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 Cs 2 CO 3 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 MoO 3 /Au layer used as top anode contact. In comparison to Cs 2 CO 3 , 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 Cs 2 CO 3 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 Cs 2 CO 3 interlayer.