Georgi Nadjakov experimentally investigated photoconducting properties of sulphur. He prepared the permanent wikt:photoelectret state of matter for the first time and published his paper in 1937  and 1938. He called the electret discovered by Mototaro Eguchi in 1919, thermoelectret and the electret discovered by him in 1937, photoelectret.
Photoelectrets were the most notable achievement of Georgi Nadjakov. It's practical application led to the invention of the photocopier by Chester Carlson some years later.