For solution-processed optoelectronics such as Organics Light-emitting diodes (LEDs), Quantum dot LEDs, Perovskite LEDs, many researchers focus on ZnO nanoparticles (NP) as electron injection layer (EIL) which has appropriate properties; it has deep valence band (~7.8 eV), large bandgap (~3.4eV), high intrinsic electron mobility (~ 200 cm2V-1S-1). However, the ZnO NPs have many defects on the surface, it results in an exciton quenching, and then it has a bad effect on the device. Moreover, due to the large barrier between the workfunction of ZnO NPs and the lowest unoccupied molecular orbital (LUMO) of emissive layer, electron cannot inject efficiently. In recent years, to reduce workfunction of ZnO, Polyethylenimine (PEI) have studied as the interfacial bilayer onto the ZnO NP films (PEI/ZnO bilayer). However, surface defects of PEI/ZnO bilayer still remain.
Herein, we suggest introduction of hybridization between ZnO NPs and Polyethylenimine (PEI:ZnO hybrid layer), called organic/inorganic hybrid materials. PEI:ZnO hybrid layer passivates the surface defect of ZnO NPs and reduces the energy barrier through dipole effect. To study how much surface defect of PEI:ZnO hybrid layer is passivated, we utilized X-ray photoelectron spectroscopy (XPS). In case of PEI:ZnO hybrid layer, lattice oxygen peak (OL, 530.2eV) is 92.7%, deficient oxygen sites peak (OV, 531.4eV) is 7.2% and ratio of oxygen vacancy to lattice of oxygen (OV / OL) is only 0.078. On the other hand, For PEI/ZnO bilayer, OL is 75.01%, OV is 19.8%, and oxygen vacancy ratio (OV / OL) is 0.264. We found that PEI:ZnO hybrid layer more passivate the surface defect than PEI/ZnO bilayer. In addition, we fabricated electron only device which use linear PEI and branched PEI and we investigate the electron injection mechanism. Linear PEI contain all secondary amines, in contrast to branched PEI which contain primary, secondary and tertiary amino groups. We show that their amino group can affect the binding of PEI:ZnO hybrid, and electron injection mechanisms can be identified. In case of breached PEI has better electron injection ability than linear PEI. As a result, it is found that branched PEI:ZnO hybrid layer has good surface defect passivation and electron injection ability.