Solvent Study Solves Solar Cell Durability Puzzle, Energy News, ET EnergyWorld

Washington: The fabrication of high-efficiency solar cells with 2D and 3D perovskite layers can be simplified by solvents that allow solution deposition of one layer without destroying the other.

It required finding the right solvent design to apply a 2D top layer of desired composition and thickness without destroying the 3D bottom layer (or vice versa). Such a cell would convert more sunlight into electricity than either layer alone, with better stability.

Chemical and biomolecular engineer Aditya Mohite and his lab at Rice’s George R. Brown School of Engineering reported in Science their success in building thin 3D/2D solar cells that offer a power conversion efficiency of 24.5 %.

It’s as efficient as most commercially available solar cells, Mohite said.

“It’s really good for flexible bifacial cells where light enters from both sides and also for cells in back contact,” he said. “2D perovskites absorb blue and visible photons, and the 3D side absorbs near infrared.”

Perovskites are crystals with cubic lattices known to be efficient light collectors, but the materials tend to be stressed by light, humidity, and heat. Mohite and many others have worked for years to make perovskite solar cells practical.

The new breakthrough, he said, largely removes the last major obstacle to commercial production.

“It’s important on many levels,” Mohite said. “The first is that it’s inherently difficult to make a solution-treated bilayer when both layers are made of the same material. The problem is that they both dissolve in the same solvents.

“When you put a 2D layer on top of a 3D layer, the solvent destroys the underlying layer,” he said. “But our new method solves this problem.”

Mohite said 2D perovskite cells are stable, but less efficient at converting sunlight. 3D perovskites are more efficient but less stable. Their combination incorporates the best features of both.

“It leads to very high yields because now, for the first time in the field, we are able to create layers with enormous control,” he said. “This allows us to control the flow of charge and energy not only for solar cells, but also for optoelectronic devices and LEDs.”

The efficiency of test cells exposed to the laboratory equivalent of 100% sunlight for more than 2,000 hours “does not degrade even 1%,” he said. Not including a glass substrate, the cells were about 1 micron thick.

Solution processing is widely used in industry and incorporates a range of techniques – spin coating, dip coating, blade coating, slot die coating and others – to deposit a material onto a surface in a liquid. When the liquid evaporates, the pure coating remains.

The key is a balance between two properties of the solvent itself: its dielectric constant and the Gutmann donor number. The dielectric constant is the ratio of the electrical permeability of the material to its free space. This determines how well a solvent can dissolve an ionic compound. The donor number is a measure of the electron donor ability of solvent molecules.

“If you find the correlation between them, you will find that there are about four solvents that allow you to dissolve perovskites and spin coat them without destroying the 3D layer,” Mohite said.

He said their discovery should be compatible with roll-to-roll manufacturing which typically produces 30 meters of solar cell per minute.

“This breakthrough leads, for the first time, to heterostructures of perovskite devices containing more than one active layer,” said co-author Jacky Even, professor of physics at the Institut national des sciences et technologies de Rennes, in France. “The dream of designing complex semiconductor architectures with perovskites is about to come true. New applications and the exploration of new physical phenomena will be the next steps.”

“This has implications not only for solar power, but also for green hydrogen, with cells capable of generating power and converting it to hydrogen,” Mohite said. “It could also enable off-grid solar power for cars, drones, building-integrated photovoltaics or even agriculture.”

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