Dutch researchers have used atomic layer deposition to build an organic solar cell with a tin oxide electron transport layer. This improved electron mobility and transparency, resulting in a record level.
A team of researchers from the University of Groningen in the Netherlands has produced an organic solar cell based on an electron transport layer (ETL) made of tin oxide (SnO2), which is said to enable the device to achieve a remarkable 79 percent duty cycle.
Currently, organic solar cells lag behind their inorganic competitors because they achieve less current, have higher open-circuit voltage drops, and have a lower duty factor, which in these photovoltaic devices usually does not exceed 80%.
The researchers explained that SnO2 is a promising alternative to zinc oxide (ZnO) for ETLs used in organic PV because its energy levels are suitable for efficient electron collection. “Compared to ZnO, SnO2 offers higher electron mobility and superior transparency to visible and UV light,” they noted, noting that ZnO is, however, more photoreactive than SnO2.
The Dutch team used atomic layer deposition (ALD) to deposit the SnO2 thin film, which they say avoids adjusting its surface through additional fabrication steps.
“Furthermore, compact layers can be deposited at a relatively low temperature (below 100 C), allowing fine-tuning of both the electrical and optical properties of the material,” it further explained, adding that the coating process shows a trade-off between the transparency of the material, which is higher at lower temperatures, and the electrical conductivity, which improves when raising the deposition temperature.
The academics built the cell from a substrate made of glass and indium tin oxide (ITO), a SnO2-based ETL, an organic active layer, a layer of molybdenum oxide (MoOx) and an aluminum (Al) metal contact. The organic absorber was prepared from two different donor-acceptor mixtures, which act as active layers. The first consists of two polymers known as TPD-3F and IT-4F, and the second of two polymers known as PM6 and L8-BO.
The solar cell constructed with PM6 and L8-BO achieved the best performance of all the tested devices by achieving a power conversion efficiency of 17.26%, an open circuit voltage of 0.872 V, a short circuit current of 25.55 mA cm−2 and a duty cycle of 79.0%. In comparison, the reference cell built with ZnO-based ETL achieved an efficiency of 16.03%, an open-circuit voltage of 0.878 V, a short-circuit current of 25.10 mA cm−2 and a fill factor of 74.0%.
“Both values are currently the highest reported values for inverted-structure OSCs and SnO2 as ETL,” the researchers said, referring to the cell’s efficiency and fill factor, respectively.
“Our goal was to make organic solar cells more efficient and use scalable methods,” says Garcia Romero. The efficiency is close to the current record for organic solar cells, which is about 19%, the research team concluded.