Barium Hydroxide as an Interlayer Between Zinc Oxide and a Luminescent Conjugated Polymer for Light-Emitting Diodes

Abstract

A study of hybrid light-emitting diodes (HyLEDs) fabricated with and without solution-processible Cs₂CO₃ and Ba(OH)₂ inorganic interlayers is presented. The interlayers are deposited between a zinc oxide electron-injection layer and a fluorescent emissive polymer poly(9-dioctyl fluorine–alt-benzothiadiazole) (F8BT) layer, with a thermally evaporated MoO₃/Au layer used as top anode contact. In comparison to Cs₂CO₃, the Ba(OH)₂ interlayer shows improved charge carrier balance in bipolar devices and reduced exciton quenching in photoluminance studies at the ZnO/Ba(OH)₂/F8BT interface compared to the Cs₂CO₃ interlayer. A luminance efficiency of ≈28 cd A⁻¹ (external quantum efficiency (EQE) ≈ 9%) is achieved for ≈1.2 μm thick single F8BT layer based HyLEDs. Enhanced out-coupling with the aid of a hemispherical lens allows further efficiency improvement by a factor of 1.7, increasing the luminance efficiency to ≈47cd A⁻¹, corresponding to an EQE of 15%. The photovoltaic response of these structures is also studied to gain an insight into the effects of interfacial properties on the photoinduced charge generation and back-recombination, which reveal that Ba(OH)₂ acts as better hole blocking layer than the Cs₂CO₃ interlayer.