Edit 2019: With the release of the Series 5, Apple’s strategies w.r.t. LTPO OLEDs are becoming clearer. LTPO OLEDS are becoming the driving force behind always-on displays with newer Apple Watches

For those who might have the keen eye, you might have observed that the new Apple Watch Series 4 has something called the LTPO-OLED. For those who might have missed it, here’s a screen-cap:

Apple’s watch comparison

Figure: Watch Feature Comparison [Source : 0]

Section 1: What are backplanes

Before we can understand what the LTPO OLED really is, we need to understand the current state of OLED displays.

When high resolution/large size OLED displays are involved, as in the case of high resolution Televisions and the increasingly common high resolution OLED displays on iPhones, there is a need to drive the large number of pixels correctly.

This is accomplished by something called the Backplane which is a Thin Film Transistor(TFT) layer that activates the OLED pixels and control the current going to the OLED layer which then determines what they show and at what brightness. This is because unlike LCD’s, OLED’s produce their own light and so the Backplanes need to take on the dual role of LCD’s Backplane and the Backlight by providing both the control voltage and the driving voltage to the OLED matrix. As a result OLED backplanes play a pivotal role in the OLED technology.

Amoled vs Amlcd

Figure: AMOLED vs AMLCD [Source : Google Images, Images might be subject to copyright]

On the other hand, LCD TFT backplanes let the light from the backlight go through and apply a controlling voltage to the Liquid Crystal Layer to determine what is shown on the screen. In essence, LCD’s backplanes have a lesser responsibility than their OLED counterpart as the ‘pixel-activation’ aspect is absent.

TL;DR Section 1:

It is important to note that Backplanes act as puppet masters for the displays that we see, silently synchronising millions of pixels a 60 or so times a second. On OLED they take on the additional responsibility of activating the OLED Pixels due to the absence of a backlight, whereas LCD screens, they only apply the controlling voltage to the Liquid Crystals. So any changes to this backplane technology plays a massive role in the overall performance and power utilisation of OLEDs.

Section 2: Current Backplane technologies

The major three Backplane technologies used in both LCD and AMOLED include LTPS, IGZO and a-Si which are respectively Low Temperature Polycrystalline Silicon, Indium Gallium Zinc Oxide, and Amorphous-Silicon.

IV Backplanes

Figure: IV Backplanes [Source : 1]

The graphs above show the IV curves of the three types of most commonly used backplanes. Backplanes include transistors and Capacitors to drive the OLED panels, but in the simplest terms,

The negative X-axis shows the current leakage when the transistor is off. The lower the value here the better the power savings as the standby drainage for switched off pixels is less.

The positive X-Axis shows the current supply to the OLED layer above the Layer when the pixel is switched on. The higher the value the more the current is supplied to the OLED layer and the better looking/performing is the display. LTPS backplanes are best in this regard (owing to the inherently high electron mobility) but however drain some current even when the transistor is off.

Apple currently uses LTPS Backplanes in its iPhones [Source : 3]

Section 3: So, what is LTPO?

Put simply, LTPO, Low Temperature Polycrystalline Oxide is essentially a hybrid of LTPS and IGZO giving the high current when the transistor is on and the low power drain when the transistor is off.

Slide from IHS

Figure: IV Backplanes [Source : 1]

Therefore, we can expect greater power savings when utilising LTPO backplanes. This is in the ballpark of 15-20%. Thus, OLED’s power consumption (already less than LCD’s) can be further reduced, leading to larger screens at same battery life or longer battery life at same display sizes.

The catch?

Remember the TFT transistors in the backplanes we talked about earlier? The new LTPO-TFT Transistors need to be slightly bigger to have these significant gains. As a result this cannot scale pretty well for high PPI displays.

This makes perfect sense in the Apple Watch. Apple is applying this technology onto Apple Watch S4 which inherently has a lower pixel count and can benefit significantly from battery gains (owing to the relatively smaller battery size).

What’s next?

Apple has three patents related to LTPO (Source : IHS):

  1. Methods of protecting semiconductor oxide channel in hybrid tft process flow [Source : 4]
  2. Organic light-emitting diode displays with semiconducting-oxide and silicon thin-film transistors [Source : 5]
  3. Displays With Silicon and Semiconducting Oxide Thin-Film Transistors [Source : 6]

It is pure speculation by me at this point, but we might see LTPO-TFT OLED screens soon on iPhones. Apple might be testing the waters with the Apple Watch Series 4.


[0] Watch comparision

[1] IHS

[2] Nathan, A., Striakhilev, D., Chaji, R., Ashtiani, S., Lee, C., Sazonov, A., . . . Milne, W. (2006). Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays. MRS Proceedings, 910, 0910-A16-01-L09-01. doi:10.1557/PROC-0910-A16-01-L09-01

[3] Cambridge Paper on Backplanes

[4] IHS on Apple’s LTPO

[5] US Patent No. US20180061867A1

[6] US Patent No.US9129927B2

[7] US Patent No.US20150055051A1

[8] Nathan, A., Striakhilev, D., Chaji, R., Ashtiani, S., Lee, C., Sazonov, A., . . . Milne, W. (2006). Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays. MRS Proceedings, 910, 0910-A16-01-L09-01. doi:10.1557/PROC-0910-A16-01-L09-01

TL;DR: LTPO Displays might bring better battery savings(15-20%) over the current LTPS-TFT backplanes. Who knows, we might even see them in iPhones soon.

P.S.: As far as possible I have linked all the resources I have used to draw conclusions. If I have made some mistake/said something which I shouldn’t have, please feel free to correct me.