By Prachi Patel
Materials Research Society/MRS Bulletin | Published: 18 February 2015
Mechanically stacking a perovskite solar
cell on a silicon cell (left) increases the efficiency of the overall
tandem cell by 50%, Stanford researchers report. The design should be
easy to integrate into today’s photovoltaic modules. The classic
monolithic tandem cell design (right) could yield higher efficiency but
is harder to manufacture. Credit: Energy & Environmental Science.
Click image to enlarge.
By stacking a semi-transparent perovskite solar cell on top of a
silicon solar cell, researchers have made tandem cells that have 50%
higher efficiency than the silicon one alone. Perovskites, inexpensive
materials with the crystal structure of calcium titanate, have recently
gained prominence for photovoltaic applications. “We’re trying to find a
way to make solar cells that are 25% or more efficient at very low
cost,” says Michael McGehee, a materials science and engineering
professor at Stanford University. Today’s commercial crystalline silicon solar cells have efficiencies ranging from 14–21%. Their cost generally goes up with efficiency. One reason for the low efficiency is that silicon and other semiconductors cannot absorb all the light that hits them. Scientists have made solar cells with efficiencies as high as 44% in the laboratory by stacking compound semiconductors such as gallium arsenide and indium gallium phosphide. Each material absorbs a separate swath of the light spectrum, boosting the cell’s light-to-electricity conversion efficiency. But these cells are prohibitively expensive for commercial use.
Perovskites are attractive for commercial tandem solar cells because they absorb the high-energy photons present in visible light, while silicon is better at absorbing lower energy near-infrared photons. Plus, perovskites are easy and inexpensive to make, typically only requiring a gentle heating of precursor salt solutions.
A common approach to making tandem cells is to grow them as layers on the same substrate. But this requires additional processing steps to connect individual cells. Instead, McGehee and his colleagues at Massachusetts Institute of Technology and at the Swiss Federal Institute of Technology in Lausanne chose a simpler approach. They made semi-transparent perovskite cells and mechanically stacked them on top of an existing silicon module.
The researchers started by depositing the perovskite, methylammonium lead iodide on a glass substrate that is coated with a transparent fluorine-doped tin oxide electrode layer. Then they pressed a polymer sheet embedded with silver nanowires on the device to transfer the nanowires onto the perovskite layer. The nanowires act as a semi-transparent electrode.
The researchers then placed this perovskite cell, which had an efficiency of 12.7%, on top of an 11.4% efficient silicon solar cell. The resulting tandem cell was 17% efficient. They also stacked the perovskite cell onto a 17% efficient copper indium gallium diselenide (CIGS) cell to make a tandem cell with an efficiency of 18.6%. The researchers published their results in Energy & Environmental Science.
The team is trying to boost efficiency further by optimizing the perovskite layer and the electrodes. Other laboratories have grown perovskites that have a higher efficiency, McGehee says, and one idea is to combine those materials with the new nanowire electrode technique.
McGehee also aims to make monolithic structures in which perovskite cells are grown on top of silicon. “We haven’t done it yet, but you could just print the perovskite over a very large area,” he says.
Supratik Guha, director of the physical sciences department at IBM Thomas J. Watson Research Center, calls the new work a good demonstration of efficiency increase in a tandem perovskite cell. However, scientists are still far from addressing the most pressing challenge of perovskites: their susceptibility to degradation under humidity. The long-term stability of perovskites will have to be proven before they can be used in commercial modules, Guha says.
Read the abstract of the study in Energy & Environmental Science.
