D-A-D-type narrow-bandgap small-molecule photovoltaic donors: pre-synthesis virtual screening using density functional theory.

A new series of D-A-D-type small-molecule photovoltaic donors are designed and virtually screened before synthesis using time-dependent density functional theory calculations carefully validated against various polymeric and molecular donors. In this series of new design, benzodithiophene is kept as D to achieve the optimum highest-occupied molecular orbital energy level, while thienopyrroledione is initially chosen as A but later replaced by difluorinated benzodiathiazole or its selenide derivative to achieve the optimum band gap. The D-A-D core is end-capped by pyridone units which could not only enhance their self-assembly via hydrogen bonds but also play a role as an acceptor (A') to form an extended A'-D-A-D-A' small-molecule donor.

N-Alkylthienopyrroledione versus benzothiadiazole pulling units in push-pull copolymers used for photovoltaic applications: density functional theory study.

Low-band-gap push-pull copolymers are promising donor materials for bulk heterojunction organic solar cells. One of the best push-pull copolymers are composed of bridged dithiophene pushing units and benzothiadiazole (BT) pulling units, but BT has no proper position to accommodate alkyl side chains introduced to enhance the solubility of the resulting copolymers in organic solvents. On the other hand, N-alkylthienopyrroledione (TPD), which has an alkyl side chain attached to its pyrrole moiety, has been combined with various bridged dithiophene pushing units to give high-solubility donor polymers whose power conversion efficiencies are higher than those of the BT-based polymers especially after a morphology control. However, our well-validated time-dependent density functional theory calculation on the intrinsic (single-chain) electronic structure, which has been proved powerful to estimate the efficiency, gives a contradictory prediction that both polymers would show essentially the same efficiency. Intrigued by this, we subsequently perform density functional theory calculations on their π-stacked-pair models in a number of stacking configurations and conclude that the enhanced performance of the TPD-based polymers is ascribed to their enhanced inter-chain interaction resulting from their enhanced dipole moments in the push-pull direction. Enhanced morphological ordering (π-stacking and π-conjugation) in their solid films, which is not considered in electronic-structure calculations, would reduce the band gap (as proved by the low-energy shoulders in UV/vis absorption spectra), improve the charge transfer (as shown by the calculated transfer integral, transfer rate, and hole mobility), and enhance the power conversion efficiencies (as observed after a morphology control).

Studies of singlet Rydberg series of LiH derived from Li(nl) + H(1s), with n ≤ 6 and l ≤ 4.

The 50 singlet states of LiH composed of 49 Rydberg states and one non-Rydberg ionic state derivable from Li(nl) + H(1s), with n ≤ 6 and l ≤ 4, are studied using the multi-reference configuration interaction method combined with the Stuttgart/Köln group's effective core potential/core polarization potential method. Basis functions that can yield energy levels up to the 6g orbital of Li have been developed, and they are used with a huge number of universal Kaufmann basis functions for Rydberg states. The systematics and regularities of the physical properties such as potential energies, quantum defects, permanent dipole moments, transition dipole moments, and nonadiabatic coupling matrix elements of the Rydberg series are studied. The behaviors of potential energy curves and quantum defect curves are explained using the Fermi approximation. The permanent dipole moments of the Rydberg series reveal that they are determined by the sizes of the Rydberg orbitals, which are proportional to n(2). Interesting mirror relationships of the dipole moments are observed between l-mixed Rydberg series, with the rule Δl = ±1, except for s-d mixing, which is also accompanied by n-mixing. The members of the l-mixed Rydberg series have dipole moments with opposite directions. The first derivatives of the dipole moment curves, which show the charge-transfer component, clearly show not only mirror relationships in terms of direction but also oscillations. The transition dipole moment matrix elements of the Rydberg series are determined by the small-r region, with two consequences. One is that the transition dipole moment matrix elements show n(-3/2) dependence. The other is that the magnitudes of the transition dipole moment matrix elements decrease rapidly as l increases.