The Sun provides approximately 1 kW of power per square meter, and a primary goal of photovoltaic research is to maximize the efficiency at which a solar cell converts this solar energy to electrical energy. High efficiency typically requires high material quality. Single crystalline silicon solar cells are the dominant technology, but the ultimate efficiency is limited by the silicon's intrinsic material properties of light absorption. Epitaxial solar cells, grown by techniques such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on crystalline substrates such as InP, GaAs, and Ge, have achieved remarkable efficiencies of nearly 45% under concentrated solar illumination.
Epitaxial growth, however, is relatively expensive and tends to be limited to fixed-area, rigid devices. Alternatively, thin film materials, such as CuIn(Ga)Se2 and CdTe, are deposited over large areas with less expensive techniques like co-evaporation and sputtering. These deposition techniques yield polycrystalline material that is typically lower material quality than single crystal material. So the challenge is to achieve high efficiency attainable from large-area, inexpensive growth techniques.
The photovoltaic research within ESTD is focused on creating high-efficiency technologies for portable power systems. This research includes the exploration of new materials and structures to realize new devices. This area of research is built on semiconductor band structure modeling enabled by unique, in-house developed software tools that predict promising new materials, which are then realized through in-house semiconductor growth and fabrication.
ESTD photovoltaic research seeks to adapt existing technologies to meet unique needs of the military. For example, we are leveraging solar cell technologies developed for space satellites to create a solar cell that operates efficiently underwater.
Going beyond solar input, ESTD research seeks to create photovoltaic technologies to exploit all forms of light input for electrical energy, such as laser power conversion and thermophotovoltaics (TPV), in which a narrow bandgap solar cell converts thermal photons to electricity.