David Garcia Romero (left) and Lorenzo Di Mario (right). Photo: University of Groningen.
David Garcia Romero (left) and Lorenzo Di Mario (right). Photo: University of Groningen.

Organic solar cells have a photoactive layer that is made from polymers and small molecules. These cells are very thin, can be flexible and are easy to make. However, their solar-to-power efficiency is still much below that of conventional silicon-based solar cells.

Applied physicists from the University of Groningen in the Netherlands have now fabricated an organic solar cell with an efficiency of over 17%, which is in the top range for this type of material. This novel solar cell takes advantage of an unusual structure produced using a scalable technique – a conductive layer of tin oxide that is grown by atomic layer deposition.

The scientists also have several ideas to further improve the efficiency and stability of this solar cell. They report their work in a paper in Advanced Materials.

In organic solar cells, polymers and small molecules convert light into charges that are collected at the electrodes. These cells are made as thin films of different layers – each with its own properties – that are stacked onto a substrate. Most important are the photoactive layer, which converts light into charges and separates the electrons from the holes, and the transport and blocking layer, which selectively directs the electrons towards the electrode.

“In most organic solar cells, the electron-transport layer is made of zinc oxide, a highly transparent and conductive material that lays below the active layer,” says David Garcia Romero, a PhD student in the Photophysics and Optoelectronics group at the Zernike Institute for Advanced Materials at the University of Groningen.

Garcia Romero and Lorenzo Di Mario, a postdoctoral researcher in the same group, worked on the idea of using tin oxide, rather than zinc oxide, as the transport layer. “Zinc oxide is more photoreactive than tin oxide and, therefore, the latter should lead to a higher device stability,” Romero explains.

Although tin oxide had shown promising results in previous studies, the best way to grow it into a suitable transport layer for an organic solar cell had not yet been found. “We used atomic layer deposition, a technique that had not been used in the field of organic photovoltaics for a long time,” says Garcia Romero. However, this deposition technique has some important advantages: “This method can grow layers of exceptional quality and it is scalable to industrial processes, for example in roll-to-roll processing.”

The organic solar cells that the researchers made with tin oxide deposited by atomic layer deposition on top displayed a very good performance. “We achieved a champion efficiency of 17.26%,” says Garcia Romero. The fill factor, an important parameter of solar-cell quality, showed values up to 79%, matching the record values for this type of structure.

Furthermore, the optical and structural characteristics of the tin oxide layer could be tuned by varying the temperature at which the material is deposited. A maximum power conversion was achieved in cells with a transport layer deposited at 140°C. This same result was demonstrated for two different active layers, meaning that the tin oxide improved efficiency in a generic way.

“Our aim was to make organic solar cells more efficient and to use methods that are scalable,” says Garcia Romero. The efficiency they achieved is close to the current record for organic solar cells, which stands at around 19%. “And we haven’t optimized the other layers yet. So, we need to push our structure a bit further.”

Garcia Romero and di Mario are also keen to try making larger solar cells. These are typically less efficient but are an essential step towards real-world applications and panels.

The new organic solar cell with an impressively high fill factor is a good starting point for further development. “It may be a bit early for industrial partners to take this on; we need to do some more research first,” says Garcia Romero. “And we hope that our use of atomic layer deposition will inspire others in the field.”

“We always strive to understand what is happening in a material and in a device structure,” says Maria Antonietta Loi, who leads the Photophysics and Optoelectronics group at the Zernike Institute for Advanced Materials. “Here, we think that there might be room for improvement. In that process, our tin oxide transport layer is a great initial step.”

This class of solar cells may make an important extra contribution to the energy transition because of their mechanical properties and their transparency. “We expect that they will be used in a totally different way than silicon panels,” says Loi. “We need to think broader and out of the box at the moment.”

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.