Earth-abundant copper–barium–thiostannate Cu₂BaSnS₄ (CBTS) based thin films have recently been reported to exhibit optoelectronic and defect properties suitable for their use as absorbers for photoelectrochemical (PEC) water splitting and the top cell of tandem photovoltaic (PV) solar cells. In this work, we synthesise CBTS thin films from precursors with different Cu compositions and find that the Cu composition of the precursor can influence the phase formation and structural preferred orientation in the resultant CBTS. Single-phase and device-grade CBTS films can be prepared using the precursor with a slightly Cu poor content of Cu/(Ba + Sn) = 0.95 and Cu/Ba = 1.81. We also find that CBTS demonstrates an intrinsic copper poor composition leading to p-type conductivity and that exposing CBTS in air can lead to a native oxide layer of SnO_x. The PEC water reduction device with the configuration fluorine-doped SnO₂/CBTS/Pt/electrolyte (pH ∼ 7) is able to generate a cathodic photocurrent of 3.9 mA cm⁻² at zero volt versus reversible hydrogen electrode under the illumination of a 300 W xenon lamp. The PV solar device with the configuration fluorine-doped SnO₂/CBTS/CdS/ZnO/aluminum-doped ZnO suffers from a severe recombination issue at the heterointerface which greatly limits the overall device performance, evidenced by detailed characterization using capacitance–voltage and impedance spectroscopy tested with various alternating-current frequencies, temperatures, and applied direct-current biases. As a consequence, the best PV device only delivers 1.6% conversion efficiency under AM 1.5 illumination, albeit with moderate heat treatments of CBTS/CdS junctions at low temperatures mitigating the interfacial recombination to some degree.