Fig. 1: Schematic of trion and exciton polaron.
From: Exciton-polaron Rydberg states in monolayer MoSe2 and WSe2

When a photo-generated exciton interacts with the Fermi sea (FS), it can excite many electron–hole pairs on the FS. The trion is a bound state between an exciton and one FS electron. The exciton-polaron state is the linear combination of a bare exciton, an exciton dressed with one FS electron–hole pair (Suris tetron), and higher-order components that represent an exciton dressed with two and more FS electron–hole pairs.