Efficient polymer light-emitting diodes with ZnO nanoparticles and interpretation of observed sub-bandgap turn-on phenomenon

Abstract

Low-temperature liquid-phase synthesized ZnO nanoparticles (NPs) are used to realize highly-efficient polymer light-emitting diodes (PLEDs) containing a poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) emissive layer. It is shown that a 77-nm-thick ZnO electron transport layer (ETL) increases the maximum luminance and current efficiency of the PLED from ∼450 to ∼13,100 cd/m2 and 0.55–4.7 cd/A, respectively. In addition, the ZnO layer shifts the peak in the electroluminescence (EL) spectrum from ∼535 nm for a PLED with a CsN3 layer or no NP layer to ∼585 nm. After ruling out the possibility of ZnO-defect-related emissions and micro-cavity or interference effects, it is argued that the sub-bandgap turn-on behavior of the PLED with a ZnO ETL is the result of emission zone transition and enhanced hole injection rate near the anode, based on the trap-filled space-charge limited conduction (SCLC) phenomenon in the current density-voltage curves and the current-dependent EL spectra.