Although it’s been nearly a century and a half since Thomas Edison flipped a switch to turn on the world’s first practical light bulb, the search for better light sources continues unabated. Many other lighting technologies have been developed since that day in 1879, bringing features such as brightness, color quality, dimming capability, and low life-cycle costs.
Organic LEDs, or OLEDs, are attracting strong interest because they can be used in lightweight, paper-thin, light-emitting panels in a variety of shapes and sizes. They can be used to create flexible or bendable lighting devices applied to a flat or curved surface area to build parts such as car tail lights and even “lighting flowers”.
But OLEDs aren’t nearly as bright or as energy-efficient as their inorganic cousins, LEDs, and so researchers at Konica Minolta, Inc. are racing to develop designs to meet growing demand. The company is a world leader in OLEDs that supports the development of cutting-edge devices for imaging and optics, often working in partnership with Japan’s leading universities.
Leiming Wang is a senior researcher at the Konica Minolta Laboratory USA in San Mateo, CA, working with a team that uses numerical simulation to analyze light-loss mechanisms in OLEDs to virtually test ways to improve designs. “Despite all their advantages, OLEDs suffer from a number of limitations we are working to minimize,” he said. “Most impactful is a complex plasmon coupling phenomenon accounting for 40% of the light lost through interactions within the device.”
HOW OLEDs WORK
OLEDs are composed of organic semiconductors sandwiched between positive (anode) and negative (cathode) electrodes. An OLED device consists of an anode made of transparent indium tin oxide (ITO), three organic layers — a hole transport layer (HTL), emitting layer (EML), and electron transport layer (ETL) — and a silver cathode. These are all fabricated on a glass substrate, which light passes through when the device is turned on.
When current is applied, electrons are injected at the cathode and holes at the anode. Electrons and holes travel toward each other through the layers, combining in the emissive layer to release energy in the form of photons. This happens quickly while current is flowing, causing a stream of continuous light.
CATCHING THE PHOTON THIEVES
But some photons never make it to the outside world. Light losses in an OLED can occur through several mechanisms, such as differences in the refractive indices of each layer that can cause light to reflect within the layers rather than traveling outward.
Wang’s team primarily explored another mode of loss, the coupling of dipole emission with surface plasmons at the interface between the cathode and the organic material. Surface plasmons are waves of oscillating electrons on the surface of a conductor. In OLEDs, light emitted from radiating dipoles (molecular excitons) in the emissive layer can couple to the electron oscillations in the cathode, resulting in the presence of waves called surface plasmon polaritons (SPPs). These travel along the cathode surface as they decay, carrying away the emitted photons rather than permitting them to radiate through the glass.
In other words, due to the presence of the metal cathode in the close vicinity of the organic emitters, some light is absorbed by the electrons in the cathode, causing the electrons to oscillate and form SPPs. These are eventually dissipated as heat, leading to significant energy loss.
Using numerical simulation in COMSOL Multiphysics® software Wang modeled light emission from the EML and the SPPs present in the system to analyze ways to prevent light loss. One promising concept included a nanograting cathode structure that disrupts the formation of the SPP mode, reducing the energy coupling between the dipole emission and the plasmons.
Wang’s simulation revealed the electromagnetic field distribution and the portion of light that escaped from the OLED for different cathode shapes. From the results, his team was able to confirm that this phenomenon accounts for significant amounts of light lost.
COMSOL® software is an important tool at Konica Minolta Laboratory because it’s not only powerful but versatile and user-friendly. Lab personnel use the software for a variety of topics under study there. “For this OLED project we were able to do everything in COMSOL®, including post-processing the data. We also imported wavelength-dependent optical properties from our own files and incorporated them into the simulation,” Wang said.
His team modeled the OLED with flat and nanograting cathodes, changing geometric parameters to determine the optimal configuration. They also performed a simulation to study the influence of different dipole orientations, studying the effect of the dipole position and wavelength on the level of light loss due to SPPs. They used a power flow analysis to calculate the portion of light emitted from the EML that actually escaped the glass.
Through their simulations, Wang’s team determined that they could reduce the plasmon losses by 50% using the optimized nanostructure surface for the cathode.
VERSATILE MODELING BRINGS BRIGHTER LIGHTING
Through his simulation work, Wang was able to offer a promising new OLED design with significantly increased efficiency. “We were able to model the OLED system and determine the optimal configuration of the cathode nanograting structure,” he concluded. “We could understand the breakdown of loss mechanisms, easily test the influence of different design constraints, and adjust our OLEDs accordingly. COMSOL® has shown us how to cut these plasmon losses in half.”
Click here to check out the 2016 edition of Multiphysics Simulation and learn how numerical analysis is being leveraged as a powerful tool in other industries.