Zinc Phosphide: a Promising Material for the Next Generation of Solar Cell Absorbers

The path towards the next generation of inexpensive and high efficiency solar cells is based on the utilization of earth-abundant, environmentally-friendly, and chemically stable materials. One promising candidate is zinc-phosphide (Zn₃P₂), with a direct bandgap of 1.5 eV, high absorption coefficient ( &gt; 10⁵ cm^-1), and excellent air stability. This work will present an overview of recent key findings related to the engineering of Zn₃P₂ absorbers for incorporation into high-efficiency solar cell devices.<br/><br/>First part of the talk will focus on three different approaches for synthesizing high crystal quality Zn₃P₂ thin films. These will include monocrystalline layer growth using molecular beam epitaxy [1], selective area epitaxy [2] and Van der Waals epitaxial growth on graphene substrates [3].<br/><br/>The second part of the talk will showcase the tunability of zinc-phosphide functional properties achieved by variation in the compositional stoichiometry [4,5]. In this case, we will show how electron and X-ray diffraction and Raman spectroscopy, along with density functional theory calculations, point to the favorable creation of P interstitial defects over Zn vacancies in P-rich and Zn-poor compositional regions. Photoluminescence and absorption measurements show that these defects create additional energy levels at about 180 meV above the valence band. Furthermore, they lead to the narrowing of the bandgap, due to the creation of band tails in the region of around 10–20 meV above the valence and below the conduction band. These results are also corroborated with the transport measurements on Zn₃P₂ devices.<br/><br/>Finally, we will discuss the device architecture for a polycrystalline Zn3P2 thin film solar cell on an InP substrate with a 4.4 % conversion efficiency. We will show the dominant recombination mechanisms in the device using different techniques and identify the key factors that limit the device efficiency. Additionally, we will provide a perspective on the next-generation Zn₃P₂-based solar cells.<br/><br/>[1] M. Zamani et al. J. Phys. Energy 3 034011 (2021)<br/>[2] S. Escobar Steinvall et al., Nanoscale Adv., 3, 326-332 (2021)<br/>[3] R. Paul et al., Cryst. Growth Des., 20, 6, 3816–3825 (2020)<br/>[4] E. Z. Stutz et al. Faraday Discuss., 239, 202-218 (2022)<br/>[5] M. Dimitrievska et al. Adv. Funct. Mater. 31, 2105426 (2021)