In Situ HeterostructureFormation of NaSbS₂ and Na₂Sb₄S₇ for Efficient PhotogeneratedCharge Separation

Sodium antimony sulfide is a recently discovered alkali metal chalcogenide that has gained considerable attention due to its enhanced efficiency, nontoxicity, and low cost as a photoabsorber. This material exists in various phases, such as NaSbS₂, NaSbS, Na₃SbS₄, and Na₂Sb₄S₇, and can be obtained only by annealing at high temperatures. However, here, we report the controlled formation of two different phases of sodium antimony sulfide, NaSbS₂, and a heterostructure of NaSbS₂/Na₂Sb₄S₇ achieved in a single successive ionic layer adsorption and reaction (SILAR) cycle without annealing procedures. Both phases were formed in two distinct colors, namely, orange (NaSbS₂) and brown (Na₂Sb₄S₇/NaSbS₂), and were found to be two different materials with different electronic properties. The band gaps for both phases were calculated to be 2.0 and 1.6 eV, which lies in the ideal band gap region for a solar absorber. Two photodetectors were fabricated, where both phases acted as the active layers with fluorine-doped tin oxide (FTO) and carbon as the other two electrodes. Both devices produced an outstanding photocurrent and photovoltage under zero-bias conditions, proving to work as excellent self-powered photodetectors. The devices were tested under 455, 525, 632 nm, and white light-emitting diode (LED) light illumination. The rise and fall times under light irradiation were as rapid as 380 and 480 ms for the NaSbS₂ device and 370 and 420 ms for the Na₂Sb₄S₇/NaSbS₂ device, respectively. The responsivity and detectivity for both the photodetectors at low intensities were found to be 0.89 and 3.5 mA/W and 8.8 × 10⁹ and 4.7 × 10¹⁰ Jones, respectively.