Ultrathin solar cells created by inkjet printing

Researchers from the King Abdullah University of Science and Technology (KAUST) have developed solar cells so thin, light and flexible that they can rest on a soap bubble.

The new cells could offer an alternative way to power novel electronic devices, such as medical skin patches, where conventional energy sources are unsuitable. They were developed by a team led by KAUST’s Derya Baran and reported in the journal Advanced Materials Technologies.

“The tremendous developments in electronic skin for robots, sensors for flying devices and biosensors to detect illness are all limited in terms of energy sources,” said postdoctoral researcher Eloïse Bihar. “Rather than bulky batteries or a connection to an electrical grid, we thought of using lightweight, ultrathin organic solar cells to harvest energy from light, whether indoors or outdoors.”

Until now, ultrathin organic solar cells were typically made by spin coating or thermal evaporation, which are not scalable and which limit device geometry. This technique involved using a transparent and conductive, but brittle and inflexible, material called indium tin oxide (ITO) as an electrode.

To overcome these limitations, the KAUST researchers applied inkjet printing. As explained by PhD student Daniel Corzo, “We formulated functional inks for each layer of the solar cell architecture.”

Instead of ITO, the team printed a transparent, flexible, conductive polymer called PEDOT:PSS, or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. The electrode layers sandwiched a light-capturing organic photovoltaic material. The whole device could be sealed within parylene — a flexible, waterproof, biocompatible protective coating.

Although inkjet printing is very amenable to scale-up and low-cost manufacturing, Corzo admitted that developing the functional inks was a challenge. “Inkjet printing is a science on its own,” he said. “The intermolecular forces within the cartridge and the ink need to be overcome to eject very fine droplets from the very small nozzle. Solvents also play an important role once the ink is deposited because the drying behaviour affects the film quality.”

After optimising the ink composition for each layer of the device, the solar cells were printed onto glass to test their performance. They achieved a power conversion efficiency (PCE) of 4.73%, beating the previous record of 4.1% for a fully printed cell. The team also showed that they could print a cell onto an ultrathin flexible substrate, reaching a PCE of 3.6%.

“Our findings mark a stepping stone for a new generation of versatile, ultra-lightweight printed solar cells that can be used as a power source or be integrated into skin-based or implantable medical devices,” Bihar said.

Images ©2020 KAUST; Anastasia Serin.

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