Fig. 2: Interfacial electrostatic potential gain by surface state-derived Fermi level alignment.
From: Surface state-induced barrierless carrier injection in quantum dot electroluminescent devices
a Modification of energy levels at a ZnO ETL–QD EML junction. Blue solid line represents Fermi level (EF). Vacuum level shift (Δvac) is estimated to be 0.5 eV. b UPS spectra of InP/ZnSe/ZnS QDs (red square) and ZnS NCs (green triangle) on ITO/ZnO substrates at low binding energy region. Inset shows the EF − EVBM values on ITO/ZnO (blue) and Au (orange) substrates. Error bars indicate an instrument resolution, 0.05 eV. c PL decay dynamics of QDs on glass (pink circle) and ITO/ZnO substrates (sky blue triangle). X− decay occurs at an early stage in the case of ITO/ZnO, with a negative trion lifetime \({\tau}_{{{{\mathrm{X}}}}^{-}}\) = 0.9 ns. Inset displays fractions of decay channels: X− (black), neutral exciton (X, white), and delayed recombination by energy transfer (ET, grey). d Band bending at the QD–HTL junction with Δvac = −0.9 eV. e HOMO levels of 2TNATA (red circle), NPB (green triangle), and TCTA (blue square) on ITO/ZnO/QDs substrates as a function of HTL thickness. f PL decays of QDs in ITO/ZnO/QDs/HTLs: without HTLs (sky blue triangle), 2TNATA (red square), NPB (green diamond), and TCTA (blue inverse triangle). Inset shows the contribution of X− (black), X (white), and ET (grey). All PL decays were acquired at the entire emission range with excitation at 488 nm.
