Parasitic absorption of zinc oxide as a transparent conductive oxide layer for silicon heterojunction solar cells

ZnO, due to its low cost, good electrical conductivity, and excellent light transmittance, is ideal for use as the transparent conductive oxide (TCO) layer of Si heterojunctions with intrinsic thin-layer (HJT) solar cells. However, it leads to a strong parasitic absorption in the ultraviolet (UV) band. Effectively utilizing the photogenerated carriers induced by UV light in ZnO is expected to enhance the UV photon conversion potential and improve the power conversion efficiency (PCE) of HJT solar cells. Here, the Silvaco software was used to numerically and theoretically investigate ZnO/a-Si(n)/a-Si(i)/c-Si(p)/a-Si(i)/a-Si(p) solar cells. By varying key parameters of the ZnO film, the performance of the HJT solar cells could be changed, and the factors affecting the utilization of the carriers generated in ZnO were investigated. It was found that the photogenerated carriers in ZnO can be utilized. Nevertheless, the diffusion of the photogenerated holes in ZnO is affected by the potential barrier at the ZnO/a-Si heterojunction. Increasing the donor concentration and decreasing the electron affinity of ZnO reduce this barrier, leading to an increase in the external quantum efficiency in the UV band and short-circuit current. Furthermore, although a thicker ZnO layer results in enhanced UV absorption, it promotes the diffusion of the photogenerated holes into Si, which is more sensitive to the width of the barrier. This study provides theoretical reference for reducing the parasitic UV absorption of ZnO as a TCO layer, which is of significance for enhancing the UV PCE of HJT cells and reducing production costs.