Tin oxide-based organic solar cell achieves 17% efficiency

Organic solar cells have a photoactive layer that is made from polymers and small molecules; the cells are thin, can be flexible, and are easy to make. However, the efficiency of these cells is still below that of conventional silicon-based ones. Applied physicists from the University of Groningen have fabricated an organic solar cell with an efficiency of over 17%, which is reportedly in the top range for this type of material.

The organic solar cell uses an unusual device structure that is produced using a scalable technique. The design involves a conductive layer of tin oxide that is grown by atomic layer deposition. The scientists also have ideas to further improve the efficiency and stability of the cell. The results have been published 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 is 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.

David Garcia Romero, a PhD student in the Photophysics and Optoelectronics group at the Zernike Institute for Advanced Materials at the University of Groningen, said the electron transport layer in most organic solar cells is made of zinc oxide, a highly transparent and conductive material that lies below the active layer. Lorenzo Di Mario, a postdoctoral researcher who worked with Romero on the idea of using tin oxide as the transport layer, said zinc oxide is more photoreactive than tin oxide and therefore, the latter should lead to a higher device stability.

Although tin oxide has 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,” Romero said. However, it has some important advantages, as the method can grow layers of exceptional quality and is scalable to industrial processes, for example in roll-to-roll processing.

The organic solar cells that were made with tin oxide deposited by atomic layer deposition on top show a good performance, with the researchers achieving a champion efficiency of 17.26%. The fill factor, an important parameter of solar cell quality, showed values up to 79%, in agreement with 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 reached in cells with a transport layer that was 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,” Romero said.

The efficiency is reportedly close to the current record for organic solar cells, which stands at around 19%. “We haven’t optimised the other layers yet. So, we need to push our structure a bit further,” Romero said.

The researchers are also keen to try making larger area cells — these are typically less efficient but are needed to step towards real-world applications and panels. The new solar cell with a high fill factor is a good starting point for further development. Romero said that it might be a bit early for industrial partners to take this on, as the team needs to do some research first. “We hope that our use of atomic layer deposition will inspire others in the field,” Garcia said.

This class of solar cells may make an important contribution to the energy transition because of their mechanical properties and their transparency.

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