Theoretical Design of Perylene Diimide Dimers with Different Linkers and Bridged Positions as Promising Non-Fullerene Acceptors for Organic Photovoltaic Cells

The intermolecular stacking and crystallization of perylene diimides (<b>PDIs</b>) has become research obstacles for small molecule acceptors (SMAs). For breaking molecular rigidity and planarity, it is an executable way to increase the distortion between two <b>PDI</b> units. A class of <b>PDI</b> dimers were designed via bridging different linkers in bay positions (1–1′ bridge) and headland positions (1–2′ bridge) to screen suitable acceptor materials for organic photovoltaic cells (OPVs). Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed to investigate their electronic structures, open circuit voltage (<i>V</i>_OC), driving forces (Δ<i>E</i>_L‑L), and some major parameters related to the short-circuit current density (<i>J</i>_SC) such as absorption spectrum and carrier transport ability. Meanwhile, the intermolecular charge transfer (inter-CT) and charge recombination (inter-CR) rates were calculated for a further analysis on charge transfer properties at donor/acceptor (D/A) interface by employing the Marcus semiclassical model. The results manifest that the investigated 1–2′ bridged molecules possess low-lying LUMO energy levels, relatively bigger Δ<i>E</i>_L‑L, bathochromic-shifted absorption, as well as the strongest maximum absorption and more effective charge transport than 1–1′ bridged molecules. Surprisingly, compared with <b>P3HT</b>/(1–1′ bridged <b>PDI</b> dimers) interface, almost constant reorganization energy (λ), higher Gibbs free energy change of exciton dissociation (Δ<i>G</i>_CT), and considerable inter-CT/inter-CR rates ratios (<i>k</i>_inter‑CT<i>/k</i>_inter‑CR) of P3HT/(1–2′ bridged <b>PDI</b> dimers) provides further evidence for that 1–2′ bridged <b>PDI</b> dimers as acceptors might perform higher efficiency in OPV device. Moreover, constructing <b>NDT</b> and <b>DPPT</b> as bridged linkers in <b>PDI</b> dimers as “push–pull” structures may rationally expect more favorable properties as acceptors in OPVs, which might provide theoretical guideline for the design and synthesis of new organic SMAs

Charge Transfer Mechanisms Regulated by the Third Component in Ternary Organic Solar Cells

For ternary organic solar cells (T-OSCs), introducing the third component (D2) can significantly enhance the efficiency of cell while still maintaining easy fabrication. However, it brings difficulty in physical understanding of the fundamental mechanism because of the more complicated photophysical processes in T-OSCs. Accordingly, how the guest donor D2 regulates the charge transfer mechanism was explored in theory using three T-OSCs containing two donors and an acceptor. The results point out that larger differences in molecular weight and/or backbone between D2 and the host donor D1 cause different charge transfer mechanisms, which hardly provide a coexisting charge transfer path. Besides, strong absorption capacity of D2 with a high oscillator strength would produce favorable regulation of the charge transfer mechanism. Therefore, this work clarifies the influence of D2 on the charge transfer mechanism in T-OSCs, which suggests that the method of improving the power conversion efficiency cannot be generalized but rather must be tailored to specific conditions

Construction of Various Supramolecular Assemblies from Rod–Coil Molecules Containing Biphenyl and Anthracene Groups Driven by Donor–Acceptor Interactions

Rod–coil amphiphilic functional molecules, comprising a rigid aromatic building block and hydrophilic oligoether dendrons as the coil segments, were synthesized. These compounds exhibit a powerful self-organizing ability to form supramolecular nanoparticles and long nanofibers in tetrahydrofuran/water solution, by controlling the intermolecular interaction of the rigid blocks. These molecules are able to form supramolecular polymers and, subsequently, to form sheetlike nanoaggregates, through charge-transfer interactions by the addition of a guest molecule, tetracyanoquinodimethane. Notably, upon addition of water-soluble 2,4,6-trinitrophenol, the self-assembly of these molecules exhibits the antagonistic effect owing to donor–acceptor and hydrophobic–hydrophilic interactions among the molecules. The experimental results reveal that various morphologies of rod–coil molecular assemblies can be obtained by tuning the molecular interaction and the hydrophilicity of guest electron-acceptor molecules. Interestingly, the cross-coupling reaction between phenylboronic acid and chlorobenzene occurs within the charge complexes of these molecular aggregates. This occurs in the nanoenvironment that affords an extremely concentrated reaction zone and reduces the activation energy barrier required for the cross-coupling reaction

Three-Dimensional Symmetry Decides the Description of the Energy of Charge-Transfer State in Organic Solar Cells

We have unraveled the corresponding relation between calculation of the energy of the charge-transfer state (ECT) and the structure of the acceptor via formula methods and bowl-nonfullerene models. The results indicate that the ground-state description of ECT is system dependent and generally limited to acceptors without three-dimensional symmetry. Whether it has LUMO/LUMO+1 degeneracy is the root judgment criterion. In contrast, there is no system limitation using the excited-state description. This work suggests nonfullerenes without three-dimensional symmetry have an incompatibility with fullerenes for predicting ECT

Nonradiative Energy Loss Generated by Two Hidden Trade-Offs in Non-Fullerene Organic Solar Cells with the Positive Correlation between <i>J</i>_SC and <i>V</i>_OC

Non-fullerene organic solar cells (OSCs) achieve the simultaneous enhancement of short-circuit current density (JSC) and open-circuit voltage (VOC), which will be defined as positive correlation in this work. However, it has not ushered in a breakthrough of power conversion efficiency (PCE). We reveal that the essence lies in two hidden trade-offs, issued from the two-sidedness of electronic coupling (Vel) and thus resulting in the competition between the increasing magnitudes of JSC and VOC. A great Vel could improve the energy of the charge-transfer state, leading to a phenomenon called positive correlation. Nevertheless, it also induces the large nonradiative energy loss and is adverse to the increasing magnitude of JSC or VOC simultaneously, limiting the great improvement in PCE. Overall, we reveal the qualitative change point of trade-offs in non-fullerene OSCs by the quantitative change of microscopic difference under the loss pathway