Advanced Synthesis and Control of Quasi-One-Dimensional Chalcohalides for High-Performance Photovoltaic Devices

In the past decade, perovskite-based photovoltaic technologies have experienced a remarkable surge, achieving conversion efficiencies exceeding 25% in a relatively short time. However, this technology faces significant challenges. One major issue is the incorporation of lead in its composition, which raises serious concerns due to its toxicity. Additionally, the inherent instability of these compounds hinders the retention of high efficiencies over extended periods. As a result, numerous research groups have focused on identifying Perovskite-Inspired Materials (PIMs), aiming to discover compounds that share similar advantageous properties, such as defect tolerance, attributed to their distinctive electronic structure.
Among the materials under investigation, ns²np⁰ (V-VI-VII) chalco-halides have garnered attention due to their suitable bandgaps, ranging from the visible to the ultraviolet spectrum, high absorption coefficients, stability, low toxicity, and composition based on elements with low supply chain risks. Similarly to perovskites, the bonding orbitals located at the conduction band minimum, coupled with the antibonding orbitals at the valence band maximum, lead to defect-induced states appearing as resonances within the bands, thereby minimizing their impact on device performance. These compounds belong to the Van der Waals (VdW) semiconductor family, characterized by VdW interactions in one or more crystallographic directions. In the case of chalco-halides, they form covalently bonded chains in one direction, while weak VdW interactions hold these chains together. This quasi-one-dimensional (Q-1D) nature imparts anisotropy in certain material properties, which, when properly oriented, can be exploited for improved charge transport and other advantageous behaviors.
The synthesis of these materials presents a considerable challenge, regardless of whether physical or chemical routes are employed, due to their tendency to form quasi-one-dimensional structures, such as nanowires, nanocolumns, or nanorods, often without controlled orientation. While some research groups have focused on vertically orienting these needles, others have sought to develop compact, thin films.
In this work, we present a comprehensive study of a novel two-step physical synthesis methodology. First, a binary chalcogen precursor layer, specifically Sb₂Se₃, is deposited via co-evaporation. This is followed by a reactive thermal process conducted at pressures exceeding atmospheric levels in a halogen-rich atmosphere to form the final compound. The study encompasses a wide range of process conditions and demonstrates the effect of various synthesis regimes on the resulting morphology, allowing for the controlled formation of different morphologies tailored to specific applications. We also propose the role of each process parameter in influencing the nucleation and growth dynamics of the layers.
Furthermore, in line with recent investigations, we demonstrate the topotactic nature of the reaction, whereby crystalline orientations are preserved between the precursor and the final product. A reaction mechanism based on the solid-liquid-vapor (SLV) model is proposed, elucidating the growth process and offering strategies to regulate it.
By synthesizing the acquired knowledge, we propose a roadmap for the fabrication of compact thin films, highlighting that the orientation of the precursor layer is critical in determining the orientation of the final layer. Moreover, advanced synthesis strategies are introduced for enhanced morphological control through precise regulation of nucleation and growth by adjusting pressure and temperature during the synthesis process. Finally, we present photovoltaic devices based on these materials, demonstrating their considerable potential, achieving open-circuit voltages of 435 mV for a bandgap of 1.6 eV.