The goal of this project is to learn to control the flow of light and the conversion of light energy into other forms of energy (and vice versa). This project is fundamental physics research that supports the mission of DOE in the areas of energy-efficient lighting, efficient solar energy utilization, and thermophotovoltaics. Research in this Project can be grouped into two sub-tasks.
Photonic Crystal Physics
Photonic band gap materials are artificially designed periodic dielectric or metallic structures with high refractive-index contrast that can be used to control light (photons) in a manner similar to that used by semiconductors to control electrons. Although this research project originated from theoretical work, its emphasis now is on physical manifestations and tests of the theory. Over the next three years research goals within this subtask will focus on:
- Wide area fabrication of photonic crystal and polymer waveguide structures using reasonable-cost soft lithography techniques. (K. Constant, W. Leung, K.-M. Ho)
- Study of fundamental photonic crystal properties including tailored thermal emission, beam steering and focusing. (R. Biswas, K. Constant, C. Soukoulis, W. Leung, K.-M. Ho)
- Development of highly efficient algorithms for design and study of devices using photonic crystals. Extension of techniques to study non-linear systems or systems with gain as well as the effects of disorder/fabrication defects on the performance of photonic crystal structures. (C. Soukoulis, K.-M. Ho)
Organic Semiconductor Physics
The goal of this subtask is to provide the fundamental physics underpinning necessary to understand and optimize the performance of organic light-emitting devices (OLEDs) at both low and high brightness. More specifically, the goal is to elucidate the interactions (particularly the spin-dependent interactions) between singlet excitons (SEs), triplet excitons (TEs), polarons, bipolarons, and trions, as they impact the optical and transport properties of these materials and devices. For example, our past experimental work has revealed the central role of TEs and polarons in quenching the SEs, thus decreasing the photoluminescence quantum yield of the films and the internal quantum efficiency of OLEDs. Indeed, these quenching processes are now recognized as the source of the decreasing efficiency of OLEDs at high injection current.
Over the next three years research within this subtask will focus on fundamental studies on novel OLED structures, including n-stacked (tandem) OLEDs, graded junction OLEDs, and hybrid polymer/small molecular OLEDs. (J. Shinar)
Tunable near-UV microcavity organic light-emitting diodes (OLEDs) that emit in the deep blue and ultraviolet light region have been developed using a novel approach. These devices address the growing need for portable compact near-UV sources for analytical tools as well as various biomedical and forensic applications. These are among the first OLEDs that emit in the near-UV region. In this new approach, the team tuned the thickness of the spacer layer of a nanometer wide microcavity, allowing them to tailor each individual OLED in the array to the desired narrow-band emission. Simulations of the OLEDs’ emissions were used for analysis of the experimental data and informed device fabrication. The fabricated device was used as the excitation source in an all-organic oxygen sensor, which improved the sensor performance in comparison to the previously used green microcavity OLEDs. Moreover, an array of nine sharp-emitting OLEDs was employed in an all-organic on-chip spectrometer, a device for measuring light intensity, paving the way to compact plastic analytical tools.
Significant LED performance improvements have been achieved by taking advantage of novel materials.An organic light emitting diode (OLED) requires at least one transparent electrode, which is most commonly indium tin oxide (ITO). While ITO is both transparent and a good electrical conductor, its light transmission differs from the other organic material layers used in the device, leading to internal reflections which reduce efficiency. Researchers replaced ITO with a special highly conductive polymer known as PEDOT:PSS. The new OLEDs have a peak power efficiency and other key properties that are among the highest reported to date. They are 44% more efficient than comparable devices made with ITO. The researchers used computer simulations to show that the enhanced performance is largely an effect of the difference of optical properties between the polymer-based electrode and ITO. Because of the improved efficiency and potentially easier processing of these ITO-free OLEDs, the results pave the way for improved commercial OLEDs at lower cost.
A novel electrode architecture has led to a new way to make transparent electrical contacts. Typical ways of attaching a conductor to a non-metallic material allow you to see the electrode. However, for many applications, like light emitting diode (LED) displays, smart windows and solar cells, transparency to visible light is a requirement that conflicts with electrical conductance. Thinner films are more transparent, but less conductive. The new architecture consists of specially patterned nanoscale-thick metallic ribbons, standing on edge, supported by a polymer matrix. Because the ribbons are only about 40 nanometers wide, light can pass between them, but their height provides enough material to ensure high conductivity. The conductivity and transmittance of the new structure rival currently used indium-tin-oxide transparent electrodes, is less brittle, and will enable flexible, large area applications. It provides an alternative to using indium, which has been identified as an “energy critical element”.