from the top to the bottom of the electrode area as the liquid fl owed through (Figure 1(vii),(viii) ). The demonstration of EL based on a moving liquid emitter indicates potential for novel device applications, such as microfl uidic devices. However, the fundamental device characteristics of liquid OLEDs are still at a very primitive stage, with low maximum luminance, low EL effi ciency, and high driving voltage. This is because of insufficient carrier injection due to the thickness of the emitting layer and unbalanced carrier recombination based on the simple single-layer structure.Here, we report OLEDs with a liquid emitting layer that exhibit a signifi cantly improved EL effi ciency, maximum luminance and driving voltage compared with previous efforts. [ 7 ] The device architecture of liquid OLEDs was improved by incorporating an electrolyte into the liquid emitting layer and a titanium dioxide (TiO 2 ) layer as a hole-blocking layer. The driving voltage was dramatically decreased by doping a small amount of electrolyte into a liquid emitting layer. Insertion of a TiO 2 hole-blocking layer improved the carrier balance. This architecture resulted in a maximum external EL quantum efficiency ( Φ EL ) of 0.31% ± 0.07% and a maximum luminance of nearly 100 cd/m 2 , which are 10 and 100 times higher, respectively, than those reported previously. [ 7 ] The chemical structures of the compounds used in the liquid emitting layer are shown in Figure 2 a . 9-(2-Ethylhexyl)carbazole (EHCz) was used as the liquid host, while 6-{5-[3-methyl-4-(methyloctyl-amino)-phenyl]-thiophen-2-yl}-naphthalene-2-carboxylic acid hexyl ester (BAPTNCE) and tetrabutylammonium hexafl uorophosphate (TBAHFP) were used as the guest emitter and electrolyte, respectively, and were doped into the EHCz layer. Liquid OLEDs composed of ITO/Poly(3,4-ethylenedioxy thiophene) poly(styrenesulfonate) (PEDOT: PSS) (40 nm)/0.1 wt% TBAHFP, 16.7 wt% BAPTNCE, EHCz (1100 ± 100 nm)/TiO 2 (X nm)/ITO. Spectral characteristics of the host and guest compounds are shown in Figure 2b . The liquid emitting layer composed of 0.1 wt% TBAHFP, 16.7 wt% BAPTNCE, and 83.2 wt% EHCz exhibited green photoluminescence (PL) with a maximum (PL max ) at 511 nm and a PL quantum effi ciency ( Φ F ) of 55%. The absorption spectrum, fl uorescence spectrum, and Φ F of the liquid emitting layer composed of EHCz and BAPTNCE were not changed by doping with 0.1 wt% TBAHFP. This is because energy transfer from EHCz and BAPTNCE to TBAHFP does not occur as TBAHFP does not absorb in the region from 300 to 800 nm. The energy level diagram of the ITO/PEDOT:PSS/0.1 wt% TBAHFP, 16.7 wt% BAPTNCE, EHCz/TiO 2 /ITO device is shown in the inset of Figure 2b .

Liquid organic light-emitting diodes (liquid OLEDs) are unique devices consisting only of liquid organic semiconductors in the active layer, and the device performances have been investigated recently. However, the device degradation, especially, the origin has been unknown. In this study, we show that material degradation occurs in liquid OLEDs, whose active layer is composed of carbazole with an ethylene glycol chain. Nuclear magnetic resonance (NMR) experiments clearly exhibit that the dimerization reaction of carbazole moiety occurs in the liquid OLEDs during driving the devices. In contrast, cleavages of the ethylene glycol chain are not detected within experimental error. The dimerization reaction is considered to be related to the device degradation.