Reducing Indium Consumption in Silicon Hetero Junction Solar Cells With TCO Stack Systems of ITO and AZO

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>J<sub>SC</sub></italic>) of AZO SHJ cells is lower than that of ITO-based cells, possibly due to a smaller optical band gap <italic>E</italic><sub>G</sub> = 3.35 eV of AZO in contrast to <italic>E<sub>G</sub></italic> = 3.71 eV for ITO, which could lead to stronger parasitic blue absorption for AZO cells. Series resistance <italic>R</italic><sub>S</sub> of pure AZO SHJ solar cells is high mainly due to high contact resistance <italic>R</italic><sub>C</sub> between silver (Ag) metallization and AZO and high <italic>R</italic><sub>C</sub> between amorphous silicon (a-Si) and the transparent AZO with low electron density <italic>n</italic><sub>e</sub>. Using ITO<sub>a-Si</sub>-AZO-ITO<sub>Ag</sub> stacks, which saves about 50% of ITO, enables <italic>R<sub>S</sub></italic> values comparable to the ITO reference group, resulting in the same efficiency as the pure ITO cells. By replacing ITO<sub>a-Si</sub> with a high <italic>n<sub>e</sub></italic> AZO<sub>a-Si</sub> the lowest <italic>R<sub>S</sub></italic> is achieved. This AZO<sub>a-Si</sub>-AZO-ITO<sub>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.

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