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

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.