Mohamed Zbiri, Lucas A. Haverkate, Gordon J. Kearley, Mark R. Johnson and Fokko M. Mulder

Abstract Organic-based photoconverters are subject to a considerable interest due to their promising functionalities and their potential use as alternatives to the more expensive inorganic analogues. We introduce the basic operational mechanisms, limitations and some ideas towards improving the efficiency of organic solar cells by focusing on probing the morphological/structural, dynamical, and electronic aspects of a model organic material consisting of charge-transfer discotic liquid — crystal system hexakis(n-hexyloxy)triphenylene/2,4,7 trinitro-9-fluorenone (HAT6/ TNF). For the electronic ground-state investigations, neutron-scattering techniques play a key role in gaining deeper insight into structure and dynamics. These measurements are complemented by Raman and nuclear magnetic resonance probes, as well as resonant Raman and UV-vis spectroscopies that are used to explore the low-lying excited states, at the vibronic level. Synergistically, numerical simulations, either classical via empirical force fields, or first-principles via density functional theory, are used for the analysis, interpretation and predictions.

6.1 Introduction

Research on organic devices is focused mainly on three concept organic photovoltaic (OPV) designs in the solid-state phase by using either: (i) vacuum-deposited mole­cules by evaporation of two or more n-and p-dopants [1], (ii) solution-processed polymeric macromolecules [2], and, (iii) the Gratzel concept [3] to build dye-sensi-

M. Zbiri (H) • M. R. Johnson

Institut Max von Laue-Paul Langevin, 71 avenue des Martyrs,

CS 20156, 38042 Grenoble, France e-mail: zbiri@ill. fr

L. A. Haverkate • F. M. Mulder

Faculty of Applied Sciences, Reactor Institute Delft, Delft University of Technology,

Mekelweg 15, 2629 JB Delft, The Netherlands

G. J. Kearley

Australian Nuclear Science and Technology Organisation,

Lucas Heights, NSW, Australia

© Springer International Publishing Switzerland 2015 109

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_6

Fig. 6.1 Schematic representation of a p-n heterojunction (a) and a bulk heterojunction (BHJ) of interpenetrating networks of p-and n-dopants (b)

tized devices based on mesoporous nanocrystalline thin-films of TiO2 covered by a dye layer. In terms of architectures, solar cells based on cases (i) and (ii) are similar and can be set either into a heterojunction or abulk heterojunction (BHJ) device (Fig. 6.1).

The former represent the “standard” way of building a p-n junction and the donor-acceptor (DA) interface at which the exciton dissociation into charge carriers should take place. The latter architecture is based on a more elaborated technique of enhancing the p-n junction of setup (i) and making it as an interpenetrating network of the n-and p-dopant materials. It should be noted that in the case of polymer-based OPV both heterojunction and BHJ setups are made by a solution process because of the impossibility of evaporation due to the large macromolecular weight. The Gratzel-type solar cell is based on a different device architecture comprising mes — oporous dye-covered TiO2 films connected through a pore-penetrating liquid electrolyte or an organic n-dopant material, to a transparent counter electrode. Each of these OPV concepts presents different advantages and limitations in terms of efficiency and production costs. The high-vacuum deposition of small molecules ensures good processability and reproducibility of the device in contrast to cases (ii) and (iii). However, it is an expensive and complex technology which limits its use. In this respect, polymer-based OPVs are better because they can be fabricated under ambient conditions. Moreover, polymers in this case can be made soluble and spray-deposited on plastic substrate, which considerably increases their flexibility with obvious benefits for commercialization [4]. The dye-sensitized (Gratzel) solar cell performs the best [5] for power-conversion efficiency (PCE), but because a heat treatment is needed for TiO2 a flexible realization of this architecture is limited.