All-perovskite tandem solar cells get a boost in efficiency via upscaling
Solar energy has a critical role to play in the transition away from fossil fuels to sustainable energy resources. Sunlight is abundantly available and solar cells can directly convert its energy into electricity—but efficiency, stability, scalability, and cost hurdles still exist.
Perovskite solar cells (PSCs) are one of the most promising semiconductor materials for solar cells because they’re fairly efficient (>25%) and less expensive than other solar cells. And tandem solar modules are emerging as an intriguing option because they feature two light-harvesting active layers, which means they can tap into the solar spectrum more efficiently than single solar cells.
A group of Karlsruhe Institute of Technology researchers led by Bahram Abdollahi Nejand, a postdoctoral researcher, and Ulrich W. Paetzold, who leads the Advanced Optics and Materials for Next-Generation Photovoltaics group, recently created a prototype to upscale high-efficiency, low-bandgap, two-terminal all-perovskite tandem solar cells with efficiencies of up to 23.5% for an active area of 0.1 cm2 into all-perovskite tandem solar mini-modules.
These tandem solar modules have an efficiency of 19.1% for an aperture area of 12.25 cm2, and prevent high-efficiency loss (merely ~2%) through the upscaling process (see figure).
To achieve this boost in efficiency, the researchers optimized light paths and reduced reflections within the solar cell architecture. They also relied on high-throughput laser scribing to enable functional tandem solar modules with two-terminal interconnected cell strips.
Stack ‘em up
Despite significant research and development, the efficiency of a single-junction solar cell is theoretically limited to <30%.
“But stacking two solar cells with different bandgaps on top of each other reveals a new path to surpass this efficiency limit by maximizing light harvesting in the device—consequently generating more power per area,” explains Nejand. “This brilliant-but-simple idea led to development of a new class of solar cells called ‘tandem solar cells,’ which can fundamentally provide efficiencies of >35%.”
Perovskite solar cells with a tunable bandgap are ideal candidates to be the tandem partner for other solar cell technologies. “To date, two terminal perovskite/silicon, perovskite/copper indium gallium selenide (CIGS), and all-perovskite tandem solar cells with efficiencies exceeding 29.8%, 25%, and 26%, respectively, are among the top well-developed perovskite-based tandem solar cells,” Nejand says.
Among these tandem cells, two-terminal all-perovskite tandem solar cells are attracting substantial attention, thanks to their low cost of fabrication, mechanical flexibility, fully solution-based processability, and flexibility in designing different architectures with different perovskite bandgaps. “They will hold a considerable share in the future market, provided the challenges of stability and scalability are met,” Nejand says. “The biggest challenge to upscaling is the deposition of narrow-bandgap perovskite on the top wide-bandgap perovskite subcell without any degradation due to diffusion of solvent.”
Several studies reported efficiencies of lab-scale all-perovskite tandem solar cells exhibiting a power conversion efficiency (PCE) of >23%, Nejand adds, and the current certified record is 26.4%.
“Considering all of these values exceed the PCEs of commercial single-junction multicrystalline silicon or CIGS cells, it was time to take the next step and develop scalable fabrication processes and interconnection methods for two-terminal all-perovskite tandem solar modules,” he says.
Interconnections
As far as an interconnection method, the established route to an efficient thin-film module interconnection uses three interconnection lines: P1, P2, and P3. The group used widths of 60, 60, and 40 µm, respectively.
Three laser scribing steps generate the interconnection between cell strips in the module to reach the highest possible power output per area. The first laser scribing line (P1) generates the layout on a transparent conductive oxide. The second laser scribing line (P2) generates the contacting path where the top contact of the side subcells is contacted to the bottom contact of the neighboring cellstrip when the rear metal contact is deposited. And finally, the third laser scribing line (P3) separates cellstrips and defines the final active area on the module when it repeats in sequence.
“The area between P1 and P3 interconnection lines is inactive, which denotes an associated area of loss that is quantified by the geometric fill factor (GFF), which in our work is 94.7%,” Nejand explains.
One of the most surprising aspects of this work is that upscaling the two-terminal all-perovskite tandem modules uses “fully scalable deposition methods—with a low drop in efficiency—and laser scribing,” says Nejand.
Perovskite devices, in general, “show outstanding progress boosting power output and scalability, but despite improvement in stability during the past decade, so far they don’t meet the required standards and criteria for the market,” Nejand says. “By resolving the stability issue, perovskite-based devices can take a big share of the market in the future.”
Next up, the researchers will try to improve the perovskite tandem cells’ efficiency and stability, as well as reach larger-area modules via scalable deposition methods to meet market needs.
Sally Cole Johnson | Editor in Chief
Sally Cole Johnson, Laser Focus World’s editor in chief, is a science and technology journalist who specializes in physics and semiconductors. She wrote for the American Institute of Physics for more than 15 years, complexity for the Santa Fe Institute, and theoretical physics and neuroscience for the Kavli Foundation.