Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/N, N’-diphenyl-N, N’-(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD) (90 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turn-on voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.
©2005 Optical Society of America
Transparent organic light-emitting devices (TOLEDs) or top-emitting organic light-emitting diodes (TEOLEDs) have become increasingly interesting in recent years because of their technological potential to multi-color self-luminous, high-resolution, high-contrast and high aspect ratio active-matrix organic light-emitting device (AMOLED), and OLED microdisplays. TOLED can be fabricated on opaque substrates such as silicon wafers and on glass with thin-film transistor active-matrix array with high aspect ratio. The transparent top electrode usually consists of transparent conducting oxides, thin metals or other combinations. But the conducting oxides, such as indium-tin-oxide (ITO) thin film prepared by sputtering will cause the radiation damage inflicted on the organic layer stack during the deposition. In order to minimize the radiation damage, a thin layer of MgAg , Ca [2,3], copper phthalocynaine (CuPc) [4,5] or SiO:Al [6,7] deposited by thermal evaporation were previously used as a protective buffer layer before sputtering ITO conducting layer. On the other hand, some semitransparent cathode materials, such as LiF/Al/Ag [8,9], Ca/Ag , Ca/Mg [11,12] and Sm , were used in TOLED or TEOLED. In order to enhance optical output, a dielectric index-matching (or antireflection) layer with optimal thickness was thermally evaporated on the top of the semitransparent metal electrode. These dielectric materials include Alq3 [8,13], ZnSe [11,12] TeO2 [9,14].
In this paper, we fabricated TOLED based on ITO glass substrate and a transparent stacked cathode of LiF/Mg:Ag. The device has structure of ITO/N, N’-diphenyl-N, N’-(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD) (90 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where TPD is the hole-transporting layer, and 80 nm Alq3 is the emissive as well as electron-transporting layer, the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer  to optimize optical output. LiF/Al is a well-known cathode for OLED. Recently, we reported improved OLED with LiF buffered low work function Mg:Ag as cathode . In this paper, a high performance TOLED with LiF/Mg:Ag will be reported. The schematic cross-sectional diagram of the TOLED structure is shown in Fig. 1. The light emits from both top and bottom side of the devices.
2. Experimental details
The TOLEDs were fabricated on lithographically patterned ITO coated glass substrate. The ITO layer was about 60 nm thick with a sheet resistance of about 50 Ω/square. The routine cleaning procedure included ultra-sonication in acetone, ethanol, rinsing in de-ionized water and isopropyl alcohol, and finally irradiated in an oxygen plasma chamber. After the oxygen plasma treatment, the ITO substrates were transferred to the main chamber under high vacuum for devices fabrication. The main chamber is equipped with ten sources, each of which is heated by a tantalum heater. The opening and closing of shutters control the deposition sequences. The deposition rate and film thickness are measured by a quartz oscillator connected to a frequency meter. In order to obtain large-area uniformity and abrupt interface, the chamber is equipped with three sets of shutters, i.e. besides the shutters for each crucibles, there is also a big shutter between the crucibles and substrates, and a small shutter under each substrate. The thickness/rate crystal sensor is installed in the center of the substrate holders, which is designed in a planetary rotation, and the rotation rate can be adjusted. The organic films, layer by layer, were deposited on the ITO substrate surface. After deposition of the organic layers, the top cathode was prepared by sequential deposition of a 0.5 nm LiF layer and a 20 nm Mg:Ag (10:1 mass ratio) overlayer without breaking the vacuum. The chamber pressure was below 2×10-4 Pa during deposition of the organic materials and the metals. The devices were transferred directly from the main chamber to the glove box without exposing to atmosphere. The devices were encapsulated in the glove box filled with nitrogen gas. EL spectra of the fabricated devices were measured with a PR650 Spectra Scan spectrometer. Luminance - current density - voltage (L-I-V) characteristics were recorded simultaneously with the measurements of the EL spectra by attaching the spectrometer to a programmable Keithley 236 voltage-current source. The transmittance of the ITO glass substrate and TOLED were characterized by UV-visible scanning spectrophotometer (UV-2501PC). All measurements were carried out at room temperature under ambient atmosphere.
The film of LiF (0.5 nm)/Mg:Ag (20 nm) was also fabricated on bare glass substrate for resistance measurement. Like the procedure for the TOLEDs, the sample was also transferred directly from the main chamber to the glove box without exposing to atmosphere. The measurement was done in the glove box with Nitrogen gas. The sheet resistance of the film is about 10 Ω/square measured by a digital multi-meter, which is about one-fifth of the ITO sheet used in this experiment.
3. Results and discussion
Figure 2 shows the photographs of the device, which were taken from bottom ITO anode side (a) and top metal cathode side (b), respectively. The device is driven by a DC voltage of 6 V. Visually, the green light emitted from ITO glass substrate side looks a little brighter than that from the top metal cathode side. The light output from top metal side can be greatly enhanced by replacing the ITO anode with a thick layer high work function, high reflectivity metals, such as Ag, Ni, et al. The light output from bottom can be reflected back by the bottom metal mirror and output from the top.
Current density-voltage (I-V) and luminance-voltage (L-V) characteristics of the TOLED are shown in Figs 3(a) and 3(b), respectively. It can be seen that both I-V and L-V curves are approaching to each other when measured the light output through ITO glass substrate and through the top cathode side. The device has a lower turn-on voltage of about 2.8 V. That is due to inserting a thin layer of LiF between Alq3 and Mg:Ag cathode, which is expected to liberate more Li because of its smaller overall heat of formation in chemical reaction of Mg+2LiF→MgF2+2Li+109.62 kJ/mol than that of Al+3LiF→AlF3+3Li+340.37 kJ/mol , and the work function of Li (2.9 eV) is much lower than that of both Al (4.3 eV) and Mg:Ag (3.7 eV), so the electron injection barrier is reduced. In the standard device with a configuration of ITO/TPD/Alq3/Mg:Ag, the hole injection barrier between ITO and TPD interface is only 0.2 eV, although Mg:Ag (10:1) alloy has a low work function of 3.7 eV. However, there still exists a higher electron injection barrier of 0.6 eV between Alq3 and Mg:Ag cathode. The injection of hole is much easier than electron. So the majority carrier is hole, and the minority one is electron in the device. In addition, the mobility of hole in TPD is two orders higher than that of electron in Alq3, so the hole current is much lager than electron current, the injection electron and hole is unbalanced, resulting in low luminance efficiency of the device. The lower turn-on voltage and higher efficiency of the device mentioned above should be due to reducing election injection barrier and enhanced electron injection, which makes the electron and hole injection current is more balanced.
At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A respectively. Figure 4 shows the current efficiency, for bottom emission and top emission, versus voltage of the TOLED. It can been seen from Fig. 4 that, the current efficiency is decreased by about 1 cd/A for top emission compared to that of bottom emission, which is due to the higher absorption of the cathode film than ITO film.
Figure 5 shows the transmittance spectra of the TOLED and ITO glass substrate used in this investigation. The transmittance spectrum of TOLED was obtained by measuring the transmittance of whole encapsulated OLED stack with that of the encapsulated ITO glass subtracted with the same multi-layers organic materials and LiF. So the TOLED transmittance in Fig. 5 refers to the transmittance of the only metal cathode, but excluding the ITO glass, organic layers and LiF. The overall transmittance of the ITO glass substrate is over 80 %, and the TOLED is about 60% in the wavelength range of 400 nm to 800 nm. At 530nm, the emission peak wavelength of Alq3, the transmittance of the ITO glass substrate and whole TOLED is 82%, and 54% respectively. These results indicate that approximately 40% of the optical output is through the top cathode side and 60% through the ITO glass substrate, which agrees with the results obtained in Fig. 4.
In conclusion, TOLEDs based on ITO glass substrate and a transparent stacked cathode of a LiF/Mg:Ag were investigated. The TOLED has lower turn-on voltage of 2.8 V and higher current efficiency are due to enhanced electron injection by using LiF/Mg:Ag cathode. The device has high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. Such a TOLED is applicable in AMOLED on glass with high aspect ratio and in OLED microdisplay using single crystalline silicon as the backplane.
The sponsorship from Honeywell Foundation is gratefully acknowledged.
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