In this contribution, molecular materials are highlighted as an important topic in the diverse field of condensed matter physics, with focus on their particular electronic and transport properties. A better understanding of their performance in various applications and devices demands for an extension of basic theoretical approaches to describe charge transport in molecular materials, including the accurate description of electron–phonon coupling. Starting with the simplest case of a molecular junction and moving on to larger aggregates of bulk organic semiconductors, charge‐transport regimes from ballistic motion to incoherent hopping, which are frequently encountered in molecular systems under respective conditions, are discussed. Transport features of specific materials are described through ab initio material parameters whose determination is addressed.

Abstractauthoren Organic solar cells are a promising technology for a large area conversion of sunlight into electricity. In particular for solar cells based on oligomers (small molecules), efficient donor materials absorbing wavelengths larger than 780 nm are still rare. Here, we investigate three aza‐BODIPY dyes absorbing in the infrared. The addition of side groups leads to a red shift of the optical gap from 802 to 818 nm. In optimized devices using these donors in a bulk heterojunction with C60, we observe a higher charge carrier mobility and a higher power conversion efficiency for the molecules without a methyl or methoxy side group lowering the molecular reorganization energy. Surprisingly, the donor–acceptor blend with the lowest energy loss during the electron transfer to the C60 yields the highest short circuit current. With increasing size of the attached side chain, the devices exhibit a larger trap density, measured by impedance spectroscopy. Based on the investigation of different blend ratios, we conclude that these traps are mainly present in the donor phase.

In a theoretical study, combining molecular dynamics simulations, quantum-chemical calculations, and charge migration simulations based on Marcus theory, we investigate the electronic structure, its fluctuations, and the charge transport of a promising organic near-infrared absorber material: 7,7-difluoro-7H-5,9diphenyldiisoindolo[2,1-c:1′,2′-f ][1,3,5,2]triazaborinine-6-ium-7-uide (Ph 2 -benz-BODI-PY), which is already successfully used as the donor material in organic solar cells. For the crystalline, defect-free phase, we find a one-dimensional hole transport characteristic with a mobility of 0.53 cm 2 /(V s) and a two-dimensional electron transport characteristic with a smaller mobility of 0.15 cm 2 /(V s). The attachment of the phenyl rings to the molecular core tends to improve the electron conduction by reducing the internal reorganization energy and by increasing the intermolecular coupling. In contrast, such functionalization tends to impair the hole transport as the highest occupied molecular orbital couples dominantly to the dynamics of the phenyl rings and the annulated benzene rings.