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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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New study unveils ultrathin boron nitride films for next-generation electronics

Posted By Graphene Council, Friday, June 26, 2020
An international team of researchers, affiliated with UNIST has unveiled a novel material that could enable major leaps in the miniaturization of electronic devices. Published in the prestigious journal Nature, this study represent a significant achievement for future electronics.

This breakthrough comes from a research, conducted by Professor Hyeon Suk Shin (School of Natual Sciences, UNIST) and Principal Researcher Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT), in collaboration with Graphene Flagship researchers from University of Cambridge (UK) and Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain).

In this study, the team successfully demonstrated the synthesis of thin film of amorphous boron nitride (a-BN) with extremely low dielectric constant as well as high breakdown voltage and superior metal barrier properties. The research team noted that this newly fabricated material has great potential as interconnect insulators in the next-generation of electronic circuits.

In the ongoing process of miniaturization of logic and memory devices in electronic circuits, minimizing the dimensions of interconencts - metal wires that link the different device components on the chip - is crucial to guarantee improved performance and faster response of the device. Extensive research efforts have been devoted to decreasing the resistance of scaled interconnects because integration of dielectrics using complementary metal oxide semiconductor (CMOS) compatible processes has proven to be exceptionally challenging. According to the research team, the required interconnect isolation materials should not only possess low relative dielectric constants (referred to as k-values), but should also be thermally, chemically, and mechanically stable.

There has been an ongoing quest to obtain materials with ultra-low-k (relative permittivity around or below 2) avoiding the artificial addition of pores in the thin film in the semiconductor industry for at least the past 20 years. Several attempts had been made to develop materials with desired characteristics, yet those materials have failed to be successfully integrated in interconnects due to poor mechanical properties or poor chemical stability upon integration, causing reliability failures.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

In this study, the joint research has succeeded in demonstrating a Back-End-ofthe-Line (BEOL) compatible approach to grow amorphous boron nitride (a-BN) with extremely low-k dielectrics. In particular, they synthesized approximately 3 nm thin a-BN on a Si substrate, using low temperature remote inductively coupled plasma-chemical vapour deposition (ICP-CVD). The resulting material showed an extremely low dielectric constant in the range of 1.78, which is 30% lower than the dielectric constant of currently available insulators.

"We found that temperature was the most important parameter with ideal a-BN film deposition occurring at 400° C," says Seokmo Hong in the Doctoral program of Natural Sciences, the first author of the study. "This material with ultra-low-k also manifests a high breakdown voltage and likely superior metal barrier properties, making the film very attractive for practical electronic applications."

Angle-dependent near-edge X-ray absorption fine structure (NEXAFS) measured in partial electron-yield (PEY) mode at Pohang Light Source-II 4D beam line was also used to investigate the chemical and electronic structures of a-BN. Their findings indicated that the irregular, random atomic arrangement causes the dielectric constant value to drop.

The new material also manifests excellent mechanical properties of high strength. Moreover, when researchers tested the diffusion barrier properties of a-BN in very harsh conditions, they found it can prevent metal atom migration from the interconnects into the insulator. This result will help resolves a long-standing issue of interconnects in CMOS integrated circuit fabrication, enabling further miniaturaization of electronic devices.

"Development of electrically, mechanically and thermally robust low-k materials (k < 2) has long been technically challenging," says Dr. Hyeon-Jin Shin from Samsung Advanced Institute of Technology (SAIT). "Our research is also a great example that shows companies and academic institutions working together to create greater synergy."

"Our results demonstrate that the amorphous counterpart of two-dimensional hexagonal BN possesses the ideal low-k dielectric characteristics for high-performance electronics," says Professor Shin. "If they are commercialized, it will be a great help in overcoming the crisis looming over the semiconductor industry."

Tags:  boron nitride  Electronics  Graphene  hexagonal boron nitride  Hyeon Suk Shin  Hyeon-Jin Shin  Samsung Advanced Institute of Technology  UNIST 

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