Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet range

Significance 

Deep UV light emitting diodes (UV-C LEDs) have numerous applications in UV curing, phototherapy, disinfection, water purification, fluorescent spectroscopy, bio-analysis and detection, sensors and monitors. Generally, the UV-C LEDs are based on AlGaN and present competitive new solid-state lighting ultraviolet sources. UV-C LEDs have numerous advantages over conventional mercury lamps. For instance, while the emission wavelength of low-pressure mercury lamps is fixed at 253.7 nm, the emission wavelength of DUV-LEDs can be tuned to various individual wavelengths across the UV spectrum. Consequently, they have earned a place in the ranks of most sought-after research areas. So far, research has shown that junction temperature (TJ) control is mandatory to improve the optical efficiency, lifetime and the wavelength accuracy of the UV-C LED light sources. Thermoelectric cooler devices have also been shown to allow a critical control of the junction temperature, based on direct measurement of the solder point temperature. Unfortunately, several fundamental shortfalls have inhibited their progress.

Research has shown that the optical performance of a UV-C LED device decreases, due to a variety of key factors such as quantum-confined Stark effect and electron leakage. In addition, excessive self-heating of the UV-C LED has also been reported to reduce the lifetime of the devices, the optical performance and yield spectral shift of the emission. Often, the range of the electrical LED current is specified by the manufacturer. If the maximal value of TJ is exceeded, the lifetime decreases substantially below the specified maximal life time value. Therefore, it would be beneficial to further assess the optical performance of the UV-LEDs devices.

In this view, a group of researchers from the University of Santiago de Chile, Professor Pablo Fredes, Professor Ulrich Raff, Professor Ernesto Gramsch, José Pascal (PhD candidate) and Javiera Cuenca, proposed to implement a PID control technique to control the voltage in the Thermoelectric Cooler Device (TEC) devices and therefore the desired range of junction temperatures. Their work is currently published in the research journal, Microelectronics Reliability.

In their study, a 285 nm LED source model: VPS171 by Nikkiso Co, LTD was used. To be precise, the researchers obtained the PID parameters with computational simulations based on physical models and experimental recordings of the solder temperature dynamics. More so, the maximal value of the junction temperature (TJ) provided by the manufacturer, which was specified for the samples used in their study was 100 °C.

The authors reported that the TEC made it possible to reduce the solder point temperature (TS), and therefore the TJ. In addition, the team observed that control of the TEC voltage allowed the temperature control of TJ. The consistency between the PID parameters obtained in the simulation and recorded temperature data, verified their junction temperature control model. Moreover, each UV-C LED device was seen to have a different thermal performance, depending on many factors, e.g. thermal conductivity of the heat dissipaters, the fan extractor velocity and operation ambient temperature.

In summary, the study by Professor Pablo Fredes and his colleagues demonstrated that the junction temperature of the UV-C LED devices could be controlled using a Thermoelectric Cooler Device applying an appropriated PID control strategy. Overall, in an interview with Advances in Engineering, Professor Pablo Fredes emphasized that every LED device can be modeled using the above developed control strategy, including a pre-cooling procedure and the voltage driving mode, owing to the fact that at lower temperature operations, optical performance was improved.

Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering

Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering
Fig. Components of the UV-C LED device A: (a) UV-C LED SMD chip, (b) MCPCB, (c) Aluminum plate, (d)Thermal isolators, (e) TEC, (f) Aluminum heat dissipater, (g) Fan heat extractor. B: All components shown in (A) are mounted.
Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering
Fig. Representations of the thermal interfaces from junction surface to ambient temperature
Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering
Fig. Visualization of a UV-C LED system, describing the series of thermal resistances and the points of the thermal interfaces, specified on the right hand side.
Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering
Fig. Schematic diagram for the temperature controller.
Thermoelectric effect can improve the optical emission in Solid State Lighting (light emmitting diodes) in deep Ultraviolet rang - Advances in Engineering
Fig. Normalized irradiance as a function of Junction Temperature.

About the author

Pablo Fredes is a Physical Engineer born in Santiago, Chile in 1983. He received his B.S. in Applied Physics from the University of Santiago de Chile, in 2009, the master’s degree in Philosophy of Science in 2015, and the Dr. degree from the same University in 2015. Fredes is Professor Instructor of Optics and Modern Physics, in the Physics Department of the Universidad de Santiago de Chile, and member of the Optics and Semiconductors Laboratory from the same department. He founded the Hydralux Company  to develop and commercialize UV Disinfection Technologies. Dr. Fredes is member of the Optical Society (OSA) since 2008 and International Ultraviolet Association (IUVA) since 2015.

About the author

Ulrich Raff He received the Ph.D. degree in Nuclear and Atomic Physics at the University of  Basle, Switzerland, in 1972. He worked from 1980 to 1996 at the School of Medicine in Denver, University of Colorado, CO, USA in radiological sciences. Dr. Raff was a Faculty Member at the University of Colorado Health Science Center from 1982 to 1996, and Associate Professor at the Department of Neurology, School of Medicine, New York University from 1999 to 2001.

.

About the author

Ernesto Gramsch He received the Ph.D. degree in Physics at the City University of New York, in 1992. Dr. Gramsch Currently is the Director of the Optics and Semiconductors Laboratory at the Physics Department, Universidad de Santiago de Chile.

.

.

About the author

José Pascal was born in Santiago, Chile. He is an Electrical Engineer graduated from Universidad de Santiago de Chile, Santiago, Chile, in 2016. He is currently pursuing a doctoral degree in the Engineering Sciences program, at Universidad de Santiago de Chile, and has actively participated in research on the development and application of identification methods for robots.

.

About the author

Javiera Cuenca, She is currently pursuing the Bachelor degree in Applied Physics at University of Santiago de Chile. Miss Cuenca is member of Optical Society (OSA) from 2013, currently is the President of the OSA Student Chapter at the same University and has actively participated in research on the development and application of ultraviolet light for disinfections processes.

.

Reference

P. Fredes, U. Raff, E. Gramsch, J. Pascal, J. Cuenca. Junction temperature control of UV-C LEDs based on a thermoelectric cooler device. Microelectronics Reliability, volume 98 (2019) page 24–30.

Go To Microelectronics Reliability

Check Also

A multibeam metamaterial backward wave oscillator