Tailoring carrier injection efficiency to improve the carrier balance of solid-state light-emitting electrochemical cells

Abstract

We study the influence of the carrier injection efficiency on the performance of light-emitting electrochemical cells (LECs) based on a hole-preferred transporting cationic transition metal complex (CTMC) [Ir(dfppz)(2)(dtb-bpy)](+)(PF(6)(-)) (complex 1) and an electron-preferred transporting CTMC [Ir(ppy)(2)(dasb)](+)(PF(6)(-)) (complex 2) (where dfppz is 1-(2,4-difluorophenyl) pyrazole, dtb-bpy is 4,4'-di(tert-butyl)-2,2'-bipyridine, ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9'-spirobifluorene). Experimental results show that even with electrochemically doped layers, the ohmic contacts for carrier injection could be formed only when the carrier injection barriers were relatively low. Thus, adding carrier injection layers in LECs with relatively high carrier injection barriers would affect carrier balance and thus would result in altered device efficiency. Comparison of the device characteristics of LECs based on complex 1 and 2 in various device structures suggests that the carrier injection efficiency of CTMC-based LECs should be modified according to the carrier transporting characteristics of CTMCs to optimize device efficiency. Hole-preferred transporting CTMCs should be combined with an LEC structure with a relatively high electron injection efficiency, while a relatively high hole injection efficiency would be required for LECs based on electron-preferred transporting CTMCs. Since the tailored carrier injection efficiency compensates for the unbalanced carrier transporting properties of the emissive layer, the carrier recombination zone would be located near the center of the emissive layer and exciton quenching near the electrodes would be significantly mitigated, rendering an improved device efficiency approaching the upper limit expected from the photoluminescence quantum yield of the emissive layer and the optical outcoupling efficiency from a typical layered light-emitting device structure.

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

“…Indeed, this high efficiency has made it a favored choice of emitter either for blue-emitting LEECs [30, 33, 53], or as the blue component in white LEECs [45, 54–56]. The performance of this emitter in white LEECs will be revisited in Sect.…”

The dynamic behavior of thin-film ionic transition metal complex-based light-emitting electrochemical cells

“…In addition, the second finding implies a significant difference in the doping density among both regions, a fact that might bear upon the very different injection barriers for electrons (1.6 eV) and holes (0.2 eV) in the present device, that may at least partly persist even in the presence of doping. 63 In view of the latter statement, the doped zone with the higher resistance should be ascribed ntype behavior, while the one with the lower resistance should be attributed p-type conductivity. With proceeding operating time, the resistance of the p-doped zone is not subjected to any significant changes which imply a virtually constant doping density throughout this region.…”

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

“…An extensive number of LEC devices were reported having yellow, orange and red light-emission with remarkable stability and efficient luminescence using cationic iridium complexes [7][8][9][10][11][12][13][14][15][16][17][18]. However the blue LECs based on iridium complexes are hard-to make because the wide optical energy gaps for blue emitters involve high-energy excitons and exhibited emissions in the bluish-green region [19,20].…”