Nanowire Based Solar Cell Increases Shockley-Queisser Efficiency Limit

The figure shows that the sun's rays are drawn into a nanowire, which stands on a substrate. At a given wavelength the sunlight is concentrated up to 15 times. Consequently, there is great potential in using nanowires in the development of future solar cells.
Credit: Niels Bohr Institute

The development of a nanowire based solar cell that increases sunlight concentration to a a factor of 15 raises the standard efficiency limit of solar cells known as the Shockley-Queisser limit.

One of the most popular and common source of renewable and sustainable energy is the Sun. Solar energy is not dependent on weather conditions such as wind power or need to be near a power source such as geothermal or hydroelectric energy producers.

Solar energy is produced by solar panels or solar cells. These cells, also known as photovoltaic cells, convert sunlight to electrical energy.

The focus on solar cell technology is raising the efficiency of the solar cell to convert solar energy to electrical energy. In solar cell production, the value that is used to gauge the efficiency of the solar cell is the Shockley-Quesser limit. This limit refers to the maximum theoretical efficiency of a solar cell using a p-n junction to collect power from the cell.

A p-n junction refers to the boundary of two semiconductors; the p-type and the n-type. The p-type semiconductor contains excess holes while the n-type contains excess free electrons.

The Shockley-Queisser limit puts the maximum solar cell efficiency at around 33.7%. This means that at most, only 33.7% of sunlight can be converted into elecrical energy. Currently, silicon based photovoltaic cells have an efficiency of 22%.

Phase Change Materials (PCM) and Nanotechnology For Faster and Efficient Computer Memory

What is a phase change material?

A phase change material (PCM) is a substance that changes from one state of matter to another at a certain temperature. It can be compared to water; when boiled (at 100 °C ) it turns to a gas and when frozen (at 0 °C)turns to ice.

PCMs are usually used in construction as a means of regulating room temperature. When the temperature in a room gets high, the PCM liquefy and absorb and store heat. Once the temperature lowers, it solidifies and releases the absorbed heat.

PCMs usually utilize the solid to liquid phase change because a phase change to gas requires a larger volume or high pressure to store the material in that state.

Current phase change materials use paraffin as its prime material. Paraffin (a wax) melts at a specified temperature and solidifies back again at a lower temperature. These are produced as microscopic wax spheres (paraffin wrapped in a glass acrylic containers called beads) integrated into building materials.

Penn researchers' study of phase change materials could lead to better computer memory

Memory devices for computers require a large collection of components that can switch between two states, which represent the 1's and 0's of binary language. Engineers hope to make next-generation chips with materials that distinguish between these states by physically rearranging their atoms into different phases. Researchers at the University of Pennsylvania have now provided new insight into how this phase change happens, which could help engineers make memory storage devices faster and more efficient.

The research was conducted by Ritesh Agarwal, associate professor in the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science, along with members of his research group. A.T. Charlie Johnson, professor in the Department of Physics and Astronomy in the School of Arts and Sciences, and Ju Li, now a professor of nuclear science and engineering at the Massachusetts Institute of Technology, also contributed to the study.

Their research was published in the journal Science.