Giant Seebeck effect over 0.1 V K-1 - is this an intrinsic phenomenon in organic semiconductors?

A good solution for energy harvesting is to generate electricity using waste heat from our bodies or living environment. Therefore, the development of flexible and lightweight thermoelectric generators (TEGs) is urgently necessary, and studies on organic thermoelectric materials have become increasingly intensive. This article will present ongoing studies about a mysterious phenomenon in organic semiconductors, the giant Seebeck effect (GSE). The GSE was first discovered with pure C60 thin films and eventually confirmed to occur in various organic semiconductors. In the thin films or single crystals of organic small-molecule semiconductors with high purity, i.e., small carrier density, huge Seebeck coefficients, >0.1 V K-1, were reproducibly observed in the temperature range near 300-400 K. The facts revealed by the experiments to date will be presented, and unresolved mysteries will be discussed.

Combining Photosynthesis and Photovoltaics: A Hybrid Energy-Harvesting System Using Optical Antennas.

A hybrid energy-harvesting system is proposed that combines photosynthesis and photovoltaics. First, the light passes through a spectrally selective solar cell, which absorbs almost all green light but absorbs almost no blue and red light. The blue and red light are absorbed by a photosynthesis executing plant. The solar cell is tailored in such a way that the photosynthetic process is almost unaffected by the generation of electrical energy. The spectrally selective solar cell consists of an array of inorganic optical antennas. By combining a spectrally selective solar cell and a photosynthetic executing plant, a hybrid energy system is formed, which absorbs almost 100% of the visible light, while the energy conversion efficiency of the solar cell reaches up to 50% of their nonspectrally selective counterparts. Guidelines are provided on how to realize both the highly efficient spectrally selective solar cells and hybrid energy-harvesting systems. The proposed solution allows for the realization of new greenhouses or gardens covered with spectrally selective transparent solar cells that produce chemical energy in the form of fruits and vegetables and electrical energy.

Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene.

Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge- and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements, and temperature-dependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature. Overall, the best performance among the approximate methodologies is observed for a system-specifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit-cell volume resulting in a pronounced overestimation of the phonon energies in the low-frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments.

Light-driven molecular switch for reconfigurable spin filters.

Artificial molecular switches and machines that enable the directional movements of molecular components by external stimuli have undergone rapid advances over the past several decades. Particularly, overcrowded alkene-based artificial molecular motors are highly attractive from the viewpoint of chirality switching during rotational steps. However, the integration of these molecular switches into solid-state devices is still challenging. Herein, we present an example of a solid-state spin-filtering device that can switch the spin polarization direction by light irradiation or thermal treatment. This device utilizes the chirality inversion of molecular motors as a light-driven reconfigurable spin filter owing to the chiral-induced spin selectivity effect. Through this device, we found that the flexibility at the molecular scale is essential for the electrodes in solid-state devices using molecular machines. The present results are beneficial to the development of solid-state functionalities emerging from nanosized motions of molecular switches.

Conducting organic frameworks based on a main-group metal and organocyanide radicals.

Reactions of the main-group cation Tl(I) with anions of 2,5-derivatives of TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have led to the isolation of a family of unprecedented semiconducting main-group-metal-organic frameworks, namely, [Tl(TCNQX(2))], (X = H, Cl, Br, I). A comparison of single-crystal and powder X-ray diffraction data revealed the existence of a third polymorph of the previously reported material Tl(TCNQ)] and two distinct polymorphs of [Tl(TCNQCl(2))], whereas only one phase was identified for [Tl(TCNQBr(2))] and [Tl(TCNQI(2))]. These new results are described in the context of the structures of other known binary metal-TCNQ frameworks that display a variety of coordination environments for the central cation, namely, four-, six-, and eight-coordinate, and different arrangements of the adjacent TCNQ radicals-parallel versus perpendicular-in the stacked columns. The halogen substituents affect the structures and the properties of these compounds, owing to both steric and electronic effects as evidenced by the semiconducting properties of crystals of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))], which correlate well with the distances of adjacent TCNQ radicals in the columns. 1D infinite Hückel model simulations of the band structures of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))] were conducted with and without consideration of the Tl(I) cations, the results of which indicate that the charge mobility does not strictly occur in one dimension. The modulations of the band structures with various assumptions of the energy difference (Δ) between the Tl(I) 6s orbital and the TCNQ LUMO orbital were calculated and are discussed in light of the observed properties.