Impedance spectroscopy: A method for determining the degree of intermixing in stacks of organic semiconductors

Abstract

One of the main difficulties occurring in printed organic electronics is the intermixing between adjacent layers. This has to be quantified to optimise printed devices. Methods like sputter X-ray photoelectron spectroscopy (XPS) are suitable for this, but also complex, expensive and destructive. We propose impedance spectroscopy as an alternative for quantifying the degree of intermixing non-destructively. Here, after measuring a device's impedance spectrum its concentration profile is determined with a fit function. For this proof of concept we produced single and double layer samples of NPD and Alq3 with different interface widths by thermal evaporation. The concentration profiles were monitored with micro balances. The material parameters were extracted from single layer spectra and then set in a fit function used to determine the degree of intermixing in double layer devices. Even slight intermixing could be evaluated with this method and complete intermixing was detected as such. Due to the imprecise production of devices and certain simplifications used for this first test, the fit yielded too small interface widths, the deviation reaching almost 50%. The variation for devices produced in the same run was only about 10%, indicating that intermixing can be reliably quantified with impedance spectroscopy if the material's properties are accurately known.