More news on the perovskite solar cell front! I’ve written a bit about these a bit in the past – they are the exciting newcomers to the photovoltaic scene. At an early stage of development, they already show up to 17% efficiency, use earth-abundant materials, and only require low-temperature solution methods to create that hold the promise of greatly lowering costs. The central perovskite used thus far is methylammonium lead halide (MAPbX3, where X is a halogen, often iodine). The crystal structure looks like this:
The A is where methylammonium sits and M is lead, which is surrounded by the halogen (X). These types of solar cells have only been studied for a few years – compare that to the decades of silicon solar cell research, and the efficiency and costs are sure to improve. They also have a high stability, giving them a leg up on organic solar cells (at least for now), large absorption coefficients (more light absorbed to excite electrons, which means more energy), and high carrier mobility (how fast the electrons/holes can move through the material to be extracted to do work), all major qualities for a good solar cell. This is exciting stuff! And now there’s new research from Science showing how performance over 10% efficiency can be achieved while removing the most expensive part of the cell! Here’s how…
Typically, a perovskite solar cell will have a hole conducting layer, which is a material that attracts holes but not electrons. This helps with the electron-hole separation that is necessary upon light hitting the cell and exciting an electron. Holes then diffuse to the hole conducting layer, which moves them on to a highly conducting gold contact that extracts them to do work (electrons do something similar in the opposite direction). In perovskite solar cells, this hole conducting layer is typically made of arylamine spiro-OMeTAD – that’s a mouthful! This is basically just an organic compound made of a bunch of aromatic carbons connected to nitrogen and oxygen atoms. Looks like this:
Imagine a bunch of these molecules mixed up together to form a thin layer in the solar cell. The key point here is that, being organic, this hole conducting layer limits the stability of the cell (organic materials degrade fairly quickly) and it’s also the most expensive part! So getting rid of this hole conducting layer would both improve stability and decrease costs. And that’s exactly what Mei et al did.Above is a schematic of the new perovskite solar cell design they used. Instead of using spiro-OmeTAD as a hole conducting layer to block electrons, they cleverly used a three-layer design that takes advantage of differences in energy offsets in the conduction band between a TiO2 (titania) layer and a ZrO2 (zirconia) layer. This takes a little bit of solid state physics to understand, but I’ll just focus on the TiO2 and ZrO2 layers to show how the cell works:
The red in the figure above indicates the perovskite – it begins as a solution that is mixed up with the mesoporous TiO2 and ZrO2 layers (grey and green in the figure). Incoming light excites electrons in the perovskite, which has the right electronic band gap to optimally absorb light, which are then quickly transferred to the TiO2. Now, at this point, the electrons and holes can move around, and we want to have them move in opposite directions so they don’t recombine – at that point, we’ve lost our excited charge and can’t gain any energy. Mei et al found that this ZrO2-TiO2 interface intrinsically allows the electrons and holes to separate. It takes a little solid state physics to understand the details, but the basic idea is that electrons would need to be given even more energy to move into the ZrO2, whereas holes would naturally move into the ZrO2 during their diffusion process. Thus, the ZrO2-TiO2 interface creates a natural barrier against electrons but openly accepts holes. Thus, it plays just the role the spiro-OMeTAD layer was doing before, but now we’ve used a cheap, stable material (ZrO2)! Electrons and holes are separated and extracted from the TiO2-ZrO2 junction to do work in an external circuit.
These are the kind of ingenious design tricks that get you a Science publication (rightfully so!) It may seem like a simple concept, but interfaces between materials can be a region of tricky, mysterious physics, and we don’t always know how electrons and holes will respond. With this cell design, Mei et al achieved a 12.8% efficiency and stability greater than 1000 hours! They also found that removing the ZrO2 layer severely reduces the cell’s performance to 4.18% efficiency, indicating its crucial role in blocking electron transport. This is a great step forward in reducing the cost of these promising new types of solar cells.
Mei, A., Li, X., Liu, L., Ku, Z., Liu, T., Rong, Y., Xu, M., Hu, M., Chen, J., Yang, Y., Gratzel, M., & Han, H. (2014). A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability Science, 345 (6194), 295-298 DOI: 10.1126/science.1254763