Role of structural isomerism in the properties of imine-based organic hole-transporting materials

Organic hole-transporting materials (HTMs) play a key role in enhancing both the efficiency and endurance of photovoltaic devices and for optoelectronic applications. In contrast to their inorganic counterparts, they offer distinct advantages such as solution processability, tunable properties, and low-cost fabrication. However, their electrical conductivity in most cases is intrinsically low and can be enhanced through doping using chemical oxidants. Doping typically involves the partial oxidation of the HTM, generating additional free charges and improved film conductivity. In this work, we investigate the effect of molecular design on the doping mechanism, with a specific focus on imine-linked, triarylamine-based compounds. Our research indicates that the effectiveness of doping and resulting conductivity are determined by the energy of the dopant-HTM complex. Through a combined approach including density functional theory (DFT) modelling, spectroscopy, and conductivity measurements, we observe that oxidation of the HTM does not guarantee doping if the generated charges are not free. This highlights the importance of imine bond orientation in the stabilisation of generated holes. Interestingly, a seemingly trivial chemical change, such as the inversion of an imine bond affects the doping of the material. Our findings show that such isomerisation can result in charge transfer complexes with stabilised holes that do not improve conductivity. This challenges many common approaches to chemical doping, where standard additives are added to newly developed HTMs without prior investigation of their efficacy for the chemical system being studied. We advocate for a tailored understanding of the doping mechanism and the use of spectroscopic techniques to enhance HTM design and characterisation.