A team of researchers from China has succeeded for the first time in converting 28.49% of sunlight with a photovoltaic cell by combining two materials synthesized in the form of perovskite, completely free of silicon.
Context. Perovskite solar cells are the most promising photovoltaic technology to replace silicon cells, but they have a problem: imperfections on the surface of this material cause a lot of energy to be lost in the form of heat, which reduces their efficiency.
To solve this problem, laboratories around the world are researching tandem cells that combine layers of perovskite with silicon to absorb different parts of the spectrum of sunlight. Following this idea, they have managed to overcome the theoretical limit of conventional cells: today the efficiency record is 33.9% thanks to perovskite. The problem is that these cells still contain silicon, the most expensive component, since perovskite is synthesized from cheap materials.
Perovskite-perovskite. While perovskite-perovskite tandem cells have already existed, they were not as efficient as silicon-perovskite ones. Until now. Researchers at Huazhong University of Science and Technology in China have demonstrated in Nature Communications an all-perovskite tandem solar cell synthesized from tin and lead with unprecedented efficiency.
The team focused on removing the imperfections in perovskite and ended up creating a treatment that rebuilds the surface of the material using two chemical compounds. Their improved cells waste less energy and can convert more sunlight into electricity.
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The treatment. The strategy basically consists of polishing and protecting the surface of the perovskites. The researchers used 1,4-butanediamine (BDA) as a polishing agent and ethylenediammonium diiodide (EDAI2) as a surface passivator.
The physical loss of efficiency of perovskites is caused by non-radiative recombination of charge carriers, a problem caused by defects on the surface of the material that form during the crystallization process. BDA helps reduce tin-related defects, which are the most problematic, and EDAI2 is responsible for passivating defects related to cation and halide vacancies.
Shimmering. Thanks to this treatment, the perovskite solar cells achieved efficiencies of 22.65% and 23.32% in monojunction, and a certified efficiency of 28.49% in two-junction tandem, a new record for this type of technology. In addition, the tandem cells retained 79.7% of their efficiency after continuous operation at full power for 550 hours.
It is the most significant advance in efficiency and stability so far for the development of perovskite-only solar cells, which have the potential to transform the sector because they are cheaper and more flexible than conventional silicon cells.
Image | Helmholtz-Zentrum Berlin / Yongyan Pan et al.
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