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

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 fluorinealt-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 approximate to 28 cd A-1 (external quantum efficiency (EQE) 9%) is achieved for approximate to 1.2 mu m 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 approximate 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.