Abstract text (incl. figure legends and references)

Molecular dopants are used to enhance the electronic properties of organic electronics. The three dimensional distribution of the dopants and whether they are present as individual dopants or in clusters determines the electrical properties. In applications, the dopant concentration varies depending on application but it can be up to several molar precents.The dopants are about 1 nm in size and the distance between them is about a few to 10 nm. A detailed information about the spatial distribution is important for the understanding of the conduction mechanisms and how the molecular dopants affect the properties of the surrounding polymer. 1,2

In this study, electron tomography is used to visualize the three-dimensional spatial distribution of individual Mo(tfd-COCF3)3 molecular dopants in the semiconducting polymer p(g42T-T) at the sub-nanometer scale.The thickness of the films is in the range of 13-17 nm and dopant concentrations are 5, 20 and 40 mol%.The acquisition parameters are chosen to obtain sufficient resolution to determine the position of individual dopant molecules in the material. A heterogenous distribution is observed. At dopant concentrations of 5 mol%, the majority of the dopants are positioned individually or in clusters consisting of less than five molecules. At higher concentrations, the dopants form larger sized clusters with increasingly elongated shapes. Despite the larger cluster sizes at higher concentrations, all of the observed dopants remain adjacent to surrounding polymer. This is a crucial aspect for the efficient charge transfer between the dopants and the surrounding polymer. Finally, electrical characterization using a four-point probe setup shows a decrease in thin-film conductivity as larger dopant clusters are formed at high dopant concentrations. 3

1 C.J. Boyle et al., Nat. Commun., 2019, 10, 1-10

2 A. Fediai et al., Phys. Chem. Chem. Phys., 2020, 22, 10256-10264

3 G. Persson et al., Nanoscale, 2022, 14, 15404-15413