Organic solar cells are highlighted as a promising future technology. They are flexible, lightweight and can be manufactured at low cost in large volumes. In recent years, their efficiency has surpassed 20 per cent. However, despite this progress, a crucial challenge remains: lifetime.

In a new doctoral thesis at Karlstad University, Suraj Prasad shows why the materials degrade and how the problem can be addressed.
“We already have high efficiency. The main challenge now is to make the solar cells last,” says Suraj Prasad.

Light and oxygen trigger degradation

In the study, organic solar cell materials were exposed to simulated sunlight in air. The results show that degradation is driven by chemical reactions in which oxygen plays a central role.

When the materials are illuminated, reactive oxygen species are formed — superoxide radicals (O₂⁻) via electron transfer and singlet oxygen (¹O₂) via energy transfer. These species react with the molecules in the solar cell and alter their structure, which changes both optical and electrical properties.

A common molecular unit causes instability

One of the thesis’ most important findings relates to the so‑called BDT unit, a building block used in many high‑performance materials. The study shows that materials containing BDT degrade more rapidly, whereas replacing BDT with a simpler structure (thiophene) improves photostability.

“This is an important result, as many of today’s best-performing materials contain this unit,” says Suraj Prasad.

UV light accelerates degradation

Another key finding is that ultraviolet light plays a central role. When the researchers blocked the most energetic part of sunlight, degradation was significantly reduced. This opens up practical solutions, such as UV filters or improved encapsulation.

Strong potential for future energy systems

Organic solar cells offer several advantages compared to conventional silicon-based cells: they can be mass-produced using printing technologies, they are lightweight and flexible, and they have low production costs. In addition, they are easier to recycle. However, improved durability is essential if the technology is to achieve widespread adoption.

“It is no longer about chasing higher efficiency, but about understanding and reducing degradation at the molecular level,” says Suraj Prasad.