Years ago my dad told me that if you wanted to photograph a TV screen your shutter speed should be 1/15th second.
Obviously, you can’t see, know or numerically choose the shutter speed on an iPhone. However, if you are attempting to iphotograph an TV, play with increasing and decreasing the phone’s distance from the TV and observe how the image changes on your screen. Using this method I was able to shoot this same shot without any black lines.