Abstract:

Recently, polymers have emerged as strong candidates for various semiconductor device applications. The physical and electrical properties of these polymer semiconductors are drastically different from those of inorganic semiconductors, and a solid understanding of these properties is necessary in order to further develop polymer electronics. This work concentrates on polymers for light emitting diode (LED) devices, which is the most promising application in polymer electronics. Understanding and control of charge injection from anode/cathode to active (light emitting) layer are crucial for high efficiency LED. To understand the charge injection efficiency, the energy band alignment and their impact on charge injection at polymer-electrode interfaces are investigated with a range of electron spectroscopies and electrical measurements.

First, electronic structure of the best known hole injecting polymer (i.e., anode), poly(3,4-ethylene-dioxythiophene) - poly(styrene-sulfonate) (PEDOT-PSS) is studied. We investigate the unique shell - like structure of the PEDOT-PSS and its impact on the electrical properties. The role of PSS surface layer in enhancing the hole injection efficiency into the active layer is discussed.

The electronic structures of two light emitting conjugated polymers, poly(9,9'-dioctylfluorene) (F8, also known as PFO) and poly(9,9'-dioctylfluorene- co -bis-N,N '-(4-butylphenyl) diphenylamine) (TFB), are also studied by various methods. The alignment of the energy levels at the anode and the cathode interfaces is investigated in detail. Different mechanisms seem to apply for the energy level alignment at the anode and the cathode. We discuss the difference in the light of different degrees of contamination at the interface which results from different processing conditions.

Finally, a modification of energetics of polymer-anode interface by doping is discussed. We propose a co-solution doping method suitable for solution processed polymer material. The polymer showed similar behavior to small molecular semiconductor upon doping. It is shown that doping induces the energy level shift and enhances the carrier injection.