New faculty Lee enhancing performance and fabrication of solar cells

A pioneer in the field of photovoltaic cell research, Professor Minjoo Larry Lee has worked on improving the efficiency and fabrication processes of solar cells for nearly 10 years. Lee recently joined the ECE Illinois and Micro + Nanotechnology Lab (MNTL) faculty, having been an electrical engineering associate professor at Yale University.

At MNTL, one of Lee’s big projects is tackling the solar energy storage problem with the support of a U.S. Department of Energy Advanced Research Projects Agency (ARPA-E) grant. Lee and his teammates aim to design a new class of solar cells that will be able to withstand temperatures of up to 400°C.

“Solar cells inherently lose energy during the energy conversion process, mostly in the form of heat,” Lee explained. “The idea is to take that heat and store it…and at 400°C you can drive steam turbines to generate electricity at night or whenever the grid demands it.”

Redesigning the solar cells to withstand such high temperatures comes with many challenges, in terms of both materials science and device design. Traditional solar modules are optimized to eliminate heat, and at high temperatures, harmful reactions between the metal and the semiconductor can occur. Furthermore, at high temperatures fundamental device physics change significantly, affecting efficiency.

“We have to accept the efficiency loss that comes with high temperature, plan for it, and redesign the solar cells accordingly,” said Lee, who aims to enable solar-thermal hybrid systems with efficiency exceeding 40%.

As a faculty member at Yale, Lee helped pioneer a dislocation engineering technique called metamorphic epitaxy, which is used to create some of the highest efficiency solar cells existing today. His group recently received a certified efficiency of 15% for a III-V solar cell grown on silicon, the best of its kind and a vital step towards Si-based multi-junction cells with efficiency exceeding 35%; current silicon cells top out at 22-25%.

Lee earned his PhD in electronic materials in 2003 at MIT, where he then held a three-year post-doctoral research position at the Microsystem Technology Lab. He made strained-germanium MOSFETs, a material that has the highest hole mobility, making it perfect for p-channel transistors.

“I’m proud of doing work that brings greater functionality to the silicon platform, which is the absolute workhorse of the semiconductor industry,” said Lee. “It is exciting and motivating work because there is commercial interest in the technology.”

In 2006, Lee joined RTI International, where he worked on using thermal gradients to harvest energy from thermoelectric materials like silicon-germanium. It was at RTI that Lee began his solar cell technology research.

According to Lee, one of his favorite things about Illinois is the current strength and historical legacy of research on electronic materials, solid-state devices, III-V compound semiconductors, and semiconductor epitaxy. “I studied these closely all throughout my schooling and read many papers that came from this institution,” said Lee.

Other factors about Illinois that appealed to Lee are the strength of its materials science and ECE departments, the expertise of MNTL support staff, faculty, and students, and the excellent fabrication capabilities.