Harnessing Quantum Dots for High Efficiency Solar Cells

Grant # 07- 03
Principal Investigator: Tim Kidd
Organization: University of Northern Iowa
Technical Area: Renewable Energy

Abstract
Cheap, reliable sources of energy have been the foundation of modern society for decades. It appears, however, that our reliance on non-renewable fossil fuels to fulfill our energy needs may soon be forcibly coming to an end. Domestic sources are becoming more expensive to exploit and place fragile ecosystems at risk. Also, given the present world political state it is unwise to continually increase America’s dependence on foreign energy sources.

One of the most promising sources of alternative energy is solar power. No matter the geographic location, only a small fraction of the sun’s energy is harnessed as usable energy. Once manufactured, solar cells can operate for long periods with little maintenance and zero pollution. They are also suitable for a wide range of applications, from large-scale generators of electricity to portable devices suitable for more personal use.

Despite the apparent advantages of solar cells as an energy source, there are many technical issues that must be addressed before they become economically viable. One of largest problems with presently available solar cells is their inefficiency. Most commercially available solar panels are approximately 10% efficient, so that panels must have a large surface area to generate even modest amounts of electricity.

A recent approach to improving solar cell efficiency is the use of quantum dots for harnessing sunlight. Semiconducting quantum dots, with sizes measuring on the nanoscale, have shown to be extremely versatile in this application. By controlling their size, a single material can be used to harness a large portion of the solar spectrum, a feat impossible in macroscopic materials. Also, they have shown the unique capability to generate multiple electronic carriers from a single photon. Solar cells incorporating quantum dots are expected to be many more times efficient than standard technologies.

Many barriers remain before such advances in efficiency can be realized in commercially viable devices even in specialized applications. Because of their small size, quantum dots are extremely sensitive to disorder and impurities. This problem is magnified by the necessity of doping, the purposeful introduction of impurity atoms, for the quantum dot systems to convert light into usable electricity. Furthermore, connecting these materials to a conducting medium to transport electricity can be a large source of loss towards the overall efficiency of the device.

In this study, quantum dots will be grown in ultra high vacuum chambers with pressures rivaling space. They will be grown by molecular beam epitaxy, a technique that allows control over the makeup of these materials at the atomic level. Self-assembly techniques will control size and constituency spontaneously without further processing. Several combinations of quantum dot materials and growth templates are to be explored to optimize possible device parameters. Once grown, their electronic and structural properties will be measured at the atomic scale using scanning probe microscopy and a variety of electron spectroscopic techniques. This will allow us to investigate what fundamental properties translate into optimum devices.

Research into the fundamental properties of semiconducting quantum wells, with emphasis on the effects of disorder and doping, should also yield insight into the broader question of how electronic and structural parameters are altered at the nanoscale. The project will focus on how these fundamental properties relate to light absorption and the creation of electronic carriers. Understanding this relationship is a necessary step along the path towards using such high efficiency devices in both specialized and more general commercial applications.