Theoretical studies on the electronic structures and optical properties of the oligomers involving bipyridyl, thiophenyl and ethynyl groups

Revealing generation, migration, and dissociation of electron-hole pairs and current emergence in an organic photovoltaic cell

Using an innovative quantum mechanical method for an open quantum system, we observe in real time and space the generation, migration, and dissociation of electron-hole pairs, transport of electrons and holes, and current emergence in an organic photovoltaic cell. Ehrenfest dynamics is used to study photoexcitation of thiophene:fullerene stacks coupled with a time-dependent density functional tight-binding method. Our results display the generation of an electron-hole pair in the donor and its subsequent migration to the donor-acceptor interface. At the interface, electrons transfer from the lowest unoccupied molecular orbitals (LUMOs) of thiophenes to the second LUMOs of fullerene. Further migration of electrons and holes leads to the emergence of current. These findings support previous experimental evidence of coherent couplings between electronic and vibrational degrees of freedom and are expected to stimulate further work toward exploring the interplay between electron-hole pair (exciton) binding and vibronic coupling for charge separation and transport.

Coherent Real-Space Charge Transport Across a Donor-Acceptor Interface Mediated by Vibronic Couplings

There is growing experimental and theoretical evidence that vibronic couplings, couplings between electronic and nuclear degrees of freedom, play a fundamental role in ultrafast excited-state dynamics in organic donor-acceptor hybrids. Whereas vibronic coupling has been shown to support charge separation at donor-acceptor interfaces, so far, little is known about its role in the real-space transport of charges in such systems. Here we theoretically study charge transport in thiophene:fullerene stacks using time-dependent density functional tight-binding theory combined with Ehrenfest molecular dynamics for open systems. Our results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for ∼200 fs and promoting charge transport within the polymer stacks. At the donor-acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports previous experimental observations of long-range ballistic charge-carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices.

Evaluación teórica de las propiedades electrónicas y estructurales que afectan a la conductividad eléctrica en copolímeros de furanotiofeno

Desde el punto de vista científico y tecnológico ha habido un gran interés en el uso de monosustituyentes de furano y tiofeno como polímeros conductores, debido a sus múltiples aplicaciones como OLED, amplificadores ópticos, nanotecnología, entre otros. Por ello, el propósito de este trabajo fue estudiar los aspectos teóricos que afectan las propiedades electroconductoras de este tipo de moléculas. Se determinaron teóricamente los aspectos estructurales y electrónicos que influyeron en la conductividad de copolímeros de furano–tiofeno monosustituidos, al utilizar grupos carboxilos, metilos, hidroxilos, ciano y fluoruros como sustituyentes en el carbono C3 y C10 de cada heterociclo. La diferencia de energía entre el LUMO y el HOMO (band gap, Eg) y el potencial de ionización (PI) fue calculada a partir de las geometrías optimizadas en DFT para el estado neutro, anión y catión. Los PI y la Eg de los copolímeros fueron obtenidos mediante la extrapolación de los valores del oligómero a (1/N) y de una cadena de longitud infinita (1/N=0), obteniéndose una correlación lineal (R=0,99), la cual se mantiene a lo largo de todos los modelos de ajuste de cada copolímero analizado en el estudio.

Influences of donor/acceptor ratio on the optical and electrical properties of the D/A alternating model oligomers: A density functional theory study

We adopted an ingenious method that cut out the DA alternating oligomers from the corresponding DA alternating copolymers. From analyzing the orbital compositions of the HOMOs and LUMOs as well as the reorganization energies, we found the level of charge transfer is increased with the increasing of D/A ratio, but ionization potentials and electron affinities show a contrary trend. Moreover, with the greater ratio, the trend in the nearness of two transitions results in broadening the absorption band in the visible range. That is why experimentally improving the ratio is beneficial for the copolymers used as the p-type materials in the BHJ solar cells. This method is impossible to take the real copolymer system, however, it could provide a strategy to avoid the limitation of the theory level and perform reliable result to study the intrinsic properties of DA alternating copolymers, which can provide a guidance to experimental works.

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

DFT and TDDFT calculations were performed to search for high-performance non-fullerene organic acceptor materials in organic solar cells.

Theoretical investigation on exciton-dissociation and charge-recombination processes of PC61BM-PTDPPSe interface

Designing and synthesizing novel electron-donor polymers with the high photovoltaic performances has remained a major challenge and hot issue in organic electronics. In this work, the exciton-dissociation (k dis ) and charge-recombination (k rec ) rates for the PC61BM-PTDPPSe system as a promising polymer-based solar cell candidate have been theoretically investigated by means of density functional theory (DFT) calculations coupled with the non-adiabatic Marcus charge transfer model. Moreover, a series of regression analysis has been carried out to explore the rational structure-property relationship. Results reveal that the PC61BM-PTDPPSe system possesses the large open-circuit voltage (0.77 V), middle-sized exiton binding energy (0.457 eV), and relatively small reorganization energies in exciton-dissociation (0.273 eV) and charge-recombination (0.530 eV) processes. With the Marcus model, the k dis , k rec , and the radiative decay rate (k s ), are estimated to be 3.167×10(11) s(-1), 3.767×10(10) s(-1), and 7.930×10(8) s(-1) respectively in the PC61BM-PTDPPSe interface. Comparably, the k dis is as 1∼3 orders of magnitude larger than the k rec and the k s , which indicates a fast and efficient photoinduced exciton-dissociation process in the PC61BM-PTDPPSe interface. Graphical Abstract PTDPPSe is predicted to be a promising electron donor polymer, and the PC61BM-PTDPPSe system is worthy of further device research by experiments.

Structures and photoelectric properties of five benzotrithiophene isomers-based donor-acceptor copolymers

In this paper, we have investigated the structures, electronic and optical properties of five conjugated copolymers (BTT1-BTz, BTT2-BTz, BTT3-BTz, BTT4-BTz and BTT5-BTz) featuring benzotrithiophene (BTT) isomers as donor units and benzothiadiazole (BTz) as acceptor units, linked through thiophene spacers, employing many-body perturbation theory (MBPT). We have explored the isomer effects by configuration of the sulfur atoms in BTT units, aimed to get insight into how the structural modifications to the conjugated backbone can influence the molecular structures and electronic properties of conjugated polymers. Using the trimer as the computational model, the calculated low and high energy absorption bands (660 and 413 nm) for BTT1-BTz agree well with the experimental ones (645 and 430 nm) with a small offset of ~15 nm. On the basis of our calculations, it is found that the backbones of these polymers display different coplanarities, with the dihedral angles between the two neighboring rings varying from 12.3° to 79.0°. Importantly, both BTT1-BTz and BTT2-BTz exhibit intense adsorption around 660 and 623 nm, indicating their promising application in solar cells, whereas BTT3-BTz and BTT4-BTz display the intense adsorption at 569 and 551 nm, which are also usable in the tandem solar cells. BTT5-BTz has narrow and weak adsorption in the visible and infrared region, implying it is not conducive to the sunlight absorption. The blue shift of about 150 nm from BTT1-BTz to BTT5-BTz is suggested to be originated from the shorter effective conjugation lengths.

Theoretical investigations of the small molecular acceptor materials based on oligothiophene – naphthalene diimide in organic solar cells

The electronic transmission paths of NDI-T3DCRD with the centroid distance from core molecule to ambient molecules marked.

Theoretical Studies on Structures and Spectroscopic Properties of a Series of Novel Cationic [trans-(C∧N)2Ir(PH3)2]+ (C∧N = ppy, bzq, ppz, dfppy)

The geometries, electronic structures, and spectroscopic properties of a series of novel cationic iridium(III) complexes [trans-(C/N)(2)Ir(PH(3))(2)]+ (C/N = 2-phenylpyridine, 1; benzoquinoline, 2; 1-phenylpytazolato, 3; 2-(4,6-difluorophenyl)pyridimato, 4) were investigated theoretically. The ground- and excited-state geometries were optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. The optimized geometry structural parameters agree well with the corresponding experimental results. The unoccupied molecular orbitals are dominantly localized on the C/N ligand, while the occupied molecular orbitals are composed of Ir atom and C/N ligand. Under the time-dependent density functional theory (TDDFT) level with the polarized continuum model (PCM) model, the absorption and phosphorescence in acetonitrile (MeCN) media were calculated based on the optimized ground- and excited-state geometries, respectively. The calculated results showed that the lowest-lying absorptions at 364 nm (1), 389 nm (2), 317 nm (3), and 344 nm (4) are all attributed to a {[d(yz)(Ir) + pi(C/N)] --> [pi*(C/N)]} transition with metal-to-ligand and intraligand charge transfer (MLCT/ILCT) characters; moreover, the phosphorescence at 460 (1) and 442 nm (4) originates from the 3{[d(yz)(Ir) + pi(C/N)] [pi*(C/N)]} (3)MLCT/(3)ILCT excited state, while that at 505 (2) and 399 nm (3) can be described as originating from different types of (3)MLCT/(3)ILCT excited state (3){[d(xy)(Ir) + pi(C/N)] [pi*(C/N)]}. The calculated results also revealed that the absorption and emission transition character can be altered by adjusting the pi electron-withdrawing groups and, furthermore, suggested that the phosphorescent color can be tuned by changing the pi-conjugation effect of the C/N ligand.