  It has been recently postulated, that the strongly correlated flat bands of
magicangle twisted bilayer graphene (MATBG) can host coexisting heavy and light
carriers. While transport and spectroscopic measurements have shown hints of
this behavior, a more direct experimental proof is still lacking. Here, we
explore the thermoelectric response of MATBG through the photo-thermoelectric
(PTE) effect in gate-defined MATBG pn-junctions. At low temperatures, we
observe sign-preserving, fillingdependent oscillations of the Seebeck
coefficient at non-zero integer fillings of the moir\'e lattice, which suggest
the preponderance of one carrier type despite tuning the Fermi level from hole
to electron doping of the correlated insulator. Furthermore, at higher
temperatures, the thermoelectric response provides distinct evidence of the
strong electron correlations in the unordered, normal state. We show that our
observations are naturally accounted for by the interplay of light and
long-lived and heavy and short-lived electron bands near the Fermi level at
non-zero integer fillings. Our observations firmly establish the electron and
hole asymmetry of the correlated gaps in MATBG, and shows excellent qualitative
agreement with the recently developed topological heavy fermion model (THF).
