Polymer-controlled growth of BiSI/Bi<sub>13</sub>S<sub>18</sub>I<sub>2</sub> thin films for photoelectrochemical applications

This study investigates, for the first time, the influence of polyvinylpyrrolidone (PVP) concen­tration in the precursor solution on the structural, morphological, and photoelectrochemical (PEC) properties of BiSI/Bi13S18I2 thin films synthesized via one-step chemical bath depo­sition. By varying the PVP content from 0 to 3.0 wt.%, a clear correlation is established bet­ween polymer-assisted precursor stabilization, phase formation pathways, and the result­ing PEC performance. SEM and XRD analyses reveal that 2.5 wt.% PVP yields dense, uniform, and highly crystalline heterophase BiSI/Bi13S18I2 films with minimized grain-boun­dary den­sity, whereas insufficient polymer leads to discontinuous coatings and excess PVP induces surface passivation and suppressed crystallization. The optimized film (2.5 wt.% PVP) exhibits the highest photocurrent density (15.7 ± 0.31 μA cm-2) and quantum efficiency (IPCE at 465 nm = 2.62 ± 0.04 %) in 0.5 M Na2SO4. To elucidate interfacial charge-transfer kinetics and operational stability, the PEC behaviour was further investigated in electrolytes with varying redox activity: Na2SO4, Na2SO3, and a mixed electrolyte of Na2SO3 + Na2SO4. The inert sulphate electrolyte reflects intrinsic semiconductor performance and provides the highest stability, whereas sulphite-containing solutions reveal defect-mediated pathways, accelerated hole extraction, and increased susceptibility to photocorrosion. The mixed electrolyte yields the highest photocurrent (25.21 ± 0.38 μA cm-2) but also demonstrates amplified dark currents and gradual degradation. These results establish a structure-property-performance relationship for Bi-based chalcohalide photoanodes and provide practical guidelines for tuning polymer concentration and electrolyte composition to enhance PEC efficiency and stability in aqueous solar-driven systems.