ZnO nanocrystals incorporating PEIE and a fluorene-based polyelectrolyte as electron transport layers for pure cesium-containing perovskite light-emitting devices

We demonstrate inverted perovskite light-emitting devices (PeLEDs) based on ZnO nanocrystals (NCs) and cesium lead bromide (CsPbBr3) film as the electron transport and emission layers, respectively. Polyethyleneimine ethoxylated (PEIE) and an ionic polyfluorene derivative containing trimethylammonium hexafluorophosphate groups (namely P2-PF6) were introduced between ZnO NCs and CsPbBr3 film. The introduction of the PEIE/P2-PF6 bilayer decreased the work function of ZnO NCs to achieve balanced charge injection into the emission layer. Furthermore, the bilayer also effectively improved CsPbBr3 coverage and morphology, thereby reducing current leakage in PeLEDs. The improved CsPbBr3 film showed better photoluminescence, owing to anti-quenching capability of the PEIE/P2-PF6 and prolonged carrier lifetime. PeLEDs with the structure ITO/ZnO NCs/ PEIE/P2-PF6/CsPbBr3 film/poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl amine) (TFB)/Au were fabricated and evaluated, employing TFB as the hole transport layer. The PeLED based on the PEIE/P2-PF6 showed a turn-on voltage of 2.8 V, a max luminance of 3,927 cd/m2 and max current efficiency of 0.2 cd/A that was significantly higher than those without PEIE/P2-PF6 bilayer.