Influence of ZnO nanoparticles on the performance of LED based on oligomer thin films

Abstract

In this work, a set of thin films of ZnO nanoparticles doped with a conjugated oligomer {9,9,9′,9′,9″,9″-hexakis(hexyl)-2,7′;2′,7″-trifluorene} has been prepared. The optical and electrical measurements on pure oligomer, ZnO nanoparticles, and blended thin films have been investigated. The absorption spectra of the pure oligomer in thin films showed a single band at 350 nm. When the thickness was increased, there was no new band detected at the end of the spectrum. This is a strong indication that the oligomer cannot be present in the dimeric state. On the other hand, the photoluminescence (PL) spectra of the oligomer showed two distinguished peaks at 400 and 420 nm due to the monomer and excimer states, respectively. Furthermore, the results showed that the PL spectrum of the oligomer was affected by the thickness. For the blended thin films with different ratios of ZnO nanoparticles, the PL spectra showed an enhancement in intensity by increasing the concentration of ZnO nanoparticles. For the electrical properties, the addition of ZnO nanoparticles to oligomer had increased the current and luminescence. The luminescences of the pure and blend film at 10 V had reached 1000 and 4200 cd/m2, respectively; and the turn-on voltage was reduced from 10.5 to 5.8 V. The light emission in the device is due to the Auger-assisted energy up-conversion process. The lifetime of the blend’s diodes measured under standard atmospheric conditions was greatly improved.