J. Kumar Jha, R. Santos-Ortiz, W. Sun, J. Du, N.D. Shepherd
University of North Texas Denton, United States
pp. 294 - 297
Keywords: AZO, surface modification, workfunction, hole injection, OLED
CFx plasma treatment and WOx interfacial layer deposition were performed on AZO films to improve its workfunction for transparent conducting oxide (TCO) anode application in the organic light emitting diodes (OLEDs). As shown in Figure 1(a), the effective workfunction of AZO films increased from 4.1 eV to 4.4 eV with CFx plasma treatment, and from 4.1 eV to 4.7 eV upon deposition of nanoscopic (~ 3 nm) WOx interfacial layers. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) studies along with density functional theory (DFT) studies indicated that CFx plasma treatment results in a fluorinated electronegative surface which provides a dipole moment that reinforces existing surface dipole, leading to increased workfunction. In contrast to CFx plasma treatment, XPS and UPS results for WOx interfacial layers deposited AZO (AZO/WOx) films indicated that higher workfunction of the WOx layers improved the effective workfunction of the AZO/WOx system. Current-voltage characterization of hole-only devices fabricated with AZO/WOx anodes demonstrated an increase in hole injection (compared to neat ITO and AZO) due to Fowler-Nordheim tunneling from gap states associated with oxygen vacancies in the WOx layers. The performance of CFx and WOx surface modified AZO anodes were compared to ITO in simple OLEDs with tris(8-hydroquinoline) aluminum (AlQ3) as the emissive and electron-transport layer, and N,N’-Bis (naphthalene-1-yl)–N,N’-bis(phenyl)-benzidine (NPB) as hole transport layer, shown in Figure 1(b). At 9 V of applied voltage, OLEDs with AZO/WOx (3 nm) anode exhibited higher luminous, power, and external quantum efficiencies of 4.7 Cd/A, 1.6 lm/W and 1.5 % respectively, compared to 2.4 Cd/A, 0.9 lm/W and 0.9% for commercial ITO. OLEDs with CFx plasma treated AZO anodes demonstrated luminous, power, and external quantum efficiencies of 2.3 Cd/A, 0.8 lm/W and 0.8 %, respectively which are higher than as-deposited AZO, but smaller than ITO. Figure 2 shows power efficiency vs current density of all the OLEDs. The improved performance of the AZO/WOx devices is ascribed to better band alignment (between the anode and NPB), leaded to enhance hole injection and improve charge balance, compared to neat AZO or ITO.