Consider an enclosed area, such as a room without windows. During the day, artificial light can provide illumination and communication thanks to advances in Visible Light Communication (VLC). Artificial lighting, however, has some drawbacks compared to using daylight in enclosed spaces. First, using sunlight consumes less power. Second, the use of natural light improves the health and comfort of the occupants. We propose a system, dubbed Sol-Fi, to provide joint illumination and communication in enclosed spaces using sunlight. Sol-Fi relies on two main components: commercial sunlight collectors and a novel transmitter to modulate ambient light. The sunlight collectors utilize optical fibers to guide natural light from open to enclosed spaces, and our transmitter modulates the incoming light providing two novel features. First, to analyze the pros and cons of the optical devices used in the literature for ambient light communication, Sol-Fi examines the properties of Liquid Crystals (LCs) and Digital Micro Mirror Devices (DMDs). Second, to investigate the trade-off between single- and multi-band communication, Sol-Fi proposes an optical design that can modulate the entire spectrum or divide it into different (individually modulated) bands. Our evaluation shows that, depending on the number of bands (single or dual) and the type of modulator (LC or DMD), Sol-Fi provides a data rate between 0.8 to 80 kbps, a range between 0.5 to 5 m, and a field-of-view between 30° to 60°.