Toward TCO‐Free Silicon Heterojunction Solar Cells: Effect of TCO Layers in Electrical Transport and Stability

The current silicon heterojunction (SHJ) cells utilize indium‐based transparent conductive oxide (TCO) layers for supporting the lateral carrier transport. However, In is a typical rare metal and its consumption in solar cell manufacturing must be minimized for sustainable production. In this work, the possibility to realize high‐efficiency TCO‐free SHJ cells, in which no In‐based material is needed, is examined. It is found that monofacial rear‐junction structure is beneficial to collect minority carriers efficiently without the help of TCO layers, regardless of the wafer polarity. In addition, the contact resistivity of locally metallized area must be minimized for efficient carrier transport. Based on these findings, a TCO‐free SHJ cell showing an efficiency of 22.1%, which is comparable to that of the benchmark SHJ cell with TCO layers, is demonstrated. However, direct metallization of amorphous silicon layers causes the degradation in the photovoltaic property after prolonged annealing, probably due to the metal diffusion into Si. This degradation can be avoided by inserting a thin barrier layer such as a SnO 2 layer. It is indicated in these results that it is possible to realize TCO‐free SHJ cells with high initial efficiencies, and the main obstacle is not the efficiency but the long‐term stability.