Transient Photocurrents and Defect States in Hierarchically Structured ZnO Nanowires

ZnO‐nanostructured thin films can be used as electron transport layers in ordered heterojunction quantum‐dot solar cells. An increased interface between electron and hole‐transporting layers allows for a fast separation of the electron–hole pairs preventing their recombination. Consequently, an improvement of the overall solar cell efficiency is expected. ZnO hierarchical nanorod thin films can provide the desired increase in interface area. In this work, their properties are studied and compared with simple ZnO nanorod films. The ZnO nanorod films are synthesized hydrothermally by placing the quartz/indium‐tin‐oxide substrates in a solution of zinc nitrate hexahydrate and sodium hydroxide at 90 °C. Hierarchical structures are obtained through a second deposition step. Scanning electron microscopy is used to characterize the size and shape of the nanorods. Simple ZnO films display hexagonal nanorods with a diameter of around 150 nm while hierarchical films show additional 40 nm diameter nanorods growing on the side surfaces of the primary 150 nm nanorods. Optical transmittance spectroscopy reveals a bandgap around 3.35 eV. Current–voltage measurements in the high‐field regime of 10 4 V cm −1 are used to determine the density of trap states N t through the application of trap‐filled limit current theory.