Diffusion Doped Organic p-n Junctions

Abstract

It is demonstrated that the conductivity type of organic semiconductors can be modfied by a mechanism analogous to impurity doping. That is, by suitable choice and control of additives, a given organic host may be rendered either n or p type. Moreover, it is shown that diffusion doping can be employed to form p‐n junctions in a variety of organic host materials. Two examples are discussed in detail to demonstrate the feasibility of doping. First, p‐type tetrathiotetracene (TTT) is shown to become n type on addition of orthochloronil (OC). The apparent cause is the formation of TTT/OC charge transfer complex molecules within the bulk of the TTT host. Secondly, the normally n‐type TTT/OC charge transfer complex is shown to become either increasingly n type or p type on introduction of metal additives. n‐type conduction is achieved with group I and IV metal additives and p‐type conduction with group II and III metal additives. It is also shown that the additives have a significant effect on the conductivity and the activation energy of conduction. Moreover, the changes in the conductivity of TTT, induced by OC additives, are indicative of a tunneling type conduction mechanism. In contrast, the effects due to metal additives can be described by a simple band model. All experiments were performed with compressed powder pellets and the pellet preparation pressure is shown to have a systematic, residual effect on the material properties. The applicability of the doping technique for the preparation of organic p‐n junction diodes is demonstrated by diffusion doping of TTT with OC, TTT/OC complex with Ag and Mg, Perylene with Br, and tetracyano‐quinedymethanol‐quinolinium (TCNQ/QUIN) complex with Ag. Rectification ratios of up to 200 were recorded using, in all cases, compressed powder diodes.