A lead-free solar cell made from tin is environmentally friendly and can be produced cheaply without any sophisticated equipment or hazardous materials.
“This is a breakthrough in taking the lead out of a very promising type of solar cell, called a perovskite,” says Mercouri G. Kanatzidis, a professor of chemistry at Northwestern University. “Tin is a very viable material, and we have shown the material does work as an efficient solar cell.”
The new solar cell uses a structure called a perovskite but with tin instead of lead as the light-absorbing material. Lead perovskite has achieved 15 percent efficiency, and tin perovskite should be able to match—and possibly surpass—that.
Kanatzidis developed, synthesized, and analyzed the material, then collaborated with Robert P. H. Chang, professor of materials science and engineering, to develop a solar cell that worked well.
“Our tin-based perovskite layer acts as an efficient sunlight absorber that is sandwiched between two electric charge transport layers for conducting electricity to the outside world,” Chang says.
The solid-state tin solar cell has an efficiency of just below 6 percent, which is a very good starting point, Kanatzidis says. Two things make the material special: it can absorb most of the visible light spectrum, and the perovskite salt can be dissolved, and it will reform upon solvent removal without heating.
“Other scientists will see what we have done and improve on our methods,” Kanatzidis says. “There is no reason this new material can’t reach an efficiency better than 15 percent, which is what the lead perovskite solar cell offers. Tin and lead are in the same group in the periodic table, so we expect similar results.”
Perovskite solar cells have only been around—and in the lab—since 2008. In 2012, Kanatzidis and Chang reported the new tin perovskite solar cell with promises of higher efficiency and lower fabrication costs while being environmentally safe.
“Solar energy is free and is the only energy that is sustainable forever,” Kanatzidis says. “If we know how to harvest this energy in an efficient way we can raise our standard of living and help preserve the environment.”
As reported in the journal Nature Photonics, the solid-state tin solar cell is a sandwich of five layers, with each layer contributing something important. The researchers were familiar with handling tin, specifically methylammonium tin iodide, which oxidizes when in contact with air.
The first layer is electrically conducting glass, which allows sunlight to enter the cell. Titanium dioxide is the next layer, deposited onto the glass. Together the two act as the electric front contact of the solar cell.
Next, the tin perovskite—the light absorbing layer—is deposited. This is done in a nitrogen glove box—the bench chemistry is done in this protected environment to avoid oxidation.
On top of that is the hole transport layer, which is essential to close the electrical circuit and obtain a functional cell. This required Kanatzidis and his colleagues to find the right chemicals that wouldn’t destroy the tin underneath. They determined a substituted pyridine molecule was best by understanding the reactivity of the perovskite structure. This layer also is deposited in the glove box. The solar cell is then sealed and can be taken out into the air.
A thin layer of gold caps off the solar-cell sandwich. This layer is the back contact electrode of the solar cell. The entire device, with all five layers, is about one to two microns thick.
The researchers then tested the device under simulated full sunlight and recorded a power conversion efficiency of 5.73 percent.
The US Department of Energy and the Institute for Sustainability and Energy at Northwestern supported the research.
Source: Northwestern University