The impact of Zr alloying on the engineering of BaHfS₃

Lead halide perovskite solar cells (LHPSCs) have rapidly gained prominence in the field of photovoltaics, boasting impressive power conversion efficiencies (PCEs) of up to 26.1% in single-junction devices. However, despite their high performance, these materials suffer from critical drawbacks, including degradation when exposed to moisture, oxygen, heat and ultraviolet light, as well as concerns regarding the toxicity of lead.

Overcoming these challenges is vital for the commercial viability and environmental safety of next-generation solar technologies, but how can we engineer stable and sustainable alternatives without compromising efficiency?

To address these limitations, my research team at the Autonomous University of Querétaro in Mexico focused on chalcogenide perovskites, particularly by alloying zirconium (Zr) into barium hafnium sulfide (BaHfS₃), as a promising alternative.

These materials offer unique properties that make them highly suitable for photovoltaic applications. They demonstrate excellent chemical stability, essential for maintaining long-term performance in real-world conditions. Furthermore, they exhibit a tunable bandgap, a high absorption coefficient for photons, and enhanced carrier mobility with p-type conductivity.

Our study explored the use of BaHfS₃ and its Zr-alloyed variants, such as BaHf_0.75Zr_0.25S₃, BaHf_0.5Zr_0.5S₃, and BaHf_0.25Zr_0.75S₃, as absorber layers in photovoltaic devices.

To evaluate and optimize their performance, we utilized SCAPS-1D (Solar Cell Capacitance Simulator in One Dimension), a simulation tool developed by Mark Burgelman at the University of Ghent. This tool enabled us to simulate real-world conditions and fine-tune key device parameters such as absorber acceptor density, defect density, and layer thickness.

Our findings, published in Materials Science and Engineering: B, show that careful optimization of these chalcogenide perovskites can significantly enhance photovoltaic performance. The results indicate a promising path toward efficient, stable, and lead-free solar cells.

This approach led to improvements in light absorption, reduced recombination losses, enhanced built-in potential, and minimized non-radiative recombination and charge transfer resistance. Additionally, we improved the band alignment between layers and strengthened interfacial properties, resulting in notable increases in PCE.

We conducted a comparative analysis of both base and optimized solar cells for all absorber compositions using techniques such as C-V profiling, Mott–Schottky analysis, C-F measurements, QE, and energy band alignment.

The enhancements in PCE were attributed to increased short-circuit current density, greater quasi-Fermi level splitting, higher carrier generation rates, stronger electric fields, improved quantum efficiency, and extended carrier diffusion lengths. Ultimately, we achieved PCEs exceeding 20% for BaHfS₃, and its Zr-alloyed forms.

Overall, our research highlights the potential of BaHfS₃ and its Hf/Zr variants (BaHf_1-xZr_xS₃) as high-performance, lead-free chalcogenide perovskite solar absorbers. We believe our work will spark further interest among materials scientists and photovoltaic researchers.

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