Doping-Induced Self-Absorption in Light-Emitting Electrochemical Cells

Abstract

We report on the quantitative effects of doping-induced self-absorption in light-emitting electrochemical cells (LECs) as a function of active material (AM) thickness and doping concentration. For state-of-the-art polymer LECs with optimized doping concentration and comprising Super Yellow as the electroluminescent (EL) polymer and poly(ethylene oxide)-KCF3SO3 as the electrolyte, we find that the self-absorption loss at the EL peak wavelength is ∼10% for a 100 nm thin AM and >70% for a 1 μm thick AM. This implies that the utilization of micrometer-thick AMs fit for fault-tolerant large-scale fabrication can be concomitant with a notable penalty in device performance, and that spatial variations in AM thickness will be manifested in a corresponding spatial light-intensity variation. Moreover, we find that inclusion of a poly(ethylene oxide)-KCF3SO3 electrolyte can inhibit the out-coupling of light and suggest that the culprit is light scattering from dispersed crystalline-electrolyte domains. Finally, we demonstrate evidence for that the selected initial salt concentration in an LEC device dictates the maximum doping concentration that can be attained at steady-state operation.

Self-absorption in a light-emitting electrochemical cell based on an ionic transition metal complex

“…We observe a significant and broad absorption band below 475 nm, which is assigned to metal-to-ligand and ligand-to-ligand charge-transfer transitions, 8 and an absorption tail that spans the reminder of the visible region. It is notable that the active material in a typical yellowemitting CP-LEC features a much stronger absorption in the visible regime, 14 as manifested in a higher absorption coefficient and a more opaque active material at the same thickness. The herein studied 95-nm thick active material is relatively transparent with a faint yellowish hue to the eye.…”

“…The doping process can cause distinct changes of the absorption profile of the doping regions, and we have recently demonstrated that this effect can result in a significant doping-induced self-absorption in CP-LECs. 14 For the measurement of this effect in an iTMC-LEC, that is the change in absorption during device operation, we have employed the setup displayed in Fig. 1(b).…”

“…13 However, it also brings a number of drawbacks and challenges, and in a recent study on CP-LECs we demonstrated that doping-induced self-absorption can be a highly performance-limiting effect, particularly if thick active materials with high levels of doping are used. 14 More specifically, as the conjugated polymer becomes electrochemically doped during the initial LEC operation, its absorption profile changes so that it begins to overlap strongly with the EL spectrum, and as a result absorb more of the photons generated in the p-n junction region. 14,15 In order to be able to design high-performance LEC devices with rational means, it is thus motivated to investigate the effects, and establish the amount, of self-absorption in iTMCLECs.…”

“…14 More specifically, as the conjugated polymer becomes electrochemically doped during the initial LEC operation, its absorption profile changes so that it begins to overlap strongly with the EL spectrum, and as a result absorb more of the photons generated in the p-n junction region. 14,15 In order to be able to design high-performance LEC devices with rational means, it is thus motivated to investigate the effects, and establish the amount, of self-absorption in iTMCLECs. In contrast to the situation in CP-LECs we find that doping-induced self-absorption is of relatively small magnitude for such devices, although it can result in an undesired drift of the emission color with time (and increasing amount of doping).…”

Fundamental Tradeoff between Emission Intensity and Efficiency in Light‐Emitting Electrochemical Cells

“…[3] In fact, in a number of recent papers doping-related quenching was proposed as cause for the efficiency roll-off. [11,12,13] Formal justification of this assignment is however lacking.…”