A team of researchers from the University of Groningen developed a more efficient organic solar cell using tin oxide, according to a study published in the journal Advanced Materials. The authors also speculate about ways to further improve these cells’ efficacy and stability.
Organic solar cells are flexible and easy to make. However, their efficiency is still below silicon-based cells. Now, a team from the University of Groningen in The Netherlands developed a solar cell with an efficiency of over 17%, which is the maximum range for this material. The devices rely on a conductive layer of tin oxide grown by atomic layer deposition.
Organic solar cells use polymers and small molecules to convert light into charges that can be collected at the electrodes. Typically, these cells are built with multiple layers of different materials, each with its own properties. The most important layers are the photoactive layer, which converts light into electricity, and the transport and blocking layer, which directs electrons toward 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,” said David Garcia Romero, a Ph.D. student in the Photophysics and Optoelectronics group at the University of Groningen. However, “zinc oxide is more photoreactive than tin oxide and, therefore, the latter should lead to a higher device stability.”
Tin oxide has been used before with promising results, but researchers are still struggling to find a way to grow it into a suitable transport layer for an organic solar cell. “We used atomic layer deposition, a technique that had not been used in the field of organic photovoltaics for a long time,” said Garcia Romero. “This method can grow layers of exceptional quality, and it is scalable to industrial processes, for example, in roll-to-roll processing.”
In the study, the researchers obtained excellent results using tin oxide deposited by atomic layer deposition. “We achieved a champion efficiency of 17.26 percent,” said Garcia Romero. In addition, the fill factor — an indicator of solar cell quality —reached almost 80%. The authors managed to obtain these values because they could fine-tune the optical and structural characteristics of the tin oxide layer by varying the temperature at which the material is deposited—they reached maximum conversion when the layer that was deposited at 140 degrees Celsius.
“Our aim was to make organic solar cells more efficient and to use methods that are scalable,” said Garcia Romero. At the moment, the efficiency is close to the current record for organic solar cells (19%). “And we haven’t optimised the other layers yet. So, we need to push our structure a bit further.” The team is keen to develop larger cells. These are usually less efficient but would be useful for many practical applications.
Di Mari, L, Romero D, Wang H et al (2023) Outstanding Fill Factor in Inverted Organic Solar Cells with SnO2by Atomic Layer Deposition. Advanced Materials, https://doi.org/10.1002/adma.202301404