  Singlet-polaron and triplet-polaron annihilation mechanisms are the most
detrimental exciton quenching processes that lower the efficiency of organic
light-emitting diodes (OLEDs) at high current densities, causing so-called
efficiency roll-off in these devices. These exciton loss mechanisms are also
the critical obstacles towards the realization of electrically pumped organic
semiconductor lasers, which require very high current densities to reach
threshold. Herein, under a relatively large external magnetic field, we
demonstrate that the efficiency roll-off at high current densities in europium
(Eu3+)-based solution-processed OLEDs can be suppressed to some extent while
the luminance is enhanced. We achieve this by reducing the Forster-type
exciton-polaron annihilation processes. Under the applied magnetic field, we
show that manipulation of the polaron-spin and exciton dynamics lead to a
quantitative roll-off suppression.
