Centimeter-scale hole diffusion and its application in organic light-emitting diodes

Abstract

In conventional organic light-emitting diodes (OLEDs), current balance between electron and hole transport regions is typically achieved by leakage of the major carrier through the devices or by accumulation of the major carrier inside the devices. Both of these are known to reduce performances leading to reduction of efficiency and operation stability due to exciton-polaron annihilation, etc. We found that hole diffusion in a centimeter-scale can be achieved in a PEDOT:PSS layer via composition and interface engineering. This ultralong distance hole diffusion enables substantially enhanced hole diffusion current in the lateral direction perpendicular to the applied electric field in typical organic optoelectronic devices. By introducing this lateral hole diffusion layer (LHDL) at the anode side of OLEDs, reduced carrier accumulation, improved efficiency, and enhanced operation stability are demonstrated. The application of the LHDL provides a third strategy for current balancing with much reduced harmful effects from the previous two approaches.

Fig. 1.. Design of the LHDL.

(A) Schematic view of carrier flows of an operating OLED driven by a DC power source; blue, red, and celeste arrows, respectively, represent directions of hole flows, electron flows, and currents; I_h and I_e are, respectively, the injected hole and electron currents at the electrode/organic interfaces. I_h′ and I_e′ are the leakage currents of hole and electron beyond the recombination zone, respectively. I_ext is the electric current across the OLED. Q_A and Q_h are hole charges at the surface of anode and in the device interior, respectively. Q_C and Q_e are the corresponding electron charges. (B) Theoretically calculated contributions of TTA and TPA to EQE loss in device C1 as a function of current density. Experimentally measured relative EQE is shown as symbol ★.(C) Side-view schematic diagram of an OLED with a lateral hole transport layer and (D) its top view. (E) Lateral resistance and (F) capacitance per unit volume characteristics of different types of PEDOT:PSS layers. (G) Top-view images of devices C1, C2, D1, and D2 under 6 V.

Fig. 2.. Impedance spectroscopy of PEDOT:PSS films.

(A) Schematic diagram of lateral real versus imaginary impedance spectra of reflective or transmissive finite-length linear diffusion and semi-infinite linear diffusion in theory and (B) their boundary conditions; here, ρ represents the charge concentration. Experimental impedance spectra of (C) PE4083/PE8000 and (D) PE4083-MeOH/PE8000 layers. The insets show their equivalent circuits and fitting parameters.

Fig. 3.. Lateral hole diffusion behaviors.

(A) Transient EL responses of devices C2 and D2 and (B) their transient current characteristics; the voltage pulse width and heights were, respectively, 48 μs and 6 V. Capacitance-voltage characteristics of (C) devices C1 and D1 and (D) devices C2 and D2, and the different colors of spheres show their EL onset, respectively. (E) Current-voltage characteristics of devices C1, C2, D1, and D2. (F) Current-voltage characteristics of single-carrier devices for device C2 and D2 (-e and -h represent electron-only device and hole-only device, respectively). (G) Schematic diagrams and (H) images of devices D2-0, D2-1, and D2-2 operating at 4 V (the substrate size is 2 cm by 2.5 cm). (I) Resistance-dominant and (J) capacitance-dominant equivalent circuit models for device D2. a.u., arbitrary units.

Fig. 4.. A working mechanism of an OLED with lateral hole diffusion.

(A) Applying bias voltage across the anode and cathode. (B) Carrier injection in injection area. (C) Asymmetric hole and electron transport abilities. (D) Hole lateral diffusion. (E) Electrical field formation in diffusion area (i.e., area outside ITO-cathode overlapping region). (F) Carrier redistribution and injection in diffusion area. (G) Exciton recombination radiation.

Fig. 5.. OLEDs with LHDL and ITO array anodes.

(A) Schematic diagram of the ITO grid array anode and the corresponding OLED. (B) Photographs (top) of operating device D2AA′ with the ITO grid array anodes. Bottom shows magnified images from the green circles marked in the top picture. (C) Schematic diagram of anisotropic PEDOT:PSS film. (D) Equivalent circuit model of device D2AA′. (E) Schematic diagram of the control devices C2LA and D2LA and (F) image of the operating device D2LA. (G) Current-voltage-luminous flux characteristics, (H) EQE-current characteristics and normalized EL spectra, and (I) normalized luminous flux degradation Φ_v(t)/ Φ_v(0) [Φ_v(0) = 0.38 lm] of devices C2LA, D2LA, and D2AA′ with an emitting area of 1.2 cm². (J) Current-voltage-luminous flux characteristics, (K) EQE-luminous flux characteristics, and (L) EL spectra of white device WDAA′ with an emitting area of 1.2 cm²; inset shows its photographs.