Mixing Time Controls Conductivity of n-Type-Doped Organic Semiconductors

Among the various n-type dopants used in organic electronics, NDMBI-H is the most common. However, precise control of the doping levels is still elusive, with reported conductivities varying by orders of magnitude for similar systems. In this study, we investigate a frequently underestimated factor: the mixing time during co-solution processing. Our focus is on a prototypical system involving NDMBI-H-doped PCBM processed in chloroform. Remarkably, we observe a decline in conductivity with prolonged mixing times that does not occur when the single components are stored individually. We establish a correlation between this behavior and characteristic signatures in the 1H NMR spectra of pristine compounds and mixtures. We attribute the decline in conductivity to the reactive nature of chloroform and extend our findings to four more organic semiconductors with different backbones. We find that choosing a suitable solvent and controlling the mixing time allow us to increase the conductivity of doped PCBM to up to 2.64 S/cm through a straightforward bulk doping process, representing a substantial improvement compared to values obtained using additives or more complex procedures.