Navigating The Complex Hole-Transport Material Additive Cocktail: Doping Methodologies for Small-Molecule HTMs

Most organic molecular hole transporters used in perovskite solar cells require effective doping to reach conductivity values necessary to achieve maximum device efficiencies. This is typically achieved by incorporating a cocktail of additives including tert-butylpyridine, ionic salts (LiTFSI, Zn(TFSI)₂) and metal complexes (FK209) which results in the partial oxidation of the semiconducting material, introducing additional free holes within the HTL and thus providing effective hole transport. However, a detailed study of the relationship between doping level and the quantity of free holes within the matrix has not been carried out. Indiscriminately adding the same additive cocktail to different HTM systems can lead to sub-optimal doping and an excess of reactive substances within the film that can catalyse degradation mechanisms. In this study, we investigate a range of HTMs possessing amide or imine linker groups. Our research highlights that oxidation and doping are not synonymous, and while oxidation generally leads to doping, this is not always the case. In addition, we reveal the intricate relationship between the different additives and the doping mechanism as a function of the chemical structures used. We conclude that a ‘one size fits all’ approach to HTM doping can lead to the misdiagnosis of effectively oxidised molecules, and therefore propose spectroscopic techniques to monitor the efficacy of doping, promoting rational HTM design and expediting the characterisation of new HTMs.