Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - iTPX (In-cavity thermophotonic cooling)
Teaser
The possibility that a light emitting diode (LED) cools down as a result of light emission has been acknowledged already half a century ago. This effect is known as electroluminescent cooling (ELC), but it has never been observed close to normal operating conditions of LEDs...
Summary
The possibility that a light emitting diode (LED) cools down as a result of light emission has been acknowledged already half a century ago. This effect is known as electroluminescent cooling (ELC), but it has never been observed close to normal operating conditions of LEDs. This is despite the fact that the material parameters reported for common III-V semiconductors are sufficient to suggest that presently available materials are suitable for ELC. Harnessing ELC would allow not only fabricating extremely efficient LEDs, but also provide a promising solid state optical heat pumping mechanism that could ideally e.g. overcome some of the efficiency limitations of Peltier coolers. This project aims at observing and developing ELC in one of the most favourable experimental setups designed for the purposes of this project: an intracavity LED structure where an LED is enclosed within the same semiconductor epistructure as a photodetector (PD) used to electrically measure the LED performance. This approach allows eliminating some of the key challenges of conventional LEDs in extracting light from a high refractive index semiconductor as well as simple means to measure the performance of the LED.
Work performed
The project started by designing, fabricating and characterizing the so called double diode structures (DDS), which are intracavity devices consisting of a double heterojunction LED and a homojunction photodetector enclosed within the same crystal structure fabricated on a GaAs substrate. The characterization of the first generation of DDS confirmed that the DDS itself is a promising platform for studying ELC, due to its small internal losses and other similar unidealities. Since the first reported DDS devices we have improved the design and fabrication methods of the device, which has allowed to increase the quantum efficiencies of the device from the initial ~50% to above 70%. The general expectation is that the threshold for ELC can be reached when the quantum efficiency is around 80-90%. Further analysis also suggests that in our most recent DDS structures the LED component itself may already have reached the ELC threshold, but the direct observation of this result still requires at least improving the efficiency of the photodetector part of the DDS.
Final results
This far the project has advanced beyond the state of the art by introducing the DDS and by producing conventional III-V LEDs with the highest directly measured quantum efficiencies. Analysis of the devices has also suggested that the fabricated LEDs have already passed the ELC threshold, but directly confirming this conclusion will still require further optimization of the structures. We have also developed simulation tools for more detailed analysis of the results, allowing more accurate studies of the limits and requirements of reaching the ELC regime. During the second half of the project we will focus on the direct observation of the ELC and on providing more insight on the effects various device parameters and operating conditions have on the performance to better understand the prospects ELC may offer for LEDs, optical cooling and the general society.