A stacked Al/Ag anode for short circuit protection in ITO free top-emitting organic light-emitting diodes

Abstract

Aluminum and silver (Al/Ag) stacked films are utilized as the anode in ITO free top-emitting organic light-emitting devices (TEOLEDs). Serious short circuit issues can be resolved since the stacked metal films can increase the crystallinity and smoothen the surface morphology to suppress the poor infiltration between pure Ag and glass substrates. Optical simulations are carried out based on a transfer matrix method and microcavity effect to guide the real fabrications of the fluorescent TEOLEDs. The stacked Al (56 nm)/Ag (44 nm) anode based TEOLEDs demonstrate a better device performance than that of the Al-only anode based devices. The proposed stacked metal electrode provides a simple and convenient way to fabricate TEOLEDs with suppressed electrical short circuits.

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Introduction

Organic light-emitting diodes (OLED) are very promising for next generation displays and solid-state lighting owing to their broad viewing-angle, fast response, low driving voltage, and flexibility.^1–3 Currently, they have been applied in mobile phones and digital electronic products. Further development towards active full color displays, silicon-based micro-displays (OLED on Silicon, OLEDoS),⁴ transparent and flexible displays,⁵ and large-area OLED⁶ lighting panels are envisaged. Currently, Active Matrix OLED (AMOLED) driven by thin film transistor (TFT) technology^7,8 is the main trend in the field of OLED displays. If conventional bottom emitting OLED are adopted, the pixel aperture rate will be reduced when the light emerges from the glass substrate side since it is blocked by the wired metal line and the opaque TFT.⁹

Top-emitting OLEDs (TEOLED) are fabricated on arbitrary substrates like silicon wafer and glass with a transparent or semi-transparent top cathode, e.g., transparent indium tin oxide (ITO) and semi-transparent Ag thin film. In existing TEOLED displays, ITO/Ag/ITO¹⁰ structure is usually taken as the transparent cathode (thin Ag film) or bottom reflecting anode (thick Ag film), which can cause some issues due to the processing complexity by two turns of ITO sputtering. In addition, indium is a very scarce and expensive element on the earth. Moreover, the high energy sputtering technology will damage the organic functional layer. Therefore, it is very important to explore abundant and cheaper electrode materials instead of ITO in OLEDs.

In visible wavelengths, Ag film has a better reflectance than Al.¹¹ However, Ag has poor infiltration on normal glass substrates. It will result in a larger film roughness compared with Al.¹² Rough Ag electrode can cause a high probability of point discharge and short circuit. Point discharge at anode is more severe than that at cathode due to the corona discharge polarity effect.¹³ To get smooth morphology, thick Al₂O₃ film is sputtered on the glass as the buffer/cushion layers for attachment enhancement using Ag in inorganic devices.¹⁴ Thick SiO₂ and/or SiO₂/ITO are usually used as cushion layer in TEOLEDs.¹⁵ Nevertheless, processing complexity with high cost is an issue. This work developed stacked Al/Ag electrode as the anode to fabricate TEOLEDs without extra processing procedure. Severe short circuit can be resolved since the stacked metal films can increase the crystallinity and smoothen the surface morphology to overcome the poor infiltration between pure Ag and glass substrate. Optical simulations are carried out based on transfer matrix method and microcavity effect to guide the real fabrication of the fluorescent TEOLEDs. Accordingly, the stacked Al/Ag anode based TEOLEDs demonstrate a better device performance than that of the Al-only anode based devices.

Experimental

The total thickness of organic functional layers with resonant wavelength λ = 528 nm is decided based on optical simulations program based on transfer matrix method. Then we select the thickness of every layer and assume that the light emitting position located on the interface between luminous organic functional layers. The device structure is listed in Fig. 1. MoO₃ works as the hole-injecting layer, N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (NPB) as the hole-transporting layer, 8-hydroxyquinoline, aluminum salt(III) (Alq₃) as the luminescence functional layer, 4,7-diphenyl-1,10-phenanthroline (BPhen) doped with Li as the electron-injecting and transporting layer for reducing the injecting barrier and enhancing the conducting ability, 10 nm BPhen as a spacing layer to induce quenching effect from Li diffusion into the light-emitting layer. Considering the transmittance and conductivity, 20 nm Ag was used as the top cathode. The samples were fabricated on normal optical glass substrates without ITO. The active area of each device is 3 × 3 mm². The current–voltage (I–V) and luminance characteristics were measured using Keithley 2400s source meter and Photo Research PR655 spectrophotometer controlled by a software system. The surface morphologies of the metal films were characterized by atomic force microscopy (AFM, MultiMode V, Veeco Instruments Inc.).

Fig. 1 Device structures of TEOLEDs. (a) Al anode; (b) stacked Al/Ag anode.

Results and discussion

Simulations

We tried stacked Al (50 nm)/Ag (50 nm) as the reflecting anode to fabricate green OLEDs of Alq₃ and found the way for avoiding the short circuit issue. Based on this strategy, the optimization of the Al (100 − x nm) Ag (x nm) thickness is carried out by transfer matrix simulation for the maximum reflectance of Ag reflecting side. The energy reflectance R, transmittance T and reflecting phase shift φ of equivalent interface can be expressed as¹¹where Y = C/B, B, C can be obtained by optical transfer matrix calculations as follows:
η₀, η_j, η_k+1 are the optical admittance of incidence dielectric, the j layer and emergence dielectric, respectively. For convenience, we only discuss the normal incidence and emergence because the optical admittance is reduced to the union form of refractive index for the s and p polarization wave. , the effective phase thickness of the j layer of dielectric, and N_jd_j, the optical thickness.

The refractive index of the metal is obtained by fitting the experimental data from the handbook of solid optical constant of Edward D. Palik's.¹⁶ The relative dielectric constant of metal can be described as¹⁷

where, ω_p is the bulk plasma frequency of metal, γ₀ the damping coefficient of free electrons, f₀ the ratio of free electrons in an atom; ω_j the intrinsic frequency of bound electrons, γ_j the damping coefficient, f_j the ratio of bound electrons, K the total kinds of bound electrons, and Z the total electrons in an atom. They meet the following condition:

The refractive index is the square root of dielectric constant and it is a complex number for metals.

We optimize the x value in Al (100 − x nm) Ag (x nm) through the MATLAB program.¹⁸ It is found that the optimized thickness is Al (56 nm)/Ag (44 nm) with the maximum reflectance of 93% on Ag inside surface at λ = 528 nm (the wavelength of Alq₃ based bottom-emitting green OLED) in Fig. 2(a). The reflectance is even larger than that of 100 nm pure Ag (92.5%) and 100 nm pure Al (87%) in Fig. 2(b).

Fig. 2 (a) The inside reflectance of Al (100 − x nm) Ag (x nm) anode, x = 0–100 nm (at λ = 528 nm); (b) the inside reflectance of 100 nm Al anode and Al (56 nm) Ag (44 nm) anode vs. wavelengths.

The optimized anode structure of glass/Al (56 nm)/Ag (44 nm)/organic is used to calculate the relative parameters for optical design and the dielectric constant of organic is assumed to be 1.7. The calculated inside reflecting phase shift is φ₁ = 2.1370 rad. The cathode structure is organic/Ag (20 nm), and the calculated inside reflecting phase is φ₂ = 1.7995 rad.

The microcavity effect exists in TEOLEDs because of a high reflectance of the top/bottom electrodes which forms a Fabry–Pérot optical resonator.¹⁹ The change of dielectric thickness of cavity is selective for emergence light color. We can enhance the luminescence intensity by regulating the cavity dielectric thickness when it matches the resonant peak wavelength of the cavity. The cavity diagram is listed in Fig. 3(a). Where, T₂ is the transmittance of top electrode, A₂ the absorbance of cathode, R₁ and R₂ the inside reflectance of two electrodes, I₀ and E₀ the luminescence intensity and electric field of the dipole in free space, I₂ and E₂ are those of existing microcavity, L₁ the optical path length from the dipole to the reflective anode, L₂ the optical path length of the dipole to the reflective cathode, L the optical path length of total microcavity. The emergence light intensity from the semi-transparent cathode can be expressed as eqn (2).

Fig. 3 Schematic diagram of (a) optical microcavity; (b) wide-angle interference; (c) multi-beam interference.

In eqn (2), the numerator represents the wide-angle interference as shown in Fig. 3(b), and the denominator represents the multi-beam interference in Fig. 3(c). Where, n_i/d_i is the refractive index and thickness of the i layer of organic material in the microcavity, n_j/d_j those of closed to the bottom anode, λ wavelength of the emergence light, φ₁(λ)/φ₂(λ) the inside reflecting phase shift of the two electrodes. From eqn (2), the emergence light reaches its maximum when constructive interference occurs. It should meet the conditions as follows:

where, m, m′ = 0, ±1, ±2…, λ is the resonant wavelength. Eqn (3) represents the constructive multi-beam interference of the total cavity, which can determine the thickness of the microcavity. Eqn (4) indicates the wide-angle constructive interference of the radiation dipole in the cavity, which can determine the position of the dipole from the two electrodes. We use φ₁, φ₂ to replace the value of eqn (3) and (4), and take m = 0 to decide the cavity length parameters of the TEOLED.

Morphology

Short circuit issue is commonly existed in Ag based TEOLEDs. We cope with this problem with great efforts. Al is pre-deposited on glass substrate for its good infiltration. Then Ag metal film is evaporated on Al film getting good crystallinity which exactly provides the smooth Ag surface to alleviate the electric field of point discharge further avoid the short circuit issue. What's more, the good reflecting performance on Ag surface of the stacked Al/Ag anode is just like pure Ag anode without weakening it.

To validate our experimental design, the surface morphology of different metal films are evaluated by AFM as shown in Fig. 4. Pure Al film is very smooth with a RMS (Root Mean Square) roughness of 1.32 nm. It denotes the good crystallinity. But Ag film is very rough with RMS roughness of 11.9 nm. The roughness of stacked Al/Ag film is greatly improved with that of 3.57 nm to overcome the short circuit problem thoroughly.

Fig. 4 AFM images of surface morphology of metal films of (a) Al (100 nm); (b) Al (56 nm)/Ag (44 nm); (c) Ag (100 nm).

OLED performance

The electroluminescence (EL) characteristics including the luminance, J–V curves, current efficiency, luminous power efficiency, and EL spectra are illustrated in Fig. 5 and 6. The fabricated device based on Al/Ag anode exhibited higher performance than that of the Al anode. In details, current efficiency increases by 15%, power efficiency increases by 25%, and driving voltage decreases from 5.2 V to 4.6 V at 40 mA cm⁻². Compared with conventional bottom-emission OLEDs (3-4 cd A⁻¹), the efficiencies are two times improved owing to the microcavity effect of TEOLEDs. There is no large difference between the work functions of Ag (4.26 eV) and Al (4.28 eV). We attributed the improved efficiency to the following two factors: (1) the conductivity of Ag is larger than that of Al, which leads to lowered driving voltage and improved power efficiency; (2) the reflectance of thick Ag is higher than that of Al, which enhances the gain of the microcavity. It can be verified from the full width at half maximum (FWHM) of the EL spectra, i.e., 49 nm for Al anode based device and 42 nm for stacked Ag/Al anode based device. By the principle of microcavity, the higher reflectance of the anode, the larger of the gain and the narrower of the FWHM are.

Fig. 5 (a) Current density–voltage and luminance characteristics; (b) current efficiency and power efficiency, of different anode based TEOLEDs.

Fig. 6 EL spectra of (a) Al anode; (b) stacked Al/Ag anode, based TEOLEDs.

Conclusions

In summary, we have demonstrated stacked Al/Ag anode based TEOLEDs for avoiding the severe short circuit issue. Stacked metal film with good surface morphology originates from the film crystallinity of Ag on Al film buffer layer. It can suppress the poor infiltration between the pure Ag and normal glass substrates. Additional buffer layer like sputtering Al₂O₃, SiO₂, ITO, etc., is not necessary in whole device fabrication process. The stacked Al/Ag anode based TEOLEDs demonstrate a better device performance than the Al-only anode based devices owing to the higher reflectance and better conductivity of Al/Ag electrode. The proposed stacked metal electrode provides a simple and convenient way to fabricate TEOLEDs.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 61177016, and 61307036). This is also a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and by the Natural Science Foundation of Jiangsu Province of China (BK20130288).

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Footnote

† Electronic supplementary information (ESI) available: The calculation program of MATLAB for the reflectance of multi-layer optical film based on transfer matrix method and the LD model of dielectric function for various metals. See DOI: 10.1039/c5ra18132a

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