Innovative Low Dimensional SbSeI Micro-Scale Devices for Photovoltaics

The recent success of the van der Waals Sb₂(S,Se)₃ system has put low dimensional semiconductors in the spotlight to develop high efficiency photovoltaics (PV). Indeed, their performance has increased consistently, triplicating in less than a decade, and they also possess many benefits compared to other PV thin film systems, including earth-abundancy, low toxicity, thermal stability, and anisotropic crystal structure – resulting in unique electrical properties when the material is correctly oriented. However, van der Waals semiconductors beyond Sb₂(S,Se)₃have been scarcely investigated, constituting an open field for research. Amongst this family of largely overlooked materials, antimony chalcohalides (<i>Sb[S,Se]X, X = Br, I</i>) are highly attractive due to their wide bandgaps in the 1.6-2.0 eV range (ideal for tandem and semi-transparent applications), customizable optical properties by using different halides or chalcogenides while retaining their quasi-1D structure, defect-tolerance, and ferroelectric properties, which might offer new opportunities (an alternative to perovskites) to improve electron-hole separation and achieve high photovoltages.<br/><br/>The first attempts to synthesize these materials for PV implementation have been essentially focused on low temperature (&lt; 300^oC) and atmospheric pressure solution-based chemical routes, although PV performance of the resulting devices has been overall low. Aiming towards a more controlled, reproducible and versatile system, in this work, SbSeI has been synthesized by a novel procedure based on the selective iodination of co-evaporated Sb₂Se₃ films at pressures above 1 atm, developing an extremely uniform morphology consisting in single-crystal micro-columnar structures, which can reach lengths up to tens of microns, and which height and density can be finely tuned by adjusting the annealing temperature and pressure. Structure and crystalline quality have been characterized by advanced microscopy imaging techniques, Raman spectroscopy and X-ray diffraction. Also, Kelvin probe microscopy has been used to determine the local work function of these micro-columnar structures, revealing a work function variation among the exposed crystalline facets.<br/><br/>In addition to the excellent crystalline quality and low dimensional structure, SbSeI crystals show an overwhelmingly high photoluminescence (PL) signal, significantly larger than that of Sb₂Se₃ thin films, suggesting that they might have the capability of developing a remarkable PV effect. Indeed, electrical and optoelectronic measurements have been performed on micro-scale devices by transferring micro-columnar crystals to a SiO₂/Si wafer, and connecting them to Pt and Au contacts, yielding a clear diode response with the Au-Au and Au-Pt electrodes, which is greatly enhanced under illumination. Memory effect, capacitance-voltage and ferroelectric measurements have also been performed. Finally, solar cell prototypes have been prepared using a substrate configuration, resulting in high open-circuit voltages around to 600 mV.<br/><br/>Overall, this work presents a general study of synthesis and characterization of novel chalcohalide-based micro-scale solar cells, showing their compatibility with scalable physical vapor deposition techniques. We report the formation of highly crystalline SbSeI micro-columnar structures, showing excellent PL response, bandgap of 1.7 eV, and diode performance of micro-scale devices. These results demonstrate the potential of low dimensional chalcohalides to be implemented into innovative architectures and advanced functionalities (such as for enhanced carrier collection and light trapping), pointing the way to develop defect-tolerant and ferroelectric absorbers with anisotropic electrical properties for the next-gen PV, photo-electrocatalysis, piezo- and thermo-electric applications.