Photoluminescence and defect evolution of nano-ZnO thin films at low temperature annealing

Abstract

Nano-ZnO thin films composed of nanoparticles with sizes of 10–16 nm on silicon substrates at low temperature were prepared by sol-gel method. By placing the nano-ZnO thin films at room temperature or annealing at 100°C in air for 10 h intermittently, within a total 70 h annealing time, the evolution of PL spectra of the nano-ZnO thin films were studied in detail. As the annealing time increases, the PL peaks shift from violet to blue and green bands. The PL peaks at violet and blue bands decrease with the annealing time, but the PL peaks at green band are opposite. The PL spectra are related to the defects in the nano-ZnO thin films. The PL peaks positioned at 430 nm are mainly related to defects of zinc interstatials (Zni), oxygen vacancies and (Vo); the ones at 420 nm to oxygen interstitials (Oi), Zinc vacancies (Vzn), Zni; and the ones at 468 nm to Vzn, Zni, and charged oxygen interstatials(Vo+). The green luminescence is related to Oi, Vo and Zni. The evolutions of PL spectra and the defects are also related to the concentrations of Zn in the thin films, the thicknesses of the films and the annealing time. For the films with 0.5 M and 1.0 M Zn concentrations, after 20 h and 30 h annealing in air at 100°C, respectively, either placing them in air at room temperature or continuing anneal in air at 100°C, the PL spectra are stable. Under the low temperature annealing, Zni decreases with the annealing time, and Oi increases. Sufficient Oi favors to keep the nano-ZnO thin films stable. This result is important to nano-ZnO thin films as electron transport layers in inverted or tandem organic solar cells.