Computational and experimental investigations into the factors influencing hole mobility in tuneable small-molecule organic hole transporters

Abstract

Perovskite solar cells (PSC) are widening the scope of photovoltaics (PVs) to applications beyond the effective capabilities of conventional silicon-based PVs. In these devices the organic hole transport material (HTM) plays a major role in controlling the overall performance and cost of PSCs. Charge recombination at the HTM/perovskite interface remains a challenge, as is the cost of producing standard HTMs, such as spiro-OMeTAD, limiting the viability of these devices. Recently, inexpensive tuneable small-molecule organic HTMs have been developed using condensation chemistry, some of which show promising charge transport properties. A better understanding of the factors governing mobility in these materials could help us move away from the conventional trial-and-error approach and enable us to design HTMs with a higher mobility. In this thesis I explored the properties of small molecule organic HTMs relevant to charge transport. In amorphous (disordered) systems such as these, where charges are localised on energetically discrete sites (molecules), charge transport has been conceptualised as a hopping process which may be described by Miller-Abrahams and Marcus rate equations. Initial quantum mechanics calculations on single molecules revealed that the best charge transport properties were exhibited by HTMs with high dipole moments. This is rather surprising, as correlated energetic disorder has been shown to scale with the dipole moment in amorphous materials and quench mobility. This led us to look further into the effects of the size and ordering of molecular dipoles on mobility using an in-house kinetic Monte Carlo code. While it has been suggested that higher dipole moments might drive favourable self-assembly during film formation and reduce the width of energetic disorder, our simulations show that mobility is rapidly quenched even at low levels of disorder. We find that increasing the dipole moment reduces the number of energetically available hopping sites, resulting in inefficient charge percolation through the film. High levels of global order are unlikely to be achieved in solution processed thin films. However, crystal structures of HTMs reveal closely packed dimers which orient antiferroelectrically in some cases. The presence of these stable supramolecules, with a zero-dipole moment, might reduce the overall energetic disorder in an otherwise disordered film. Molecular dynamics simulations show that our systems based on high performing molecules have a greater proportion of these dimers and in kMC simulations the mobility increases sharply with the population of zero dipole dimers. While these results suggest that it may be beneficial to design molecules with a low dipole moment, polar HTMs may be better at binding to and passivating the perovskite layer, which would increase the power conversion efficiency (PCE) and long-term stability of PSCs. In addition, it has been shown that suitable alignment of dipoles generates a giant surface potential (GSP) across organic semiconductor films, which could be exploited to enhance charge extraction and transport. Based on ii our insights, we set out to control disorder in high dipole HTMs via dimer formation by developing and synthesising a series of HTMs with different H-bonding capabilities. These consisted of both symmetric and asymmetric secondary and tertiary amides, as well as a urea compound. The intermolecular interactions and charge transport properties of these molecules was investigated. Results show that molecules with highly available H-bonding sites readily form H-bonded dimers in the crystal structure and in solution, by proton NMR spectroscopy. These molecules also show favourable charge transport properties when compared to the methylated derivative which is unable to dimerise via intermolecular H-bonding. Our results indicate that it may be possible to use bonding to tune the width of the energetic disorder in high dipole amorphous HTM films. This could be achieved by the formation of short-range ordered domains with quenched dipoles, made up of closely packed favourably oriented monomers, and to a lesser extent through better ordering of dipoles in the film as a whole