Effect of plasma and thermal annealing on optical and electronic properties of SnO2 substrates used for a-Si solar cells

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The sensitivity to various types of annealing treatments of three commercially available textured SnO2 substrate materials was investigated using optical transmission and Hall effect measurements. The treatments included H2 plasma and annealing in H2/Ar, Ar, or air from 100 to 400 °C. With both types of H2 treatments, the mobility of the SnO2 having the lowest carrier density (<2×1020 cm−3) doubles from ∼30 to ∼60 cm2/V s while the mobilities of SnO2 materials having a higher carrier density were unaffected. There was no significant change in carrier density with either treatment for any SnO2. The visible transmission degrades significantly with H2 plasma at 200 °C but it is unaffected by H2/Ar annealing up to 400 °C. A 20 nm layer of sputtered ZnO is sufficient to protect the SnO2 from plasma-induced damage while still allowing the factor of 2 improvement in SnO2 mobility. The resistivity of bare SnO2 can be decreased by a factor of 2 without any loss of transmission using H2/Ar or Ar annealing at 400 °C without a protective ZnO layer. The changes were stable over 2 years of storage in air. Results are consistent with thermal desorption of oxygen from grain boundaries in a reducing ambient. An optimum SnO2 product for superstrate a-Si solar module fabrication should be deposited with a low carrier density, <2×1020 cm−3, then annealed in H2/Ar or Ar at 300–400 °C to increase the mobility without increasing the absorption losses.