Paper link, includes videos of the prototypes:
https://assets-eu.researchsquare.com/files/rs-3909633/v1_cov...
"Sunlight-powered hovering of an ultra-light micro aerial vehicle" (PDF)
https://www.researchgate.net/publication/378206931_Sunlight-...
...but that's still paywalled.
Here's the unlisted demo video: https://www.youtube.com/watch?v=LBoee1l4OXo