We measure the quantum
efficiency (QE) of individual dibenzoterrylene
(DBT) molecules embedded inp-dichlorobenzene at
cryogenic temperatures. To achieve this, we combine two distinct methods
based on the maximal photon emission and on the power required to
saturate the zero-phonon line to compensate for uncertainties in some
key system parameters. We find that the outcomes of the two approaches
are in good agreement for reasonable values of the parameters involved,
reporting a large fraction of molecules with QE values above 50%,
with some exceeding 70%. Furthermore, we observe no correlation between
the observed lower bound on the QE and the lifetime of the molecule,
suggesting that most of the molecules have a QE exceeding the established
lower bound. This confirms the suitability of DBT for quantum optics
experiments. In light of previous reports of low QE values at ambient
conditions, our results hint at the possibility of a strong temperature
dependence of the QE.