Fig. 3: Comparative performance of NIR QD-LEDs based on TFB and crosslinked TFB:CBPV hole transport layers (HTLs).

The scheme (a) and cross-sectional TEM image (b) of the QD-LEDs with direct structure. The scale bar in (b) is equal to 50 nm. The thicknesses of the ZnMgO layer (electron transport layer), QD layer, crosslinked poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))-diphenylamine:4,4′-bis(3-vinyl-9H-carbazol-9-yl)−1,1′-biphenyl (TFB:CBPV, HTL) and poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, hole injection layer) were 38, 20, 12 and 17 nm, respectively. c Flat energy level diagram of QD-LEDs. The highest occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels of various functional layers were obtained using UPS and optical measurements. The conduction band (CB) and valence band (VB) energy levels of InAs/InP core QDs, bulk ZnSe and ZnS are shown in red, yellow and sky blue, respectively. Inset: the molecular structure of CBPV. d EL spectra of NIR QD-LEDs using TFB (light coral) and crosslinked TFB:CBPV (sky blue) as HTLs. The applied voltage was 4.0 V. Evident parasitic emission from the HTLs can be observed in the TFB-based devices (light coral, inset), which was effectively suppressed in TFB:CBPV-based devices (sky blue, inset). e Current density and radiance as a function of voltage for NIR QD-LEDs based on TFB HTLs (light coral) and TFB:CBPV HTLs (sky blue). f External quantum efficiency (EQE) as a function of voltage for NIR QD-LEDs based on TFB HTL (light coral) and TFB:CBPV HTL (sky blue). The inset shows corresponding peak EQE histograms of more than 45 QD-LEDs based on TFB:CBPV HTLs.