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Enhancing the Performance of PSCs with ‘Graphene Armor’

Posted By Graphene Council, Thursday, July 2, 2020
An electrode has been developed that will greatly improve the stability of the “Perovskite Solar Cell”, which is attracting attention as a next-generation solar cell due to its high efficiency and low cost. This is because transparent, flexible and highly conductive graphene is inserted to prevent the decomposition of the metal electrode used in the past.

A research team, led by Professor Hyesung Park in the School of Energy and Chemical Engineering at UNIST has developed a high-performance metal-based flexible transparent electrode with an interlayer of graphene. By using graphene with excellent impermeability, the metal-induced decomposition phenomenon, which has been identified as a chronic problem of metal-electrode-based perovskite solar cells, was suppressed to significantly improve stability. In addition, the efficiency and mechanical stability of the perovskite solar cell were significantly increased by using graphene’s excellent electrical conductivity and mechanical durability.

A transparent and electron-transfer electrode is included in the’photoelectric device’ that converts light energy into electrical energy (solar cell) or converts electrical energy into light energy (display device). Until now, metal oxide-based electrodes (ITO) were used, but they were hard and easily broken, making them difficult to apply to wearable devices. In particular, when this electrode is applied to a perovskite solar cell, there is a problem that the halogen element contained in the perovskite (photoactive layer) moves toward the metal oxide and the metal electrode and the photoactive layer are decomposed simultaneously.

The research team solved this issue by inserting a graphene layer. Graphene has high electrical conductivity and allows electrons to pass through well, but it has an’impermeability’ that prevents atoms from moving. When graphene is inserted as an intermediate layer between the metal transparent electrode and the perovskite photoactive layer, electrons (charges) flow well but halogen elements cannot move. In addition, graphene itself is transparent and flexible, so it is also suitable for use as an electrode for photoelectric devices.

The research team applied a “metal-graphene hybrid flexible transparent electrode” with an interlayer of graphene to perovskite solar cells. The perovskite solar cell made in this way had a photoelectric conversion efficiency of 16.4% and maintained over 97.5% of the initial efficiency even after 1,000 hours. In addition, after 5,000 bending tests, it showed excellent mechanical durability such as maintaining 94% of the initial efficiency, and thus it was applicable to next-generation wearable devices.

“The new method of inserting graphene interlayer has significantly improved the efficiency and stability of the perovskite solar cells,” says Professor Park. “We expected that this will greatly help in the development of various next-generation flexible photovoltaic devices based on perovskite, such as LEDs and smart sensors, as well as solar cells.”

Tags:  Graphene  Hyesung Park  photoelectric  solar cell  UNIST 

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Microcavities save organic semiconductors from going dark

Posted By Graphene Council, Monday, December 2, 2019
More and more electronics manufacturers are favoring organic LED displays for smartphones, TVs and computers because they are brighter and offer a greater color range.

The organic semiconductors that drive these devices are highly flexible and easily controlled. They also have the potential to be mass produced more readily than inorganic semiconductors such as silicon, which require higher temperatures for processing.

But there is a dark side to purely organic LEDs: They can be incredibly wasteful, losing up to 75% of their energy because organic semiconductors have a tendency to enter “dark states” in which they don’t emit light. These states sometimes even lead to the devices breaking down. Researchers have been looking for ways to either harness these dark states or jettison them altogether.

A collaboration led by Andrew Musser, assistant professor of chemistry and chemical biology in the College of Arts and Sciences, and Jenny Clark of the University of Sheffield, United Kingdom, has found a way to keep these organic semiconductors from going dark. Musserused tiny sandwich structures of mirrors, called microcavities, to trap light and force it to interact with a layer of molecules, forming a new hybrid state, known as a polariton, that mixes light and matter.This approach could lead to brighter, more efficient LEDs, sensors and solar cells.

The team’s paper, “Manipulating Molecules with Strong Coupling: Harvesting Triplet Excitons in Organic Exciton Microcavities,” published in Chemical Science.

“In the LED world, people are putting huge efforts into designing these vast libraries of molecules and testing them in different device configurations to see if, by tweaking the bonds or changing energy levels, they can harvest these dark states more efficiently,” Musser said. “It’s a cumbersome, difficult battle because it’s really hard to design molecules. And you don’t necessarily know how to make them do what you want.

“So what we’ve done here is address that problem with a standard molecule, purely by putting it between these mirrors and tuning the way it interacts with light,” he said. “This suggests that, for some phenomena, we can bypass a lot of this cumbersome synthetic exploration and tune the molecules at a distance.”

Musser’s interest in polaritons began while he was studying the ways organic semiconductors can improve light-harvesting efficiency in solar cells. In that case, molecules undergo a process called singlet fission, in which they absorb one photon and split that energy into two “packets” – essentially two excited electrons – thereby doubling the photon current efficiency in the solar cell.

Musser began investigating how the reverse process can also occur, with two packets of energy combining into a single, high-energy state that can emit a high-energy photon. That led him to microcavities and the ways these simple optical structures can have a profound effect on organic material through light.

In addition to manipulating a molecule’s electronic properties for enhanced brightness, recent research has demonstrated that these structures also can be used to target specific bonds and change their chemical reactivity.

Musser said different molecules interact with light in the microcavities in different ways, and further research is needed to explore the rules that underpin their behavior.

“Right now, it serves to show that when you have these complex materials and you do something even more complicated with them – putting them between these mirrors – weird and wonderful things can happen,” Musser said.

“This work literally sheds light on dark states,” said Clark. “We’ve shown that we can use polaritons to force dark states to emit light. Apart from immediate applications for LEDs, this offers a new method for studying organic semiconductors more broadly, using previously unavailable techniques.” 

Tags:  Andrew Musser  Chemical Science  energy  Graphene  Jenny Clark  LED  Semiconductor  silicon  smartphones  solar cell  U.S. Department of Energy  University of California  University of Cambridge  University of Kentucky  University of Sheffield 

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