3, Lamar University, Beaumont, Texas, United States
1, Arizona State University, Tempe, Arizona, United States
S. Mohsin, D. Quispe, A. Leilaeioun, Z.J. Yu, and Z.C. Holman
Transparent conductive oxide (TCO) electrodes are commonly found at the front—and sometimes the rear—of thin-film solar cells and modules, including those composed of CdTe, CIGS, and perovskites. In this role, the TCOs are responsible for transporting charge laterally to the nearest scribe line, which are spaced 5–10 mm apart, and for transmitting photons into the cell that are shorter than the bandgap wavelength, which is usually 700–1100 nm for the cells of interest. Indium tin oxide, which can have carrier densities of over 1021 cm-3 and usually has a mobility of 20–30 cm2/Vs, is most commonly used in these cells, and typical layers have sheet resistances of 10–20 Ω/sq.
Tandem solar cells that have a narrower-bandgap bottom cell, such as silicon, also require TCOs—both at the front surface and possibly between the sub-cells, to act as a recombination junction—but the optimal TCO has considerably different properties. In particular, the TCO must remain transparent out to 1200 nm, which means that the carrier-density-to-mobility ratio must be small according to the Drude model, and the sheet resistance can be as large as 100 Ω/sq if closely spaced metal fingers are employed, as in silicon solar cells.
In this contribution, we deposit and characterize indium zinc oxide (IZO) layers and compare these to ITO layers. The IZO layers are sputtered in atmospheres with varying oxygen gas partial pressures, leading to varying carrier density. This, in turn, tunes the sheet resistance of approximately 100-nm-thick layers from 30 Ω/sq to over 10,000 Ω/sq, but the mobility remains nearly constant for all layers at the high value of 45–65 cm2/Vs. Absorption measurements of films on glass confirm what Hall measurements reveal: for identical thicknesses and sheet resistances, an IZO layer can have more than 5% less parasitic absorption than its ITO counterpart.