An international team of scientists have come together to build solar cells with high efficiency using a new blend of materials. The materials used allow the cells to convert sunlight into electrical energy without the addition of dopants. The design, termed DASH, uses molybdenum oxide and lithium fluoride.
Doing away with doping
Most of the solar cells use silicon wafers in the crystalline form. The wafer, along with the layers of materials deposited on it are doped or injected with special impurity atoms that either introduce free electrons or create electron deficiencies called holes. The presence of these extra electrons or holes increases the electrical conductivity of the material. However, the impurities introduce certain complexities within the crystalline structure, which bring down the performance.
Since solar cells are all about increasing the performance of these devices, researchers have been looking at means to eliminate the doping process. The international team has made available a cell prototype with a simple architecture. The journal Nature Energy has published an article on the design of the solar cell. James Bullock, a faculty member of Australian National University and a team member is the principal author of the paper. He has been a visiting researcher at Lawrence Berkeley National Laboratory and UC Berkeley as a part of the project.
Bullock explains that the simple structure of the cell designed by them would cut down the production and operational costs considerably, thereby enhancing the efficiency. The silicon cell, free from doping impurities designed by the team is termed DASH, which is the acronym for dopant-free asymmetric heterocontact. The efficiency of this product is 19%, which exceeds that of other dopant-free cells. The efficiency of previous cells of this category did not exceed 14%.
Special properties of the contact materials
The researchers applied several layers of amorphous silicon over a wafer of crystalline silicon. This was overlaid with very thin films of molybdenum oxide on the sun exposed surface and lithium fluoride at the bottom one. These two external layers are only a few nanometers thick and act as contacts for the holes and electrons. Ali Javey, a team member and a professor of Electrical Engineering and Computer Science at UC, Berkeley, explains that both molybdenum oxide and lithium fluoride have been selected for making up the contacts because of their special properties. The materials form transparent layers at this thickness. Molybdenum oxide has several imperfections in its crystalline structure that allow it to act as an effective hole contact.
Likewise, the defects in the lithium fluoride structure allow it to be a useful electron contact. Stefaan de Wolf, another team member has described in the article how the molybdenum oxide and lithium fluoride layers work as effective contacts when used with a combination of amorphous and crystalline silicon. In addition, these materials have allowed the scientists to come up with remarkable variations in their properties with different thicknesses.
The scientists used the thermal evaporation technique at room temperatures to apply the coatings. Javey said that the researchers intend to use the material combination for other semiconductor applications like transistors to improve their performance.