Qing-Qing Pan

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.

Theoretical characterization on photoelectric properties of benzothiadiazole- and fluorene-based small molecule acceptor materials for the organic photovoltaics

AbstractThe upper efficiency of heterojunction organic photovoltaics depends on the increased open-circuit voltage (Voc) and short-circuit current (Jsc). So, a higher lowest unoccupied molecular orbital (LUMO) level is necessary for organic acceptor material to possess higher Voc and more photons absorbsorption in the solar spectrum is needed for larger Jsc. In this article, we theoretically designed some small molecule acceptors (2∼5) based on fluorene (F), benzothiadiazole, and cyano group (CN) referring to the reported acceptor material 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (1), the crucial parameters affecting photoelectrical properties of compounds 2∼5 were evaluated by the density functional theory (DFT) and time dependent density functional theory (TDDFT) methods. The results reveal that compared with 1, 3 and 4 could have the better complementary absorption spectra with P3HT, the increased LUMO level, the improved Voc, and the decreased electronic organization energy (λe). From the simulation of transition density matrix, it is very clear that the excitons of molecules 3 and 4 are easier to separate in the material surface. Therefore, 3 and 4 may become potential acceptor candidates for organic photovoltaic cells. In addition, with the increased number of CN, the optoelectronic properties of the molecules show a regular change, mainly improve the LUMO level, energy gap, Voc, and absorption intensity. In summary, reasonably adjusting CN can effectively improve the photovoltaic properties of small molecule acceptors. Graphical AbstractStructure–property relationship of small molecule acceptors could be rationally evaluated in the article. The changes of conjugate length and CN are important strategies to alter the photovoltaic properties of small molecule acceptors. Therefore, taking the K12/1 as a reference, we have theoretically designed a series of small molecule acceptors (2–4). The calculated results by means of DFT and TDDFT manifest that molecules 3 and 4 have the better complementary absorption spectra with P3HT, the increased LUMO level, the improved Voc, the decreased electronic organization energy and the easier separation in the material surface than 1. In summary, reasonably increasing conjugate length and decreasing CN can effectively improve the PCE, which will provide a theoretical guideline for the design and synthesis of new small molecule acceptors.

A theoretical exploration of the effect of fluorine and cyano substitutions in diketopyrrolopyrrole-based polymer donor for organic solar cells

A series of polymer donor materials 1-5 based on diketopyrrolopyrrole and thiophene unit which have been widely used in organic solar cells (OSCs) were investigated based on quantum chemical calculations. The effect of fluorine and cyano substitutions in polymer donor materials was focused on. Based on the investigation on electronic structures and optical properties of the reported molecules 1 and 2 and the analysis on some parameters relevant to charge dissociation ability at donor/acceptor interface constituted by 1 and 2 with PC61BM such as intermolecular charge transfer and recombination, driving force and Coulombic bound energy, we explained why fluorine substitution can improve OPV efficiency through strengthening eletron-withdrawing ability from a theoretical perspective. Then we designed cyano-substituted polymers 3-5 with the aim of obtaining better photovoltaic donor materials. The results reveal that our attempt to design donor materials which can balance large open-circuit voltage (Voc) and high short-circuit current (Jsc) in OSCs has worked out. It is worth noting that the substitutions of fluorine and cyano groups synergistically reduce energy gap and HOMO energy level of polymers 3 and 4. Moreover, 3/PC61BM and 4/PC61BM heterojunctions show over 107 and 104 times higher than 1/PC61BM on the ratios of intermolecular charge transfer and recombination rates (kinter-CT/kinter-CR). Thus, our work here may provide an efficient strategy to design promising donor materials in OPVs and we hope it could be useful in the future experimental synthesis.

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.