Doping Evolution and Junction Formation in Stacked Cyanine Dye Light-Emitting Electrochemical Cells

Abstract

Cyanine dyes are fluorescent organic salts with intrinsic conductivity for ionic and electronic charges. Recently ( J. Am. Chem. Soc. 2013 , 135 , 18008 - 18011 ), these features have been exploited in cyanine light-emitting electrochemical cells (LECs). Here, we demonstrate that stacked, constant-voltage driven trimethine cyanine LECs with various counteranions develop a p-i-n junction that is composed of p- and n-doped zones and an intrinsic region where light-emission occurs. We introduce a method that combines spectral photocurrent response measurements with optical modeling and find that at maximum current the intrinsic region is centered at ∼37% away from the anode. Transient capacitance, photoluminescence and attenuance experiments indicate a device situation with a narrow p-doped region, an undoped region that occupies ∼72% of the dye layer thickness and an n-doped region with a maximum doping concentration of 0.08 dopant/cyanine molecule. Finally, we observe that during device relaxation the parent cyanines are not reformed. We ascribe this to irreversible reactions between doped cyanine radicals. For sterically conservative cyanine dyes, this suggests that undesired radical decomposition pathways limit the LEC long-term stability in general.

Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing Arylazole Ancillary Ligands

“…A wide range of organic materials has been used in the manufacture of LECs. 7,8,9,10,11 However, the best efficiencies have been achieved with ionic transition-metal complexes (iTMCs), 12,13,14,15 and, in particular, with iridium(III) complexes due to their efficient spin-orbit coupling, which ideally allows to harvest 100% of the excitons generated in the device.…”

Visible light-emitting host-guest electrochemical cells using cyanine dyes

“…LECs [12][13][14][15][16][17][18][19]. They are considered as an easy-to-purify [12] and low-cost [13] alternative to the more investigated polymer and iTMC materials.…”

“…The number of mobile charges within the intrinsic region at 10 mA cm -2 amounts to ~10 16 cm -3 assuming a charge carrier mobility of 10 -5 cm 2 V -1 s -1 [39] and an electric field across the intrinsic region of 70 V µm -1 . Using chemical doping of cyanine dyes it was shown that for a substantial PL quenching a doping concentration of some percentage is required [15].…”

Light-Emitting Electrochemical Cells: A Review on Recent Progress

“…It should, however, be acknowledged that a vast majority of the subsequent research on iTMC-LECs have employed an additional electrolyte (often an ionic liquid) in order to attain a reasonably fast turn-on time [4,5]. The LEC field has up until now been dominated by these two principal classes of compounds, although new groups of functional materials, in the form of non-ionic luminescent small molecules [6][7][8][9][10][11][12][13][14], ionic fluorescent small molecules [15][16][17], quantum dots [18,19], and perovskites [20,21], have emerged more recently. Figure 1c presents the chemical structures of a CP, an iTMC, and a luminescent small molecule, as well as a number of electrolytes that have been utilized in recent high-performance LEC devices.…”