Zirconium-doped indium oxide electrodes for heterojunction solar cells

A European research group led by Italy’s University of Catania has tested the use of zirconium (Zr)-doped indium oxide (In2O3) as a transparent conductive film in the silicon heterojunction (HJT) solar cells developed by module manufacturer 3Sun, a unit of Italian energy company Enel.

“Reduction of the use of indium can be achieved by decreasing the thickness of the transparent conductive oxides (TCOs) films,” explained the researchers. “The main issue for a TCO with reduced thickness is related to the consequent sheet resistance increase. Thus, the key point is the doping of indium oxide with impurities which can improve the conductivity.”

The scientists tested the film in several configurations, with varying Zr atomic concentrations and thickness. They decided to dope the In2O3 with Zr, as it is a type-III transition metal (TM) that can be realized with high carrier mobility and concentration.

All the films in this experiment were deposited at room temperature and underwent a post-deposition thermal treatment at 200 C in the air. “That makes the synthesis procedure compatible with industrial fabrication processes and applications in flexible devices with polymer substrates,” they highlighted.

Zirconium-doped indium oxide thin films (IZrO) were deposited in a 4 cm x 4 cm area. While the standard value of the thin film in commercial crystalline silicon solar cells is usually 100 nm, in the first experiment, the IZrO was 40 nm thick. At the same time, Zr content ranged from 0.6 at.% to 0.9 at.%.

With a Zr atomic concentration of 0.6%, films were created with different thicknesses of 15 nm, 40 nm, and 90 nm. “The thinnest film of 15 nm was compared to other ultra-thin TCO films (ITO-20nm and AZO-20nm) showing the lowest sheet resistance,” said the academics.

Finally, the group tested the IZrO electrodes on Sun's cells. They deployed 90 nm IZrO films as both front and back contacts of the bifacial silicon HJT devices. The performance of IZrO electrodes was found to be “comparable” to those of standard counterparts based on indium tin oxide.

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“These results suggest that ultra-thin IZrO films may be successfully used to reduce costs and the amount of Indium used in Indium-based transparent conductive oxide layers for solar cells,” the research group stated.

The new films were presented in the study “Zirconium doped indium oxide thin films as transparent electrodes for photovoltaic applications,” published in Solar Energy Materials and Solar Cells. The research was conducted by scientists from Italy’s University of Catania, the National Research Council, Enel Green Power, as well as the Dutch institute AMOLF Physics of Functional Matter, and the University of Amsterdam.