Absolute quantum efficiency measurements of electrochemiluminescent devices through electrical impedance spectroscopy

AC-operating electrochemiluminescent devices (ECLDs) have recently shown a substantial increase in brightness, making them potentially relevant for applications in lighting and displays. To further improve the performance of these liquid-state light-emitting devices, it is essential to reliably assess the absolute quantum efficiency of electrochemiluminescence and the electrochemical reactions leading to exciton formation. However, significant non-faradaic currents occurring under the AC operation schemes make it challenging to quantify the number of charges involved in the electrochemical reactions. Here, faradaic and non-faradaic currents in sandwich-type ECLDs are analysed by electric impedance spectroscopy and to assess the absolute quantum efficiency of the electrochemiluminescence (ΦECL). An equivalent circuit model enables analysis of the current at different voltage amplitudes and operating frequencies. The analysis reveals that non-faradaic currents stem primarily from capacitive currents associated with the formation of electric double-layers near liquid/electrode interfaces. Their contribution to the total current increases at high operating frequencies and low voltages. For ECLDs operating based on an exciplex-formation and energy transfer pathway, the estimated ΦECL and resulting exciton formation efficiency are 0.53% and 27%, respectively. Reverse redox reactions are identified as a significant loss factor, thus indicating potential avenues for future improvements.