Lead Free Two-Dimensional Mixed Tin and Germanium Halide Perovskites for Photovoltaic Applications

Recently, inorganic–organic hybrid perovskites have gained immense attention in view of its outstanding performance in solar cell devices. Due to the toxicity of lead, lead-free perovskites are highly desirable in order to achieve comparable or superior photovoltaic performance. In this study, we have investigated the structural, electronic, and optical properties of lead free two-dimensional CH₃NH₃Sn₍₁ – ₓ₎GeₓI₃ (0 ≤ x ≤ 0.5) perovskites. These perovskites are direct band gap semiconductors and possess band gap in the region of 1.38–1.61 eV, which are highly desirable for solar energy absorption. On the application of tensile strain, they remain direct band gap semiconductors with larger band gap as compared to free ones. These perovskites show excellent optical absorption (∼10⁵ cm–¹) in the entire UV–vis region and finally achieve a high photoconversion efficiency of up to 25.76%, making them competitive with previously reported perovskite-based solar cells. Our findings will stimulate the experimentalists for designing such lead-free perovskite semiconductors for high-efficiency solar cell devices.