Controlled p-doping of pigment layers by cosublimation: Basic mechanisms and implications for their use in organic photovoltaic cells

We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopa nt. The films are characterized in situ by temperature dependent Seebeck an d conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states wit h increasing dopant concentration. We thus conclude that doping has the pot ential of both reducing the series resistance and increasing the photovolta ge of organic solar cells. As a first step to exploit this potential, we pr esent two different ways of preparing diodes with rectification ratios in e xcess of 10(4) using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which wor k already in the strict absence of oxygen and are stable in air. To increas e the efficiency of charge carrier generation in photovoltaic cells, we nee d to use photo active donor-acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominal ly undoped layers. (C) 2000 Elsevier Science B.V. All rights reserved.