Investigation of impact of band alignment in lead-free double-layer perovskites solar cells: Numerical simulation with SCAPS-1D

Abstract

This study numerically investigates the effects of band alignment on the photovoltaic performance of lead-free double-perovskite solar devices using the SCAPS-1D software. The device's general configuration of FTO/TiO 2 /absorber/CuSbS 2 /back-contact incorporating Cs 2 TiBr 6 , Cs 2 TiI 2 Br 4 , Cs 2 ScAgI 6 , Cs 2 TiI 6 , Cs 2 AgBiBr 6 , and Cs 2 PdBr 6 , with TiO 2 serving as the electron transport layer (ETL) and CuSbS 2 as the designated hole transport layer (HTL), was investigated. The work functions of the selected back contact materials were 5.27 eV for platinum (Pt) and 5.56 eV for iridium (Ir). By systematically optimizing thicknesses of each layer, quantum efficiency and J-V characteristics were obtained. The best-performing simulated device configuration was FTO/TiO 2 /Cs 2 ScAgI 6 /CuSbS 2 /Pt, achieving a power conversion efficiency (PCE) of 20.66%, with a fill factor (FF) of 8.86%, short-circuit current density (J sc ) of 26.30 mA cm -2 , and open-circuit voltage (V oc ) of 8.87 V. Meanwhile, the Ir-based device with configuration FTO/TiO 2 /Cs 2 TiBr 6 /CuSbS 2 /Ir exhibited lower PV performance, with a PCE of 19.88%, a J sc of 25.13 mA cm -2 , a V oc of 9.63 V, and an FF of 8.22%. Impedance characteristics derived from both Bode and Nyquist plots indicate that, among the absorber E g investigated in the range of 1.20–2.00 eV, the device with a 1.55 eV E g exhibits increased charge-transfer resistance at lower band gap (E g) . Consequently, the analysis of the Bode plots (Zphase and Zmag) and Nyquist plots (Real [Re(Z)]-Imaginary [Im(Z)] plane of the complex impedance) provides important insights into the critical role of E g -tuning and appropriate work-function selection in optimizing solar cell performance.