Solar power is a green, renewable energy source that is currently seeing a lot of research interest. While panels today are usually made of silicon solar cells, other materials are being explored that might improve efficiency and cost.
Another material, however, is starting to look a lot more attractive for solar technology. Halide perovskites are cheap, easy-to-make, and have a high power conversion efficiency (PCE); it’s now high enough to match the efficiency of silicon cells.
They’re still in need of improvement, however. The halide perovskite MAPbI3 (methylammonium lead tri-iodide) degrades rapidly under normal conditions. In the paper Mechanisms for light induced degradation in MAPbI3 perovskite thin films and solar cells (Abdelmageed et al. 2016), this problem is addressed. How does MAPbI3 degrade, and can we prevent it?
How Perovskite Cells Work
Silicon solar cells rely on altering the silicon so that it can conduct electricity. In the case of perovskite thin films like MAPbI3, the technology relies on conduction band theory. That means it uses different conduction bands within the molecule that electrons can jump across. Electricity, after all, is electrons in motion.
We aren’t entirely sure why materials like MAPbI3 are so efficient at capturing sunlight, but studies suggest that conduction band theory plays a role. It looks like light excites sub-band gaps, harnessing sunlight into electricity. It’s a powerful tool, but it also degrades quickly when exposed to the elements. To solve that problem, this study evaluates how MAPbI3 degrades on an environmental and molecular level.
Following the Clues
In MAPbI3, we have an ideal material for solar cells. It also degrades quickly. How would a scientist go about solving this problem? First, we have to answer a few questions in the lab.
When does degradation occur?
When you expose different materials to ultraviolet light, they absorb different wavelengths. The resulting UV absorbance spectra can be used to identify materials.
UV absorbance is one of many spectroscopy methods, and there are others used in this study. But for now, it answers generally: do we have MAPbI3, or do we have the degraded version?
In Fig. 1, MAPbI3 is exposed to three conditions: (1) dry air and light, (2) dry air and dark, and (3) nitrogen gas and light. In this way, we can control for two variables. Does oxygen degrade it? Does light do the trick? The spectra show no change in MAPbI3 after exposure to just light or just oxygen (in air). However, when both are present, the spectra show an identity shift. Degradation must require both.
What does degradation change?
MAPbI3 has changed – but how? A second spectrographic tool, FT-IR, can give more detail. In Fig. 2, we compare the FT-IR spectra of MAPbI3 and the degraded product made above. This data is specific enough to notice not just a general change in the molecule, but also changes in the actual molecular structure.
For example, this data shows changes in nitrogen/hydrogen bonds, and carbon/hydrogen bonds. We can now see how amine and methyl groups change after degradation. These chemical clues tell us how MAPbI3 breaks down. And that means we can identify the chemical reactions that cause it.
The Chemistry of Degradation
These experiments leave us with two clues. We know that oxygen and light are both necessary to degrade MAPbI3. We also know what chemical groups change in MAPbI3 after degradation.
Now, we want a mechanism. What are the chemical reactions that break down MAPbI3?
Based on the evidence of the first two experiments, the authors knew that a deprotonation occurred on an amine group (a hydrogen was taken away). From their test on environmental conditions, light and air are both necessary for this.
To get a closer look at the reaction, they set up a chemical solution and exposed it to light, tracing UV absorbance over time. And over time, they noticed that iodide ions in the solution, when they are excited by light, release electrons. Those electrons then interacted with oxygen and carbon dioxide in the air, producing free radicals.
We’ve discussed free radicals before and the destruction they bring to living things. In this case, the free radicals are what tear hydrogen protons off of MAPbI3. This, then, causes it to degrade.
In other words: light excites iodide ions → electrons are released → electrons react with air to make free radicals → free radicals deprotonate MAPbI3. And there you have it – the material has degraded, and can no longer be used to harness sunlight for energy.
In a solar panel, MAPbI3 acts as a thin film for capturing sunlight and converting it to energy.
By adding another, protective film, to the MAPbI3, the scientists were able to protect it from degradation and keep PCE (power conversion efficiency) high throughout their experiment. This data is in Fig. 4, and it shows how we can rescue MAPbI3 devices for solar panel technology.
In the end, this paper highlights some ways that physicists evaluate materials for use in the everyday world. In this case, MAPbI3 is something we’d like to use for better solar panels and greener energy. But the film has some shortcomings. By testing MAPbI3 in different conditions and noting how it changed, the physicists were able to understand where the material’s weaknesses were coming from. With that key information, they could find a way to fix it.
Abdelmageed, G., Jewell, L., Hellier, K., Seymour, L., Luo, B., Bridges, F., Zhang, J. Z., & Carter, S. (2016). Mechanisms for light induced degradation in MAPbI3 perovskite thin films and solar cells. Applied Physics Letters, 109(23). https://doi.org/10.1063/1.4967840