August 22, 2024
RIKEN
Tosoh Corporation
Nagoya University
– Achieving hole transport properties comparable to conventional primary materials using only carbon and hydrogen –
This research was carried out by Kenichiro Itami, a principal investigator at the Itami Molecular Creation Laboratory, RIKEN Exploratory Research Center (and principal investigator at the Institute of Transformative Bio-Molecules (WPI-ITbM) at Nagoya University), along with Yuta Morinaka, a principal investigator at the Organic Materials Research Institute of Tosoh Corporation (currently a principal researcher at the Advanced Materials Research Institute, Advanced Interdisciplinary Research Center, Research Center at the time of the study).Joint Research Groupteeth,Heteroatoms[1]Without utilizing any substituents,Organic EL[2]ofHole Transport Materials[3]We have identified a hydrocarbon-based hole transport material that functions as a
The findings of this research are as follows:Molecular Nanocarbon Science[4]” Hydrocarbon-based materials developed fromOrganic Electronic Devices[5]This represents a significant advance in the field of interdisciplinary research connecting organic electronic devices and molecular nanocarbon science, where only a few examples have been verified.
The joint research group has now shown that organic electroluminescent devices using non-planar hydrocarbon-based materials (HBTs), a type of nanocarbon, as hole transport materials have significant potential.Triarylamines[6]The characteristics of the HBT that underpin this performance were clarified through quantum chemical calculations and analysis of solid-state films.
This research was published in the scientific journalApplied Chemistry International Edition in the online version on August 13th.
Hydrocarbon-based hole transport material identified in this study
Background
The hole transport layer, which is a component of organic electronic devices like organic solar cells and organic electroluminescent (EL) devices,is crucial for achieving effective performance. However, the hole transport materials used in these layers have predominantly been triarylamine compounds that contain nitrogen atoms. While efforts to develop hole transport materials that do not rely on triarylamines have been reported thus far, no hydrocarbon-based materials composed solely of carbon and hydrogen atoms have shown performance on par with that of triarylamines.
Previously, the joint research group successfully synthesized hydrocarbon-based materials with a broad range of structures, and this time, they focused on a molecule called “HBT,” a form of non-planar hydrocarbon material, to analyze its molecular characteristics and explore its application in organic electroluminescence.
Research Methods and Results
The joint research group examined the properties of HBTs through a combination of single-crystal X-ray structure analysis and quantum chemical calculations (Figure 1). In the crystal structure of HBT, individual molecules are arranged in a staggered manner, forming a one-dimensional columnar (vertical) arrangement.Transfer Integral[7]Analysis revealed that each HBT molecule interacts not only in the column direction but also with neighboring molecules in adjacent columns. Strong intermolecular interactions of the HBTs were also evident in the solid film created by evaporating HBTs. Additionally, the highly twisted structure of the HBTs enhances the film’s stability, making itamorphous[8].
Figure 1. Various analytical results of HBT
- (A)Single crystal X-ray structure analysis confirmed the development of a one-dimensional columnar structure for HBT.
- (B)The transfer integral of the HBT molecule is numerically presented based on quantum chemical calculations of the single crystal X-ray structure. The intermolecular interactions of HBTs are highlighted in the areas indicated by arrows; with larger values signifying stronger interactions.
- (C)A solid film formed on a quartz substrate through vacuum deposition was heated in air. In contrast to the comparative compound DBC, a substructure of HBT that crystallizes readily upon heating, HBT preserved its amorphous nature due to its highly twisted configuration.
Subsequently, the HBT solid film was analyzed usingAirborne photoelectron yield spectroscopy[9], which indicated that the HBT solid film exhibited characteristics similar to those of triarylamine solid films.HOMO level[10]Quantum chemical calculations revealed that the key aspect of HBT’s organic synthesis lies in theAPEX (annulative π-extension)[11]approach, which effectively utilizes the HOMO level to attain the desired HOMO level for hole transport materials (Figure 2).time-of-flight method[12]Results from NMR measurements demonstrated hole mobilities comparable to those of triarylamines, while electron mobilities were not detected. These findings underscore the potential of HBTs as hole transport materials in organic electronic devices.
Figure 2. Comparison of linear/cyclized and APEX approaches for the HOMO levels of polycyclic aromatic hydrocarbons
The HOMO levels of each model compound were established through quantum chemical calculations, demonstrating that the APEX approach substantially elevates the HOMO level compared to linear or cyclization methods.
Finally, HBT was utilized as a hole transport material in organic electroluminescence devices, which demonstrated that devices with a hole transport layer made from HBT outperformed those using conventional triarylamine materials (α-NPD, TCTA). This marks the first report of a material made solely from carbon and hydrogen, devoid of any heteroatoms or substituents, exhibiting performance comparable to triarylamines.
Figure 3. Element evaluation results of an organic EL device using HBT
- (A)The device’s configuration, band diagram, and materials employed in the organic EL device fabricated during this study. The numbers under the material labels indicate film thickness, while those above and below the band signify experimentally measured LUMO (Lowest Unoccupied Molecular Orbital) and HOMO levels, respectively.
- (B)Emission spectrum. The emission color purity is consistent across all cases.
- (C)Voltage-current density curve. A preferred device allows a substantial current to pass at low voltage when voltage is applied, necessitating a lower driving voltage.
- (D)Luminance vs. external quantum efficiency curve. Higher external quantum efficiency is desirable.
- (E)Time-brightness decay curve. The degradation time was assessed through an accelerated test by continuously illuminating the device, estimating the element’s lifespan. Devices capable of maintaining brightness over extended periods are preferable.
Future Expectations
This research has demonstrated that nanocarbon materials developed from molecular nanocarbon science, which are fundamentally distinct from traditional triarylamines, can be applied in the realm of organic electronic devices. It has also been found that nanocarbon materials composed solely of carbon and hydrogen exhibit performance on par with materials that incorporate heteroatoms and substituents.
As pioneering materials that leverage heteroatoms and substituents have historically propelled significant advancements in organic electronic devices, it is anticipated that nanocarbon materials stemming from molecular nanocarbon science will similarly contribute to the ongoing evolution of organic electronic devices in the future.
Additionally, many of the synthesized nanocarbons have not yet been explored for application in organic electronic devices. The joint research group intends to continue fostering interdisciplinary research between molecular nanocarbon science and organic electronic devices by verifying the applicability of these nanocarbons in such devices.
Supplementary Explanation
- 1.Heteroatoms
Refer to atoms other than carbon and hydrogen. In organic materials for electronic devices, structures and substituents containing nitrogen, oxygen, sulfur, etc., are extensively utilized. Particularly, nitrogen atoms are frequently employed as critical components that considerably alter the energy levels of compounds based on their bonding configurations. - 2.Organic EL
In OLEDs, holes and electrons are injected from the electrodes surrounding the organic layer, subsequently recombining in the organic layer’s center to cause light emission. To realize practical performance, materials specifically tailored for each component of the operational mechanism—”injection,” “transport,” “recombination,” and the “emission” of organic materials—are utilized, enabling functional separation. Most organic layers current on the market feature a multi-layer structure. - 3.Hole transport material, hole transport layer
A hole refers to the vacancy created when a molecule loses an electron. In organic electronic devices, holes are treated as carriers flowing in the opposite direction to electrons. The hole transport layer is one of the organic layers that facilitate the movement of holes necessary for light emission in organic EL devices, and the materials suitable for this layer are known as hole transport materials. - 4.Molecular Nanocarbon Science
Nanocarbon science synthesizes structurally pure nanocarbon materials, previously mixed, while clarifying the relationship between structure and physical properties for various applications. Principal Investigator Itami and his team are pioneering a new research field termed “molecular nanocarbon science,” grounded in organic and synthetic chemistry. - 5.Organic Electronic Devices
This term broadly designates devices wherein the electrode interval is filled with organic compounds. Typical examples include organic electroluminescence (EL), dye-sensitized solar cells, perovskite solar cells, and organic field-effect transistors. The hole transport layer investigated here is commonly utilized in organic EL and perovskite solar cell research. - 6.Triarylamines
A class of compounds where all three substituents bonded to a central nitrogen atom consist of aromatic hydrocarbon groups (aryl groups). Their unique traits, including strong donor properties, excellent hole mobility, heat resistance, and amorphous character, have led to their use for decades as a fundamental molecular structure in hole transport materials. - 7.Transfer Integral
A parameter indicating the degree of molecular orbital overlap between molecules; a larger absolute value of the transfer integral (between occupied orbital levels such as HOMO) signifies a higher ease of hole transfer. - 8.Amorphous
Also referred to as amorphous, this state indicates non-crystallization and is characterized by the absence of the order seen in crystals. It is generally challenging to obtain high performance using crystalline films in organic EL devices, leading to a preference for materials capable of forming amorphous films. - 9.Airborne photoelectron yield spectroscopy
A technique for measuring thin films’ work function and ionization potential, developed by Dr. Masayuki Uda, associated with RIKEN. In this study, ionization potential corresponds to the HOMO level, employing terminology favored by organic chemists. - 10.HOMO level
The highest energy level in a molecular orbital occupied by electrons, recognized in organic electroluminescent devices as the level most favorable for hole flow. Molecules with elevated (shallow) HOMO levels typically excel in transporting holes compared to those with lower (deeper) HOMO levels. - 11.APEX (annulative π-extension)
This term encompasses reactions employing simple compounds as starting materials, allowing one-step π-extension while condensing; it is effective in efficiently synthesizing extended π-conjugated molecules such as nanographene. This innovative synthesis methodology was proposed and developed by Principal Investigator Itami and his team in 2015. - 12.Time-of-flight method
A method for measuring mobility by detecting the transit duration of carriers generated when light irradiates an organic film under an applied electric field. This approach additionally facilitates observation of the bulk mobility of amorphous films.
Joint Research Group
RIKEN Exploratory Research Center Itami Molecular Creation Laboratory
Chief Researcher: Kenichiro Itami
(Principal Investigator, Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University)
Tosoh Corporation
Institute of Organic Materials
Principal Investigator (at the time of the study): Yuta Morinaka
(Currently Chief Researcher, Advanced Materials Research Institute, Advanced Fusion Research Center, Research Headquarters)
Chief Researcher (during the study): Yohei Ono
(Currently Chief Researcher, Advanced Materials Research Institute, Advanced Fusion Research Center, Research Headquarters)
Patent Office
Director: Tsuyoshi Tanaka
Nagoya University Graduate School of Science
Associate Professor Hideto Ito
Nagoya University Institute of Transformative Bio-Molecules (WPI-ITbM)
Professor Takeshi Yanai
Specially Appointed Associate Professor Kazuhiro Fujimoto
Research Support
This research was funded by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (Specially Promoted Research) “Creation of Unexplored Molecular Nanocarbons (Principal Investigator: Kenichiro Itami)” and by the Japan Science and Technology Agency (JST) Strategic Basic Research Programs (ERATO) “Itami Molecular Nanocarbon Project (Principal Investigator: Kenichiro Itami).”
Original Paper Information
- Yuta Morinaka, Hideto Ito, Kazuhiro J. Fujimoto, Takeshi Yanai, Yohei Ono, Tsuyoshi Tanaka, Kenichiro Itami, “Nonplanar Nanographene: A Hydrocarbon Hole-Transporting Material That Competes with Triarylamines”, Applied Chemistry International Edition, 10.1002/anie.202409619
Presenter
RIKEN
Pioneering Research Department, Itami Molecular Creation Laboratory
Chief Researcher: Kenichiro Itami
(Principal Investigator, Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University)
Tosoh Corporation Organic Materials Research Laboratory
Principal Investigator (at the time of the study): Yuta Morinaka
(Currently Chief Researcher, Advanced Materials Research Institute, Advanced Fusion Research Center, Research Headquarters)
Kenichiro Itami
Yuta Morinaka
Press Contact
RIKEN Public Relations Office Press Officer
Contact form
Tosoh Corporation Public Relations and Investor Relations Office
Tel: 03-6636-3712
Research Promotion Division, Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Tokai National Higher Education and Research System
Tel: 052-789-4999
Email: press [at] itbm.nagoya-u.ac.jp
※The above[at]Please replace with @.
Inquiries Regarding Industrial Use
August 22, 2024
RIKEN
Tosoh Corporation
Nagoya University
– Realization of hole transport capabilities comparable to those of conventional main materials using only carbon and hydrogen –
The study was conducted by Kenichiro Itami, a principal researcher at the Itami Molecular Creation Laboratory, RIKEN Exploratory Research Center, and Yuta Morinaka, a principal researcher at the Organic Materials Research Institute at Tosoh Corporation. This research group has discovered a hydrocarbon-based hole transport material that functions effectively in organic electronic devices without using any substituents or heteroatoms. The findings highlight the potential of this material to drive advancements in organic electroluminescence (EL) devices.
Background
The hole transport layer (HTL) is a crucial component in organic electronic devices, including organic solar cells and organic EL devices. Traditionally, these layers have relied on triarylamines containing nitrogen atoms, posing limitations on material diversity. The recent research demonstrates the capabilities of hydrocarbon-based materials, specifically non-planar structures like HBT, offering a promising alternative that does not depend on heteroatoms or substituents.
Research Methods and Results
The research team employed advanced techniques such as single-crystal X-ray structure analysis and quantum chemical calculations to analyze the properties of HBTs. The results indicated that HBT molecules are arranged in a staggered one-dimensional columnar structure, facilitating strong intermolecular interactions that enhance hole mobility. The findings emphasize the significance of these interactions for maintaining stability and achieving high performance in organic electronic devices.
Hydrocarbon-based hole transport material discovered in this study
One of the remarkable discoveries includes the HOMO (highest occupied molecular orbital) levels of HBTs, achieved through an innovative approach known as APEX (annulative π-extension). This methodology enhances the HOMO levels necessary for effective hole transport, competing with traditional triarylamine materials.
Figure 1. Various analysis results of HBT
- (A)Single crystal X-ray structure analysis confirmed a one-dimensional columnar structure.
- (B)Transfer integral analysis reveals strong intermolecular interactions.
- (C)The highly twisted HBT structure facilitates amorphous film formation, crucial for stability.
The team tested HBT as a hole transport material in organic EL devices, finding that HBT exhibited superior performance compared to traditional triarylamine compounds like α-NPD and TCTA, marking a significant shift in material innovation.
Figure 3. Element evaluation results of an organic EL device using HBT
- (A)Device configuration with materials and band diagram.
- (B)Emission spectrum showing color purity.
- (C)Voltage-current density curve indicative of low driving voltage.
- (D)Luminance vs. external quantum efficiency, showing high performance.
- (E)Time-brightness decay curve indicating long element life.
Future Expectations
This groundbreaking research anticipates that nanocarbon materials, with a unique composition of only carbon and hydrogen, will lead to significant advancements in organic electronic devices. The study invites future investigations into various synthesized nanocarbon structures yet to be explored for such applications.
Supplementary Explanation
- 1.Heteroatoms
Atoms other than carbon and hydrogen often used in organic materials. - 2.Organic EL
Devices that emit light through organic materials; require optimized components for efficient function. - 3.Hole transport materials
Essential for facilitating carrier movement in organic electronic devices. - 4.Molecular Nanocarbon Science
Concerned with the synthesis and application of unique nanocarbon structures. - 5.Organic Electronic Devices
Devices utilizing organic compounds for functionality, including organic ELs and solar cells. - 6.Triarylamines
Traditional hole transport materials with strong donor properties. - 7.Transfer Integral
Measures the overlap of molecular orbitals between adjacent molecules. - 8.Amorphous
Describes materials lacking crystalline order, vital for performance in EL devices. - 9.Airborne photoelectron yield spectroscopy
A measurement method for analyzing the properties of thin films. - 10.HOMO level
The energy level through which the majority of holes flow, crucial for transportation efficiency. - 11.APEX (annulative π-extension)
A synthetic methodology to enhance molecular structures effectively. - 12.Time-of-flight method
A technique used to measure carrier mobility in organic films.
Joint Research Group
RIKEN Exploratory Research Center Itami Molecular Creation Laboratory
Chief Researcher: Kenichiro Itami
(Principal Investigator, Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University)
Tosoh Corporation
Institute of Organic Materials
Principal Investigator (at the time of the study): Yuta Morinaka
(Currently Chief Researcher, Advanced Materials Research Institute, Advanced Fusion Research Center)
Chief Researcher (at the time of the research): Yohei Ono
(Currently Chief Researcher, Advanced Materials Research Institute, Advanced Fusion Research Center)
Patent Office
Director: Tsuyoshi Tanaka
Nagoya University Graduate School of Science
Associate Professor Hideto Ito
Nagoya University Institute of Transformative Bio-Molecules (WPI-ITbM)
Professor Takeshi Yanai
Specially Appointed Associate Professor Kazuhiro Fujimoto
Research Support
This research was supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid and the Japan Science and Technology Agency (JST) Strategic Basic Research Programs (ERATO).
Original Paper Information
- Yuta Morinaka, Hideto Ito, Kazuhiro J. Fujimoto, Takeshi Yanai, Yohei Ono, Tsuyoshi Tanaka, Kenichiro Itami, “Nonplanar Nanographene: A Hydrocarbon Hole-Transporting Material That Competes with Triarylamines”, Applied Chemistry International Edition, 10.1002/any.202409619
Presenter
RIKEN
Pioneering Research Department, Itami Molecular Creation Laboratory
Chief Researcher: Kenichiro Itami
Tosoh Corporation Organic Materials Research Laboratory
Principal Investigator (at the time of the study): Yuta Morinaka
Kenichiro Itami
Yuuta Morinaka
Press Contact
RIKEN Public Relations Office Press Officer
Contact form
Tosoh Corporation Public Relations and Investor Relations Office
Tel: 03-6636-3712
Research Promotion Division, Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Tokai National Higher Education and Research System
Tel: 052-789-4999
Email: press [at] itbm.nagoya-u.ac.jp
※The above [at] Please replace with @.