High-efficiency Cs₄CuSb₂Cl₁₂ double perovskite solar cells enabled by carbon nanomaterial-derived charge transport layers: interfacial engineering and optimization

Lead-free, vacancy-ordered Cs₄CuSb₂Cl₁₂ nanocrystals have recently garnered considerable attention as promising absorber materials for double perovskite solar cells (DPSCs), owing to their exceptional optoelectronic properties, including a direct bandgap, high charge mobility, large grain size, strong light absorption, and enhanced stability. In this study, a novel all-inorganic Cs₄CuSb₂Cl₁₂-based DPSC architecture featuring engineered carbon-based charge transport layers (CTLs), structured as FTO/graphene oxide (GO)/Cs₄CuSb₂Cl₁₂/graphene quantum dots (GQDs)/carbon, was proposed and simulated using SCAPS-1D software. Specifically, Cs₄CuSb₂Cl₁₂ (CCAC) was employed as the absorber layer, GO as the electron transport layer (ETL), GQDs as the hole transport layer (HTL) and carbon as the back contact. To the best of our knowledge, this is the first reported integration of all-carbon based optimized CTLs within a CCAC-DPSC device. These carbon-derived CTLs were selected for their favorable energy level alignment with CCAC, along with their excellent surface defect passivation, thermal stability, environmental compatibility, and cost-effectiveness. A rigorous numerical simulation, compared against the reported experimental data for a representative case, yielded theoretically optimized thicknesses of GO, CCAC, and GQDs layers as 6 nm, 1.2 μm and 6 nm, respectively, with the corresponding doping concentration as 10¹⁵, 10¹⁷ and 10¹³ cm^-3. Defect analysis revealed that the GQDs/CCAC interface was more sensitive to defects than the CCAC/GO interface, exhibiting tolerable interfacial defect densities below 10⁷ cm^-2 and 10¹¹ cm^-2, respectively, due to slower hole extraction at the former interface. Under optimized conditions, the proposed device demonstrated the potential to achieve a theoretical power conversion efficiency (PCE) of 29.87%, exceeding the PCE predicted for non-carbon based CCAC-based DPSCs reported in the literature. These simulation-guided predictive trends and design guidelines can support the design, fabrication and interfacial engineering of CCAC-DPSC devices, thereby enabling the development of efficient, affordable, and sustainable lead-free DPSC technologies.