Quantum dots have offered an attractive option for photovoltaics. Multijunction solar cells made from colloidal quantum dots (CQD) have been able to achieve around 7-percent conversion efficiency in the lab. While figures like this may not seem too impressive when compared to silicon solar cells, their promised theoretical conversion efficiency limit is an eye-popping 45 percent. This is possible because when a single photon is absorbed by a quantum dot, it produces more than one bound electron-hole pair, or exciton, thereby doubling normal conversion efficiency numbers seen in single-junction silicon cells.
Now researchers at the Massachusetts Institute of Technology (MIT) have raised the bar for quantum dot-based solar cells by producing one that changes light to electricity with 9-percent conversion efficiency. Furthermore, says the MIT team, it can be produced using an inexpensive production method that promises to keep manufacturing costs down.
The researchers, who published their findings in the journal Nature Materials, hit upon a way to produce quantum dot solar cells through a solution processing technique that doesn’t require high temperatures or a vacuum atmosphere to achieve stability for the solar cells when they are exposed to air. By using ligand treatments, which involve molecules or ions that bind to a central metal, the researchers were able to align the bands of the quantum dot layers, improving the performance of the films.
“Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented,” said graduate student Chia-Hao Chuang in a press release.
The processing technique for the quantum dot layers allows for the dots to do what they do well individually and also to work together in the transport of electrical charge to the edges of the film where it can then be collected to provide an electrical current.
Nine-percent efficiency may still seem low to casual observers, but the development of quantum dots for photovoltaics has been so rapid that researchers are impressed by the latest development.
“Silicon had six decades to get where it is today, and even silicon hasn’t reached the theoretical limit yet. You can’t hope to have an entirely new technology beat an incumbent in just four years of development,” said professor Vladimir Bulović in the release.
The researchers still need to determine why these films are so stable and there’s still a long way to go before they are commercially viable. But they now hold the National Renewable Energy Lab (NREL) record for quantum dot solar efficiency.