer donor PBTZF4 was characterized using a combination of molecular dynamics (MD) simulations, X-ray scattering and spectroscopic techniques. MD provided a rationalization for the observed trends among a series of PBTZF4-R devices with alkyl chains R of different length, elucidating the subtle equilibrium between the conjugated backbone rigidity and the folding enhancement provided by longer alkyl chain substituents. The results established a relationship between the single polymer chain behavior, its aggregation in solution and the subsequent formation of domains in the BHJ. This study shows how side chain engineering can be used to fine tune the BHJ morphology without significantly changing the HOMO-LUMO levels of the polymer backbone.<br/>We also performed a systematic investigation of a series of novel ITIC-derived acceptors. We used both electronic structure methods and molecular dynamics simulations to investigate how changes in the acceptor molecular structure influence both electronic properties (i.e., the reorganization energy) and molecular packing, correlating them to their photovoltaic efficiency. In particular, the effects of fluorination and end-group conjugation extension were studied.<br/>Finally, a series of ITIC-derived NFAs having alkyl chain substituents of different length was also investigated by means of quantum chemical calculations, MD simulations, X-Ray diffraction and other spectroscopic measurements. It emerges that the alkyl chain length has a dramatic effect on OPV performances. Side chains effectively mediate intermolecular interactions by preventing excessive aggregation and crystallization of the acceptor while allowing efficient π-stacking in the amorphous and semi-crystalline domains.<br/>

Optimization of Donors and Acceptors for Organic Photovoltaics Guided by Molecular Simulations

2019 Fall National Meeting

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