Charge-Carrier Injection and Transport in Poly-p-Phenylene-Vinylene Light-Emitting Diodes

Copyright © (1997) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

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 rho(r) of about 10**(6) 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 u(h) is found to be of the order of 10**(-5) cm**(2)/Vs.

By: S. Karg (Univ. Bayreuth, Germ), M. Meier (Univ Bayreuth, Germ), W. RieB

Published in: Journal of Applied Physics, volume 82, (no 4), pages 1951-60 in 1997

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