No abstract

Charge generation and recombination processes at interfaces between electron donating (donor, D) and accepting molecules (acceptor, A) are mediated by intermolecular charge-transfer (CT) states. Since organic photovoltaic and photodetecting devices rely on D–A interfaces, an understanding of the molecular and morphological aspects governing CT state properties is crucial. In this paper, we synthesize a novel series of bi­(thio)­pyranylidene donor molecules and show how the interplay of molecular structure and energy levels in a D–C60 blend affect the line shape of the CT absorption cross section. By rationally designing the molecule 2,2′,6,6′-tetra-(2-methylthienyl)-4,4′-bithiopyranylidene, we achieve a 2 times stronger CT absorption peak than the literature-known molecule 2,2′,6,6′-tetraphenyl-4,4′-bipyranylidene when blended with C60. The low CT state energy combined with relatively strong CT absorption of this new material blend is exploited by fabricating near-infrared, cavity enhanced narrowband detectors. The photodetectors cover an impressive wavelength range from 810 to 1665 nm with line widths between 30 and 50 nm.

The low-energy edge of optical absorption spectra is critical for the performance of solar cells, but is not well understood in the case of organic solar cells (OSCs). We study the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs. We simulate the temperature dependence of the excitonic density of states and low-energy absorption features, including low-frequency molecular vibrations and multi-exciton hybridisation. For model donor-acceptor blends featuring charge-transfer excitons, our simulations agree very well with temperature-dependent experimental absorption spectra. We unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction, causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially limit the OSC's efficiency. Strategies to reduce these vibration-induced voltage losses are discussed for a larger set of systems and different heterojunction geometries.

The use of ab initio methods for accurate simulations of electronic, phononic and electron-phonon properties of molecular materials such as organic crystals is a challenge that is often tackled stepwise based on molecular properties calculated in gas phase and perturbatively treated parameters relevant for solid phases. In contrast, in this work we report a full first-principles description of such properties for the prototypical rubrene crystals. More specifically, we determine a Holstein-Peierls type Hamiltonian for rubrene including local and nonlocal electron-phonon couplings. Thereby, a recipe for circumventing the issue of numerical inaccuracies with low-frequency phonons is presented. In addition, we study the phenyl group motion with a molecular dynamics approach.