A hot-air balloon heated directly by sunlight is called a solar balloon.
A solar balloon uses solar radiation to warm the air inside its envelope. The envelope is usually dark on the outside so it absorbs more sunlight, while the heated internal air becomes less dense than the surrounding atmosphere and produces lift. This allows the balloon to rise without a conventional burner.
Solar balloons are generally simpler and quieter than burner-heated recreational balloons, but they depend strongly on sunshine. Clouds, nighttime, and changing solar intensity reduce the available heating. Their temperature and lift also vary with the envelope’s color, size, and material.
A solar balloon is not the same as a gas balloon. Gas balloons obtain lift from helium or hydrogen, whereas solar balloons use the density difference created by heating ordinary air with sunlight.