Measuring the internal luminescence quantum efficiency of OLED emitter materials in electrical operation

One major performance parameter for organic light-emitting diodes (OLEDs) in display and illumination applications is the overall efficiency of the device, which is directly affected by the emitter's internal luminescence quantum efficiency q. It is very desirable to determine q of the emitter in-situ, i.e. in electrically driven operation, since photoluminescence measurements of q provide a rather rough (over-)estimation. In a layered system (LS), the value q of the emitting material (EM) is associated with the coupling probability of emitted radiation into the different modes provided by the LS (air, substrate, guided, surface plasmon). We show the in-situ determination of q by optical means from a relative comparison of current efficiencies of OLEDs with varying emitter-cathode distance. As a prerequisite, we outline procedures for a complete characterization of the passive and active optical properties of the LS and the EM, respectively. Then, precise optical simulation allows determining q without additional assumptions.