There are many mantras for the Sun, as the Sun has many names like Surya, Savitar, Aditya, Ravi, Mitra, Varuna, Aryaman, Pushan, Indra, Agni, and so on.
The main name mantra is: OM SURYAYA NAMAHA. Others are OM SAVITRE NAMAHA; OM ADITYAYA NAMAHA; OM RAVAYE NAMAHA. The main bija (seed) mantra is OM SUM (Pronounced soom). These mantra should be repeated at least 108 times a week, preferably on a Sunday, during the day (as at dawn, noon or sunset).
Another good seed mantra for the Sun is RAM (pronounced rahm), the name of the avatar Rama. This increases our connection with the Divine light generally. Om itself is said in the Vedas and Upanishads to be the sound of the Sun. The Sun is the essence of all vowels.