Hong Kong Baptist University SPIE-OP-07, Organic Light Emitting Materials and Devices XI, 6655-58 Carrier injection and bipolar transport in NPB for single-layer OLEDs Appl. Phys. Lett. 90, 213502 (2007). S.C. Tse, K.K. Tsung and S.K. So Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University Introduction & Aims • Single-layer organic-light-emitting diode (OLED) is very appealing because of simplified device structure. N N • We demonstrate below that N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB), a well-recognized hole transporter, can be used in single-layer OLEDs. The following three issues will be addressed: a) Bipolar transport of NPB, b) Single-layer OLEDs based on NPB, and c) Nearly Ohmic injection from PEDOT:PSS to NPB. Fig.1 Chemical structure of NPB 3 Photocurrent (a.u.) A. Bipolar transport of NPB 290K B. Single-layer OLEDs based on NPB (a) * PEDOT : PSS τ O O * n Electron and hole mobilities of NPB were evaluated 2 50kV/cm 1 Single-layer OLEDs was fabricated with a configuration of : by time-of-flight (TOF) measurement. Follows are the ITO/ PEDOT:PSS (50nm)/ NPB (120nm)/ Ca/ Ag. Current- * S n* SO3H experimental details: 1 0.1 voltage (JV) characteristics of the single-layer device was 10 100 N Al ITO compared to the theoretical hole space-charge-limited current N2 laser N (337.1nm) h Organic 0 (JSCL) of NPB: DSA-Ph h (d µm) CRO 0 20 40 60 80 Time (µs) 9 F2 where µ0 and β were LUMO J SCL = µ0ε 0ε r exp(0.89β F ) h d evaluated from TOF. (b) 2.4eV Fig. 2 Electron TOF transient for NPB under an 8 d 2.7eV V applied field of 50kV/cm. The film thickness was R 15.3µm. To confine the recombination zone, DSA-Ph was Ca PEDOT NPB NPB (2.9eV) Operating pressure: < 10-3 torr -3 intentionally doped in the middle of the single-layer OLEDs 10 NPB (µe) :PSS with 10% concentration. The doped device structure was ITO/ (5.15eV) • Set up a electric-field with reverse bias voltage V. PEDOT:PSS (50nm)/ NPB (40nm)/ NPB:DSA-Ph (40nm)/ Mobility (cm /Vs) • Optical generation of free carriers from ITO (electron) NPB (µh) NPB (40nm)/ Ca/ Ag. 5.4eV HOMO NPB:DSA-Ph 2 or semi-transparent Al (hole). -4 10 Results and discussions Fig. 4 (a) Chemical structure of PEDOT:PSS • Current was monitored by CRO and charges collected and DSA-Ph. (b) An energy diagram of the 10 -6 Alq3 (µe) For applied voltage < 4V, undoped and DSA-Ph-doped NPB single-layer by a counter electrode at a distance d. devices. • Flight time τ was determined from transient plot and 10 -7 N • JV curves of single-layer OLEDs match with the O O evaluate the carrier mobilities from: Al N Alq3 (µh) computed hole JSCL. Thus, the conduction of NPB 10 3 d2 -8 O Undoped µ = 10 Current density (mA/cm ) N devices are hole-dominated and bulk-limited. 2 DSA-Ph-doped 4 (Figure 2) τ ⋅V 2 10 luminance (cd/m ) 10 2 • Mobilities follow the Poole-Frenkel dependence: 0 400 800 • NPB has a nearly Ohmic hole injection from 1 1/2 PEDOT:PSS. 10 d 3 [Electric field (V/cm)] ulate µ = µ0 exp(β F ) 0 Sim J CL 10 10 hole S where µ0 is zero-field mobility and β is PF slope. Fig. 3 Electron and hole mobilities of NPB and 4V, For applied voltage > 4V, Alq3 at 290K. The inset is Alq3 structure.  -1 2 • JV curves of NPB devices acquire 10 10 different shape due to electron injection -2 10 C. Nearly Ohmic injection from PEDOT:PSS to NPB from the Ca cathode. -3 10 1 In order to probe the contact condition between PEDOT:PSS DISCLC measurement 10 • The turning point of the JV curve is at -4 and NPB, admittance spectroscopy (AS) and dark-injection space- PEDOT:PSS Ag ~4V and coincides with the starting 10 charge-limited current (DISCLC) transient were performed on voltage of LV curves, at which the 0 2 4 6 8 10 ITO NPB Organic hole-only devices with configuration of ITO/ HIL/ NPB /Ag, h detectable light emission can be observed. Voltage (V) h (d µm) (4.11µm) CRO where HIL can be ITO or PEDOT:PSS. h • The turning point of the JV curve Fig. 5 JVL characteristics of NPB devices. The filled (■) V and opened (□) symbols represent the undoped and DSA- corresponds to the onset of electron Ph-doped devices, respectively. The solid line is the When HIL = PEDOT:PSS, a well-defined minimum was R injection from the Ca cathode. simulated hole JSCL of NPB.  observed from the capacitance plot in AS measurement. Meanwhile, in DI experiment, the DI signal exhibits an ideal Operating pressure: < 10-3 torr characteristic with a distinct τDI. The above results indicate that 4 Conclusions Current density (mA/cm ) the contact between PEDOT:PSS and NPB is very close to Ohmic. 2 • NPB, an important organic material, was found to possess bipolar transporting τDI abilities by TOF measurement. AS measurement 1.3 Ag Hole injection layer (HIL) • To demonstrate the bipolar property, NPB was employed as the host material in PEDOT:PSS ITO 1.2 2 single-layer OLEDs, which attained an appreciable performance. ITO NPB PEDOT:PSS h (3.7 µm) Hole injection layer (HIL) h 1.1 • Intentional doping was applied to NPB device to confine the recombination C / Cgeo h ITO PEDOT:PSS zone. The device performance has a notable improvement upon doping. 1.0 • Clear AS and DISCLC results indicate that PEDOT:PSS forms an quasi-Ohmic Vac Vdc 0 Impedance analyzer 0 20 40 60 80 contact with NPB. 0.9 Vdc = 30 V Time (µs) Fig. 7 Experimentally determined Acknowledgments References 0.8 Fig. 8 Room temperature DI signals of ITO/  Appl. Phys. Lett. 89, 262102 (2006). frequency dependent capacitances of 10 3 10 4 10 5 Support of this research by Research Committee of HKBU HIL/ NPB (4.11µm)/ Ag devices using ITO/ HIL/ NPB (3.7µm)/ Ag devices under Grant #FRG/05-06/II-41, and the Research Grant  Appl. Phys. Lett. 90, 213502 (2007). Frequency (Hz) different HIL materials. The applied field using different HIL materials.  Council of Hong Kong under Grants #HKBU/211205E  Org. Electron. 7, 474 (2006). strength was 0.09MV/cm.  and #HKBU/211707E are gratefully acknowledged.  J. Appl. Phys. 100, 063708 (2006).