Silicon plus perovskite solar reaches 34 percent efficiency

As the price of silicon panels has continued to come down, we've reached the point where they're a small and shrinking cost of building a solar farm. That means that it might be worth spending more to get a panel that converts more of the incoming sunlight to electricity, since it allows you to get more out of the price paid to get each panel installed. But silicon panels are already pushing up against physical limits on efficiency. Which means our best chance for a major boost in panel efficiency may be to combine silicon with an additional photovoltaic material.

Right now, most of the focus is on pairing silicon with a class of materials called perovskites. Perovskite crystals can be layered on top of silicon, creating a panel with two materials that absorb different areas of the spectrum—plus, perovskites can be made from relatively cheap raw materials. Unfortunately, it has been difficult to make perovskites that are both high-efficiency and last for the decades that the silicon portion will.

Lots of labs are attempting to change that, though. And two of them reported some progress this week, including a perovskite/silicon system that achieved 34 percent efficiency.

Boosting perovskite stability

Perovskites are an entire class of materials that all form the same crystal structure. So, there is plenty of flexibility when it comes to the raw materials being used. Perovskite-based photovoltaics are typically formed by what's called solution processing, in which all the raw materials are dissolved in a liquid that's then layered on top of the panel-to-be, allowing perovskite crystals to form across its entire surface. Which is great, except that this process tends to form multiple crystals with different orientations on a single surface, decreasing performance.

Adding to the problems, perovskites are also not especially stable. They're usually made of a combination of positively and negatively charged ions, and these have to be present in the right ratios to form a perovskite. However, some of these individual ions can diffuse over time, disrupting the crystal structure. Harvesting solar energy, which involves the material absorbing lots of energy, makes matters worse by heating the material, which increases the rate of diffusion.

Combined, these factors sap the efficiency of perovskite solar cells and mean that none lasts nearly as long as a sheet of silicon. The new works tackle these issues from two very different directions.

The first of the new papers tackles stability by using the flexibility of perovskites to incorporate various ions. The researchers started by using a technique called density functional theory to model how different molecules would behave when placed into a spot normally occupied by a positively charged ion. And the modeling got them excited about a molecule called tetrahydrotriazinium, which has a six-atom ring composed of alternating carbon and nitrogen atoms. The regular placement of nitrogens around the ring allows it to form regular interactions with neighboring atoms in the crystal structure.

Tetrahydrotriazinium has a neutral charge when only two of the nitrogens have hydrogens attached to them. But it typically grabs a charged hydrogen (effectively, a proton) out of solution, giving it a net positive charge. This leaves each of its three nitrogens associated with a hydrogen and allows the positive charge to be distributed among them. That makes this interaction incredibly strong, meaning that the hydrogens are extremely unlikely to drift off, which also stabilizes the crystal structure.

So, this should make perovskites much, much more stable. The only problem? Tetrahydrotriazinium tends to react with lots of other chemicals, so it's difficult to provide as a raw material for the perovskite-forming solution.

High efficiency

So, the researchers involved in this work, based in Saudi Arabia and Turkey, decided to put raw materials that can form Tetrahydrotriazinium into the perovskite-forming solution. The reasoning is that the chemical would form in solution and immediately get incorporated into a perovskite crystal, after which it wouldn't have a chance to react with anything else. And it worked. The team used density functional theory to predict what the absorption spectrum of the material should look like and found that the perovskites produced using this process closely matched the prediction.

The initial crystals had some defects caused by uneven distribution of the other ions in the crystal. However, the researchers tried various conditions for the crystal-forming reaction and found one that largely eliminated these imperfections.

So, the team went ahead and layered it on top of silicon and got efficiencies in the area of 33 to 34 percent. They also sent a sample to a European test lab, which came out with an efficiency of 33.7 percent. The researchers have a few ideas that should boost this to 35 percent, but didn't attempt them for this paper. For comparison, the maximum efficiency for silicon alone is in the area of 27 percent, so that represents a very significant boost and is one of the highest perovskite/silicon combinations ever reported.

The crystals were reasonably stable when simply exposed to light. But the combination of light and heat caused a more significant decay in performance. The researchers say that "devices maintain ≥90 percent of their initial performance up to 1,000 hours," but a decay of up to 10 percent in about three months is not ready for commercial deployment. So, still some work to do there.

Better crystals

The second paper focuses on the fact that solution processing tends to produce a large number of individual crystals, with the faults between them allowing atoms to leach out of the perovskite structure. Fixing this requires a balancing act: exerting greater control over the crystallization process without increasing the time and cost of it so much that it erases some of perovskite's advantages.

To control this process, the researchers focused on using something called an anti-solvent, which basically reduces the solubility of other chemicals in solution. The one they used was essentially a long hydrocarbon chain linked up with an ammonium and bromine atom, both of which are typically components of perovskites. Adding that to the solution could control the formation of perovskite crystals with a wide variety of compositions. The result was a more robust crystal with fewer of the defects that affect performance and stability.

When combined with a silicon photovoltaic layer, these devices reached efficiencies in the area of 30 to 33 percent—again, significantly higher than silicon alone. Durability, however, remained a problem, with similar performance to the material we mentioned above at elevated temperatures. But, at room temperature, the material had over 98 percent of its original efficiency at 100 days. Room temperature operations are unlikely, though, and that's still not going to be good enough for commercial use.

The nice thing about this work is that it tackles two different aspects of perovskite performance: crystal composition and crystal formation. So, it might be possible to combine the two and get even greater performance. Still, as one of the papers suggests, "it is evident that addressing the photothermal stability of perovskite/silicon tandem solar cells is a multifaceted challenge that necessitates unraveling various complexities including the interfaces, contacts, electrodes, and encapsulants." So, while we're seeing progress, we've not yet seen an approach that can balance all those complexities into a commercially viable product.

Science, 2024. DOI: 10.1126/science.ado9104, 10.1126/science.adp1621  (About DOIs).