From a “molecular” point of view, each electronic state of the organic molecule has a well-defined potential energy hyper-surface (PES), with a minimum corre­sponding to the molecular geometry. There is therefore also a set of characteristic molecular vibrations and the electronic relaxation process is mediated by the molecular vibrations of the corresponding electronic state. One aspect of the energy losses concerns the hot-carrier relaxation of the molecule through excited elec­tronic-states, and the excited state via which the electronic charge is finally trans­ferred to the neighbouring molecule. The electric potential related to an exciton is greater for the higher excited states. In analogy with relaxation to the ground state, relaxation between electronic states is driven by the molecular vibrations in the corresponding electronic states. The carrier relaxation-dynamics are of fundamental importance to understanding the processes underlying carrier transport in both organic and inorganic devices. The advantage of an organic system as a model for research is the rich molecular vibrational-spectrum that can be studied computa­tionally and experimentally. Determining the PES of ground and excited electronic — states would provide insights into the variation in molecular geometry and vibra­tions in these states, with a view to ascertaining whether such changes are signif­icant and measureable and further, whether the structure and vibrations can be chemically tailored to improve the performance of OPVs.

Organic systems, like hexa-peri-hexabenzocoronene with a perylene dye have been successfully used in photodiodes with efficiencies as high as 34 %. However, these systems are too complex for high-level, molecular-modelling studies of vibrations in ground and excited electronic-states. These vibrations play a key role in properties such as: light-harvesting, exciton transport, primary charge-separation, and electron/hole conduction. In this context, the columnar discotic liquid-crystals based on triphenylene derivatives like hexakis(alkyloxy)triphenylenes (HATn), are attractive representative model systems for organic photovoltaic devices. Works on ground-state (valence-band) electron-phonon coupling of HAT6 (HATn with n = 6) have shown that structure and dynamics play a central role in the charge-transfer process of this material, and it was noted that the dynamics of the aromatic cores and the alkyl tails have an important effect on the electronic properties [14-18]. Perhaps surprisingly, it was found that the electronic properties are affected not only by the dynamics of the aromatic cores, but also by the dynamics of the alkyl tails. HAT6 is a convenient model system (Fig. 6.8) because it has the optimum side — chain length giving the broadest mesophase range. Between five and seven carbons in the alkyl tail are required for the columnar phase to form. Further increase or decrease of the tail length reduces the temperature range of the hexagonal columnar mesophase formation. From a practical point of view, it is generally thought that the

BHJ setup leads to optimal performance of DLC solar cells. An interesting design route to achieve this device architecture is to dilute the columnar liquid crystalline phase with a non-discogenic electron acceptor, such as 2,4,7-trinitro-9-fluorenone (TNF) (Fig. 6.8).

A HAT6/TNF diamond-like carbon (D-LC) charge-transfer (CT) system is then realized. The following sections present selected results that highlight the use of neutron powder diffraction (NPD) and quasielastic neutron scattering (QENS) measurements, as well as simulations based on classical molecular-dynamics (MD) and ab initio density functional theory (DFT), to probe structure and dynamics of the HAT6/TNF D-LC-CT system. We also show that this type of study, in which both the nuclear and electronic aspects of structure and dynamics are mixed, extensive work from other techniques must be considered. Consequently, whilst the main thrust is neutron scattering, results from nuclear magnetic resonance, Raman, resonant-Raman, UV-visible, and infrared (IR) measurements are also presented and discussed. We emphasise here that neutron-scattering methods can bring interesting structural and dynamical information for HAT6 systems, largely because of their abundant H-atoms. In contrast to X-rays, neutron scattering from H atoms is strong and by deuterating the HAT6 sample (HAT6D) the related sites and their dynamics are highlighted differently.