Perovskite solar cells have attracted much attention since they offer efficient energy conversion and potentially cheaper solar power. However, the toxicity of lead and the volatility of the organic ions in perovskite light absorbers are major obstacles to their development and eventual commercial use. In a new study by researchers from Brown University and University of Nebraska–Lincoln, a new lead-free perovskite material that can be used as an alternative to silicon in solar cells has been demonstrated.
As perovskites, a class of materials with a certain crystalline structure that makes them useful for energy, usually contain lead, as well as organic materials that can be unstable when exposed to the environment, the team devised a new titanium-based material for producing lead-free, inorganic perovskite solar cells that holds promise. This is particularly true for tandem solar cells where the perovskite cells are positioned on top of silicon or another material, facilitating overall efficiency. Titanium is a viable alternative to lead as it is abundant and robust, as well as being a biocompatible element. As reported in the journal Joule [Chen et al. Joule (2018) DOI: 10.1016/j.joule.2018.01.009], the resulting material also has favorable properties for solar applications that can be tuned.
“Tandem cells are the low-hanging fruit when it comes to perovskites. We're not looking to replace existing silicon technology just yet, but instead we're looking to boost it.”Nitin Padture
Using computer simulations, it was predicted that a class of perovskites with cesium, titanium and a halogen component (bromine or/and iodine) was a good candidate, before a solar cell was developed with the material and its properties assessed. A semi-transparent perovskite films with a bandgap of 1.8 electron volts, ideal for tandem solar applications, and a conversion efficiency of 3.3%, well below that of lead-based cells, was produced.
A high-temperature evaporation method was used to prepare the films, and the material's relatively large bandgap compared to silicon also means that as the upper layer in a tandem solar cell it would work to absorb the higher-energy photons from sunshine that the lower silicon layer can't absorb. In addition, lower energy photons would pass through the semi-transparent upper layer to be absorbed by the silicon, helping to increase total absorption capacity. As senior author Nitin Padture said “Tandem cells are the low-hanging fruit when it comes to perovskites. We're not looking to replace existing silicon technology just yet, but instead we're looking to boost it.”
As the method for making the thin films is scalable, it could be used to deposit over a large area. The team hope to further develop a new low-temperature, solution-based process to depositing thin films of this material to bring the cost of cell fabrication down even further.