  We explore the electroluminescence efficiency for a quantum mechanical model
of a large number of molecular emitters embedded in an optical microcavity. We
characterize the circumstances under which a microcavity enhances harvesting of
triplet excitons via reverse intersystem-crossing (R-ISC) into singlet
populations that can emit light. For that end, we develop a time-local master
equation in a variationally optimized frame which allows for the exploration of
the population dynamics of chemically relevant species in different regimes of
emitter coupling to the condensed phase vibrational bath and to the microcavity
photonic mode. For a vibrational bath that equilibrates faster than R-ISC (in
emitters with weak singlet-triplet mixing), our results reveal that significant
improvements in efficiencies with respect to the cavity-free counterpart can be
obtained for strong coupling of the singlet exciton to a photonic mode, as long
as the singlet to triplet exciton transition is within the inverted Marcus
regime; under these circumstances, we show the possibility to overcome the
detrimental delocalization of the polariton states across a macroscopic number
of molecules. On the other hand, for a vibrational bath that equilibrates
slower than R-ISC (i.e., emitters with strong singlet-triplet mixing), we find
that while enhancemnents in photoluminiscence can be obtained via vibrational
relaxation into polaritons, this only occurs for small number of emitters
coupled to the photon mode, with delocalization of the polaritons across many
emitters eventually being detrimental to electroluminescence efficiency. These
findings provide insight on the tunability of optoelectronic processes in
molecular materials due to weak and strong light-matter coupling.
