Metal-Organic Framework for Efficient Electron Injection

A 3D metal-organic framework (MOF) [(H3O)Zn(HCOO)(3)](infinity) is developed as a highly efficient and air-stable solution-processed electron injection layer (s-EIL). The [(H3O)Zn(HCOO)(3)](infinity) film can be easily obtained by annealing a spin-coated [Zn(HCOO)(2)(H2O)(2)](infinity) film at 120 degrees C in air, which is facilely prepared by the reaction of ZnO powder with formic acid and the aqueous solution of ammonium. The un-encapsulated electron-only device with [(H3O)Zn(HCOO)(3)](infinity) as the s-EIL exhibits significantly enhanced electrical property and air stability, compared to that with vacuum-evaporated Cs2CO3 as EIL. Such improvements can also be observed in inverted organic light-emitting diodes. The device with [(H3O)Zn(HCOO)(3)](infinity) s-EIL demonstrates an operating voltage of 6.8 V at 1000 cd m(-2), much lower than that with Cs2CO3 EIL (8.4 V). And the power efficiency is increased about 18%. The enhanced electron injection property of [(H3O)Zn(HCOO)(3)](infinity)-modified indium-tin oxide (ITO) is attributed to its reduced work function compared to bare ITO, which is confirmed by ultraviolet photoelectron spectroscopy analysis. This approach is anticipated to be applied in other optoelectronic devices for efficient electron injection.