Unveilingthe potential of Cs₂ AgBiBr₆ double perovskite solar cells: a detailed study of the optoelectronic and photovoltaic properties via DFTand SCAPS-1D frameworks

The Cs₂AgBiBr₆ double halide perovskite has catered for the problem related tolead toxicity and stability in perovskite solar cells (PSCs). Unfortunately, fundamental structural, optical, and electronic properties of Cs₂AgBiBr₆ are still not yet fully understood, couple with the unstable hole transport layer (HTL) such as spiro-OMeTAD, limited diffusion length of charge carriers, interface defects, and inferior hole extraction capability results to poor performance. Here, we delve into thorough assessmentofCs₂AgBiBr₆ asafeasible absorber materialbydeploying density functional theory (DFT)as acalculation tool. A band gapof 1.639 eV was obtained with contribution on Ag-4d and Br-4p orbitals. From the optical results, there was an observation of low optical losses, maximal absorptivity, larger refectivity at the ultraviolet-visible regions, superior conductivity and suitable refractive index for application in opto-electronic devices. Guided by DFT investigation, we proposeda novel design ofCs₂AgBiBr₆ graphene-based PSC with graphene intercalated between the HTL and the absorber. Numerical optimizationof HTLs such asspiro-OMeTAD, CuO, Cu₂O, CuSbS₂, CBTS and Mg-CuCrO₂ using solar capacitance simulation tool (SCAPS-1D), shows Mg-CuCrO₂ as the optimal HTL. The graphene modifed device demonstratesanimproved performance. Moreover, there wasa better band matchinginthe graphene modifed PSC. In comparison to the graphene-free PSC that shows PCE of6.70%,FF of37.50%, J_sc of 21.56 mA c m ^{- 2} and V_oc of 0.83 V,the optimized graphene-based PSC shows the following metric performance PCE = 30.83%,FF=90.87%J_sc = 21.72mAcm^-2 andV_oc of1.58V.The improvedperformanceis attributedtothe minimal recombination losses and improved suppressionofnon-radiative interfacial recombination. The outcomesof the present study have paved the way for the developmentof aless expensive and high performing PSC with graphene layer bymanaging and controlling defect states and non-radiative recombination.