Effect of ZnO seed layer thickness on hierarchical ZnO nanorod growth on flexible substrates for application in dye-sensitised solar cells

ZnO nanorod (NR) arrays are considered to be suitable for application in flexible photovoltaic devices due to the high surface-to-volume ratio provided by the one-dimensional nanostructure. Hier- archical ZnO NRs were grown on flexible ITO/PEN substrates by sputtering a compact ZnO seed layer followed by chemical bath deposition. The effect of ZnO NR growth with the variation of the seed layer thickness (50, 100, 300, 500 and 800 nm) was studied. It has been found that by varying the seed layer thickness, the individual rod diameter, density and alignment can be controlled. The SEM images confirmed that relatively thin seed layers give rise to more dense films, whereas thick seed layers result in less dense films. The applications of flexible ZnO NR electrodes were tested by employing them in dye- sensitised solar cells (DSSC). The performance of flexible DSSCs was evaluated by studying the key cell parameters. The effect of the seed layer thickness on DSSC performance was investigated. It has been found that the overall cell efficiency increased when the seed layer thickness was varied from 50 to 500 nm, whereas sharp decrease in efficiency was observed when the thickness was further increased to 800 nm. It was found that a seed layer thickness of 500 nm gave the highest overall efficiency of 0.38 % and incident photon-to-electron conversion efficiency of 6.5 %. As well as having good electrical properties, ZnO NR films grown on ITO/PEN by this method have excellent reproducibility, and NR growth is readily controllable. This shows that these films have a wide range of potential applications including flexible energy harvesting and electronic devices.