Spray coating solar cells improves efficiency

NYU Tandon School of Engineering

A NEW technique for manufacturing perovskite solar cells improves their efficiency and solves previous fabrication challenges.

Perovskite solar cells are a low-cost alternative to conventional silicon-based cells, but so far have only been produced on a small scale as they are difficult to manufacture. A team of researchers from the US and China has now developed a scalable method to create perovskite solar cells by spraying on the electron transport layer (ETL).

Most solar cells are arranged in layers. When light hits the ETL, electrons are excited in the negatively-charged material and move through a light-trapping intrinsic layer to a positively-charged hole transport layer (HTL). The layers can be arranged as positive-intrinsic-negative (p-i-n), with the perovskite as the intrinsic layer, or they can be inverted to n-i-p.. However, it is difficult to effectively apply a uniform ETL layer over the crystalline surface of the perovskite.

"Very little research has been done on ETL options for the planar p-i-n design," said André Taylor, an associate professor in the NYU Tandon School of Engineering's chemical and biomolecular engineering department. "The key challenge in planar cells is, how do you actually assemble them in a way that doesn't destroy the adjacent layers?"

Conventionally, the ETL layer, using the compound [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), is applied via spin casting, where the ETL fluid is applied over the cell as it is spun. Spin casting is limited to small surfaces and can’t easily be scaled up for larger solar panels. It also requires a high concentration of PCBM solution to ensure the perovskite is fully covered, which leads to a higher cost. Another limitation of this method is that the gaps between the perovskite grains leave openings in the surface of the ETL, which can allow hot metal atoms to permeate and form direct contact with the perovskite layer, therefore stopping the energy flow.

Taylor and his team have developed a method to spray on the PCMB ETL coating, and by controlling the spray coating parameters such as spray rate, pressure, and solution concentration, they can optimise the film morphology. This method covers up the gaps between the perovskite grains, creating a more efficient charge extraction. The spray-coated solar cell is 30% more efficient compared to previous methods.

"Our approach is concise, highly reproducible, and scalable,” said Taylor. “It suggests that spray coating the PCBM ETL could have broad appeal toward improving the efficiency baseline of perovskite solar cells and providing an ideal platform for record-breaking p-i-n perovskite solar cells in the near future."

Nanoscale http://doi.org/csng