Wood Materials Can Create Reliable Organic Solar Cells

The team from Linköping University and the Royal Institute of Technology (KTH) have now shown that untreated kraft lignin can be used to make organic solar cells even more environmentally friendly and reliable. The new research is detailed in a paper in the journal Advanced Materials.

Organic solar cells are already in use, mainly for indoor applications. They can also replace batteries in sensors and similar low-energy devices.

Traditional solar cells made from silicon are efficient but have an energy-demanding and complicated manufacturing process, which may lead to hazardous chemical spills.

Organic solar cells have, therefore, become a hot research area thanks to their low production cost, lightweight, and flexibility, and hence, many applications, such as indoor use or attachment to clothing, to power personal electronic devices.

However, one problem is that organic solar cells are made of plastic or polymers derived from oil. So, although organic, they are not as environmentally friendly as they could be.

The researchers developed a solar cell where part of the electron transport layer connected to the cathode in the solar cell is made of what is known as kraft lignin, sourced directly from wood pulp.

Although only a small part of the solar cell is currently made of lignin, the long-term goal is to build a solar cell almost entirely made from wood materials.

“We want to build efficient, reliable, cheap, and environmentally friendly solar cells. This study shows that this is possible and a first step towards replacing today’s oil-based materials with wood-based alternatives,” explained Mats Fahlman, professor at the Laboratory of Organic Electronics (LOE) at Linköping University.

In previous studies, chemically modified wood-based materials have been used to increase reliability or stability in both organic solar cells and solar cells made from the crystalline material perovskite.

In their recently published study, the Linköping researchers used a ‘raw’ version of lignin, called kraft lignin, directly extracted from the wood in the paper manufacturing process. Together with KTH, they analysed which molecular composition of lignin is best suited for the purpose.

Qilun Zhang, principal research engineer at LOE, said: “We have created a material, or composite, from kraft lignin which is to constitute the cathode interface layer.

“It turned out that this made the solar cell more stable. The advantage of kraft lignin is that it has the ability to create many hydrogen bonds, which helps to stabilise the solar cell.”

Fahlman concluded: “Organic solar cells will never be the most efficient. But their advantage is that they are non-toxic, sustainable and cheap.

“If they have a 15-20% efficiency, that is more than enough for most applications.”

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