ZnO/Tris-(8-hydroxyquinoline)aluminum (Alq3)bilayer structure based ultraviolet photodetector with improved recovery time

Abstract

Tris-(8-hyroxyquinoline)aluminum (Alq3) film coated onto the solution processed spin-coated ZnO films absorbs the ultraviolet (UV) light and reduces the transmittance of ZnO/Alq3 bilayer structure. The band gap of spin-coated ZnO film (Eg = 3.28 eV) slightly decreases (Eg = 3.22 eV) upon the deposition of thermal evaporated Alq3 thin film onto the ZnO film, exhibiting the UV absorption of Alq3 film. The PL spectra showed the band-edge emission (386 nm) with a broad defect emission peak in the visible region for ZnO film of thickness ~150 nm and it is decreased for the decreased thickness of the ZnO film (~90 nm). When ZnO/Alq3bilayer structure is excited with UV light of 360 nm, the band edge emission of ZnO is suppressed by the characteristic emission (518 nm) of Alq3, confirming its UV absorption. The SEM images reveal that the rough, root-like surface feature of spin-coated ZnO film is smoothened by the deposited Alq3 film. The ZnO/Alq3bilayer structure exhibits the increased intensities of the characteristic XRD peaks of ZnO due the internal reflections in the Alq3 medium. The ZnO (~150 nm thick)/Alq3 bilayer structure based ultraviolet (UV; 365 nm, 250 mW/cm-2) photodetector exhibited the higher current density due to increased crystallite size with improved crystalline property as observed in the XRD studies. The Alq3 film onto the spin-coated ZnO film improves the recovery time by (18.51s from 20.27s) by trapping of charge carriers from ZnO, resulting in the decreased persistent photocurrent current (PPC) and hence the improved recovery time.