Analysis and discussion of OLED from four types

  1. Classification according to the formation process of the light-emitting layer

As the organic material of the light-emitting layer, it can be classified into two types according to the molecular weight of the organic compound used: a low molecular type compound and a high polymer type compound. There is no difference between these two types of organic compounds in terms of light-emitting principle, but there are some differences in the preparation process of electroluminescence (EL) layer.

1.1 Types of small molecules

The light-emitting principle of organic molecules was first discovered from low-molecular type organic materials. After early experimental research, it has entered the stage of practical application. Through the method of masking, the vacuum thermal evaporation process is used to make small molecular organic material thin films. Layers are deposited on the desired area. Figure 1 shows a multilayer structure of fluorescent and phosphorescent OLEDs, however, in practice, this method requires higher temperatures to convert the small molecule type of organic material to be deposited into a gaseous state, During this process, the metal mask is thermally expanded, resulting in a potential non-uniformity problem in the organic thin film layer formed. The effect of temperature is more severe for larger panel sizes (requiring larger metal masks). Therefore, at present, a method for enlarging an EL panel using a small-molecule-type organic material is considered to be difficult to achieve.

Figure 1 - Vacuum Thermal Evaporation Process Deposition
Figure 1 – Vacuum Thermal Evaporation Process Deposition

1.2. Polymer type

On the other hand, for organic materials of high polymer type, the mainstream preparation processes of EL panels include ink jet method and spin coating method by taking advantage of the high solubility of these organic materials in liquid state. Figure 2 shows the device structure of a polymer organic light-emitting diode (P-OLED). This inkjet method applies organic materials to the desired pixels, which can be precisely controlled down to micron-scale thin film layers. Compared with the method used for low molecular type organic materials, the method has higher material utilization rate, lower cost, and is more suitable for preparing large-sized light-emitting panels.

Figure 2- Device structure of (P-OLED)
Figure 2- Device structure of (P-OLED)
  1. Classification according to the luminescent mechanism of luminescent materials

Fluorescent OLEDs can only utilize singlet-to-singlet electron transitions to emit light, which account for up to 25% of the injected electrons. Therefore, 75% of the electrons undergo a non-radiative recombination decay process, which has a negative impact on the stability of the device.

In recent years, phosphorescent organic light-emitting diodes (PHOLEDs) have played an increasingly important role in the field of OLEDs, due to their superior luminous efficiency, making them more suitable for high-performance high-brightness displays and solid-state lighting, fluorescent small-molecule organic devices The quantum efficiency upper limit of 5% has been broken through the collection of singlet and triplet excitons of PHOLEDs to generate large numbers of photons. Iridium(III) and platinum(II) complexes are well known as phosphorescent light-emitting devices. Iridium(III) complexes have been shown to be the most effective triplet dopants for the preparation of highly efficient OLEDs, and Figure 3 shows the phosphorescent emission mechanism of phosphorescent OLEDs. To obtain phosphorescent OLEDs with high quantum efficiency, the excitation energy to excite phosphorescent light-emitting devices must be confined within the device itself. This has been achieved with multilayer device structures with wide-bandgap host and carrier transport materials as electron/hole injection and transport layers.

Figure 3 - Phosphorescence emission mechanism of phosphorescent OLEDs
Figure 3 – Phosphorescence emission mechanism of phosphorescent OLEDs
  1. Classification according to driving method

3.1. Passive matrix

OLED On a passive matrix OLED (PMOLED) panel, the current-carrying conductor wires are laid in the horizontal (X electrode) and vertical directions (Y electrode), and the current timing passes through the intersection of the two axes to make the OLED of the pixel light up, As shown in Figure 4. The passive matrix type, also known as the simple matrix type, has a relatively simple structure, thereby reducing the production cost of the panel, but it also has disadvantages. Since the time of supplying current is periodic and its brightness is limited, it is necessary to provide a Larger current to maintain a certain brightness. In addition, PMOLEDs have high power consumption, and have certain limitations in realizing high-resolution displays due to their inherently huge parasitic capacitance and high-impedance electrodes. The larger screen size and higher number of traced wires result in a significantly reduced duration of light emission per pixel, which leads to a number of problems, including the inability to guarantee the required lifetime. However, the PMOLED driving method is useful on panels with small size and low resolution (fewer scan lines).

Figure 4 - Low cost passive matrix array OLED, Figure 5 - Traditional pixel circuit for AMOLED including two thin film transistors and a capacitor
Figure 4 – Low cost passive matrix array OLED, Figure 5 – Traditional pixel circuit for AMOLED including two thin film transistors and a capacitor

3.2. Active matrix OLED

The active matrix driving scheme requires the construction of complex circuits. Compared with the passive matrix type, it has faster response time and higher resolution. It also has the advantages of lower driving voltage and lower power consumption, and even larger screen size. , and it provides a longer service life.
Active-matrix OLEDs (AMOLEDs) combine transistors to control each pixel individually and have a wide range of applications in high-resolution small to large-area displays. To power AMOLED, additional power lines and power control transistors are required at each pixel [18-8910 Therefore, at least two transistors and a storage capacitor (Cr) are required for each pixel of AMOLED, as shown in Figure 5 . The function of the thin film transistor (TFT) switch is to digitally control the input signal, and the function of the driving thin film transistor is to modulate the power supply of the OLED. The brightness of AMOLED increases monotonically with increasing current supplied by the TFT driver.

  1. Classification according to optical channel

4.1. Bottom or top glow

Bottom-emitting device structures use a transparent or translucent bottom electrode to pass light through a transparent substrate. The top light-emitting device structure uses a transparent or translucent top electrode to emit light directly. A major disadvantage of bottom-emitting device structures is that the light-emitting aperture and the electronics at the bottom share the same substrate, thus limiting the pixel aperture. The top light emitting device structure can avoid this disadvantage, because for the top light emitting device, ① the light escapes the device through the transparent cathode electrode and the encapsulation layer, ② allows a larger pixel aperture, ③ all electronic circuits can be placed on the bottom, top light emitting device Structured OLEDs are more suitable for active-matrix driving applications because they can be more easily integrated on an opaque transistor substrate.

4.2. Transparent OLED

Transparent OLEDs (TOLEDs) are the use of transparent or translucent contact layers on the bottom and top of the device to create displays that emit light from both the bottom and the top (transparent displays). Transparent OLED can greatly improve the contrast ratio, making it clearer in bright environments. The technology could be used in head-up displays, smart windows, or augmented reality displays. In 2010, at the annual global display exhibition held in Tokyo, Japan, a high-performance transparent white OLED display panel made of luminescent materials from Novale d of Germany was displayed. At the 2011 Consumer Electronics Show (CES-2011) in Las Vegas, Novale d and Universal Display demonstrated high light quality, color rendering index (CRI) of 95, transparent, bendable lighting and display products. Sony Corporation has invented a bendable and transparent OLED display. Transparent Active Matrix OLED (AMOLED) can be used for high-resolution, full-color display.

4.3. Inverted OLED

A unique selling point of OLEDs is the potential for highly transparent devices in the visible spectrum. This is due to the large Stark shift and low intrinsic absorption loss of organic semiconductor materials. As a result, new applications for displays and lighting become a reality, such as the integration of OLEDs into automotive windshields and ceilings. Compared with conventional OLEDs, the anode is placed on the substrate side, while for inverted OLEDs (IOLEDs), a bottom cathode is used, which can be directly connected to the depletion region of n-channel thin film transistors (TFTs). Transparent OLED device structures are particularly For low-cost amorphous silicon thin film transistor backplanes, it is widely used to make AMOLED displays. Above the bottom cathode are the electron transport layer, the light emitting layer, the hole transport layer/hole injection layer, and finally the anode. As shown in Figure 6, the light of the inverted OLED is collected by the semi-transparent cathode and then output. The main technical challenge in realizing fully transparent devices is the deposition of the upper electrode. To obtain uniform light emission over the entire viewing angle and reduce series resistance, transparent conductive oxide films (TCO) such as ITO are also commonly used as the top electrode. However, sputter deposition of ITO on top of the organic layer can cause damage to the organics due to the introduction of energetic particles and UV radiation. Meyer et al. reported a method for the fabrication of inverted OLEDs with effective protection of organic layers during RF magnetron sputtering of ITO. In the visible light region, its average transmittance reaches 80%. The structure of the inverted top-emitting OLED is shown in Figure 7. Chen et al. reported a novel inverted top-emitting OLED structure with Al q3/L i F/Al or Ag as the bottom electrode. To integrate OLED technology into standard silicon-based driver circuits, light emission from the top transparent conductive electrode is considered necessary.

Figure 6 - Structure of an inverted bottom-emitting OLED
Figure 6 – Structure of an inverted bottom-emitting OLED
Figure 7 - Structure of an inverted top-emitting OLED
Figure 7 – Structure of an inverted top-emitting OLED