Combinatorial Synthesis of BaCu₂Se₂ Thin Films: Effect of Composition on Crystal Structure and Optoelectronic...

The BaCu₂Se₂ semiconductor compound with earth-abundant elements is currently studied for application in thermoelectrics. The properties of this emerging material, however, are still not well established, while being mostly investigated on powders or pelletized powders. In this work, BaCu₂Se₂ thin films are prepared, and the chemical space of Ba-rich and Cu-rich compositions is explored. Ba-Cu-Se thin film combinatorial libraries with lateral compositional gradients with [Cu]/([Ba]+[Cu]) (CBC) atomic ratios between 0.15 and 0.98 are synthesized on areas of 12 × 50 mm². Using XRF and XRD analysis, composition-resolved phase fractions are determined, which identify the orthorhombic α-BaCu₂Se₂ as the main phase for CBC between 0.62 and 0.82. Optical measurements reveal for α-BaCu₂Se₂ a direct bandgap of 1.89 eV and a high absorption coefficient ∼10⁵ cm^-1. Surface photovoltage and Seebeck coefficient measurements show p-type conductivity of the films irrespective of composition. Combined Kelvin probe, photoelectron yield, and optical-pump terahertz-probe spectroscopies demonstrate tunability of charge carrier concentrations over several orders of magnitude. The absolute photoluminescence hyperspectral imaging reveals a bright luminescence centered at 1.91 eV and an implied open-circuit voltage of 1.3 eV. Ba-rich conditions are found to be appropriate for the synthesis of films for photovoltaics, while Cu-rich conditions are suitable for the preparation of this material for thermoelectrics.