Discovering non-toxic more stable semiconductor for solar

Materials World magazine
29 Aug 2019

Engineers have discovered a lead-free semiconductor for solar cells using supercomputers. Shardell Joseph reports.

Data analytics and quantum-mechanical calculations have aided the discovery of lead-free perovskite semiconductors for solar applications. Engineers from Washington University, St Louis, USA, claim the semiconductor is more stable and less toxic due to a novel double mineral.

According to Washington University Department of Mechanical Engineering and Materials Science Assistant Professor, Rohan Mishra, the inorganic, double perovskite oxide semiconductor – KBaTeBiO6 – was the most promising of the materials screened from 30,000 bismuth (Bi)-based oxides with the double perovskite structure. Because of this, the material could be used in place of lead-halide perovskites, providing benign and stable properties for various semiconducting applications.

Mishra claimed that the material is suitable for solar applications due to its stability and electronic performance, including high absorbance, low effective mass of carriers and a band gap comparable to Bi-based halide double perovskite alternatives.

Alternatives to lead

Solar cells made from lead-halide perovskites have shown potential to provide a substantial increase in power conversion efficiency, recorded to reach 24.2%, rendering this an ideal material for solar energy generation. However, the toxicity of the lead combined with its instability makes lead-halide perovskite semiconductors problematic.

Mishra explained that Bi-based double perovskites may present a promising alternative to the toxic lead-halide versions, but the research leading from this has focused on Bi-based halide double perovskites, which contain chlorine, bromine and iodine.

‘This is a limited composition space and the promising candidates have mostly been identified,’ Mishra told Materials World. ‘The Bi-based halide double perovskites suffer from issues related to environmental stability – shorter device lifetime – and electronic properties that are under optimised for solar cell absorbers.’

The paper, KBaTeBiO6: A lead-free, inorganic double-perovskite semiconductor for photovoltaic applications, argued that using Bi-based oxide instead of halide offers the ability to change both the cation combination and stoichiometry to achieve desired electronic properties.

‘With interest from the community, one can tune their properties to make them desirable for semiconducting applications beyond solar cells, such as for photocatalysis and LEDs,’ said Mishra.

The band gap impact

The stability of the semiconductor lies in the band gap. Referring to the energy difference between the top of the valence band and the bottom of the conduction band, the band gap measures the capacity of electrons to jump from one band to another. Mishra explained that oxides are generally very stable, but most oxides have a wide band gap, means they appear transparent to most photons in the solar spectrum.

For solar cells to function, the semiconductor needs to absorb those photons. ‘We found that KBaTeBiO6 has good combination of both stability and lower band gap compared to most oxides.’

According to the paper, KBaTeBiO6 has an experimental indirect band gap of 1.88eV. For comparison, conventional semiconductors average at 1-1.5eV, but materials with wide band gaps such as silicon dioxide can reach up to 9eV.

‘The band gap of KBaTeBiO6 is not ideal,’ Mishra added. ‘However, it is comparable to other Bi-based halide double perovskite alternatives. Given the large composition space offered by oxides, one can potentially lower the band gap by playing with cation substitution.

Non-toxic solar cells

Mishra said these are first-generation solar cells of this type and will need more fine-tuning of the band gap, but it is a good first step toward non-toxic solar cells.

‘This shows that we can go away from these lead-halide perovskites,’ Mishra said. ‘This opens up a really big space for designing semiconductors not just for solar cell applications but also for other semiconductor applications, such as LCD displays.’

Mishra confirmed that this process is not yet ready for commercialisation, as many of its inherent materials properties remain to be investigated, including the defects and dopability. The team will now investigate the role of defects, gain better control over composition, identify dopants, and find a scalable method for large-scale production of this or related oxide materials.