His solo recordings include a collection of Chopin transcriptions as well as Ligeti's complete works for piano. Moreover, his first volume of Ligeti's piano works, released in 1996, contained the premiere recording of the second book of Ligeti's Études. Ullén plans to record all of Sorabji's Études transcendantes (100), which will be the first time they were all recorded by one person. Future recording projects mentioned on his website include the complete piano works of Alfred Schnittke, Arvo Pärt, and Charles Ives.
In addition to his career as a performer, he is involved in scientific research on music and the brain. Since 2010, he has been Professor of Cognitive Neuroscience at Karolinska Institutet.