Light-emitting diodes based on poly-p-phenylene-vinylene: I. Charge-carrier injection and transport

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Abstract
Detailed investigations of the device characteristics of poly-p-phenylene-vinylene (PPV) light-emitting diodes are reported. We analyze the influence of various hole- and electron-injecting electrodes on the current–voltage (I–V) characteristics and electroluminescence behavior. Our studies reveal that thermal conversion of the prepolymer on indium–tin–oxide (ITO) substrates leads—in contrast to conversion on Au and other high-work-function metals—to a p-type doping of PPV and, additionally, to the formation of an ohmic hole-injecting contact at the ITO/PPV interface. Hence, devices fabricated with low-work-function metals acting as the electron injecting contact (for example, Al and Ca) display Schottky behavior. These Schottky diodes are distinguished by a high rectification ratio ρr of about 106 and display electroluminescence at bias voltages as low as 1.5 V for ITO/PPV/Ca light-emitting diodes. The I–V characteristics can be quantitatively described within the modified Shockley equation, taking into account the voltage drop on a serial resistance, yielding ideality factors n ranging from 1.6 to 2.4 for different devices. At high current densities, space-charge-limited currents determine the device characteristics. The hole mobility μh is found to be of the order of 10−5 cm2/V s.