We use delayed electroluminescence and photoluminescence measurements to study triplet-polaron-quenching (TPQ) mechanism in phosphorescent organic light emitting devices. Results show that the TPQ mechanism is mainly caused by charges within the bulk of the emission layer (EML) rather than by charges in the hole transport layer (HTL) or at the HTL/EML interface. Furthermore, charges on the guest rather than those on the host are found to be the most efficient in quenching excitons, revealing that guest polaronic species are the most detrimental to device efficiency. The results also show that direct injection of holes from the HTL into the guest material reduces device efficiency.
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