While many love OLED displays, they are by far not the most efficient displays possible. Even if you remove E-Ink and Mirasol displays from the list, they are still not the most efficient, not even close.
The most efficient display type is an LCD of all things, but a specific subset of LCDs. Those being standard transflective and transflective memory LCDs. And within that subset, WOLED backlight. But there is even a step beyond that increases efficiency by orders of magnitude.
Being both a smartwatch and watch guy, I pull many things learned from the watch world over to my approach on smartwatch design. The DarkVision display concept is one of those. The DarkVision display utilizes the benefits of WOLED backlight, transflective LCDs and something somebody that's not a watch guy would never think of. That being the use of lumino-phosphorescent material (A.K.A. lume).
The addition of lume allows for high contrast, large font information to remain visible for an hour or more with absolutely no backlight. If no power is used light the display, the advantages are fairly obvious. You can either reduce the battery size, (making the watch thinner), or increase battery life.
The physical design of such a display is as follows;
LCD > Lume > WOLED backlight
WOLED backlighting is used for it even lighting and broad range of levels. WOLED backlight can also be made as thin as you standard OLED display which offsets the thickness of the lume layer.
The lume as applied directly to the read surface of the LCD panel at a thickness of approximately 0.5mm and sits between the backlight and LCD. When the backlight is used it charges the lume layer causing it to glow. This glow acts as a backlight for the LCD in low and complete darkness environments. The average amount of backlight on time to achieve about one hour of usable glow is approximately 10-15 seconds using off the shelf lume. Something along the lines of Super-Luminova should achieve better results.
Another advantage by using a WOLED backlight, the backlight could periodically be used to recharge the lume layer throughout the night. There are two ways this can be achieved. The first is a long low level on period every 30 minutes or so. Another is have the backlight do shorter brighter cycles ever hour. Either way, this uses far less battery than what Ambient mode does on current AMOLED smartwatches.
While not having the proper facilities to do this, I can demonstrate it using a simple brushed on method to a standard LCD. The drawback here is consistency over such a large area is difficult and the standard backlight causes hotspots.
All the images except the first one are 15s f/2 800ISO exposures every 10 min. This specific exposure was used as it's still well above the limit of dark adjusted vision for the average person. The images are unadjusted. To get them to match what I physically see with dark adjusted eyes, they would actually be brighter than what's seen here.
The face used is probably the worst I could have used do to it's fine details. The advantage is it shows that even fine details remain visible for a pretty long time.
The color cast can be compensated for by using a color balancing layer between the LCD panel and lume layer. Without access to a color meter and means to make a proper filter, it does have a slight green cast when backlit.
I used an Omate Racer for this as it one of only three transflective display smartwatches I own. The others being a Sony SW2 and an inWatch Z.