Yong Wu

A comparative study of a fluorene-based non-fullerene electron acceptor and PC61BM in an organic solar cell at a quantum chemical level

The importance of developing non-fullerene acceptors has been emphasized to overcome the drawbacks of fullerene derivatives such as limited light absorption, poor solubility and high cost. In this study, we present a comparative theoretical study on the organic solar cell (OSC) performance of a fluorine-based non-fullerene acceptor FENIDT and the representative fullerene derivatives, PC61BM, based on the dependent/time-dependent density functional theory (DFT/TDDFT) calculations. Remarkably, according to the investigation on relevant parameters with OSC performance, FENIDT not only shows a much higher charge separation rate and lower recombination rate at the donor/acceptor interface, but also possesses a larger open circuit voltage and better electron mobility than that of PC61BM, which suggests that FENIDT might be more suitable as an acceptor material. Some speculations were provided to explain the superiority of PC61BM in experiments such as the simplified model method. Moreover, according to the advantages of FENIDT, a series of acceptors were designed. The results illustrate that the designed molecule 3 with a p-phenylenediamine unit based on the FENIDT shows better performance than others and may act as a promising acceptor material for OSCs. Finally, we hope our investigations in this study could provide a theoretical guideline of further optimization of the acceptor materials for OSCs.

Theoretical design of three-dimensional non-fullerene acceptor materials based on an arylenediimide unit towards high efficiency organic solar cells

Two non-fullerene acceptors with a three-dimensional structure based on the reported DBFI-T molecule (an arylenediimide-containing system) were theoretically modelled to study their performance as acceptor materials in organic solar cells (OSCs). Many performance indices were employed to judge the molecules designed by us on the basis of density functional theory/time-dependent density function theory (DFT/TDDFT). Compared with DBFT-T, the modelled molecule 2 has a larger density of states in the lowest unoccupied molecular orbitals and more low lying excited states in the anion, which greatly favors the charge separation process in OSCs. The comparison of charge separation/recombination rates (kCS/kCR) evaluated by considering the influence of low-lying excited states at the PSEHTT (donor)/acceptor interface suggests that molecules 1 and 2 have a higher short-circuit current density (Jsc) than DBFI-T, since they have higher kCS and lower kCR. Moreover, many other important parameters, such as the open circuit voltage, energetic driving force and absorption spectrum were also provided, which further illustrates the efficacy of molecules 1 and 2 in OSCs.

Exploring the influence of aggregation on the structure and fluorescent properties of a tetraphenylethylene derivative: a theoretical study

The case that aggregation has a large influence on the structure and fluorescent properties of 5-(4-(1,2,2-triphenylvinyl)phenyl)thiophene-2-carbaldehyde (P4 TA) is investigated in detail herein by employing quantum mechanics and molecular mechanics. Besides the isolated molecule, the aggregated molecule in water and in the crystalline state was studied by focusing on the comparison of photoelectronic properties, including the geometrical and electronic structures at ground and excited states, emission and internal conversation properties. For the aggregation state, the intermolecular interaction was used to explain the difference in structure, emission color and intensity of different polymorphs. The noticeable contribution from low-frequency region, corresponding to the four phenyl rings twisting vibration, to the Huang-Rhys factor and reorganization energy, as well as the possible potential energy surface crossing between S0 and S1 states for isolated molecules was considered as the reason of its aggregation-induced emission (AIE) performance. Importantly, the aggregation process in water simulated at the same time helps us to have a deeper understanding of the AIE behavior of P4 TA, which also provides another perspective to explore the AIE phenomenon in theory.

Theoretical investigation on the effect of fluorine and carboxylate substitutions on the performance of benzodithiophene-diketopyrrolopyrrole-based polymer solar cells

Three donor–acceptor (D–A) polymers 2–4 were designed and investigated based on the reported polymer 1 with benzo[1,2-b:4,5-b′]dithiophene (BDT) as D fragment and diketopyrrolopyrrole (DPP) as A fragment. The fluorine substitutions on the BDT unit in molecule 2 have less influence on the lowest unoccupied molecular orbital (LUMO) compared with the carboxylate substitutions on the BDT unit in 3 and 4. The charge transfer rate (kinter-CT) of molecule 4 is the largest, which determines that molecule 4 has a priority in the interfacial process among these investigated molecules with the same acceptor PC61BM. The designed molecules 2–4 show larger open-circuit voltages (Voc), relatively narrower bandgaps and higher value of kinter-CT/kinter-CR than 1. Moreover, the results demonstrate that fluorine and carboxylate substitutions on molecule 4 show a synergistic effect on the FMO energy levels and electron interfacial process, which is expected to help the further understanding of the design rules for polymer donor materials in polymer solar cells.