If you've ever tried to use a smartphone camera in less than optimal conditions, you'll have noticed they tend not to like shooting in low light conditions. This is partly due to the small sensors deployed in smartphone cameras, and that most lack the sort of powerful flash which was typical on compact cameras.
There are many pocketable lighting options which promise smartphone-friendly illumination. They range from basic LED lights to higher-end models which can be controlled by, and synced with, your smartphone.