Low dark current and Type-II band Alignment in Double Perovskite Single Crystal Cs 2 AgBiBr 6 / Cs 3 Bi 2 I 9 Nanocrystals Heterojunction Enable High-Performance Photodetection

Abstract

We report the design and characterisation of a heterojunction photodetector based on Cs2AgBiBr6 single crystal (DPSC) and Cs3Bi2I9 (CBI) nanocrystals (NCs), demonstrating enhanced charge collection efficiency and significantly reduced dark current due to the built-in electric field at the junction interface. Highly crystalline, cubic-shaped CBI NCs were synthesised via a mechanochemical approach and subsequently deposited onto the double perovskite single crystal through a drop-casting method, forming a Cs2AgBiBr6/Cs3Bi2I9 perovskite–perovskite heterojunction. The heterojunction device exhibited a marked increase in photocurrent and suppression of dark current, attributed to the enhanced generation of photocarriers and efficient interfacial charge transfer facilitated by the built-in electric potential. The resulting HS device showed an improved photocurrent of 4.3×10-7 A and a reduced dark current of 5.5×10-9 A at a given voltage of -5V. The HS device showed enhanced responsivity of 0.98 A/W and a high detectivity of 2.97 × 1012 Jones. Compared to the pristine Cs2AgBiBr6 SC device, the heterojunction configuration yielded a ~30-fold improvement in the on/off current ratio and an approximate six-fold enhancement in responsivity, reflecting superior photodetection performance. The detectivity of the HS device improved by two orders of magnitude (~148-fold) compared to the pristine DPSC device. Photophysical analyses, including steady-state photoluminescence (PL), time-resolved PL, and Raman spectroscopy, confirmed effective photoinduced charge transfer from CBI NCs to the Cs2AgBiBr6 matrix. Ultraviolet photoelectron spectroscopy (UPS) revealed a type-II band alignment at the heterointerface, favouring charge separation and transport across the junction. Complementary density functional theory (DFT) calculations further corroborated significant electron transfer from the CBI NCs to the Cs2AgBiBr6 SC, in agreement with the observed device characteristics. The resulting HS device exhibited excellent environmental stability (~4 months) and operational stability (>2000 s) under extreme exposure conditions. These findings underscore the potential of all-inorganic, lead-free halide perovskite heterojunctions in advancing high-performance photodetector technologies.