Organic photonics deals with sustainable light generation, modulation, and detection using organic materials to create versatile and tunable optical devices such as organic photodetectors, solar cells, and OLEDs. This article provides an overview of organic photonics, offering insights into the current state of research and its diverse applications.
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Organic Photonics: An Overview
Organic photonics is an emerging interdisciplinary field in modern optics and electronics research, focusing on developing optoelectronic devices and components using organic materials.
Organic materials used in photonics applications are primarily semiconducting small molecules or polymers containing extensive carbon-carbon and carbon-hydrogen bonds that can be synthesized and processed via low-cost manufacturing.
The most popular organic materials used are π-conjugated small molecules or polymers. These materials contain alternate single and double carbon-carbon bonds that facilitate charge transport and light emission.
Chemical tuning of the molecular structure of these π-conjugated organic semiconductors allows researchers to optimize their electrical and optical properties. For instance, bandgap and emission wavelength can be adjusted, conductivity can be modulated across twelve orders of magnitude, and solubility in different solvents can be controlled. This incredible fine-tuning ability of molecular design distinguishes organic photonics from its inorganic counterparts.
The optoelectronic characteristics of organic photonics materials and their amenability to simple, low-temperature solution-based processing techniques like spin coating and inkjet printing make them extremely attractive for emerging applications.
As a result, academic and industrial research groups actively develop organic photonics components targeting novel technologies like flexible display panels, conformal image sensors, biodegradable photovoltaics, printable optical communication circuitry, and bio-integrated wearable light-emitting devices.
Research and Application Areas
Organic photodetectors generate electrical signals upon absorbing light. They offer large detection areas, broad spectral sensitivity, mechanical flexibility, and cost-effective and lightweight imaging.
Mass-producible organic photodetectors for enhanced imaging
In a study published in Optica, researchers designed highly sensitive transparent green-light-absorbing organic photodetectors with CMOS fabrication methods.
The study addresses the challenges of photodetectors' mass production by introducing a mixed buffer layer, enhancing stability, efficiency, and detectivity. The resulting photodetectors demonstrate comparable performance to conventional silicon ones, operating stably at high temperatures for extended periods (150 °C for 2 hours and 85 °C for 30 days).
The researchers plan to customize these photodetectors further for applications in mobile and proximity sensors, wearable sensors, and fingerprint-on-display devices.
Organic lasers incorporate high-gain 'active' media made of organic materials. Their flexibility, low-cost fabrication, and potential integration into wearable devices make them promising for diverse fields, albeit with ongoing challenges in improving efficiency and stability for commercialization.
Electrically driven organic semiconductor laser
A study published in Nature has proposed an electrically driven organic semiconductor laser. Unlike traditional lasers made from rigid semiconductor crystals, this organic semiconductor laser is flexible, based on carbon, and emits visible light.
This study eliminates the need for an external laser to power it, opening up possibilities for integration with OLED displays and applications in communication or spectroscopy for disease and environmental pollutant detection.
The researchers anticipate this development could lead to more energy-efficient manufacturing and broader applications across the visible spectrum.
Organic Light-Emitting Diodes (OLEDs)
Organic light-emitting diodes (OLEDs) use thin films of organic semiconductor materials such as polymers and small molecules that emit light when electricity is applied. The tunability of these molecules allows for adjustment in color and efficiency.
OLED displays and lighting have become widely accessible in the commercial market, and the advent of flexible/stretchable OLEDs is unlocking new possibilities, integrating lighting or displays into wearable electronics and bio-integrated health monitors by conformally wrapping devices with soft organic electronics.
OLED with unprecedented low voltage for blue light
In a study published in Nature Communications, researchers designed a novel OLED with an ultralow turn-on voltage of 1.47 V for blue emission, significantly lower than conventional blue OLEDs.
The new OLED operates through upconversion (UC), where electrons and holes are injected into acceptor and donor layers, recombining at the interface to form a charge transfer state. This state's energy is then selectively transferred to the first triplet-excited states of the emitter, resulting in blue light emission through triplet-triplet annihilation.
The UC mechanism and triplet-triplet annihilation reduce the applied voltage required for exciting the emitter, achieving a 100 cd/m2 luminance at just 1.97 V. This development is a significant step toward meeting commercial requirements for efficient blue OLEDs.
Enhancing OLED efficiency and color stability
A study published in Nature Photonics has proposed a method to couple light and matter, enhancing the brightness and color of OLED displays without sacrificing efficiency.
OLEDs inherently exhibit broad emission spectra, limiting color space and saturation for high-end displays.
To address this, the researchers used the strong coupling of light and matter, similar to a microcavity, without the typical viewing angle dependence. They created polaritons by placing the OLED stack between metallic mirrors and adding a light-absorbing film, enabling improved color purity and stability under varying view angles.
The resulting polariton-based OLEDs demonstrated application-relevant efficiency and brightness.
"With a performance in the same range as OLEDs used in commercial displays, but with much-improved color purity and color stability under varying view angles, our polariton-based OLEDs could be of great value to the display industry." Professor Malte Gather, the study's lead investigator.
Organic Photovoltaic Cells
Organic photovoltaic (OPV) or solar cells use carbon-based materials as active layers to convert sunlight into electricity. Unlike traditional silicon-based cells, they are flexible, lightweight, and can be produced using cost-effective manufacturing methods. Ongoing research aims to improve their sustainability, efficiency, scalability, and shorter lifespan compared to silicon cells.
Wood-derived enhancements for solar cells
In a study published in Advanced Materials, researchers improved the stability of the solar cell using untreated kraft lignin, an abundant organic compound derived directly from wood pulp.
The researchers incorporated kraft lignin into the electron transport layer connecting to the cathode. While lignin is currently used in a small portion of solar cells, the researchers aim to develop a solar cell almost entirely composed of wood components, contributing to the development of efficient, reliable, low-cost, and environmentally friendly solar cells.
Organic photonics has advanced significantly, achieving performance comparable to established inorganic technologies while offering unique benefits like flexibility, printability, and sustainability.
Continued molecular design, synthesis, and nanoengineering advancements will further improve these technologies, leading to more efficient organic LEDs, affordable solar cells for developing regions, and integrated biomedical devices.
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References and Further Reading
Park, S., Lim, Y., Heo, C. J., Yun, S., Leem, D. S., Kim, S., ... & Park, K. B. (2022). Transparent organic photodiodes for high-detectivity CMOS image sensors. Optica, 9(9), 992-999. https://doi.org/10.1364/OPTICA.449557
Yoshida, K., Gong, J., Kanibolotsky, A. L., Skabara, P. J., Turnbull, G. A., & Samuel, I. D. (2023). Electrically driven organic laser using integrated OLED pumping. Nature, 621(7980), 746-752. https://doi.org/10.1038/s41586-023-06488-5
Izawa, S., Morimoto, M., Fujimoto, K., Banno, K., Majima, Y., Takahashi, M., ... & Hiramoto, M. (2023). Blue Organic Light-Emitting Diode with a Turn-on Voltage at 1.47 V. Nat Commun. https://doi.org/10.1038/s41467-023-41208-7
Mischok, A., Hillebrandt, S., Kwon, S., & Gather, M. C. (2023). Highly efficient polaritonic light-emitting diodes with angle-independent narrowband emission. Nature Photonics, 17(5), 393-400. https://doi.org/10.1038/s41566-023-01164-6
Zhang, Q., Liu, T., Wilken, S., Xiong, S., Zhang, H., Ribca, I., ... & Fahlman, M. (2023). Industrial Kraft Lignin Based Binary Cathode Interface Layer Enables Enhanced Stability in High Efficiency Organic Solar Cells. Advanced Materials, 2307646. https://doi.org/10.1002/adma.20230764