The goal of this research project is to explore new methods to alter electronic band structure and therefore the optical properties of semiconductor materials so as to transform them into more efficient solar energy converters. The electronic band structure will be modified by optical hyperdoping via femtosecond laser irradiation in the presence of various gases. This process leads to higher levels of doping than are possible using other methods. To better understand and optimize the dynamics of optical hyperdoping, the team will develop new mathematical tools for modeling the hyperdoping process, focusing on the quantum mechanics associated with the unusual band structure, as well as methods for understanding and controlling the non-equilibrium process itself. The multidisciplinary team of PIs will integrate expertise in mathematics and continuum modeling of optical hyperdoping, theoretical chemistry and modeling at the quantum level, materials science of optoelectronic materials, and chemistry of surfaces and interfaces to make a concerted attack on creating and understanding new materials with transformative potential and broader impacts. NON-TECHNICAL SUMMARY: Meeting the challenge of harvesting solar energy with Earth-abundant materials such as Si and TiO2 require transformative approaches to increase efficiency, lower manufacturing cost, and reduce material requirements. While these materials have been widely studied, a multidisciplinary team from Harvard University brings a new approach to modifying the properties of semiconductors so as to open the door to more efficient solar cells. In addition, the multidisciplinary nature of the research provides unique training for students and postdocs involved in the project, with a synergistic experimental program combining materials science and chemistry, intimately coupled to a theoretical program combining mathematical analysis of the materials processes and quantum mechanical calculations of the band structure. This project is co-funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences.
We created an interdisciplinary team of scientists and engineers from Harvard, MIT, and abroad with a simple goal: to create new materials capable of converting the sun’s light into energy in more efficient ways than current technology is capable of. Our strategy was to take common, abundant materials and transform them into special alloys that absorb more light from the sun. At Harvard, Eric Mazur’s research group focused on creating two of these special alloys: silicon mixed with sulfur, and titanium dioxide mixed with metals. The silicon alloy might be used to make high-efficiency photovoltaics (which make electricity out of sunlight) and that the titanium dioxide alloy can be used to make high-efficiency photocatalysts that use sunlight to split water into oxygen and hydrogen. (The hydrogen can then be used as a fuel.) To create these alloys, Mazur's group used a special laser that produces ultra-intense pulses of light that last only around a millionth of a billionth of a second. These laser pulses melt the target material (silicon or titanium dioxide), and the rest of the alloy materials mix in the melt. The material then resolidifies very quickly – fast enough to trap the alloy materials in a way that could not be achieved under ordinary circumstances. Mazur's group observed in real time the alloy mixing process, which happens incredibly quickly – in about a thousandth of a billionth of a second. To view this incredibly fast process, they used the ultrafast laser pulses – which are about 1000 times faster than the mixing process – to image the process as it occurs. In this way, they directly observed the alloy mixing process in real time. This helps understand the mixing process so the making of these alloys can be improved. After creating the alloys, Mazur's group worked with the group of Cynthia Friend, a chemist at Harvard, along with the group of Tonio Buonassisi, a solar cell specialist, and Silvia Gradecak, a materials scientistat MIT, to measure how well the new alloys are in coverting light to electricity and splitting water. To understand how these devices work and to improve their performance, the team studied their ability to absorb light and conduct electricity, as well as the exact alloy composition needed to make useful technology. They also studied how their performance can be further improved by using special heat treatment techniques. In addition to the experiments the team collaborated withtheoretical physicists and mathematicians to model how these devices work on a microscopic scale and how to make even better devices in the future. The team gained significant insights into these exciting new alloys, aimed, ultimately at making clean energy from sun light.
