In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In₂Se₃ has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys, and initial experiments on the growth and characterization of γ-In₂Se₃ thin films. The predicted band gap of 1.84 eV for γ-In₂Se₃ is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In₂(Se_1-xTe_x)₃ alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In₂Se₃/γ-In₂(Se_1-xTe_x)₃ bilayer could be employed in tandem solar-cell architectures absorbing at E_g ≈ 1.8 eV and at E_g ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys to the available toolbox of materials for solar cells and other optoelectronic applications.