As the price of silicon panels continues to fall, the cost of building a solar power plant is becoming smaller. This means that it may be worth spending more to buy panels that convert more sunlight into electricity, since you’ll get more return on the money you pay to install the panels. But silicon panels are already approaching the physical limits of their efficiency, which means that to significantly improve the efficiency of panels, your best bet may be to combine silicon with additional photovoltaic materials.
Currently, the most excitement is in combining silicon with a type of material called perovskite. By layering perovskite crystals on top of silicon, you can create a panel with two materials that absorb different parts of the spectrum. What’s more, perovskites can be made from relatively cheap raw materials. Unfortunately, it’s been difficult to make a perovskite that’s both efficient and lasts for the same decades as the silicon part.
But a number of labs are trying to change that, and this week two of them reported some progress, including a perovskite/silicon system that achieved an efficiency of 34 percent.
Improving perovskite stability
Perovskites are a family of materials that all form the same crystal structure, so there is a lot of flexibility when it comes to the raw materials used. Perovskite-based photovoltaics are typically formed using a method called solution processing, where all the raw materials are dissolved into a liquid and then layered on top of what will become the panel, allowing perovskite crystals to form across the entire surface of the panel. This is great, but the process also tends to result in multiple crystals of different orientations on one surface, reducing performance.
To make matters worse, perovskites aren’t particularly stable. They’re typically made up of a combination of positively and negatively charged ions, which must be present in the right ratio to form a perovskite. But some of these ions can diffuse over time and disrupt the crystal structure. Harvesting solar energy requires the material to absorb large amounts of energy, which makes things worse, as it heats up the material and increases the rate of diffusion.
These factors combine to make perovskite solar cells less efficient and less durable than sheets of silicon. The new research tackles these problems from two very different directions.
The first paper in the new paper takes advantage of perovskites’ flexibility by incorporating different ions to study their stability. The researchers began by using a technique called density functional theory to model how different molecules would behave if they were placed in the space normally occupied by positively charged ions. This modeling led the researchers to intrigue a molecule called tetrahydrotriazinium, which has a six-atom ring with alternating carbon and nitrogen atoms. The regular arrangement of nitrogen atoms around the ring allows them to form regular interactions with neighboring atoms in the crystal structure.
Tetrahydrotriazinium is charge neutral when only two of the nitrogens have hydrogens bonded to them. But it will grab a normally charged hydrogen (essentially a proton) from solution, giving it a net positive charge. This causes each of the three nitrogens to remain bonded to a hydrogen, and the positive charge is shared between them. This makes the interaction very strong, making it extremely unlikely for the hydrogen to drift away, and also stabilizing the crystal structure.
This should therefore make the perovskite much more stable. The only problem is that tetrahydrotriazinium tends to react with many other chemicals, making it difficult to provide as a raw material for the perovskite-forming solution.
