Photoanodes and Thin-Film Solar Cells Based on SbSBr, SbSeI and BiSeI Absorbers

Emerging quasi-1D (Q1-D) van der Waals materials, (Sb,Bi)(S,Se)(Br,I), hold the potential to be a breakthrough in photovoltaic (PV), photocatalytic (PC), and photoelectrocatalytic (PEC) applications, aiming to address some of the main issues affecting even the more mature PV, PC, and PEC technologies. Although this materials family has rarely been explored for these applications, mixed chalco-halides fulfill the urgent need for innovative and renewable energy sources based on earth-abundant, low-toxicity, thermally stable, and defect-tolerant compounds.<br/><br/>This materials class has the peculiarity of possessing a crystallographic structure in which the atoms are held together by strong covalent bonds along one of their crystallographic directions and weak van der Waals bonds along the other two directions. When properly oriented, it results in a highly anisotropic material composed of nano/micro-ribbons with unique conductive properties. The high tunability of these materials and the results obtained in the present work place these semiconductors in the spotlight for a wide range of applications, including next-generation PV tandem and transparent applications, photocatalytic hydrogen production, and oxygen and hydrogen evolution reactions (OER, HER) among others.<br/><br/>The nano/micro-ribbon structure, coupled with the extremely uniform single-crystal phases, leads to an increased number of photons that reach the space charge region, and the light-trapping effect reduces reflection. This morphology enhances the optoelectronic properties of SbSBr and BiSeI-based solar cells, the OER of Q1-D van der Waals-based photoanodes, and the photocatalytic activity of such-based photocatalysts. The dense coverage of properly oriented nano/micro-ribbons increases the surface area, a key parameter for improving photons and reactant absorption, enabling the synthesis of efficient, photoactive, photostable, and nontoxic photoanodes and photocatalysts. Furthermore, the band gaps of the obtained materials range between 1.25 and 2 eV, making them active to visible light and allowing an optimal usage of the solar spectrum.<br/><br/>The above-mentioned reasons are indicative of what can be achieved with this relatively unexplored technology. To date, no mixed chalco-halide compounds have been synthesized using physical vapor deposition (PVD) techniques, primarily due to the complexity of the material and the varying vapor pressures involved. This work marks the very first successful attempt to synthesize and investigate Q1-D van der Waals-based semiconductors using an innovative methodology that involves the co-evaporation of chalcogenides followed by high-pressure annealing under a halogen atmosphere. Various characterization techniques (XRD, XRF, SEM, EDX, PDS, UV-Vis, Raman spectroscopy, and DFT calculations) have demonstrated the high tunability of the morphology of this compound family by modifying the annealing conditions, confirming the possibility of synthesizing high-quality single-phase material with PVD techniques for the first time.<br/><br/>The versatility of the system allows for the optimal distribution of nano/micro-ribbons in terms of height, thickness, density, and orientation, according to the final device's use. An array of operational Q1-D van der Waals-based solar cells and photoanodes, each derived from distinct stack combinations, will be showcased and thoroughly examined, together with some preliminary results about photocatalytic hydrogen production under visible light. Notably, solar cells have exhibited open-circuit voltages reaching up to 600 mV, and photoanodes, even when utilizing non-protected absorbers, have achieved a photoanode current of nearly 2 mA/cm-2. This study demonstrates how these novel compounds represent a promising development in advanced energy conversion applications.