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 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 fl uorescent emissive polymer poly(9-dioctyl fl uorine‐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 effi ciency of ≈ 28 cd A − 1 (external quantum effi ciency (EQE) ≈ 9%) is achieved for ≈ 1.2 μ m thick single F8BT layer based HyLEDs. Enhanced out-coupling with the aid of a hemispherical lens allows further effi ciency improvement by a factor of 1.7, increasing the luminance effi ciency 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 backrecombination, which reveal that Ba(OH) 2 acts as better hole blocking layer than the Cs 2 CO 3 interlayer.