Carrier injection and bipolar transport in NPB for single-layer OLEDs
2007
We studied the carrier injection and transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-
4,4'diamine (NPB), a common hole transporter for organic light-emitting diodes (OLEDs). NPB was found to possess
significant electron mobility from time-of-flight (TOF) measurement. With bipolar transporting ability, NPB was used to
fabricate single-layer devices with a configuration of ITO/ PEDOT:PSS/ NPB/ Ca/ Ag. PEDOT:PSS was demonstrated
to form a quasi-Ohmic contact to NPB by admittance spectroscopy (AS) and dark-injection space-charge-limited current
(DISCLC) measurements. From current-voltage ( JV ) characteristics, single-layer NPB devices exhibited a bulk-limited
hole current in low-voltage region. Electron injection was clearly observed at a turn-on voltage of about 4V, which
coincided with the luminance-voltage measurement. In order to confine the recombination zone, dye-doped NPB layer
was inserted into single-layer devices. This intentional doping technique made a notable improvement in current
efficiency. The mechanisms of the doped devices were also addressed.
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