胡書華 connects IXO's low-temperature process with the trade-offs that extend into material circulation

Flexible solar cells aim to place devices on substrates that are thinner, lighter and even bendable. A transparent electrode must simultaneously transmit light, carry current, survive later processing and remain conductive after deformation.

When 胡書華 of Solar Applied Materials introduced IXO targets at the forum, he began with process temperature. Conventional transparent conductive films are often annealed after deposition to improve crystallinity and optoelectronic performance. That may not pose a problem on glass, but it narrows the options for plastic substrates, organic materials and device stacks with a low thermal budget. In the talk, 胡書華 defined “room-temperature deposition” here in concrete terms: about 50°C or below.

Solar Applied Materials offers an IXO-01 to IXO-35 target series. Its public proposition is that sputtered IXO can combine conductivity and transparency without high-temperature annealing. For the 100 nm films listed on the product page, IXO has a sheet resistance of 37.9 Ω/sq, compared with 58.3 Ω/sq for ITO. At a wavelength of 550 nm, the respective transmittances are 90.9% and 85.0%. Change the film thickness or wavelength and both the values and the difference will change.

Some customers receive a no-anneal material and still anneal it out of habit. 胡書華 reminded the audience: “We are making solar cells now, not displays.” In a tandem cell, after visible light passes through the upper perovskite layer, part of the spectrum must continue to the silicon cell below. An annealing practice established for displays may not preserve the long-wavelength transmission needed by solar cells.

This is why reducing a product table to one universal improvement percentage removes the conditions that make the result useful. Film thickness, sheet resistance, wavelength and heat treatment have to remain together before the advantage has a chance of surviving in another device.

The talk next turned to nodules on the target surface and residues after oxalic-acid etching. Solar Applied Materials states that IXO neither forms nodules during sputtering nor leaves residues after etching. The first affects whether a target can sputter steadily over time; the second affects whether the transparent electrode can be patterned cleanly. Neither appears directly in a solar-cell efficiency number, yet each can become maintenance or defects on a production line first.

The bending issue came with an unusual exchange. 胡書華 recalled that a customer first told him ITO would crack in a flexible device. He did not initially believe it. Back in the laboratory, testing revealed cracks that were difficult to see with the naked eye and the risk that they would continue to grow with changes in temperature.

Solar Applied Materials' product data states that IXO retains its material properties after 200,000 R3 bending cycles. At the forum, 胡書華 explained that R3 refers to a 3 mm bending radius. The test object is an IXO film. A complete perovskite device is also affected by the absorber, contacts and encapsulation. The result therefore addresses whether the transparent electrode can withstand repeated bending; the lifetime of the full cell still depends on the other device layers. It also converts the earlier crack problem into a repeatable test condition.

If a single film still cannot provide conductivity, transparency and mechanical performance at once, structure offers another option. In the IXO/metal/IXO multilayer described by 胡書華, a thin metal layer carries part of the current while the oxides on both sides preserve optical and surface functions.

After the film discussion, the customer's questions were not over. 胡書華 said that in the past, a target presentation largely ended after optoelectronic properties, conductivity and transmittance. Today, customers continue by asking where the raw material comes from.

He used display manufacturing to explain why recycling belongs to the same problem. When a batch of ITO enters sputtering, some remains in the spent target and some is deposited inside the chamber. Material that reaches the glass or substrate may later be etched away during patterning. Recycling begins during target use and film deposition, long before the final product is discarded.

There is an external backdrop to that question. The USGS Mineral Commodity Summaries 2026 estimates that China produced about 760 metric tons of refined indium in 2025, out of a global total of roughly 1,100 metric tons—close to 70% by that measure. Indium is generally not supplied by dedicated indium mines; it is recovered as a by-product of zinc processing. Its supply is therefore tied simultaneously to the zinc industry, refining geography and recycling capacity.

USGS describes refined production and by-product origin. The demand and recycling discussed by 胡書華 also involve material use per device, process loss and recovery rate. Together they show why a customer might move from film specifications all the way to raw-material origin. IXO lowers the thermal burden of device fabrication; it does not remove dependence on indium. Solar Applied Materials operates both a target business and precious- and rare-metal recycling, which is why 胡書華 placed the two subjects in one talk.

The low temperature, transparency and bending results on the performance sheet already explain why IXO is worth testing. Whether it becomes a long-term option depends on what happens after a target has been used: where the indium goes, and how much can return to the next round of manufacturing.

Sources and further reading

Talk and organizer material

Company product and circularity material

Official mineral data