Effect of front TCO on the performance of rear-junction silicon heterojunction solar cells: Insights from simulations and experiments

In this study we make a detailed comparison between indium tin oxide (ITO), aluminum-doped zinc oxide (ZnO:Al) and hydrogenated indium oxide (IO:H) when applied on the illuminated side of rear-junction silicon heterojunction (SHJ) solar cells. ITO being the state of the art material for this application, ZnO:Al being an attractive substitute due to its cost effectiveness and IO:H being a transparent conductive oxide (TCO) with high-mobility and excellent optical properties. Through numerical simulations, the optically optimal thicknesses for a double layer anti-reflective coating system, consisting of the respective TCO and amorphous silicon oxide (a-SiO2) capping layers are defined. Through two-dimensional electrical simulations, we present a comparison between front-junction and rear-junction devices to show the behavior of series resistance (Rs) in dependence of the TCO sheet resistance (Rsh) and the device effective lifetime (τeff). The study indicates that there is a τeff dependent critical TCO Rsh value, above which, the rear-junction device will become advantageous over the front-junction design in terms of Rs. Solar cells with the respective layers are analyzed. We show that a thinner TCO optimized layer will result in a benefit in cell performance when implementing a double layer anti-reflective coating. We conclude that for a highest efficiency solar cell performance, a high mobility TCO, like IO:H, is required as the device simulations show. However, the rear-junction solar cell design permits the implementation of a lower conductive TCO in the example of the cost-effective ZnO:Al with comparable performance to the ITO, opening the possibility for substitution in mass production.