Optoelectronic properties of diathiafulvalene-functionalized diketopyrrolopyrrole-fullerene molecular dyad.

Single component molecular dyad donor-acceptor junction is an important type of organic solar cells. Understanding the optoelectronic properties of molecular dyad plays the critical role to develop active layer materials for such kind of solar cells. Here, diathiafulvalene-functionalized diketopyrrolopyrrole-fullerene (DFDPP-Ful) was selected as the representative system, and the geometries, electronic structures and excitation properties of DFDPP-Ful monomer and dimer were systematically investigated based on extensive quantum chemistry calculations. The transition configurations and molecular orbitals show that the effective electron donor and acceptor are DFDPP and fullerene moieties, respectively. It also found the light harvesting is dominated by local excitation in DFDPP moiety. Meanwhile, the hybridization and quasi-degeneration between charge transfer (CT) and local excitation exist. The dimer data suggest that the increased excited states contribute to the expanding of absorption spectra, and the excitations exhibit both the intermolecular and intra-molecular CTs. Also, the remarkable CT energy differences among the different dimer models for the lowest CT excited states support the strong interface and energy disorder in such system. Therefore, the suggestions for developing molecular dyad of single component organic solar cells would be the combination of increasing light absorption, enhancing CT and local excitation hybridization, as well as suppressing energy and interface disorder by the aid of molecular design.

The bis-dimethylfluoreneaniline organic dye sensitizers for solar cells: A theoretical study and design.

Development of novel dye sensitizers with suitable optoelectronic properties is effective to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). Considering the effectiveness of conjugate bridges in modification of optoelectronic properties, based on the dye sensitizers C201, C203, C204 and C205, five kinds of organic dye sensitizers are designed with different thiophene-based moieties and the functionalized graphene flakes (GFs) as conjugate bridges. The performances of these dye sensitizers are analyzed in terms of the calculated geometries, electronic structures and excitation properties. The transition configurations and molecular orbitals of dye sensitizers suggest that bis-dimethylfluoreneaniline is effective electron donor, and the transitions of optical absorption in visible region are charge transfer excitations. The conjugate lengths, energy level alignments, light harvesting capabilities, excitation character, and transition properties, as well as the free energy variations for electron injection and dye regeneration support that the designed dye sensitizers are effective to be applied in DSSCs. Particularly, introducing the functionalized GF into conjugate bridges significantly elongate conjugate length, reduce orbital energy gap, lead to denser distribution of orbital energy, generate red-shift of absorption spectra, enhance light harvesting capability, increase absorption bands and coefficients. Therefore, introducing the functionalized GF into conjugate bridges is effective, and the designed panchromatic dye sensitizer C20x-GF-BTD must be better than other designed dye sensitizers for DSSCs.

Assessment of Ab Initio and Density Functional Theory Methods for the Excitations of Donor-Acceptor Complexes: The Case of the Benzene-Tetracyanoethylene Model.

The understanding of the excited-state properties of electron donors, acceptors and their interfaces in organic optoelectronic devices is a fundamental issue for their performance optimization. In order to obtain a balanced description of the different excitation types for electron-donor-acceptor systems, including the singlet charge transfer (CT), local excitations, and triplet excited states, several ab initio and density functional theory (DFT) methods for excited-state calculations were evaluated based upon the selected model system of benzene-tetracyanoethylene (B-TCNE) complexes. On the basis of benchmark calculations of the equation-of-motion coupled-cluster with single and double excitations method, the arithmetic mean of the absolute errors and standard errors of the electronic excitation energies for the different computational methods suggest that the M11 functional in DFT is superior to the other tested DFT functionals, and time-dependent DFT (TDDFT) with the Tamm-Dancoff approximation improves the accuracy of the calculated excitation energies relative to that of the full TDDFT. The performance of the M11 functional underlines the importance of kinetic energy density, spin-density gradient, and range separation in the development of novel DFT functionals. According to the TDDFT results, the performances of the different TDDFT methods on the CT properties of the B-TCNE complexes were also analyzed.

The Electronic Structures and Optical Properties of Alkaline-Earth Metals Doped Anatase TiO2: A Comparative Study of Screened Hybrid Functional and Generalized Gradient Approximation.

Alkaline-earth metallic dopant can improve the performance of anatase TiO2 in photocatalysis and solar cells. Aiming to understand doping mechanisms, the dopant formation energies, electronic structures, and optical properties for Be, Mg, Ca, Sr, and Ba doped anatase TiO2 are investigated by using density functional theory calculations with the HSE06 and PBE functionals. By combining our results with those of previous studies, the HSE06 functional provides a better description of electronic structures. The calculated formation energies indicate that the substitution of a lattice Ti with an AEM atom is energetically favorable under O-rich growth conditions. The electronic structures suggest that, AEM dopants shift the valence bands (VBs) to higher energy, and the dopant-state energies for the cases of Ca, Sr, and Ba are quite higher than Fermi levels, while the Be and Mg dopants result into the spin polarized gap states near the top of VBs. The components of VBs and dopant-states support that the AEM dopants are active in inter-band transitions with lower energy excitations. As to optical properties, Ca/Sr/Ba are more effective than Be/Mg to enhance absorbance in visible region, but the Be/Mg are superior to Ca/Sr/Ba for the absorbance improvement in near-IR region.

The adsorption of α-cyanoacrylic acid on anatase TiO2 (101) and (001) surfaces: a density functional theory study.

The adsorption of α-cyanoacrylic acid (CAA) on anatase TiO2 (101) and (001) surfaces, including adsorption energies, structures, and electronic properties, have been studied by means of density functional theory calculations in connection with ultrasoft pseudopotential and generalized gradient approximation based upon slab models. The most stable structure of CAA on anatase TiO2 (101) surface is the dissociated bidentate configuration where the cyano N and carbonyl O bond with two adjacent surface Ti atoms along [010] direction and the dissociated H binds to the surface bridging O which connects the surface Ti bonded with carbonyl O. While for the adsorption of CAA on (001) surface, the most stable structure is the bidentate configuration through the dissociation of hydroxyl in carboxyl moiety. The O atoms of carboxyl bond with two neighbor surface Ti along [100] direction, and the H from dissociated hydroxyl interacts with surface bridging O, generating OH species. The adsorption energies are estimated to be 1.02 and 3.25 eV for (101) and (001) surfaces, respectively. The analysis of density of states not only suggests the bonds between CAA and TiO2 surfaces are formed but also indicates that CAA adsorptions on TiO2 (101) and (001) surfaces provide feasible mode for photo-induced electron injection through the interface between TiO2 and CAA. This is resulted from that, compared with the contribution of CAA orbitals in valence bands, the conduction bands which are mainly composed of Ti 3d orbitals have remarkable reduction of the component of CAA orbitals.

Understanding the electronic structures and absorption properties of porphyrin sensitizers YD2 and YD2-o-C8 for dye-sensitized solar cells.

The electronic structures and excitation properties of dye sensitizers determine the photon-to-current conversion efficiency of dye sensitized solar cells (DSSCs). In order to understand the different performance of porphyrin dye sensitizers YD2 and YD2-o-C8 in DSSC, their geometries and electronic structures have been studied using density functional theory (DFT), and the electronic absorption properties have been investigated via time-dependent DFT (TDDFT) with polarizable continuum model for solvent effects. The geometrical parameters indicate that YD2 and YD2-o-C8 have similar conjugate length and charge transfer (CT) distance. According to the experimental spectra, the HSE06 functional in TDDFT is the most suitable functional for describing the Q and B absorption bands of porphyrins. The transition configurations and molecular orbital analysis suggest that the diarylamino groups are major chromophores for effective CT excitations (ECTE), and therefore act as electron donor in photon-induced electron injection in DSSCs. The analysis of excited states properties and the free energy changes for electron injection support that the better performance of YD2-o-C8 in DSSCs result from the more excited states with ECTE character and the larger absolute value of free energy change for electron injection.