“Charge Injection and Transport in Organic Semiconductors”
Yulong Shen, DuPont Displays
The operation of organic optoelectronic devices involves charge injection at metal/organic interfaces and charge transport in the bulk of the organic semiconductor. The main part of the talk will discuss how different factors like mobility, doping density, etc. affect injection. The model system studied was MDPs (molecularly doped polymers) such as PC:TPD.
The current injected at an ITO contact was measured as a function of the hole mobility. The contact was found to be injection limited and the injected current was proportional to mobility. The dependence of charge injection at metal/organic interface on doping density was also studied. The contact resistance is found to decrease dramatically upon doping beyond what is expected from former theory seen in crystalline semiconductors. This phenomenon is understood in terms of broadening of the transport manifold in the organic semiconductor, induced by the dopants.
The effect of doping on the conductivity of a molecularly dispersed polymer was also studied. The conductivity was found to be thermally activated and exhibited two regimes: From lower to higher doping levels the conductivity was found firstly increased linearly and then superlinearly with doping, while at the same time the activation energy decreased from about 0.2eV to around 0.12 eV. This behavior was understood in terms of transport manifold filling, Coulombic trapping of charge, and broadening of the transport manifold due to dipolar disorder induced by the dopants.