Interface Engineering Toward High‐Efficiency CIAS Thin‐Film Solar Cells Using a Cadmium‐Free CuAlSe ₂ Buffer Layer: A SCAPS‐1D Investigation

Cu(In,Al)Se ₂ (CIAS)‐based thin‐film solar cells remain largely unexplored regarding buffer layer and interface engineering. This work reports a SCAPS‐1D investigation of CuAlSe ₂ (CASe) as a cadmium‐free alternative to CdS, starting from a validated Glass/Mo/CIAS/CdS/ZnO/ITO/(Ni/Al)/MgF ₂ model reproducing experimental results. Replacing CdS with CASe maintains constant short‐circuit current density while improving band alignment at the buffer/absorber interface, reducing interfacial recombination and raising open‐circuit voltage ( V _OC ), without additional parasitic absorption losses. A parametric analysis identifies an optimal CASe thickness of 0.01 µm nm and an optimal conduction band offset of −0.25 eV at the CASe/CIAS interface, yielding a simulated efficiency of 25.03%. Integrating a NiO hole transport layer suppresses back‐contact recombination, raising efficiency to 26.39% ( J _SC  = 33.06 mA/cm ² , V _OC  = 1.1 V, FF = 73.77%), versus 16.9% ( J _SC  = 36.00 mA/cm ² , V _OC  = 0.621 V, FF = 75.5%) for the CdS‐based reference device. In addition, an unconventional thermal behavior is revealed within the 280–360 K range, where V _OC and efficiency increase with temperature, attributed to a progressive improvement of the CASe/CIAS interfacial band alignment that outweighs intrinsic thermally activated recombination losses. These results identify CASe as a promising, previously unexplored cadmium‐free buffer material for CIAS solar cells, warranting experimental investigation.