Investigating the effect of materials processing on ZnO nanorod properties and device performance

Abstract

This project explores the effect of materials processing on the optical, morphological, and electrical properties of ZnO nanorods synthesised using the low-temperature (90˚C) aqueous chemical technique. A highly-alkaline (pH 11) growth solution fabricated nanorods that exhibit morphological sensitivity to the anneal atmosphere used. This was attributed to unreacted precursors trapped throughout the nanorod bulk and near the surface. A significant increase in the c-axis peak intensity post-annealing and evidence of nitrogen-doping in all annealed nanorods, confirmed the precursors were present prior to the annealing process. In addition, intense green photoluminescence was observed under UV excitation and was shown to be dependent on the anneal atmosphere. The origin of this emission was related to zinc vacancy defects that were energetically favoured during the oxygen-rich synthesis and anneal conditions according to first principle calculations. To better study the electrical properties of the ZnO nanorods they were incorporated into p-n heterostructures using p-type CuSCN. An alternative spray-coating method was developed for depositing CuSCN that was a significant improvement over previous methods as demonstrated by the high hole mobility 70 cm2/V.s. The current popularity of conductive polymers led to the comparison of hybrid inorganic-organic (ZnO-PEDOT:PSS) and purely inorganic (ZnO-CuSCN) devices. These were tested as UV photodetectors and differences in device structure were shown to have a significant impact on the device response time and responsivity. A rectification ratio of 21500 at ±3 V was achieved for these ZnO-CuSCN devices. The inorganic ZnO-CuSCN device exhibited photovoltaic behaviour at zero-bias, which highlighted it as a suitable choice for self-powered UV photodetection. The effect of processing on the photodetector performance was investigated for two sets of nanorods; pH 6 and pH 11. Consequently, a maximum photocurrent response of 30 μA (for 6 mW cm-2 irradiance) was achieved for nitrogen-annealed pH 11-nanorods with a rise time of 25 ns. The high response was assigned to fewer zinc vacancies acting as electron trap states and the introduction of N-related donor defects.