Exploring The Impact of Physical Vapor Synthesis Regime on The Properties of Pnictogen Chalco-Halides for Solar Cell Applications

Recent advances in the study of van der Waals (vdW) materials have led to an increase in the effort devoted to their development for energy applications. In particular, the photoconversion efficiencies of Sb₂(S_1-xSe_x)₃ compounds highlight the potential of vdW semiconductors as photovoltaic absorbers. Significantly, their unique structure based on covalently-linked chains in the z-direction (low-dimensional structure) gives rise to anisotropic optoelectronic properties, which can result in enhanced charge carrier transport when the material is correctly oriented in the (00l) crystallographic direction. Sb-chalcogenides also have high absorption coefficients, optimal bandgap for solar energy conversion, and they are constituted by earth-abundant low-toxic elements. Similarly, pnictogen chalcohalides are recently gaining a lot of attention, showing great potential as PV candidates. In addition to the aforementioned properties originating from vdW structure, the antibonding character of their valence band together with the conduction band consisting of an extension of p-states, results in a highly dispersive character for both bands, which can lead to defect tolerant properties, similar to those of Pb-halide perovskites. However, the particular vdW structure also leads to a complicated morphology consisting of randomly oriented needle-shaped crystals, which is clearly inadequate for the design of conventional thin film solar cells. Therefore, it is necessary to explore new alternatives to obtain more compact and uniform morphologies.<br/>In this work, the formation dynamics of SbSI and SbSeI compounds from the selective halogenation of precursors Sb₂S₃ and Sb₂Se₃ has been investigated. In particular, the effects of different reaction pathways are analyzed in detail. These reactions are divided into convection-controlled and diffusion-controlled processes. For the first case, a series of experiments are carried out in high pressure and large volume tubular furnaces, where the effects of pressure and temperature as well as duration have been studied. In this case, high pressures are necessary to prevent the decomposition of the resulting compound at high temperatures. In the second case, the same reactions are carried out in the reduced volume of a close-space sublimation (CSS) reactor. A thin film deposition system is innovatively used as a reaction medium. In this case, the control of the decomposition is facilitated by the saturation of the atmosphere with the halide source (i.e., SbI₃). In addition to the effect of convection or diffusion-controlled processing, other synthesis routes have been investigated, including the reaction of previously deposited chalcogen and halide layers (solid-solid or solid-liquid reaction), which offers interesting possibilities for very high-pressure systems where evaporation is disabled and a fully diffusive solid-state reaction is favored.<br/>The differences between the two reaction pathways are clear, confirming that the diffusion-controlled process has much slower kinetics. Indeed, it has been observed that for processes of the same duration, the reaction is not complete in the diffusion-controlled case, suggesting that longer processes are necessary. The study is supported by microscopy and energy-dispersive X-ray spectroscopy analyses, which demonstrate the formation of large and compact micro-columns. The evolution of the reaction has been studied by X-ray diffraction in order to evaluate the reaction kinetics. In addition, photovoltaic devices have been developed using both methodologies. Temperature-dependent current-voltage and external quantum efficiency measurements have been performed.