Mobility enhancement of hole transporting layer in quantum-dot light-emitting diodes incorporating single-walled carbon nanotubes

Abstract

Polymeric hole-transporting layers (HTLs) in quantum-dot light-emitting diodes (QLEDs) usually exhibit lower charge carrier transport properties (i.e. carrier mobility) than those of electron-transporting layers (ETLs) which consist mainly of metal oxide nanoparticles, such as TiO2 or ZnO. The difference of carrier transport properties between HTLs and ETLs creates an unbalanced charge injection in the QLED, which results in electron charging of the quantum-dot (QD) and the subsequent non-radiative Auger recombination process, attributing to detrimental effects on QLEDs. In this study, we incorporate small amounts (0.001 to 0.010 wt.%) of single walled carbon nanotubes (SWCNTs), which have ultrafast carrier transport properties, with the HTL material in the QLED to overcome the intrinsic hole-mobility limitation of the HTL material. We adopt poly (N-vinylcarbazole), which has a considerably low hole-mobility of 2.5 x 10(-6) cm(2) V-1 s(-1), as the HTL of our QLED. This intentionally-created unbalanced charge injection of our QLEDs allows us to take transient electroluminescence (TEL) measurements, a dynamic method to determine minor carrier mobility while maintaining overall device structure. We observe a reduced delay time for TEL onsets in the presence of SWCNTs in the HTL, which is clear evidence for hole-mobility enhancement. Compared with our normal device, 55% enhancement of zero-field hole mobility from is recorded from 0.005 wt.% SWCNT-incorporated QLED. Simultaneously, luminance and efficiency of devices are also maximized when 0.005 wt.% SWCNTs are incorporated with PVK HTL (from 13,962 cd/m(2), 1.32% to 16,922 cd/m(2), 2.54%, respectively for luminance and current efficiency). Consequently, A linear correlation between mobility enhancement and luminance is obtained, clearly demonstrating that these performance enhancement is due to the enhancement of hole mobility of the HTL. Based on our study, we expect that incorporation of SWCNTs in the HTL have a potential to further the improvement of QLED displays.