Exploring Thin-Film CaZn₂P₂ and SrZn₂P₂—Optoelectronic Properties for Next-Generation Tandem Solar Cells

Tandem solar cell architectures offer a transformative approach to overcoming the inherent efficiency limitations of single-junction solar cells, which are capped by the Shockley-Queisser limit at ~33.5%. By coupling a top cell with a higher band gap (1.5 eV - 2.3 eV) and a bottom cell with a complementary lower band gap absorber, these tandem configurations maximize solar spectrum utilization, theoretically achieving power conversion efficiencies (PCEs) of up to ~47%. This enhanced performance has significant implications for advancing the photovoltaic industry and meeting global renewable energy demands. In this study, we focus on the potential of thin-film Zintl compounds CaZn₂P₂ and SrZn₂P₂ as top cell layers for tandem solar cell applications. These materials exhibit band gaps of approximately ~1.8 eV, placing them within the optimal range for efficient light absorption and energy conversion in tandem architectures. We performed room-temperature photoluminescence (PL) spectroscopy and confirmed their direct band gaps as 1.95 eV and 1.81 eV, respectively, highlighting their suitability for photovoltaic applications. Complementary studies on Raman spectroscopy further elucidated their vibrational modes, including Eg and A1g, with results closely matching theoretical predictions. Additional characterization techniques were employed to provide a holistic understanding of the properties of these materials. FTIR spectroscopy on the thin films offered detailed insights into the atomic arrangements and bonding characteristics of these compounds, revealing vibrational features that support their structural integrity. UV-Vis reflectance and transmittance spectroscopy identified their optical absorption edges and transparency, critical for light management strategies in tandem cell designs. Expanding on these findings, we also performed current-voltage (I-V) measurements and photocurrent response analyses on the thin films to directly evaluate their electrical performance. The results showcased efficient light-to-current conversion providing compelling evidence of their viability as active photovoltaic layers. These optical properties, combined with their favorable electronic behavior, position CaZn₂P₂ and SrZn₂P₂ as highly promising materials for photovoltaic applications, offering not only performance enhancements but also aligning with sustainability and material abundance principles—key considerations for large-scale deployment.
Through a comprehensive combination of optical, vibrational, and electrical characterizations, this work establishes a robust framework for understanding the optoelectronic behavior of CaZn₂P₂ and SrZn₂P₂ thin films. This study contributes to the broader efforts of advancing solar energy technologies and addresses the critical need for sustainable, high-efficiency solar materials by highlighting their potential integration into next-generation photovoltaic technologies, .