Visualizing electroluminescence process in light-emitting electrochemical cells.

Electroluminescence (EL) generally occurs via the fundamental recombination reaction of electron and hole, but the dynamics measurement of the reaction process has not been achieved so far. Here, we explore the operation dynamics of organic EL-devices in ionic liquid (IL)-based light-emitting electrochemical cells (LECs), by means of multi-timescale spectroscopic measurements synchronized with the LEC operation. Bias-induced absorption measurements under steady state conditions reveal that the migration of IL-electrolyte ions proceeds on a time scale of tens of seconds. The bias-modulation (BM) spectroscopic measurements for injected holes allow us to observe the behavior of p-doped layers varying with the bias-magnitude from growth to saturation and to recession. Simultaneous BM measurements with the EL intensity and current in a wide bias range show that, in the bias of the highest EL conversion efficiency, holes are not used for the doped layer formation but are efficiently consumed in the recombination process. The dynamics of LEC operation is directly visualized by time-resolved BM spectra, which leads to the following findings. Electron injection occurs more slowly than hole injection, causing delay of EL with respect to the p-doping. The EL of LEC starts with an insufficiently grown n-doped layer under a sufficient p-doped layer formation. N-doping then progresses gradually with the recession of the p-doped layer while the EL intensity remains constant. With the growth of n-doped layer, hole injection is suppressed gradually due to charge balance, leading to the accumulation of hole on the anode, and through them, the LEC operation reaches equilibrium. These spectroscopic techniques allow for studying the operation dynamics of LEC and can be an effective evaluation tool applicable widely to organic EL-devices.