Reduced graphene oxide (rGO) grafted zinc stannate (Zn2SnO4) nanofiber scaffolds for highly efficient mixed-halide perovskite solar cells

Abstract

Electron transporting materials based on ternary metal oxides (TMOs) are the best electron transport layers (ETLs) for perovskite solar cells (PSCs). In the present investigation, reduced graphene oxide (rGO) grafted highly porous zinc stannate (Zn₂SnO₄) (ZSO) nanofiber scaffolds have been synthesized by a single step electrospinning technique and successfully used as ETLs for mixed halide PSCs whose perovskite material is composed of MAPb(I_1−xBr_x)₃ and (FAPbI₃)_0.85(MAPbBr₃)_0.15 (MA: methyl ammonium and FA: formamidinium). The fabricated optimized perovskite solar cells having FTO/Bl-ZSO/rGO–ZSO_0.7–MAPb(I_1−xBr_x)₃/PTAA/Au devices exhibited a 13.41% power conversion efficiency (PCE) with an open circuit voltage (V_OC) of 1.036 V, a current density (J_SC) of 19.62 mA cm⁻² and a fill factor (FF) of 0.66 under AM 1.5G sunlight (100 mW cm⁻²) which is higher than that of bare Zn₂SnO₄ nanofiber (η = 7.38%) based PSCs. The optimized conditions were further used for formamidinium lead halide (FAPbI₃)_1−x(MAPbBr₃)_x (x = 0.15) perovskite and our optimized results show η = 17.89% PCE (J_SC = 22.50 mA cm⁻², V_OC = 1.046 V, FF = 0.76) for the FTO/Bl-ZSO/rGO–ZSO_0.7–(FAPbI₃)_0.85(MAPbBr₃)_0.15/PTAA/Au device configuration. The role of rGO grafting and electron transfer mechanisms are investigated with complementary characterization, including photoluminescence (PL) and time-resolved photoluminescence (TRPL) decay measurements. The TRPL results revealed that the grafting of rGO in ZSO scaffolds reduces the slow decay lifetime which facilitates efficient electron injection from the perovskite conduction band (CB) to the rGO Fermi level to the CB of ZSO compared to the bare ZSO ETL. Furthermore, the stability of these devices based on various configurations has been discussed. This improvement is achieved due to the high conductivity of rGO and grafting with high porosity Zn₂SnO₄ nanofibers which make them promising new ETLs for the fabrication of highly efficient PSCs.