06 December 2023

One of nature’s most common organic materials – lignin – can be used to create stable and environmentally friendly organic solar cells. Researchers at Linköping University and KTH Royal Institute of Technology have now shown that untreated kraft lignin can be used to improve organic solar cells further. The study has been published in the scientific journal Advanced Materials.

Two persons behind large lab equipment.
The long-term goal for Mats Fahlman and Qilun Zhang is to build a solar cell almost entirely made from wood materials. Thor Balkhed

Sunlight currently seems to be one of the main sustainable energy sources. 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, light weight and flexibility, and hence many applications, such as indoor use or attached to clothing to power personal electronic devices.Portrait of Mats Fahlman.Mats Fahlman, professor at LOE. Photo credit Thor Balkhed

But 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.

Directly from wood pulp

Researchers at Linköping University and KTH have now developed an organic 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 enables us to show that this is possible and a first step towards replacing today’s oil-based materials with wood-based alternatives,” says 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. Portrait of Qilun Zhang.Qilun Zhang, principal research engineer at LOE. Photo credit Thor Balkhed 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.

“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,” says Qilun Zhang, principal research engineer at LOE.

 Non-toxic, sustainable and cheap

Organic solar cells are already in use, mainly for indoor applications. They can also replace batteries in sensors and similar low energy devices. According to Mats Fahlman, this is the first step into the market for organic solar cells. This technology can then be scaled up for larger applications such as pure energy supply.Lab equipment with wires.To test the stability and efficiency of the solar cell in the lab an artificial sun is used. Photo credit Thor Balkhed And building them from wood materials would make the entire solar cell more environmentally friendly.

“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 percent efficiency, that is more than enough for most applications,” says Mats Fahlman.

The study was funded mainly by the Knut and Alice Wallenberg Foundation through the Wallenberg Wood Science Centre. Funding was also received from the Swedish Energy Agency, the Swedish Research Council, the Swedish Foundation for International Cooperation in Research and Higher Education – STINT, and the Swedish Government Strategic Research Area on Functional Materials – AFM – at Linköping University.

Article: Industrial Kraft Lignin Based Binary Cathode Interface Layer Enables Enhanced Stability in High Efficiency Organic Solar Cells; Qilun Zhang, Tiefeng Liu, Sebastian Wilken, Shaobing Xiong, Huotian Zhang, Iuliana Ribca, Mingna Liao, Xianjie Liu, Renee Kroon, Simone Fabiano, Feng Gao, Martin Lawoko, Qinye Bao, Ronald Österbacka, Mats Johansson, Mats Fahlman; Advanced Materials 2023, published online on 9 October 2023. DOI: 10.1002/adma.202307646

A small pile of brown powder (kraft lignin) next to an organic solar cell.Kraft lignin improves the stability in organic solar cells thanks to its ability to form hydrogen bonds that acts as a sort of glue. Photo credit Thor Balkhed



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