Effect of hole transport layer thickness on OLED performance

Since the first use of aromatic diamine derivatives as hole transport materials and 8-hydroxyquinoline aluminum (Alq3) as the light-emitting layer material in 1987 [1], high-efficiency, high- brightness and low- driving- voltage organic light - emitting diodes ( OLEDs ) have attracted great attention due to their low power consumption, high brightness, wide viewing angle, fast response speed and many other characteristics. The research on organic electroluminescence (EL) has become one of the hot topics in the current luminous display field. People have conducted a lot of research work from luminescent materials, preparation processes, to luminescence mechanisms, device structures and other aspects. The photoelectric performance of the device has been significantly improved, but factors such as the device's luminous efficiency and brightness are still one of the bottlenecks hindering the commercialization of OLEDs.

In order to improve the optoelectronic performance of luminescent devices , this paper uses polyvinyl carbazole ( PVK ) as a hole transport material and Alq3 as an electroluminescent/electron transport material to prepare an OLED device with a structure of indium tin oxide (ITO)/PVK/Alq3/Mg:Ag/Al. The effect of the thickness of the hole transport layer on the optoelectronic performance of the device is studied, the thickness matching of the device functional layer is optimized, and an OLED device with optimized structure is obtained.

1 Experiment

1.1 Materials

In the experiment, ITO conductive glass (15 Ω/□) was selected as the anode material of the OLED device, high-purity metal magnesium (99.9%), silver (99.9%) and aluminum (99.999%) were selected as the cathode material of the device, PVK was selected as the hole transport layer material of the device, and Alq3 was selected as the light-emitting layer and electron transport layer material of the device. These organic materials were purchased from Aldirch Company in the United States, and their molecular structures can be found in the literature [1-3].

1.2 ITO substrate surface treatment

Before preparing the OLED device, the ITO substrate was cleaned with detergent, acetone solution, NaOH solution, ethanol solution, and deionized water ultrasonically for 20 minutes each, then dried with high-purity nitrogen gas, placed in the pretreatment chamber of the OLED-V organic multifunctional film-forming equipment, and treated with oxygen plasma at a voltage of 250 V for about 30 minutes.

1.3 OLED device preparation

PVK/chloroform solutions with different concentrations (2, 3, 6, 9 mg/ml) were spin-coated on clean ITO substrates at a speed of 4 000 r/min (time of 60 s) to obtain PVK films of different thicknesses (4, 15, 30, 60 nm). All PVK films obtained by spin coating were baked under vacuum conditions for about 30 min to remove residual solvents in the films. Finally, at a vacuum degree of 10?4 Pa, the organic layer Alq3, the alloy cathode layer Mg:Ag (10:1) and the metal layer Al were deposited in sequence by thermal evaporation. The alloy Mg:Ag was prepared by dual-source co-evaporation technology, and the evaporation rate and film thickness were monitored by a quartz crystal oscillator. The evaporation rates of the organic layer and the metal layer are 0.2~0.4 nm/s and 2~4 nm/s, respectively. The thickness of each functional layer and the structure of the fabricated OLED device are ITO/PVK (0~60 nm)/Alq3 (60 nm)/Mg:Ag (100 nm)/Al (150 nm).

1.4 OLED device performance Test

The voltage, current, brightness and spectrum of all unpackaged OLED devices were tested under atmospheric and room temperature (25°C) conditions using a KEITHLEY-4200 semiconductor tester, ST-86LA screen brightness meter and OPT-2000 spectrophotometer.

2 Results and Discussion

2.1 Effect of hole transport layer thickness on device current-voltage characteristics

Figure 1 shows the current-voltage (JV) characteristic curves of devices with different hole transport layer thicknesses. In the figure, letter A represents a single-layer device, and letters B, C, D, and E represent OLED devices with PVK thicknesses of 4, 15, 30, and 60 nm, respectively. As shown in Figure 1, the current density of all devices increases steadily with the increase of the driving voltage, and the current density does not decrease monotonically with the increase of the thickness of the hole transport layer PVK. When the forward bias is less than 8 V, the change in current density is not obvious as the applied voltage increases; when the applied voltage increases to a certain extent, the current increases rapidly. When V = 15 V, the current density J of devices A, B, C, D, and E are 7.2, 31.0, 30.3, 22.7, and 24.6 mA/cm2, respectively. At the same voltage V, the magnitude relationship of the current density J passing through the device is B>C>E>D>A. Since PVK is a hole transport material with excellent performance, its introduction effectively improves the recombination of carriers in OLED devices, making the current density of double-layer devices significantly greater than that of single-layer devices. At the same time, although the introduction of the hole transport layer PVK in double-layer OLEDs helps to improve the recombination of carriers in the device, the increase in the thickness of PVK increases the series resistance of the device. Therefore, only when the thickness of PVK is appropriate, the current density of the device is the highest.

Figure 1: JV curves of devices with different hole transport layer thicknesses

2.2 Effect of hole transport layer thickness on device LV characteristics

图2为所有OLED器件的L-V特性曲线，很显然，空穴传输层厚度明显影响器件的启亮电压和发光亮度。由图可以看出，器件A、B、C、D、E的启亮电压分别为7.2、4.5、4.4、4.7、4.6 V，其中器件C的启亮电压最小，而器件A的启亮电压最大。当正向偏压小于8 V时，器件的发光亮度L随外加电压V的变化不显著；当外加电压V大于8 V时，器件C、D的发光亮度L随V增大而迅速增强，但器件A的发光亮度变化不够明显。当外加电压V相同时，器件的发光亮度L存在较大的差别，如当V =15 V时，器件A、B、C、D、E的发光亮度L依次为75.7、1 805.4、2 408.1、1 503.8、1 722.6 cd/m2，相同电压时OLED发光亮度的大小依次为C＞B＞E＞D＞A。容易发现：器件C具有最高的发光亮度，当V =20 V时，L的值接近5557.5 cd/m2，远远高于其他的OLED器件。

Figure 2: LV curves of devices with different hole transport layer thicknesses

2.3 Effect of hole transport layer thickness on device η-V characteristics

The η-V characteristic curves of devices with different hole transport layer thicknesses are shown in Figure 3. It can be seen that the current efficiency η of all double-layer devices is significantly better than that of single-layer devices. When the applied voltage V = 13.5 V, the η values ​​of devices A, B, C, D, and E are 0.33, 1.77, 2.29, 1.96, and 2.08 cd/A, respectively, which means that the current efficiency η of the double-layer device is 5 to 6 times that of the single-layer device.

In addition, it can be seen from the figure that the overall change trend of η is that it increases rapidly with the increase of V. When V increases to a certain extent, η reaches the maximum value, and then η gradually decreases with the increase of V. The maximum current efficiency ηmax of devices A, B, C, D, and E are 0.39, 1.77, 2.42, 1.99, and 2.11 cd/A, respectively. Among them, device A has the smallest ηmax and C has the largest. The curve of the effect of the thickness of the hole transport layer PVK on the device ηmax is shown in Figure 4.

Figure 3: η-V curves of devices with different hole transport layer thicknesses

Figure 4: Curve showing the effect of hole transport layer PVK thickness on device ηmax

表1比较了各器件的主要性能指标，可以看出，一方面双层器件的性能明显优于单层器件，另一方面空穴传输层的厚度对OLED器件的光电性能具有显著影响。综合而言，器件C的性能最好，它具有最低的启亮电压、最高的发光亮度和发光效率。

Table 1: Comparison of performance parameters of various devices (click on the image to enlarge)

For bipolar injection OLED devices, the luminous intensity L is proportional to the number of electron-hole pairs, that is:

Where N and P are the electron and hole concentrations, respectively; qη is the efficiency of electroluminescence; and r is the proportionality coefficient. It can be seen that the greater the difference between N and P, the smaller the luminous brightness L; the closer N and P are, the greater the luminous brightness L; and when N=P, the luminous brightness L reaches the maximum value. According to the device energy level diagram shown in Figure 5 [4-10], for the device ITO/Alq3/Mg:Ag/Al, its electron injection barrier is 0.5 eV, while the hole injection barrier is 0.9 eV. Therefore, when the device is working, the injection of electrons and holes is extremely unbalanced, and the values ​​of N and P differ greatly. From formula (1), it can be seen that the photoelectric performance of this type of device is poor. However, for the device ITO/PVK/Alq3/Mg:Ag/Al, after the PVK hole transport layer is introduced, on the one hand, the hole injection barrier is reduced (0.7 eV), which improves the hole injection ability. On the other hand, a high blocking barrier is formed at the Alq3/PVK interface, which is conducive to limiting the electrons injected from the cathode in the light-emitting layer (Alq3) and effectively recombine with holes, thereby improving the recombination efficiency of carriers in the device and improving the performance of OLED. Therefore, the photoelectric performance of this type of device is significantly better than that of the device ITO/Alq3/Mg:Ag/Al.

Figure 5: Energy level structure diagram of the device

3. Conclusion

本文制备了一系列结构为ITO/PVK/Alq3/Mg：Ag/Al的有机发光器件，通过测试和分析器件的光电性能，研究了空穴传输层厚度对OLED器件性能的影响，优化了器件功能层的厚度匹配。实验结果表明，双层器件的光电性能明显优于单层器件，同时空穴传输层厚度对其光电性能也具有显著的影响。当空穴传输层厚度为15 nm时，双层器件有较佳的器件性能，其起亮电压最低，发光亮度和发光效率最高。