As discussed in Sect. 6.2, one of the loss mechanisms to which OPVs might be subject can be via the strong electron-phonon coupling inherent to the molecular nature of the device, which limits the efficiency of charge separation. Upon photo­excitation, strongly bound exciton-states are formed that first need to dissociate before charge-transport to the electrodes can occur. Dissociation takes place at the DA interface, where intermediate CT states are formed with the hole on the donor, and electron at the acceptor molecule. It is established that charge separation is mediated by the higher lying vibronic-states of the CT manifold [38, 39]. As introduced in Sect. 6.3.1.3, fundamental knowledge about the electronic and vibrational properties of the excited-state levels and relaxation pathways is a key topic in further improving the working of OPVs.

For self-assembled aggregates such as DLCs and DLC-CT complexes, the characterization of photo-induced electron-transfer and relaxation processes is in its infancy. The addition of electron acceptors has been shown to increase the con­ductivity of DLCs. On the other hand, it has been proposed that recombination processes limit the hole photocurrent in such DLC-CT compounds [40].

An important step towards the characterization of the influence of molecular vibrations on the charge-carrier relaxation in self-assembled DLCs can be made by studying the prototypical discotic electron-donating discoid HAT6 and its 1:1
mixture with electron acceptor TNF, combining synergistically Raman, resonant Raman, UV-visible and nuclear magnetic resonance techniques, as illustrated in reference [41]. From this one can draw the following conclusions:

(i) The lowest electronic-transition in the CT complex is due to charge-transfer from the HAT6 core to TNF, with a strong involvement of the nitro groups.

(ii) There are strong indications for a weak ground-state electron-transfer in the HAT6-TNF complex.

(iii) Both nuclear magnetic resonance chemical-shift changes and Raman fre­quency-shifts are seen and are consistent with a weak electron-transfer from the HAT6 core to TNF leading to a delocalized redistribution of the charge on TNF.

(iv) The resonant Raman spectra of both HAT6 and the CT-complex show a strong enhancement of the modes related to the benzenes forming the tri- phenylene core. This is characterized by a doubly-degenerate electronic — state which is vibronically coupled (pseudo-Jahn-Teller) to a nondegenerate state. It should be noted that this Jahn-Teller mode provides a significant contribution to the reorganization energy of HATn, being a limiting factor for hole transport along the columnar stacks [42].

(v) The hot-carrier relaxation processes in the CT-band in the visible-light region are relatively slow compared to the fast relaxation within the original UV absorption band of pure HAT6, which will be relevant concerning the efficient separation of charge in organic PV-devices.

6.4 Conclusions

Inorganic photovoltaics possess a major advantage over organic solar cells by pro­viding the highest power-conversion efficiencies. But in terms of the fabrication flexibility, production costs and market accessibility, organic photovoltaics are also potential candidates for the future generation of solar cells. This requires under­standing the dynamic and electronic properties so that optimization routes can be devised that will increase their efficiency and boost their performance. This chapter illustrated how neutron scattering, in combination with other techniques, can be used to extract this fundamental aspect of organic photovoltaics working principles. In this chapter different structural and dynamical aspects of an organic molecular-model for photovoltaic applications are presented, including an evaluation of the known limi­tations. The chapter provides an example of how the logical sequential advance of experiment, theory and understanding lead towards improvements. Different exper­imental techniques and theoretical methods are used, in a quite complementary way— if not synergetic. Neutron-scattering techniques are shown to play an important role in this field by probing efficiently structure and dynamics in the presented hydrogenated materials. Numerical simulations are almost mandatory in this field to aid the inter­pretation and analysis of the different measurements including the other experimental techniques (IR, UV, Raman, and nuclear magnetic resonance methods).