The sun represents the most abundant potential source of sustainable energy on earth. Solar cells for producing electricity require materials that absorb the sun's energy and convert its photons to electrons, a process called photovoltaics. Lowering the cost per watt for solar photovoltaic energy conversion systems is a long-standing goal that could enable more widespread adoption of solar energy. In particular, thin-film solar cells can be made cheaper than crystalline silicon-based solar cells if the right combination of material properties for high solar energy conversion efficiency can be found. The goal of this project is to investigate new layered structures for thin-film solar photovoltaics that potentially offer both low-cost processing and high solar energy conversion efficiency. These new layered structures, based on a metal-insulator-semiconductor sandwich of electronic materials, have behavior at their respective material boundaries that may favorably change the overall electronic structure and properties of the solar cell, resulting in improved performance. The innovative aspect of this research is that advanced techniques will be used to deposit these layers on top of one another with atomic level precision so that these properties can be more carefully and insightfully studied. The educational activities associated with this project include the development of a community outreach program with a local science center and the production of videos that animate the effects of physics behind the operation of photovoltaic devices.
The overall goal of this research is to identify the underlying mechanisms that induce barrier height modifications and other interfacial electronic changes by insertion of dielectric tunnel layers in the context of metal-insulator-semiconductor photovoltaics (PV). Metal-insulator-semiconductor structures will be fabricated by film deposition and interface modification techniques that allow for an unprecedented level of interfacial control. This level of control will enable investigation of the fundamental behavior of fixed charges, molecular surface functionalization, atomic layer deposition (ALD) chemistry, hydrogen treatment, and ALD bilayers in MIS structures. The specific influence of these phenomena on barrier heights and interfacial electronic figures of merit relevant for improving PV devices will be quantified. Dipoles within bilayers of dissimilar metal oxides will also be used to control barrier heights. The impact of fixed charge on electronic properties will be investigated by varying fixed charge density and insulator thickness experimentally, and comparing these experimental results with theoretical simulations. Molecular surface functionalization and hydrogen at interfaces provide additional synthetic control, and their ability to minimize interfacial electronic defects will be determined. By comparing electronic measurements, low-energy ion scattering, and photoelectron spectroscopy measurements, critical relationships between layer mixing, dipole strength, and interface trap densities will be elucidated. Thus, the research will advance fundamental understanding of the underlying physical mechanisms while improving energy conversion figures of merit in a new generation of metal-insulator-semiconductor, thin-film solar PV devices.
