In the solar technology space, perhaps the most sought-after goal is the scalability and commercial viability of perovskite solar cells (PSCs), which are said to hold the key to much higher efficiency PV compared to just silicon — and can also be paired in tandem with existing solar cells to boost efficiency limits. Last week brought word of two significant breakthroughs in this quest.
Tandem cell research shows promise of 29.5% efficient PERC cell
Tandem cells made of silicon and perovskite are able to convert the broad energy spectrum of sunlight into electrical energy more efficiently than the respective single cells. Now, for the first time, two teams from HZB and ISFH Hameln have succeeded in combining a perovskite top cell with a so-called PERC/POLO silicon cell to form a tandem device.
This is an important achievement, since PERC silicon cells on p-type silicon are the “workhorse” of photovoltaics, with a market share of about 50% of all solar cells produced worldwide. They are largely optimized, long-term stable and temperature stable. Therefore, it is particularly interesting for the commercialization of a perovskite-silicon tandem technology to develop a “perovskite tandem upgrade” for PERC cells. The cooperation took place within the framework of the joint project P3T, which is funded by the Federal Ministry of Economics and coordinated by HZB.
The team at ISFH used an industry-compatible PERC process for the backside contact of the silicon bottom cells. On the front side of the wafer, another industrializable technology was used, the so-called POLO contact, which was adapted here for the small-area proof of concept cells.
The following process steps took place at HZB: A tin-doped indium oxide recombination layer was applied as a contact between the two subcells. On top of this, a perovskite cell was processed with a layer sequence similar to that in the current world record tandem cell on n-type silicon heterojunction cells, made by HZB. The first perovskite PERC/POLO tandem cells produced in this way achieve an efficiency of 21.3% on an active cell area of about 1 cm². This efficiency is thus still below the efficiency of optimized PERC cells in this feasibility study.
“However, initial experimental results and optical simulations indicate that we can significantly improve the performance through process and layer optimization,” explains Dr. Lars Korte, the corresponding author of the study.
PCE estimated at 29.5 % | The experts estimate the Power Conversion Efficiency (PCE) of these perovskite/silicon tandem solar cells with PERC-like sub-cell technology at 29.5%. The next steps for further efficiency increases are already clear: Dr. Silvia Mariotti from the HZB team had identified the coverage of the silicon surface by the perovskite as potential for improvement:
“For this purpose, one could adapt the surface of the silicon wafers and thus quickly increase the efficiency to about 25%,” says Mariotti. This is then already significantly higher than the efficiency of PERC single cells.
Damp-heat testing success
In a recently published Science paper, KAUST researchers reported the first-ever successful PV damp-heat test of PSCs. The damp-heat test is an accelerated and rigorous environmental aging test aimed at determining the ability of solar panels to withstand prolonged exposure to high humidity penetration and elevated temperatures. The test is run for 1,000 hours under a controlled environment of 85% humidity and 85 degrees Celsius. It is meant to replicate multiple years of outdoor exposure and evaluate factors such as corrosion and delamination.
The harshness of the test is in line with commercialization requirements of photovoltaic (PV) technology needing to cover 25 to 30 years of warranty for conventional crystalline-silicon modules. In order to pass the test, the solar cell has to maintain 95% of its initial performance.
Passing the test | The trouble with perovskite cells is that they a thin-film technology and is prone to not holding up well or for very long in real-world conditions. But, unlike silicon wafers, perovskites can be coated directly on a glass substrate, using a precursor solution. The solution is made with a solvent that gets crystallized into a solid state.
One of the significant advantages is that precursor materials can be made without the need for expensive facilities and energy-intensive environments of over 1,000 degrees, which is typical for more traditional semiconductors such as silicon.
“It’s a very simple way to make solar cells. Also, while the optoelectronic properties are not unique, they are excellent. They’re on-par with very high-quality traditional semiconductors. That’s quite remarkable,” explained De Wolf. By altering the composition, it’s also possible to tune the spectral sensitivity across the solar light spectrum from UV up to infrared. This is quite attractive for certain applications.
Led by Randi Azmi, a postdoctoral fellow in Stefaan De Wolf’s KAUST Photovoltaics Laboratory, their research had to overcome an enduring weakness in encapsulated PSCs to prevent packaging leakage. This vulnerability of 3D perovskite films is what allows the unwanted infiltration of atmospheric agents and offers limited resilience against heat. The solution found by the KAUST researchers is the engineering and introduction of 2D-perovskite passivation layers to simultaneously enhance the power conversion efficiencies and lifetime PSCs.
The remaining challenge, after performance and stability, is scaling. Most solar cell applications are focused on utility-scale sectors as well as rooftop panels.
“The market is silicon-based, and it will be silicon-based for the next 20 years at least,” said De Wolf. “So we are mainly focused on improving the performance of perovskites solar cells in order to advance more efficient ‘tandem’ solutions pairing both traditional silicon and perovskites, where the current findings will aid much in increasing the reliability of such perovskite/silicon tandem solar cells.”