Cheap, flexible, and efficient, perovskite solar cells are the rising star in the field of photovoltaics. However, their low durability has kept them from the consumer market.
But now, KACST researchers, in collaboration with École Polytechnique Fédérale de Lausanne, Switzerland, have unravelled some fundamental electronic processes, which could be useful to improve the performance of these light-harvesting devices.
The researchers tested and compared two types of perovskites. The perovskites general formula is ABX3, where A is a larger cation (positively charged ion), B a smaller metallic cation, and X a halide anion (a halogen atom that is negatively charged). The team used a frame of lead bromide with either formamidinium (FA) or caesium (Cs) as A-site cations, creating FAPbBr3 and CsPbBr3, respectively.
“Stability is still an issue in perovskite solar cells, particularly in humid and hot conditions. We used a frame of lead bromide, because it has higher stability compared to others, such as lead iodide. Then, we thought that optimizing the A cation could further improve the efficiency of these devices,” says Mohammad Hayal Alotaibi, assistant professor at the Materials Science Research Institute, and a member of the team. “The size of A cations can play an important role. For example, if they do not fit properly in their positions, they may cause poor functioning.”
Compared to the cell made with caesium (CsPbBr3), the cell made with formamidinium (FAPbBr3) has more favourable photocurrent, photovoltage, and fill factor: three photovoltaic parameters that determine the power conversion efficiency of a solar cell.
Perovskite solar cells were built in layers, starting with fluorine doped tin oxide glass, followed by titanium dioxide (TiO2), perovskite, then spiro-OMeTAD as hole transport material (HTM) and finally gold. Photons of the sunlight are absorbed by the perovskite film, where they excite some electrons and cause them to migrate to the TiO2 layer. A hole is created where each electron was formerly bound. This hole also participates in conduction by moving to the HTM. The scientists used various techniques to measure how free electrons roam within the device, to generate current, and end their journey by recombining with the holes.
Several analyses confirmed that electrons in devices made with formamidinium can move quicker across the TiO2-perovskite interface, than in those made with caesium. The devices of caesium are more subject to power loss as they have higher internal resistances for charge transport, and lower shunt resistance, which provides an alternate current path for the light-generated current. Caesium cells are also limited by mechanisms that facilitate a quicker recombination between electrons and holes.
The team is planning to introduce other A cations of various sizes, as well as different anions.