A new lead‑free perovskite based on cesium, sodium, iron, and bromine (Cs₂NaFeBr₆) could provide a solar cell efficiency of up to 32%. According to calculations, this efficiency is achieved with an absorber layer thickness of only 5 µm. This is the conclusion reached by scientists from the Bangladesh University of Engineering and Technology and the University of Dhaka, who used computer modelling to study the properties of the new material.
Perovskites are considered one of the most promising materials for solar energy: they absorb light well and allow very thin photoactive layers to be created. The efficiency of the best perovskite solar cells has already reached 26.7%. However, the majority of the most efficient materials of this class contain lead, whose toxicity remains an obstacle to their widespread use. Researchers are therefore looking for lead‑free compounds that could retain the advantages of perovskites. Cs₂NaFeBr₆ is one such candidate.
The material itself has not yet been synthesized by the researchers. Its properties were studied using quantum‑mechanical calculations: the scientists modelled the crystal structure, checked its stability, and then calculated the electronic, mechanical, and optical characteristics of the compound. The results showed that Cs₂NaFeBr₆ can form a stable cubic crystal lattice and should not spontaneously decompose or change its structure. In addition, calculations indicate a relatively soft and plastic structure, which could be useful for making thin films.
One of the key characteristics of Cs₂NaFeBr₆ is its band gap which is equal to 1.09 eV. This parameter determines how much energy an electron needs to transition to a state in which it can participate in generating an electric current. For comparison, silicon has a band gap of about 1.1 eV. The obtained value lies in the range suitable for efficient conversion of sunlight into electricity. Calculations also showed that the material absorbs visible light well, and that electrons can move relatively freely within it, which is important for charge transport.
Cs₂NaFeBr₆ works particularly efficiently even in a very thin layer. At a thickness of just 1 µm, the calculated solar cell efficiency was 30.6%, and at 5 µm – 32.09%. The latter figure is practically the same as the Shockley‑Queisser limit, specifically, the theoretical maximum efficiency for a single‑junction solar cell with such a band gap, which is about 32.2%.
The high calculated figures are also related to the material’s ability to absorb light well and transport the generated charges. In addition, Cs₂NaFeBr₆ has a high dielectric constant, which reduces interaction between positive and negative charge carriers. This can reduce their recombination, i.e., the process by which charges recombine and the energy obtained from light is lost instead of being converted into electric current.
However, these are only theoretical calculations. Scientists still have to synthesize Cs₂NaFeBr₆ and verify its characteristics experimentally. In a real solar cell, efficiency will depend on the quality of the material obtained, the number of defects, charge losses, and stability against moisture, oxygen, heat, and prolonged light exposure.
