Heterovalent tin ion-regulated bromobismuth double perovskite-based fully-inorganic solar cells

Abstract

Recently, lead-free halide double perovskites have evolved to be a promising photo-absorber for photovoltaic applications due to their high thermal and moisture stability and negligible toxicity. However, their photovoltaic performance is still limited owing to improper film quality, inappropriate band gap, and high defect density. Herein, we report Cs₂AgBiBr₆ double perovskite (DP)-based fully-inorganic solar cells with enhanced photovoltaic performance accomplished via heterovalent Sn²⁺ doping. The band gap of the DP photo-absorber is reduced by carefully controlling the Sn²⁺ doping concentration and subsequently its charge extraction capability is improved. In addition, Sn²⁺ incorporation has improved the surface morphology, carrier lifetime, defect states and electron mobility of the DP layer. Meanwhile, an upshift of the valence band maximum energy of Cs₂AgBiBr₆ (from −6.41 eV to −6.32 eV) with Sn²⁺ incorporation is observed via ultra-violet photoelectron spectroscopy (UPS) analysis. This re-alignment of energy levels between the perovskite absorber layer and the hole transport layer has created an efficient pathway for hole extraction and transport at the interface. As a result, the optimized Sn-doped device exhibits a remarkably enhanced photo conversion efficiency (PCE) of 2.74% with an excellent open-circuit voltage (V_oc) of 1.02 V. The Sn-doped device also shows higher stability over the pristine device for several weeks in ambient atmosphere without encapsulation. This work demonstrates that doping Cs₂AgBiBr₆ photo-absorber with Sn²⁺ is an effective approach for efficiency enhancement in PSCs and paves new ways for high performance lead-free stable photovoltaic applications.