Passivation technique reduces defects in kesterite solar cells to achieve 11.51% efficiency

Over the past few decades, solar cells have become increasingly widespread, with a growing number of individuals and businesses worldwide now relying on solar energy to power their homes or operations. Energy engineers worldwide have thus been trying to identify materials that are promising for the development of photovoltaics, are eco-friendly and non-toxic, and can also be easily sourced and processed.

These include kesterite-based materials, such as Cu₂ZnSnS₄ (CZTS), a class of semiconducting materials with a crystal structure that resembles that of the naturally occurring mineral kesterite. Kesterite solar cells could have various advantages over the conventional silicon-based photovoltaics that are most used today, including lower manufacturing costs, a less toxic composition and greater flexibility.

Despite their potential, kesterite solar cells developed to date attain significantly lower power conversion efficiencies (PCEs) than their silicon counterparts. This is in great part due to atomic-scale defects in kesterite-based materials that trap charge carriers and prompt non-radiative recombination, a process that causes energy losses and thus reduces the solar cells’ performance.

Researchers at Shenzhen University and University of Rennes recently introduced a new passivation technique that could help to suppress defects in CZTS and other kesterites, which could in turn boost the performance of solar cells based on these materials. Their proposed technique, outlined in a paper published in Nature Energy, was found to result in solar cells with a certified efficiency of 11.51%, without the use of any further additives to improve the materials’ properties.

“CZTS is a competitive photovoltaic material, especially for multijunction solar cells,” wrote Tong Wu, Shuo Chen and their colleagues in their paper. “However, the device power conversion efficiency has remained stagnant for years. Deep-level defects, such as sulfur vacancies (V_S), cause serious non-radiative recombination of charge carriers. We propose a passivation strategy for V_S through the heat treatment of the CdS/CZTS heterojunction in an oxygen-rich environment.”

The passivation strategy devised by this team of researchers entails heating the CdS/CZTS heterojunction, which is the interface between the kesterite material (i.e., CZTS) and a cadmium sulfide (CdS) buffer layer, all within an oxygen-rich environment. Buffer layers are intermediate layers in solar cells that are placed between absorber materials, in this case CZTS, and a transparent conductive material.

“In this process, V_S are occupied by oxygen atoms, suppressing V_S defects,” explained Wu, Chen and their colleagues. “In addition, the diffusion of Cd ions to the CZTS absorber layer, and the formation of positive Na–O and Sn–O complexes can passivate related defects. These effects led to a reduced charge recombination and favorable band alignment.”

To demonstrate the potential of their passivation approach, the researchers applied it to real CZTS-based solar cells and then assessed these cells’ performance in a series of tests. They found that their strategy improved the cells’ PCEs, without the use of any additives or extrinsic doping strategies.

“We demonstrate a certified efficiency of 11.51% for air-solution-processed CZTS solar cells (bandgap of 1.5 eV) without any extrinsic cation alloying,” wrote Wu, Chen and their colleagues. “The study offers insights into defect passivation and performance improvement mechanism of kesterite solar cells.”

In the future, the recent study by Wu, Chen and their colleagues and the new passivation strategy they devised could be perfected further and applied to other kesterite-based solar cells. Eventually, it could contribute to the advancement of these solar cells, which could in turn facilitate their real-world deployment.

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