In this blog post, we will take a closer look at the concept of “aperture ratio,” which has a significant impact on the performance of OLED displays, and the problems that arise when the aperture ratio is low.
OLED (Organic Light Emitting Diode) refers to an organic material that emits light of a specific color when electrical energy is applied to the light-emitting layer of an LED. OLED has been attracting a lot of attention in recent display technology due to its excellent image quality, low power consumption, and thin and flexible display. In addition, OLED is a self-illuminating display that does not require a separate backlight, enabling thinner and lighter designs.
The most basic RGB-OLED consists of three subpixels that emit the three primary colors of light, red, green, and blue, to form a single pixel. The subpixels form a multilayer structure in order, consisting of a cathode that injects electrons, a light-emitting layer where electrons and holes meet to produce light, and an anode that injects holes. A thin-film transistor (TFT), which acts as a kind of valve for each subpixel, is located on the anode (+) side to block or allow the passage of current and regulate the amount of current. When all subpixels are turned off, black is produced, and when all are turned on, white is produced. By adjusting the amount of current to the subpixels and combining the amount of light appropriately, various colors can be expressed.
So what is the principle behind the light emitted from the light-emitting layer? The state in which the energy is lowest and the electrons are stable is called the “ground state.” When energy above a certain level is applied to the ground state, the electrons move from their original positions and become highly energized, which is called the “excited state.” Electrons in the excited state have a tendency to stabilize, so they return to the ground state. At this point, the electrons emit energy equal to the difference in energy between the excited state and the ground state, i.e., the energy that was applied to raise them from the ground state to the excited state. When TFTs allow current to flow, the electrons in the excited state move toward the positive electrode, while the holes move toward the negative electrode, and they meet in the light-emitting layer. In the light-emitting layer, the electrons combine with the holes and stabilize to become in the ground state, and most of the energy difference between the excited state and the ground state is converted into light energy.
The color of light emitted by each subpixel is determined by the “band gap” of the organic material in the light-emitting layer. Band gap refers to the difference in energy levels between the highest energy orbit (HOMO) filled with electrons and the lowest energy orbit (LUMO) that can be filled with electrons. When energy greater than the band gap is applied to electrons in the ground state in HOMO, the excited electrons move to LUMO and combine with holes. The electrons then release energy and return to the ground state, emitting light with a wavelength corresponding to the band gap. The larger the band gap, the more energy is required to emit light, so organic materials with large band gaps have shorter lifespans than those with small band gaps.
OLEDs are divided into backlighting and front lighting depending on where the light generated in the light-emitting layer in the middle is emitted. When the light is emitted toward the anode, it is called backlighting, and when it is emitted toward the cathode, it is called front lighting. In the case of backlighting, the cathode must act as an electron injection and reflection layer, so a mixture of silver and magnesium, which have low work functions and are opaque, is used. On the other hand, a compound of indium and tin oxide (ITO), which has the opposite properties, is used for the anode. However, when light passes through the TFT located at the cathode, some of the light is blocked by the TFT and cannot escape, resulting in a decrease in the aperture ratio. The aperture ratio is the ratio of the area from which light can actually escape to the total area of the unit pixel. When the aperture ratio is high, the amount of light emitted is greater when the same current flows, resulting in higher brightness. For this reason, a decrease in the aperture ratio leads to a decrease in brightness, and in order to achieve a certain image quality, it is necessary to emit light that is brighter by the amount of brightness lost, which has a negative effect on the life of the organic material.
In order to increase the aperture ratio, front-emitting light that emits light toward the cathode without TFTs requires the use of metals such as gold or platinum, which have high work functions and can act as a reflective layer, on the anode, and a highly transparent material on the cathode. However, when ITO is used on the cathode, the work function is high, making it difficult to easily release electrons. Ultimately, a metal with low work function and high transparency must be used for the cathode, and to increase transparency, the metal must be made thin. However, if the cathode is thinner than a certain thickness, the surface resistance increases, and when the resistance increases, the voltage generated at each position of the panel becomes different, resulting in a side effect of reduced screen uniformity.
A typical solution to this problem is to use the micro-resonance phenomenon. Some of the light generated in the light-emitting layer escapes through the translucent cathode, but some is reflected by the cathode toward the anode, where it is reflected again. The reflected light interferes with each other, causing a micro-resonance phenomenon. Due to the micro-resonance phenomenon, when waves with the same phase meet, constructive interference occurs, strengthening the intensity of the waves, and when waves with opposite phases meet, destructive interference occurs, weakening or eliminating the waves. Through this micro-resonance phenomenon, the intensity of light increases, resulting in higher brightness. As a result, high currents are not required to improve brightness, which extends the life of OLEDs. In addition, only wavelengths that match the conditions are reinforced, and wavelengths that do not match the conditions are canceled out, narrowing the spectrum and increasing color purity.
