This study investigates the structural stability, electronic properties, and optical absorption of α₁-CaAl₂S₄, α₁-InGaSe₂, and the heterojunctions formed by these two materials by first-principles calculations. The CaAl₂S₄/InGaSe₂ heterostructures exhibit type-II and type-I band alignments. The CaAl₂S₄/InGaSe₂ heterostructures exhibit enhanced optical response, tunable bandgap under strain, and efficient charge separation.

Hexagonal α₁-CaAl₂S₄ and Janus α₁-InGaSe₂, featuring unique physical and chemical attributes, stand out as exceptional contenders for optoelectronic implementations. However, on account of weak absorption of visible and UV light, α₁-CaAl₂S₄ faces limitations in optoelectronic device applications. Constructing a heterojunction with α₁-CaAl₂S₄ and α₁-InGaSe₂ can significantly enhance photon absorption in both the visible and UV domains. This research employs first-principles simulations to scrutinize the optical and electrical properties of α₁-CaAl₂S₄, α₁-InGaSe₂, and the heterojunctions formed by these two materials. The output of the calculations shows that CaAl₂S₄/InGaSe₂ heterojunction demonstrates a remarkable enhancement with respect to light collection efficiency across the visible and UV span. The CaAl₂S₄/InGaSe₂ heterojunctions with different stacking structures exhibit type-I and type-II alignment modes, respectively. Furthermore, the bandgap value and type of heterojunctions can be effectively controlled by modulating the interlayer spacing and applying biaxial strain, resulting in a variety of band alignments and light absorption properties. These findings provide new material options and technological pathways for developing high-efficiency photovoltaic cells, photoresponsive devices, solid-state lighting elements, and novel photocatalytic and integrated optoelectronic devices.