Phone screens being hard to see when the sun is out really shouldn’t be a problem in this day and age, especially if you bought an expensive phone. However, it still happens often because the screen just doesn’t get bright enough.
Screen brightness is measured in NITS. And your phone needs at least 3000 NITS to beat the sunlight on its own. If you’ve set your screen brightness to the maximum and you still can’t see what’s on the screen while outdoors, there’s a good chance your device is near to or below that 3000 NITS mark.
If that is the case, one of the best things you can do is install a paper-like screen protector. These protectors help reduce glare on the screen, which in turn can make it more readable while outdoors, as this page shows.
The matte finish of the protector will also change how the screen feels to the touch. As the name implies, the screen will feel paper-like, similar to the screen on an eReader.