Crystal and Electronic Structure of Bismuth Thiophosphate, BiPS₄: An Earth-Abundant Solar Absorber

Abstract

The opto-electronic properties of crystalline BiPS4 are described for the first time for solar energy conversion. Detailed structural-analysis is extracted from XRD refinement of powders synthesized by solid-state method. BiPS4 exhibits a rather unusual 3-dimensional orthorhombic structure with two distinctive Bi sites with octahedral coordination distorted by 6s 2 lone-pairs. High-resolution TEM imaging clearly shows the two Bi-Bi interatomic distances in close agreement with the XRD analysis. BiPS4 displays a complex Raman spectrum under near-resonant conditions which is rationalized by Density Functional Perturbation Theory. Hybrid-functional-DFT calculations show significant spin-orbit-coupling effects in Bi-6p bands, affecting not only on the band dispersion but also lowering the conduction band minimum by approximately 0.5 eV. Optical properties of BiPS4 powders are dominated by a direct transition at 1.72 eV, closely matching the calculated band gap. Electrochemical experiments revealed n-type conductivity with a flat band potential located at 0.16 V vs RHE. We also show a remarkable agreement between the position of the band edges estimated from first-principles calculations and electrochemical measurements. Time-resolved Photoluminescence transient revealed a carrier-lifetime of approximately 1 ns, manifesting as strong potential and wavelength dependent photocurrent responses. Finally, the nature of structural defects responsible for the relatively short lifetime is briefly discussed.

Single-Crystal Bismuth Thiophosphate, BiPS4, as a Nontoxic and Mechanically Robust X-ray Detector

“…X-ray powder diffraction (Figure d) shows phase-pure peaks that can be indexed to the orthorhombic structure of BiPS 4 , which are consistent with reported data. , No characteristic reflection peaks originating from possible Bi 2 S 3 -related secondary phases was observed.…”

“…The crystals appeared translucent with a dark red color under a strong light source. The band gap was determined to be 1.69 eV via optical transmission spectra (see Supporting Information Figure S3), confirming the reported band-gap energy of 1.72 eV (measured on polycrystalline samples) …”

“…Previously, a Pb-based wide-band-gap semiconductor Pb 2 P 2 Se 6 was investigated as a radiation detection material. In this selenophosphate, inclusion of the light element phosphorus increases the material’s band gap without significantly compromising its radiation stopping power. − Following this approach, a ternary bismuth thiophosphate BiPS 4 compound has been explored for room-temperature ionizing radiation detection.…”

“…The BiPS 4 compound was first discovered by Glatzel (1893), and its crystal structure was reported by Zimmermann et al (1975) . Some physical properties (stability, electrochemistry) of BiPS 4 , , associated with its application as a solar absorber, have been reported for the polycrystalline powder sample . No effort has been taken to study the single-crystal behavior of BiPS 4 because of the challenges in purifying and controlling the volatile elements (S and P) in high-temperature melt crystal growth processes.…”

“…In this selenophosphate, inclusion of the light element phosphorus increases the material's band gap without significantly compromising its radiation stopping power. 16−19 Following this approach, a ternary bismuth thiophosphate BiPS 4 compound 20 Due to the higher ratio of light elements, BiPS 4 is expected to exhibit a lower stopping power against ionizing radiation. However, at an energy range below 500 keV, BiPS 4 has a radiation attenuation length comparable to the benchmark radiation detector, Cd 0.9 Zn 0.1 Te (Figure 1).…”