Influence of imide side-chain functionality in the doping characteristics of naphthalenediimide derivatives as electron transport materials

Achieving effective doping in n-type organic molecular charge transport materials is critical for the development of high-performance optoelectronic devices. However, the role of side-chains in doping reactions remains incompletely understood in some systems. This study focuses on naphthalenediimide (NDI) derivatives, which offer simple synthetic protocols and potentially lower costs compared to traditional fullerene-derived materials. In particular, we explore two functionalised NDI derivatives, comparing one with polar ethylene glycol side-chains (NDI-G) to a non-polar variant with branched alkyl side-chains (NDI-EtHx). Our results show that the effectiveness and speed of the doping reaction with (4-(1,3-dimethyl-2,3-dihydro-1<i>H</i>-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) is much higher with the more polar NDI-G derivative. We postulate that this arises partly from the closer interactions between the dopant and the NDI molecule, facilitated by the polar glycol side-chains. As a result, thin films reach conductivities exceeding 10^-2 S cm^-1. We additionally demonstrate their incorporation into efficient perovskite solar cells, demonstrating the effectiveness of the doping process. We investigate this process with a combination of spectroscopy and density functional theory (DFT) modelling, showing that a complex is likely formed between the resulting N-DMBI cation and the NDI radical anion which then promotes electron transfer to a neutral NDI molecule, thereby generating free charge in the film. These findings underscore the importance of synthetic design on the doping behaviour, with the incorporation of ethylene glycol side-chains emerging as an effective strategy to achieve better electrical conductivity for NDI based systems.