Singlet fission may boost photovoltaic efficiency by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission has been disputed for decades and several mechanisms have been proposed. Using time- and angle-resolved photoemission spectroscopy, we record an ultrafast movie of the formation and fission of excitons in crystalline pentacene. Our results reveal the orbital character of the excited states providing unprecedented insight into the mechanism of the singlet fission process.

Neef et al., Nature 616, 275 (2023). See also related News & Views by Musser and Stern.