Reducing Indium Consumption in Silicon Hetero Junction Solar Cells With TCO Stack Systems of ITO and AZO
Philipp Schmid, Winfried Wolke, H. Nagel, Leonard Tutsch, Vasileios Georgiou-Sarlikiotis, Anamaria Steinmetz, S. Pingel, J. Rentsch, Martin Hermle, Martin Bivour
Abstract
This article reports on the reduction of indium consumption in bifacial rear emitter n-type silicon heterojunction (SHJ) solar cells by substituting the transparent conducting oxide (TCO) indium tin oxide (ITO) with aluminum doped zinc oxide (AZO). AZO, ITO, and stacks of both TCOs are sputtered at room temperature and 170 °C on both sides of SHJ solar cells and glass samples. The short circuit current density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J<sub>SC</sub></i> ) of AZO SHJ cells is lower than that of ITO-based cells, possibly due to a smaller optical band gap <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> = 3.35 eV of AZO in contrast to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E<sub>G</sub></i> = 3.71 eV for ITO, which could lead to stronger parasitic blue absorption for AZO cells. Series resistance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> of pure AZO SHJ solar cells is high mainly due to high contact resistance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> between silver (Ag) metallization and AZO and high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> between amorphous silicon (a-Si) and the transparent AZO with low electron density <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> . Using ITO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a-Si</sub> -AZO-ITO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ag</sub> stacks, which saves about 50% of ITO, enables <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R<sub>S</sub></i> values comparable to the ITO reference group, resulting in the same efficiency as the pure ITO cells. By replacing ITO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a-Si</sub> with a high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n<sub>e</sub></i> AZO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a-Si</sub> the lowest <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R<sub>S</sub></i> is achieved. This AZO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a-Si</sub> -AZO-ITO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ag</sub> structure saves about 70% ITO. Damp heat tests on cell and glass samples reveal a clear advantage of TCO stacks over AZO single layers.