ECE 443 Course Project Highlights: Portable device charging applications, high wall plug efficiency innovations, and directed energy applications
Every spring, the ECE 443 LEDs and Solar Cells course encourages the class to address a grand societal challenge by putting the students’ semester-long learnings into the creation of a project. The objective of the class is to explore energy conversion devices so as to enable energy-efficient and scalable light emitting diodes and solar cells as solutions to the grand challenges in energy, communication, and health.
The MicroLink Devices Best Project Award was given to MatSE senior Karen Yang for her project entitled "High Efficiency Triple-Junction Solar Cell for Portable Device Charging Applications."
Traditionally, high efficiency solar cells based on III-V materials have been limited to space-exploration applications due to their high cost. However, with the technological maturity of silicon-based solar panels, using these III-V cells in terrestrial applications is a possible next step towards widespread solar adoption. Specifically with high reliance of the current generation of children and adults on portable electronic devices such as iPads for entertainment and iPhones for GPS navigation, integrating solar energy into these devices would remove parental stress related to bringing five different charging cables to charge devices and the potential to be stranded without GPS guidance.
Thus, Yang proposed her design for an iPad case containing an array of triple-junction solar cells to provide direct power to the device while it is on and storage of electricity in an attachable battery when the device is powered off. The surface area available for solar panels on an iPad is extremely limited and is not enough for a solar array based on commercial silicon panels to power the device, as the panels only have an efficiency of around 20%. With a triple-junction solar cell design based on III-V materials, however, array efficiencies have been shown to exceed 40%, reducing the amount of space needed by a factor of two. While the solar array will not be able to instantaneously supply the power needed to constantly charge the device, the cell simulated here with CrossLight TCAD software was designed such that the operational condition is at the maximum power point while matching the output current and voltage to that of a lithium-ion battery. The layer thicknesses and doping levels were adjusted to satisfy the criteria previously mentioned. Simulations were done for the standard AM1.5 solar spectrum, and a brief financial feasibility analysis was conducted to determine how much expense the attached solar array would add to the price of an iPad case.
The Crosslight Best Project Awards were given to ECE graduate students Jaekwon Lee and Robert Kaufman. Lee was recognized for his project entitled "2D Optical-Thermal-Electrical Simulation for High Wall Plug Efficiency Cubic GaN Green LED on Silicon."
Despite the relative success of the phosphor-converted Solid-State Lighting (SSL), the need for high efficiency green LED, especially at high current densities, has been emphasized over the past two decades. Cubic InGaN-based LEDs on silicon are a promising platform to realize high-efficiency green emission. Their superior properties over their hexagonal counterparts are apparent in the literature, with no doubt dictating higher Internal Quantum Efficiency (IQE) than the conventional InGaN-based green LEDs.
However, there are almost no studies on the cubic GaN LED itself, and especially there are no studies on their aspects other than the IQE: light extraction, junction temperature, peak emission wavelength, etc. These properties can actually be pointed out as weaknesses of cubic GaN LEDs compared to hexagonal ones, which deems Lee's study critical for the realization of high wall plug efficiency(WPE) green LED. Here, the 2D optical-thermal-electrical modeling of cubic GaN green LED on silicon is suggested, and several designs are simulated to solve the issue of light extraction, junction temperature, and peak emission wavelength, which are mostly limited by the current crowding. As a result, it is found that the DOE 2025 goal for green LEDs can be achieved theoretically by the flip-chip vertical LED design.
Kaufman was recognized for his project entitled "HgCdTe Long-Wave Infrared Photovoltaics for Directed Energy Applications."
Free-space optical beam propagation in the long-wave infrared has been a field explored primarily for communication applications but may also provide utility in power-delivery, often termed as directed energy transfer. One critical issue that any of these systems must deal with is the absorption and perturbation of light in the atmosphere. However, there are certain wavelength ranges, notably in the mid-wave and longwave infrared (MWIR and LWIR), where the atmosphere is almost completely transmissive. With mature laser systems operating at LWIR wavelengths, such as quantum cascade lasers and CO2 lasers, it may be practical to pair these sources with efficient LWIR photovoltaics (PV) to create directed energy systems for free-space power delivery. This could be utilized to network power supply between satellites or for powering electric drones and allowing continuous flight.
Kaufman's work presents a rudimentary design for a HgCdTe based LWIR photovoltaic to act as the receiver in such a system and utilizes Crosslight APSYS to perform electrical simulations. The design is optimized to convert the 10.6 μm optical emission of CO2 lasers and multiple versions designed to operate a range of temperatures from 77 K to 300 K are investigated. Beyond this, the effects of incident power density is also explored. The best performing design and conditions produced a simulation result of 17.68% conversion efficiency at 77 K with 10 kW/m2 incident optical power density. Finally, the limitations with using HgCdTe photodiodes as the power conversion device is reviewed such as its dependence on active cooling solutions.
ECE 443 Best Projects can be found here.