Reaction‐Time pH Stabilization by Poly(acrylic acid) with Ethanol‐Mediated Growth Control for Efficient Hydrothermal Sb 2 (S,Se) 3 Solar Cells

Hydrothermal Sb 2 (S,Se) 3 deposition proceeds through the time‐dependent release of Sb species, chalcogenide precipitation, and changes in proton balance, making solution pH a dynamic variable rather than a fixed initial condition. Here, a carboxylate buffering strategy that operates throughout the hydrothermal reaction is introduced to reduce voltage loss. The central design rule is that the buffer must operate within the actual pH trajectory of the solution during growth. Poly(acrylic acid) (PAA) satisfies this requirement through its COOH/COO − equilibrium and suppresses reaction‐induced pH fluctuations. Control experiments based on initial pH adjustment and alternative carboxylic acids show that the open‐circuit voltage ( V OC ) gain originates from effective buffering in the solution. PAA‐mediated pH stabilization reduces the Sb‐O related surface disorder and improves grain‐boundary electronic homogeneity, reducing localized recombination that can limit quasi‐Fermi‐level splitting and contribute to V OC loss. However, PAA‐induced acidification also raises the availability of reactive Sb species. A higher ethanol content moderates this excess Sb supply and the deposition rate, coupling voltage‐loss reduction with efficient charge collection. This combined strategy of pH buffering and growth control improves morphology, local photoresponse, and device‐level trap characteristics, driving a simultaneous improvement across all photovoltaic parameters and raising the power conversion efficiency from 8.95% to 10.25%.