Effective Near-Infrared Light Management in Silicon Heterojunction Solar Cells for Perovskite/Silicon Tandem Solar Cell Application

Perovskite/silicon tandem solar cells are intensively being investigated as a promising candidate for next generation solar cells combining low costs and high efficiency. Currently, the silicon heterojunction (SHJ) bottom cell technology, which provides high open-circuit voltage values of up to 750 mV, dominates in perovskite/silicon tandem research, and also the latest perovskite/silicon tandem world records are based on SHJ bottom cells [1]. Despite this exciting performance, the tandem solar cell was still limited not only by a relatively low top cell voltage output but also by the bottom-cell photocurrent. For instance, the total photocurrent density of the world record perovskite/silicon tandem solar cell was only 39.6 mA cm⁻² [1]. For comparison, 42.7 mAcm⁻² were reached in the world record SHJ cell [2], which is very close to the theoretical maximum for silicon of about 43.3 mAcm⁻² [3]. An important optical loss mechanism in this tandem cell, in comparison to an optimized single-junction silicon cell, is the increased reflection and reduced absorption in the near infrared (NIR) part of the spectrum, hence reduced photocurrent in the silicon bottom cell. In particular, on the rear side of SHJ solar cells, an unoptimized transparent conducting oxide (TCO)/metal reflector is the origin of significant losses in the NIR part of the spectrum. These losses come from plasmonic absorption at the TCO/metal interface and from free carrier absorption in the TCO [4,5]. Therefore, the NIR light management becomes increasingly important as short-circuit current in the NIR region has to be incereased owing to the increasing interest of using SHJ solar cells as bottom cells in perovskite/silicon tandem solar cells. This contribution demonstrates an improved NIR response of the tungsten-doped indium oxide (IWO)/Ag rear contact in rear-junction SHJ solar cells. The free-carrier concentration and the thickness of the rear IWO layer are optimized in order to minimize the free-carrier and the plasmonic absorption losses without detrimentally affecting the selectivity and the electrical transport properties of SHJ solar cells. In detail, the NIR reflectance of IWO/Ag rear contacts deposited on random pyramid textured silicon wafers can be enhanced by increasing the thickness of IWO layers because of reduced plasmonic absorption at a IWO/Ag interface by increasing the thickness of IWO layers. In addition, 160-nm-thick IWO layers fabricated with an O₂-rich O₂/Ar process gas during remote plasma deposition of the IWO layers, demonstrate the largely reduced free-carrier and plasmonic absorption and the enhanced NIR reflectance of IWO/Ag rear contacts. Furthermore, measured SHJ solar cell parameters such as open-circuit voltage, short-circuit current and fill factor, are shown to be impacted by free-carrier concentration in IWO layers within the investigated ranges. As a result of these optimizations, a significant reduction of the parasitic absorption loss in the NIR part of the spectrum is obtained, leading to a champion SHJ solar cell with a short-circuit current density of up to 40.8 mA/cm² and an efficiency of 22.4%. This work was supported by NEDO.<br/> <br/><b>References</b><br/>[1] P. Tockhorn, J. Sutter, A. Cruz, et al., <i>Research Square</i>; 2022. DOI: 10.21203/rs.3.rs-1439562/v1.<br/>[2] K. Yoshikawa, H. Kawasaki, W. Yoshida, et al., <i>Nature Energy</i>, 2017, <b>2</b>, 17032.<br/>[3] A. Richter, M. Hermle, S. W. Glunz, <i>IEEE Journal of Photovoltaics</i>, 2013, <b>3</b>, 1184.<br/>[4] F.-J. Haug, T. Söderström, O. Cubero et al., <i>Journal of Applied Physics</i>, 2008, <b>104</b>, 064509.<br/>[5] Z. C. Holman, M. Filipič, A. Descoeudres et al., <i>Journal of Applied Physics</i>, 2013, <b>113</b>, 013107.