From HTL to HTL-free: Experimental and numerically modelled performance dynamics of Cs2AgBiBr6 double perovskite solar cells

Abstract

For addressing the global energy crisis, this experimental and numerical study investigates HTL-free Cs₂AgBiBr₆ (CABB) double perovskite solar cells (DPSCs), highlighting their lead-free, non-toxic, and thermally stable properties. The FTO/TiO₂/CABB/Carbon-structured DPSCs showed >95 % PCE retention (1.63 % to 1.57 %) after 200 h. The underlying charge transport dynamics reveal a high R_Rec (1625  Ω), long τ_e (0.226  s), and efficient η_cce (89.65 %). Additionally, the estimated L_n (3.86  µm) and D_eff (27.33  µm²/s) support the observed retention in device performance, confirming that interfacial stability and minimal charge recombination govern the long-term durability of the HTL-free DPSCs. However, the low photovoltaic (PV) performance (J_SC = 9.41 mA/cm², V_OC = 456.13 mV, FF = 0.38, and PCE = 1.63 %) is concerning for the futuristic technological development. Therefore, numerical simulations for optimizing the HTL and HTL-free DPSCs consisting of four different ETLs (TiO₂, SnO₂, WO₃, and ZnO) were conducted using SCAPS-1D, revealing ZnO as the optimal ETL for both HTL and HTL-free DPSCs. For HTL-DPSCs, ZnO achieved the highest performance (PCE: 27.30 %, J_SC: 23.83 mA/cm², V_OC: 1.29 V, FF: 0.89), followed by TiO₂ (25.48 %), WO₃ (25.42 %), and SnO₂ (22.63 %). Additionally, the HTL-free DPSCs showed overall ∼4 % reduced efficiency due to higher interfacial recombination and limited charge extraction, with ZnO again leading (PCE: 23.09 %, J_SC: 23.80 mA/cm², V_OC: 1.15 V, FF: 0.85). This study highlights the optimization strategy that could bridge the simulation-experiment performance gap, positioning CABB as a leading lead-free double perovskite material for efficient and sustainable PV solutions.