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 the Royal Institute of Techno­logy (KTH) have now shown that untreated kraft lignin can be used to make solar cells even more environ­mentally friendly and reliable. So far, organic solar cells are made of polymers derived from oil. So, although organic, they are not as environ­mentally friendly as they could be.

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. 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 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 from the labora­tory of organic electronics 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. The Linköping researchers used a raw version of lignin – kraft lignin – directly extracted from the wood in the paper manu­facturing process. Together with KTH, they analyzed 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 stabilize the solar cell,” says Qilun Zhang. 

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. 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 to 20 percent efficiency, that is more than enough for most appli­cations,” says Mats Fahlman. (Source: Linköping U.)

Reference: Q. Zhang et al.: Industrial Kraft Lignin Based Binary Cathode Interface Layer Enables Enhanced Stability in High Efficiency Organic Solar Cells, Adv. Mat., online 9 October 2023; DOI: 10.1002/adma.202307646

Link: Laboratory of Organic Electronics, Dept. of Science and Technology (ITN), Linköping University, Norrköping, Sweden