Organic Mip-diodes by p-doping of amorphous wide-gap semiconductors: CV and impedance spectroscopy

Organic thin film materials are widely investigated for applications in electronic and optoelectronic devices. For OLEDs with low operation voltage, controlled doping of the charge transport layers is particularly important. We have recently shown that starburst like amorphous materials (m-MTDATA) can be doped by strong molecular acceptors like F4-TCNQ (tetrafluoro-tetracyano-quinodimethane). These amorphous materials have the advantage of being stable up to 100 °C and forming smooth surfaces. Very well blocking Mip-type Schottky structures (rectification ratio 104–105) allow to determine doping profiles using CV (capacitance–voltage) spectroscopy. The interface between the doped and the undoped layer can be clearly seen as an abrupt change of the capacitance. The impedance spectra can be understood in terms of an equivalent circuit model. The deduced parameters agree well with the nominal thicknesses of the i- and the p-layer and the independently measured bulk conductivity.