I use my phone outside a lot for work, so I've done plenty of research on this topic. The main problem you're dealing with in this situation is glare — light from the sun is reflecting off your phone's screen, blocking the display.
One way to counteract that issue is by making your screen brighter. If I leave my phone on its auto-brightness setting, it never gets as bright as it can be. The auto mode avoids going to full brightness for some reason. Setting the brightness to the maximum manually often helps me — see if your phone has a similar feature.
Another option is to get a pixel screen protector. As this website shows, there are many types of screen protectors out there, and matte screen protectors can be used to reduce glare on the screen, making it more visible.
There are other options. Using polarized sunglasses may help reduce screen glare. You can also invest in a sun hood for your phone, which protects the screen from glare at the expense of making the phone bulkier.
