An in-depth investigation of lead-free KGeCl3 perovskite solar cells employing optoelectronic, thermomechanical, and photovoltaic properties: DFT and SCAPS-1D frameworks

Potassium germanium chloride (KGeCl₃) has emerged as a promising contender for use as an absorber material for lead-free perovskite solar cells (PSCs), offering significant potential in this domain. In this study, we conducted a density functional theory (DFT) investigation to analyze and assess the structural, electronic, thermomechanical, and optical characteristics of the cubic KGeCl₃ absorber. The positive phonon dispersion curve confirmed the dynamical stability of KGeCl₃. The elastic constant satisfied the Born criteria, validating the mechanical stability and ductility of solid KGeCl₃. The electronic band structure and density of states (DOS) confirmed that the KGeCl₃ material is a semiconductor with a direct band gap of 0.754 eV (GGA) and 0.803 eV (mGGA-RSCAN). The study identified key optical parameters, including absorption, conductivity, reflectivity, dielectric function, refractive index, and loss function, revealing the potential suitability of KGeCl₃ for solar applications. The Helmholtz free energy (F), internal energy (E), entropy (S), and specific heat capacity (C_v) are computed based on the phonon density of states. Additionally, we investigated twenty-four configurations comprising different combinations of electron transport layers (ETLs) and hole transport layers (HTLs) in SCAPS-1D software. For this purpose, ETLs such as Ws₂, ZnSe, PCBM, and C₆₀ and HTLs such as CBTS, CdTe, CFTS, Cu₂O, P3HT, and PEDOT:PSS are employed. The highlighted structure, ITO/CBTS/KGeCl₃/Ws₂/Ni, demonstrates remarkable performance with an efficiency of 22.01%, a V_oc of 0.6799 V, a J_sc of 41.439 mA cm⁻², and a FF of 78.12%. To analyze photovoltaic (PV) performance, we chose the top four solar cell (SC) configurations. Moreover, a comprehensive examination was conducted to assess the impact of various factors, including the thickness of different layers, capacitance, Mott–Schottky behavior, series and shunt resistance, temperature, and generation–recombination rates, as well as J–V (current–voltage density) and quantum efficiency (QE) characteristics.