16 October 2007 Carrier injection and bipolar transport in NPB for single-layer OLEDs
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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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S. C. Tse, K. K. Tsung, S. K. So, "Carrier injection and bipolar transport in NPB for single-layer OLEDs", Proc. SPIE 6655, Organic Light Emitting Materials and Devices XI, 66551Q (16 October 2007); doi: 10.1117/12.733775; https://doi.org/10.1117/12.733775

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