Swedish researchers develop ‘transparent wood’ for solar cells

31 Mar 2016

Project at Royal Institute of Technology cuts lignin, nano-engineers transparency for new PV and architectural materials.

Windows and solar panels in the future could be made from one of the best — and cheapest — construction materials known: wood. Researchers at Stockholm's KTH Royal Institute of Technology have developed a new transparent wood material that's suitable for mass production.

Lars Berglund, a professor at Wallenberg Wood Science Center at KTH, says that while optically transparent wood has been developed for microscopic samples in the study of wood anatomy, the KTH project introduces a way to use this material on a large scale. The work has been published in the American Chemical Society journal, Biomacromolecules.

”Transparent wood is a good material for solar cells, since it's a low-cost, readily available and renewable resource,” Berglund said. “This becomes particularly important in covering large surfaces with solar cells.”

Transparent wood panels could also be used for windows, and semitransparent facades, where the intention is to let light in but maintain privacy. The optically transparent wood is a type of wood veneer in which the lignin, a component of the cell walls, has been chemically removed.

Nanoscale tailoring

"When the lignin is removed, the wood becomes beautifully white. But because wood isn't not naturally transparent, we achieve that effect with some nanoscale tailoring," Berglund added. The white porous veneer substrate is impregnated with a transparent polymer and the optical properties of the two are then matched, he says.

"No one has previously considered the possibility of creating larger transparent structures for use as solar cells and in buildings," he said. “Among the development work to be done next is enhancing the transparency of the material and scaling up the manufacturing process. We also intend to work further with different types of wood."

"Wood is by far the most used biomass-based material in buildings. It's attractive that the material comes from renewable sources. It also offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity." This research and development project is financed by the Knut and Alice Wallenberg Foundation.

Technical details

A brief extract from the Biomacromolecules article explains how the KTH team has developed the transparent wood:

"Optically transparent wood (TW) with transmittance as high as 85% and haze of 71% was obtained using a delignified nanoporous wood template. The template was prepared by removing the light-absorbing lignin component, creating nanoporosity in the wood cell wall. Transparent wood was prepared by successful impregnation of lumen and the nanoscale cellulose fiber network in the cell wall with refractive-index-matched prepolymerized methyl methacrylate.

"During the process, the hierarchical wood structure was preserved. Optical properties of TW are tunable by changing the cellulose volume fraction. The synergy between wood and PMMA was observed for mechanical properties. Lightweight and strong transparent wood is a potential candidate for lightweight low-cost, light-transmitting buildings and transparent solar cell windows."