Organic solar cells stand the test of time

Creating electricity from sunlight is a promising route to renewable and carbon-free energy, yet the processes to produce this electricity need to be sustainable. A team at KAUST has worked with international colleagues to make one group of emerging materials more efficient, durable and stable.

Silicon underpins the prevailing commercial solar-cell technology, however emerging alternatives made from organic materials, or organic–inorganic hybrids, hold potential if they can be produced more sustainably. These alternatives can be light, flexible and even transparent, making them useful in a much wider range of practical applications.

The Organic/Hybrid Materials for Energy Applications Laboratory (OMEGALAB) focusses on the development of these organic solar cells. Headed by Derya Baran, the lab aims to develop sustainable electronic materials and devices using low-energy processes and with minimal impact on the environment.

Organic solar cells neatly fit this ambition. They can be manufactured using so-called roll-to-roll printing, which may be less expensive and less energy-intensive than traditional solar cells. This, together with the fact that organic materials can be environmentally friendly and abundantly available reduces the technology’s impact.

The efficiency of organic solar cells has been improving and is now as high as 20 percent in the lab: so the scientists at OMEGALAB are also working towards another important goal of making the devices more durable.

One factor that limits the performance of an organic solar cell lies in its morphology, or the arrangement and structure of the different organic components. Stress in the device caused by temperature can alter the morphology and thereby degrade efficiency over time.

The team worked with colleagues at Jianghan University, China, and the University of the Basque Country, Spain, to show that the thermostability of organic photovoltaics can be significantly improved by introducing so-called thermoset materials^[1].

“A cross-linked thermoset matrix is a type of polymer network formed by chemical bonds that create a three-dimensional structure,” explains Jianhua Han, a former postdoctoral fellow from KAUST, now at Julius-Maximilians-Universität Würzburg, in Germany.

“In this matrix, the polymer chains are interconnected through covalent bonds, known as cross-links, which prevent the material from softening or melting when exposed to heat,” explains Han. Using these materials, the team doubled the energy generated by their organic solar cell during an eleven-week, outdoor test.

A further challenge is that some of the changes to device architecture that aim to improve efficiency have had the unintended consequence of worsening stability. So there is a trade-off between the two^[2], demonstrated the group—working with colleagues from Flinders University, in Australia, and Middle East Technical University, in Turkey.

In this work, the team looked at the role of self-assembled monolayers. Previously, these have been shown to improve organic solar-cell operation by minimizing absorbance at the interface with the indium tin oxide (ITO) electrical contact, or anode.

They showed that the molecules in the self-assembled monolayer are not photochemically stable, and that they degrade and decompose under light exposure. “We establish direct correlation between the properties of so-called self-assembled monolayers in organic solar cells and their performance and stability—an area that has been largely unexplored in the field,” explains KAUST research scientist, Anirudh Sharma. The team showed that inserting a nickel oxide layer in between the ITO and the self-assembled monolayer reduced the problem.

Baran believes that KAUST is the perfect place to do this type of important research. “The campus location is close to the equator, so it is exposed to plenty of direct sunlight,” she says. “It is also hot and humid, conditions where even the performance and reliability of commercial silicon solar cells need to be improved. So, any technology proven at KAUST should also be useful elsewhere in the world.”

Checking this world-wide applicability of their devices is the team’s next step. “We will conduct a global ‘round robin’ study to understand how organic photovoltaics work in different parts of the world and compare it with perovskites,” Baran says. “This will be a collective effort in collaboration with several international institutions in a study that is the first of its kind.”