In situ measurement of spectrum, emission zone, and dipole emitter orientation in OLEDs
The optical features of the internal dipole emission have major impact on the radiation pattern and overall device efficiency of organic light-emitting diodes (OLEDs). In recent years, the characterization of OLED emitter properties by optical analysis of far-field radiation patterns of OLEDs in electrical operation was established as an in situ investigation method. However, in order to observe the internal features of the dipole emission in the OLEDs far-field accurately, well adapted devices should be utilized to optically enhance the feature of interest. Although this is a crucial point, the potential of adapted devices to OLED characterization has not been investigated universally yet. In our contribution, we provide general directives how the OLEDs layered stack is to be designed in order to enable for precise measurements of the active optical properties of the emissive material (internal electroluminescence spectrum, profile of the emission zone and dipole moment orientation) by radiation pattern analyses. Basically, we utilize the fact that the distance of the emissive sites to the metal cathode is most crucial to enhance or suppress certain dipole contributions to the far-field. A model layered system is discussed and universal emitter positions suitable to determine the internal feature of particular interest at most accuracy are deduced.