PORTLAND, Ore. — Solar cell designs today pursue performance at the lowest possible cost, neglecting the dimensions of thin-and-lightweight, according to Massachusetts Institute of Technology (MIT) researchers who aim to design the world's thinnest solar cells.
For mobile electronics, thin-and-lightweight are prime design goals, but solar cells have aimed instead at the highest efficiency. Today, making solar cells thinner and lighter would be welcome for applications in aviation, space exploration, and in remote areas where transportation costs are high, according to MIT. In the future, as materials become more scarce, the conservation achieved with ultra-thin solar cells could cost-reduce even urban installations.
"Our predictions are for what may very well be the thinnest solar cells possible, ones out of only two layers of materials," professor Jeffrey Grossman told EE Times. Grossman performed the work with post-doctorate researcher Marco Bernardi and Maurizia Palummo, a visiting researcher from the University of Rome.
"There are indeed applications where weight is crucial, where the thinnest possible amount of active layer material with minimal encapsulation may change the installation game, because it could get us onto [thinner, more durable] substrates," Grossman said. "In addition, this gets to the heart of what I think is an important question: namely, what is the most power we can squeeze out of each and every atom or bond of a given material?"
MIT researchers use computer simulations to shuffle through different materials in the search for the thinnest possible solar cells. (Source: MIT)
MIT estimates that its ultra-thin solar cell films -- essentially two-dimensional (2D) layers as thin as one nanometer -- can deliver 1,000 times more energy-per-pound than conventional solar cells. The tradeoff is that their efficiency is lower, requiring about 10 times the area of a conventional solar cell to produce the same amount of energy, since ultra-thin solar cells have an efficiency of up to 2 percent, compared with up to 20 percent for conventional photovoltaic (PV) solar cells. However, the researchers have plans for stacking the ultra-thin 2D solar cells in layered structures to improve their efficiency.
"These two-sheet stacks we predict could have efficiencies of 1 to 2 percent. However, it is certainly possible to make stacks that consist of more than just two layers, and in that case the efficiency would go up," said Grossman. "There is no reason efficiencies of cells made from 2D materials couldn't be just as efficient as current 'traditional' PV -- in the 10 to 20 percent range."
The ultra-thin solar cell design is still in simulation while the researchers decide which material to use for prototypes. In detailed simulations, various topologies of stacked sheets use atomically thin graphene, molybdenum-disulfide, and molybdenum-diselenide. The best of these designs not only provide a weight advantage over conventional solar cells, but are also immune to oxygen, ultraviolet radiation, and moisture in the environment -- the three killers of long-term stability in conventional solar cells -- giving the new ultra-thin designs the additional advantage of eliminating the need for glass covers or standoff mounting, which consumes over half the cost of conventional PV installations.
"Ultralight solar cells (with extremely high power/weight in our case) have the potential to reduce installation costs. Current solar modules based on silicon are heavy and made heavier by the glass protecting them. Their installation amounts to 60 percent of the total cost of a solar array, largely due to the high weight," said Bernardi. "By finding ultra-thin and mechanically flexible materials, the hope is to make very light solar cells, which can be encapsulated with plastics rather than glass, and hence create new paradigms for photovoltaic installation."
The material cost for ultra-thin solar cells would be minimal, compared to conventional solar cells, but the researchers have yet to create prototypes in the lab or to work on making the materials manufacturable in high volume. Next they plan to test their formulations in the lab by measuring the efficiency and long-term stability of various formulations and stacking structures.