PEROVSKITES Tin halide: Paving the way for lead-free perovskites

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There’s a huge buzz surrounding perovskite solar cells (PSC), or ‘perovskites’. Referred to by many as the next generation of solar photovoltaic technology, they promise to bring impressive gains in terms of cost, stability, and efficiency.

These gains come at a cost, however; perovskites are made using lead, a highly toxic material that remains a key concern for regulators despite assurances that encapsulation technology and recycling schemes will offset any risks to the environment posed by this emerging technology.

Why are perovskites so highly regarded?

Perovskites are so often referred to as a “next-generation” technology due to their great optoelectronic properties. This leads to characteristics like good carrier diffusion length, strong light absorption, and low defect density, meaning that perovskite solar cells are much more stable and efficient than their more expensive silicon cousins. The technology is so good, in fact, that the PCE of lead perovskite solar cells has increased from a mere 3.8 % to a huge 25.5 % in the last decade.

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However, the inclusion of toxic lead in perovskite solar cells constantly shadows over its potential because of these environmental risks. In addition to this, the peak efficiency of a lead-based perovskite solar cell is thought to be around 32 % due to its relatively wide bandgap of above 1.5 electronvolt (eV).

For researchers working on perovskite solar cell technology concepts, getting rid of lead and finding an alternative material has therefore become a key target. Materials that have been explored as a potential alternative to lead include gold, tin, and bismuth, and while significant progress has been made with using these materials in recent years, they lag far behind lead and lead-containing materials in terms of efficiency and stability.

The most promising alternative candidate material

Tin halide perovskite is currently thought of as the most promising alternative candidate to lead for developing solar cells. It is far less toxic, has a similar electron configuration to lead, and has a lower bandgap which increases the potential efficiency gains. This has triggered an intensive research effort as more and more researchers are working to make tin halide perovskite solar cells (TPSCs) a reality.

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While promising, tin halide has limitations that need to be overcome. For example, it’s very easy to oxidize tin halide due to its unique electronic structure. Several works are currently focusing on the development of methods to reduce this oxidation by adding antioxidant additives or using low-dimensional structures.

One example of recent work which focused on solving this problem saw a team of researchers build a stable amorphous-polycrystalline structure which was able to block outside oxygen, moisture, and suppress ion diffusion. This design enabled the researchers to produce a stable TPSC that maintained 95 % of its initial power conversion efficiency and obtain quasi-steady state efficiency of over 10 % for TPSCs from an accredited certification institute.

Another study led by Helmholtz-Zentrum Berlin (HZB) and the Institute of Functional Nano and Soft Materials at Soochow University in China introduced an additive to the perovskite material which led to a more ordered crystalline structure and a PSC that performs stably over an extended period of time. While the efficiency of current TPSCs remains well below what might be considered commercially viable and far behind what has already been achieved with lead-based perovskites, researchers are convinced that tin is the most promising alternative material for the future and that it will open up new doorways in the development of highly efficient PSCs.

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