There are two ways to make LCD screens more visible in the presence of bright light (sunlight, etc): Reflect the light, or overpower the light with even brighter light.

A purely reflective LCD is pretty difficult to see in the dark, so there are not many of these other than your typical alarm clock, etc. "Transmissive" displays are the most common - these are backlit, and what you find on most products with color displays (iPhones, 688/633, PIX-E, etc).

Many marine displays use "transflective" displays, which reflect a portion of the incident light back at you as well as transmit the backlight through (about 20% reflective and 80% transmissive). These have mostly fallen out of favor because their poor display in all conditions. The colors often look very washed out, and they suffer from very poor off-axis fidelity. Newer transflective displays simply are not made. Most of the transflective screens available are larger, lower-resolution displays. A good portable video monitor needs to be small, have accurate color representation, be high resolution and have great off axis viewing.

The other way to deal with this issue is to use a transmissive display and increase the backlight power. This works to a point, as eyes respond logarithmically. So if you want 10dB more light, you'd need to put about 10x more power into the backlight. There are a few issues with this: the display would get very thick to accommodate such LEDs, it would get hot such that you might need a cooling system (especially in a small chassis), it would be expensive, and the batteries in the unit would not last very long!

OLED displays are fundamentally different than LCDs: each pixel is a small LED. While they offer superior contrast ratios and color compared to LCDs, they are much dimmer and wash out in sunlight.

The bottom line is that the perfect display has yet to be invented. Said display would be mostly reflective, with great color fidelity, fast-response pixels, low power, reasonable cost, and small. That said the PIX-E5 IPS LCD offers a great overall performance with its 500nit brightness, 1920x1080 (441ppi) resolution, and great off axis viewing.