Our research focuses on engineering micro/nano structured materials to enhance device performance and functionality in energy, advanced electronics, and biomedical technology.
I. Inexpensive, High-efficiency Solar Cells
Sunlight is an abundant renewable energy source, providing 10,000 times more energy than the world’s current total energy use. A key metric in photovoltaics (PV) is power conversion efficiency, the ratio of the electrical output of a solar cell to the incident sunlight. Next generation solar cells use novel device architectures and inexpensive PV materials. For instance, an array of radial junction solar cell enables decoupling of the direction of light absorption to photo-generated carrier collection, enhancing its PV performance in comparison to planar controls.
Thin film solar cells are an exciting PV technology, where a thin PV layer (< 5 µm) can absorb over 90% of sunlight. This attractive technique, however, has shown a module efficiency of 15 %, still well below the theoretical maximum value of 30 % for CdTe. The grain boundaries and the quality of the p-n junctions are one of the primary sources for the limited performance. We investigate local PV properties of individual microstructures and correlates the inhomogeneity to device performance. (read more)
II. Development of In-situ, Versatile Microscopy Techniques
The capability to access individual micro/nano structures and to measure their properties is critical for advanced technologies. We develop multi-probe techniques in conjunction with quantitative analysis to reveal fundamental carrier dynamics at the level of individual nanostructures. An electron beam irradiating on a specimen produces characteristic signals in proximity, which makes it ideal for local characterizations. As a quantitative probe, electron beam induced current (EBIC) has been widely used in various semiconductor devices, where a single primary electron generates a few thousands of electron-hole pairs within a specific volume. (read more)
III. Interface Engineering for Advanced Nanoelectronic Devices
The use of self-assembled monolayer in advanced (opto)electronic devices provides a whole new degree of freedom to integrate hetero-structural nanomaterials in 3D. An example includes crossed-nanowire molecular junction devices, where the integration strategy combined a high yield parallel assembly of individually addressable devices with a technique that does not damage the active molecular units during the fabrication. Furthermore, the robust device platform allows in-situ electrical and spectroscopic characterizations to correlate between the electrical properties of the device and the structure of molecules participating in the charge transport using Raman and inelastic electron tunneling spectroscopies (IETS).
We are interested in electrical characterizations of a metal-organic framework (MOF) materials. By adding a tetracyanoquinodimethane (TCNQ) molecule to their framework, the electrical conductivity of the MOF increases by over six orders of magnitudes. This development could have profound implications for the future of electronics, sensors, energy conversion and energy storage. (read more)