Abstract
AbstractEfficiencies of organic solar cells have practically doubled since the development of non‐fullerene acceptors (NFAs). However, generic chemical design rules for donor‐NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ≈75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency.
Highlights
We can summarize the known and new chemical design rules that can help to narrow the search of efficient donor– acceptor combinations for organic solar cells
Rigid elongated planar cores of non-fullerene acceptors (NFAs) favor the formation of spatially extended domains, about 10–30 nm in size.[36]
Acceptor molecules are well-aligned within these domains, which leads to a narrow distribution of electron affinities, with half-widths on the order of 0.1 eV, facilitating good electron mobilities
Summary
We begin by showing that the A–D–A molecular architecture of the acceptor is essential for the NFA design, because it lowers the barrier for the dissociation of the CT state into the CS state. To show that the A–D–A molecular architecture provides the required increase of the CT state energy, we adopt a simple model, leaving involved atomistic-level calculations to Section 6. Which explicitly depends on the spatial coordinate r because of the concentration gradient of the acceptor in the interfacial region. The introduced bias potential, BeA, is the manifestation of the charge-quadrupole interactions and the interfacial concentration gradient. A qualitative comparison of bias potentials of different acceptors is still possible: in Note S5, Supporting Information we show that the bias potential is (approximately) proportional to the solid-state contribution to EA, BeA.
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