More efficient, multi-junction solar cells are costly to produce. They stack layers on top of each other, so each layer efficiently captures a certain part of the spectrum of sunlight. However, a team from the Massachusetts Institute of Technology (MIT) and the Masdar Institute have created a new solar cell design that may be both highly efficient and less costly, according to an article from the Masdar Institute.
Silicon is a low-cost material often used in solar cells. However, by itself, it is not very efficient. At most, Silicon cells can convert 15 to 20 percent of the energy from sunlight to electricity. The relatively narrow bandgap of silicon makes it less efficient at converting the higher energy part of the sunlight spectrum including blue, green, and yellow light. Silicon is most efficient at converting red light to electricity.
The team’s unique multi-junction solar cell employs a “step-cell” design approach combines a gallium arsenide phosphide-based solar cells with low-cost silicon. The team’s innovative “step-cell” design features a larger area of silicon beneath a smaller area of GaAsP. The result is that the silicon is partially underneath the GaAsP layer and partially exposed to direct sunlight.
If the design were just a conventional multi-junction solar cell without the step-effect, the wide bandgap GaAsP top layer would efficiently absorb the blue, green, and yellow light of the spectrum. Then, only the remaining red part of the spectrum would reach the bottom silicon layer. While this is the case for the silicon underneath the GaAsP, the ‘step’ that does not reside beneath gets the entire spectrum of sunlight.
The researchers also etched the top GaAsP layer to increase its surface area exposed to direct sunlight.
The new solar cell can reach practical efficiencies of 35% and theoretical efficiencies of up to 40%, according to Dr. Eugene Fitzgerald, MIT’s Merton C. Flemings – SMA Professor of Materials Science and Engineering, and Masdar Institute’s Dr. Ammar Nayfeh, Associate Professor of Electrical Engineering and Computer Science. The professors intend to start a company to commercialize the promising solar cell.
Dr. Fitzgerald previously launched AmberWave Systems Corporation, Paradigm Research LLC, and 4Power LLC.
Dr Fitzgerald says that the step-cells could be ready for the PV market within two years or less.