Understanding LTPO OLED Display
Recently there is news that the high-end iPhone 12s series released later this year will not only use a 120Hz high refresh rate screen but also use an LTPO OLED panel which can dynamically adjust the display refresh rate.
LTPO is not a new technology, as early as in the Apple Watch Series 4 smartwatch, Apple used the adaptive refresh rate LTPO OLED panel from LG for energy-saving and AOD function. The high refresh rate on cell phones has become popular, but the more power-efficient adaptive high refresh rate display panel has been slow to catch on, with only Samsung’s own top flagship Galaxy Note 20 Ultra and the recently released S21 Ultra in use.
Is it that hard to achieve a dynamic variable refresh rate display on a phone’s OLED panel?
Before talking about LTPO technology, let’s take a brief look at LTPS technology, which is very mature in OLED panels today.
The LTPS is called Low-Temperature Poly-Silicon, which is essentially a semiconductor material (technology).
OLED is a current driver device, in the birth of a period, the pixel density (PPI) is not particularly large, to enhance the panel resolution in a limited space, it is necessary to increase the open-state current of the amorphous TFT device to drive OLED in a limited space, but this, in turn, will increase the size of the TFT device – and the LTPS material allows the TFT device to be driven by the current. The LTPS material allows for higher mobility of the TFT device, which allows for miniaturization of the TFT device while enhancing the open-state current release capability.
Besides, a high refresh rate can be achieved by increasing the charging speed of the capacitor inside the TFT device. However, the LTPS material TFT device has another major problem – the capacitor is prone to leakage and needs to be constantly charged to compensate for the loss of potential during the display process, so the LTPS OLED is not able to achieve low-frequency drive from the hardware, which also increases the power consumption of the whole panel to some extent.
On the other hand, if you want LTPS OLED to achieve a higher refresh rate and higher resolution, it must increase the panel’s panel drive burden for each pixel (high refresh rate means shorter charging speed for each capacitor, larger resolution means more capacitors need to be charged at the same time), so the LTPS OLED panel that can achieve both high resolution and high refresh rate There are not many LTPS OLED panels that can achieve both high resolution and high refresh rate.
LTPO can be seen as an advanced version of LTPS, which can solve the aforementioned shortcomings to a certain extent.
The full name of LTPO is Low-Temperature Polycrystalline Oxide, low-temperature polycrystalline oxide, but is a low-temperature polycrystalline silicon and oxide hybrid technology, which has the advantages of LTPS but also has the advantages of oxide materials.
From a structural point of view, simply put, LTPO is the LTPS and oxide semiconductor integrated into the same pixel, LTPS and oxide these two TFT devices, the former mainly for OLED drive, while the latter is mainly used for switching, both to retain the high mobility characteristics of the original LTPS, oxide semiconductor TFT devices and weaken the LTPS The former is mainly used for driving OLEDs, while the latter is mainly used for switching.
In the structure of LTPO technology, LTPS devices and oxide devices can be flexibly switched at different stages to maximize the advantages of each, and switching to oxide devices in the light-emitting stage will enable low-frequency driving that cannot be achieved by ordinary LTPS.
And in the OLED panel display static screen, switch to oxide semiconductor devices can maintain the electrical signal in the capacitor (basically no leakage), when the screen changes, you can switch to LTPS devices, the electrical signal in the capacitor continues to LTPS devices, without frequent replenishment, to achieve a dynamic refresh rate display. While avoiding the LTPS high refresh rate display low frame. That enables dynamic refresh rate display and avoids the unnecessary power consumption caused by frequent charging of capacitors when LTPS displays low frame rates.
However, the LTPO process is relatively more complex than LTPS, and at this stage, the related OLED panels are hard to manufacture, expensive, and have a low yield, so not many models can be used. Although LTPO technology is not exclusive to Samsung, today it is Samsung’s process is the most mature, and currently on the market using LTPO OLED panel cell phones are few, Galaxy S21 Ultra is the first one this year. In addition to the future iPhone 12s series, which other manufacturers this year, especially Chinese companies can get Samsung to display “care” the first to use the LTPO OLED panel? We’ll see.