Different functional layers in different organic semiconductor devices request a variety of special characteristics.
For instance organic thin-film transistors (OTFTs) need high mobility materials in their active channel. Transparent circuits, such as transparent OTFTs require that the high mobility organic material also comprises a wide electronic band gap; the electric injection of holes and/or electrons must be still provided.
OLEDs require transparent transport layers, with high conductivity. The transparency is necessary in those opto-electric devices to avoid non desired absorption of the light. These so called “window” materials can be used as transport layers, exciton or charge blocking layers. The thickness of the layers made with the window materials is used to adjust the micro cavity of the OLEDs in such a way that the outcoupled emission of the OLED is a maximum. The non-optically active layers of all kinds of semiconductor devices can be exchanged for window materials in order to fabricate fully transparent components and circuits (e.g US20060033115). The functionality and nomenclature of the layers are typical as used in the field. Further explanation can be found in US2006244370.
Electronic devices also need high stability towards temperature, meaning that the intrinsic properties of the amorphous organic semiconducting materials, such as triphenyl amine derivatives, or phenantronine derivatives, must include a high glass transition temperature (Tg) and high temperature stability in the device.
The performance characteristics of (opto)electronic multilayered components are determined by the ability of the layers to transport the charge carriers, amongst others. In the case of light-emitting diodes, the ohmic losses in the charge transport layers during operation are associated with their conductivity. The conductivity directly influences the operating voltage required and also determines the thermal load of the component. Furthermore, depending on the charge carrier concentration in the organic layers, a bending of the band in the vicinity of a metal contact results which simplifies the injection of charge carriers and can therefore reduce the contact resistance.
By electrically doping hole transport layers with a suitable acceptor material (p-doping) or electron transport layers with a donor material (n-doping), respectively, the density of charge carriers in organic solids (and therefore the conductivity) can be increased substantially. Additionally, analogous to the experience with inorganic semiconductors, applications can be anticipated which are precisely based on the use of p- and n-doped layers in a component and otherwise would be not conceivable. The use of doped charge-carrier transport layers (p-doping of the hole transport layer by admixture of acceptor-like molecules, n-doping of the electron transport layer by admixture of donor-like molecules) in organic light-emitting diodes is described in US2008203406 and U.S. Pat. No. 5,093,698.
US2008227979 discloses in detail the doping of organic transport materials, also called matrix, with inorganic and with organic dopants. Basically, an effective electronic transfer occurs from the dopant to the matrix increasing the Fermi level of the matrix. For an efficient transfer in a p-doping case, the LUMO energy level of the dopant must be more negative than the HOMO energy level of the matrix or at least slightly more positive, not more than 0.5 eV, to the HOMO energy level of the matrix. For the n-doping case, the HOMO energy level of the dopant must be more positive than the LUMO energy level of the matrix or at least slightly more negative, not lower than 0.5 eV, to the LUMO energy level of the matrix. It is furthermore desired that the energy level difference for energy transfer from dopant to matrix is smaller than +0.3 eV.
Typical examples of doped hole transport materials are: copperphthalocyanine (CuPc), which HOMO level is approximately −5.2 eV, doped with tetrafluoro-tetracyanoquinonedimethane (F4TCNQ), which LUMO level is about −5.2 eV; zincphthalocyanine (ZnPc) (HOMO=−5.2 eV) doped with F4TCNQ; a-NPD (N,N′-Bis(naphthalen-1-yl)-N,N-bis(phenyl)-benzidine) doped with F4TCNQ.
Typical examples of doped electron transport materials are: fullerene C60 doped with acridine orange base (AOB); perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) doped with leuco crystal violet; 2,9-di(phenanthren-9-yl)-4,7-diphenyl-1,10-phenanthroline doped with tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten (II) (W(hpp)4); naphthalene tetracarboxylic acid di-anhydride (NTCDA) doped with 3,6-bis-(dimethyl amino)-acridine; NTCDA doped with bis(ethylene-dithio)tetrathiafulvalene (BEDT-TTF).
There is a technical challenge to provide electron transport materials (ETM) and emitter host (EMH) materials that have a sufficiently low laying LUMO level so that they can be doped, and still have a high enough laying LUMO level which can efficiently transfer charge to emitter host (in case of an ETM) and transfer energy to the emitter dopant (in case of EMH). The limitation for high laying LUMO level of the ETL is given by the dopability, since the n-dopants with very high HOMO tend to be unstable; also the injection is difficult for very high LUMO of the ETL.