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The properties of field effect transistors with organic insulator and semiconducting regions, fabricated with a top-gate architecture, have been investigated. Thin films (d≈30 nm) of regioregular poly(3-dodecylthiophene) were employed as the active semiconductor and the gate insulator was formed by a 500-nm-thick layer of poly(4-vinylphenol). Both were solution-processed on top of poly(ethylenetherephthalate) films, which were used as substrates. The output characteristics show a pronounced saturation behavior with an unconventional nonquadratic saturation current dependence on the gate voltage. Hence the (hole) mobility of 0.002–0.005 cm2/Vs has been estimated from the linear region of the transfer characteristics. The transistor turn-on occurs at a threshold voltage of approximately Vth=0 V, and the device can be operated with a supply voltage of between 15 and 20 V. As is usually observed for organic transistors, the inverse subthreshold slope (S) is very high, in our case S≈7 V/dec, by contrast with S≈200 mV/dec obtained for the similar material poly(3-octylthiophene) (P3OT) with silicon dioxide (SiO2) as an insulator. Furthermore, the subthreshold current depends on the drain voltage even though the transistor is electrically a long channel device with L=2 μm, notwithstanding the fact that this channel length is rather small for the present organic devices. To clarify these peculiarities numerical simulations have been carried out with a systematic variation of the relevant material parameters and assuming the existence of interface or bulk trap states. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained by recharging of trap states either at the interface or in the bulk. Considering the difference to the P3OT device with SiO2 as insulator it is proposed that interface traps are responsible for these effects, although one excludes the possibility that the film formation either on an organic substrate or on SiO2 leads to different bulk properties.

Trap-limited electron transport in disordered semiconducting polymers Mandoc, M. M.; de Boer, B.; Paasch, G.; Blom, P. W. M.

The electric eld distribution in organic hetero-layer light-emitting devices based on N,N 0 -diphenyl-N,N 0 -bis(1-naphtyl)-1,1 0 -biphenyl-4,4 0 -diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq 3 ) has beeninvestigated under di erent bias conditions using capacitance-voltage measurements. Although this method yields primarily information on the di erential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density o f 6 :8 10 11 e=cm 2 at the NPB/Alq 3 interface at large reverse bias. This leads to a jump of the electric eld at the interface and a non-uniform eld distribution in the hetero-layer device.A major breakthrough in the development of organic electroluminescent devices for display applications was the demonstration of high e ciency and brightness at moderate voltages by m e a n s o f a n organic multi-layer device 1,2]. The underlying idea was to separately optimize injection and transport of holes and electrons and their radiative r e c o m bination in di erent organic layers. A common starting point for device optimization is the considera-

I n the local density approximation (LDA) the density of a many-electron system is expressed as a functionof the spatially varying potential. Here a modifiedLDA is derived applicable for a potential with a high step a t some plane which is e.g. a model for the band edges in an inversion layer a t the semiconductor-insulator interface. The local density of states shows oscillations and decreases t o zero a t the interface as a consequence of quantum mechanical reflection. Simple expressions for the density are obtained for the strongly degenerate and for the non-degenerate system. The comparison with exact results for a model system (triangular potential) shows that the modified LDA yields good results for a wide range of parameters corresponding to real inversion layers.Die Naherung der lokalen Dichte (LDN) liefert die Dichte eines Vielelektronensystems als Funktion des Potentialverlaufes. Es wird eine modifizierte LDN abgeleitet, die fur ein Potential gilt, das an einer Ebene einen groden Sprung aufweist. Dieses System modelliert z. B. den Bandkantenverlauf in einer Inversionsschicht an der Halbleiter-Isolator-Grenzflache. Als Folge der quantenmechanischen Reflexion zeigt die lokale Zustandsdichte Oszillationen und fallt zur Grenzfiche hin auf Null ab. Besonders einfache Ausdrucke fur die Dichte ergeben sich fur die Grenzfalle der Entartung bzw. Nichtentartung. Der VeIgleich mit exakten Ergebnissen fur ein Modellsystem (,,Dreieckpotential") zeigt, daB die modifizierte LDN gut ist fur einen weiten Parameterbereich, der den tatsachlichen Inversionsschichten entspricht.