Position:  PhD Candidate

Current Institution:  University of Toronto

Abstract:  Colloidal Quantum Dots for Solar Energy Conversion

Increasing global energy demand drives the development of clean energy sources that will help reduce the consumption of fossil fuels. Solar energy, the most abundant renewable source, is converted to electricity using photovoltaic devices. The photovoltaics market has witnessed rapid growth in the past decade, and today, many photovoltaic strategies aim at low-cost, solution-processed manufacture. Colloidal quantum dots (CQDs), emerging semiconductors, have attracted attention in view of their spectral tunability. The bandgap of CQDs is readily tuned to harvest infrared solar energy. This could enable both full-spectrum devices and also tandem solar cells that can be integrated with wider-bandgap semiconductors. Unfortunately, a high density of surface-associated trap states, low carrier mobilities, and an inhomogeneous energy landscape have previously limited CQD photovoltaic performance. Three strategies were developed to address these problems. Through new materials processing approaches, I succeeded in increasing charge extraction, reducing bandtail states, and lowering the barrier to carrier hopping in CQD solids. The benefits from enhanced charge extraction were demonstrated in a double-sided junction architecture enabled by the engineering of an electron-accepting layer. A solution-phase ligand-exchange method was then developed to create CQD inks that can be deposited as an active layer in a single step. The resulting CQD films exhibited a flattened energy landscape that increased the carrier diffusion length and contributed to solar cells having certified efficiencies of 11.3 percent. After this, a hybrid material system was designed through combining CQDs with epitaxially-grown inorganic metal halide perovskites. The matrix-passivated CQD films showcased a two-fold increase in carrier mobility and superior thermal stability compared to pristine CQDs. My work provides promising pathways to achieve more fully the potential of CQD solids and to showcase these advances in improved performance for CQD solar cells.

Bio:
Mengxia Liu is a PhD candidate in the Department of Electrical and Computer Engineering at the University of Toronto, working in in Professor Edward H. Sargent’s group.  She received a bachelor’s degree in materials science and engineering from Tianjin University in China in 2014.    Her research focuses on colloidal quantum dot optoelectronic devices, which have shown the promise for low-cost, high-efficiency solar energy harvesting.  She devised an approach to produce printable colloidal quantum dot inks that enabled a record certified quantum dot solar cell in 2017.  After completing her PhD, she will move to Cavendish Laboratory in the University of Cambridge as a research associate.  She received the Chinese Government Award for Outstanding Self-Financed Students (2018) Award, the Hatch Graduate Scholarship (2015-2018), and China National Scholarship (2014).