SASOLAR13 · Self-assembly strategies towards optimal morphology in small molecule organic solar cells

Self-assembly strategies towards optimal morphology in small molecule organic solar cells

Self-assembled structures have inspired much research interest due to their exciting electronic and optical properties. Many non-covalent interactions can be employed to build such assemblies, as for example p-p stacking, hydrogen bonds, halogen bonds and metallophilic interactions. During this research period we have explored the influence of supramolecular chemistry in solution processed organic photovoltaics (OPVs) and in charge transport processes. Particularly, hydrogen bonds together with p-p stacking interactions have been introduced into p-conjugated organic semiconductors to achieve the described ideal “checkerboard” morphology (Fig. 1a) and in a second part of the project, metallophilic (platinum-platinum) together with p-p stacking interactions have been used to build supramolecular architectures formed by small semiconducting segments to study charge transport processes depending on the packing mode of the constituent molecules (Fig. 1b). In particular, interactions between d8 metal ions (platinum ions) are progressively being recognized as building blocks thanks to their square planar geometry and high stacking tendency. This geometry results in the overlapping of dz2 orbitals, providing a driving force for the anisotropic growth of nanostructures. Furthermore, ground state metallophilic (Pt···Pt) interactions can be established when the distance between the complexes is below 3.5 Å, leading to the formation of new excited states, namely metal-metal-to-ligand charge transfer (MMLCT) transitions, that can be conveniently used to enhance the emission properties and to induce a large bathochromic shift of both, emission and excitation. Such metallophilic interactions are weaker than covalent bonds but they can reach binding strengths similar to hydrogen bonds. This means that metallophilic interactions can be disrupted by other non-covalent forces, even though they are usually present together with other self-assembly motifs, yielding supramolecular entities with different structures and properties according to the packing interplay of all the coexisting interactions. Several cases of the combination of Pt interactions with other non-covalent interactions have been reported. However, most of these studies have been done using single crystals, which usually is not the most representative sample if we consider an entire device. In this project, we have studied systems based on discrete molecules forming assemblies on thin films and directly on the real devices. We have explored a library of oligothiophene containing platinum (II) complexes (Scheme 1) as the constituents of the supramolecular assemblies on films. Their thin film morphology and their packing mode have been studied, establishing a relationship between these results and their charge transport properties. The influence on the packing of the number of thiophene rings has been explored, as well as the effect of the introduction of solubilizing alkyl tails and the presence of fused aromatic systems. A detailed study of their photophysical properties (absorption, emission, quantum yield and lifetime) in solution and solid state, their morphology in thin films, their packing mode with powder X-ray diffraction and the fabrication of charge transport devices demonstrate that small, rigid molecules coordinated to platinum centers enhance the molecular packing resulting in better charge mobility values. Particularly, platinum complexes containing a single thiophene ring or a benzothiophene moiety show better results than complexes containing longer oligothiophene segments or even powerful semiconducting segments. In conclusion, we have demonstrated that self-assembled platinum complexes can arrange semiconductors in organized architectures using metallophilic interactions in combination with p-p stacking. This synergy of non-covalent forces results in the enhancement of electronic properties, especially in small, rigid conjugated semiconductors, which are able to pack in a more efficient manner yielding higher values of mobility. With these results we expect that self-assembly strategies can lead to the use of simple discrete molecules in energy technologies and demonstrate that an improvement in efficiency can be achieved.

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