Probing the charge transfer mechanisms in type-II Cs₂AgBiBr₆-CdSe composite system: ultrafast insights

Lead-free halide-based double perovskites (DPs) have established themselves as the emerging nontoxic alternatives for photovoltaic (PV) applications thus substituting the long-standing lead halide perovskites. Among the prospective lead-free DPs, Cs₂AgBiBr₆has gained immense popularity owing to the fascinating properties demonstrated by them including low carrier effective mass and microsecond lifetime for electron-hole recombination. Nevertheless, the large, indirect bandgap remains the prime hurdle that restrains commercialization of the Cs₂AgBiBr₆DPs based PV devices. A rational solution could be designing its heterostructure with another suitable material that could mitigate the inadequacies of Cs₂AgBiBr₆DPs. With this line of thought, herein we synthesized a composite of Cs₂AgBiBr₆DPs with CdSe NCs and then performed transient absorption (TA) spectroscopic measurements to introspect its photophysical aspects. Executing excitation energy-dependent studies clearly reveal the carrier transfer efficiency to be strongly pump-dependent. Upon exciting with 350 nm pump, in compliance with the energy band alignment and tendency of both the constituents to be photoexcited across their bandgap, there is a bidirectional transfer of hot electrons anticipated in the composite system. Nevertheless, the TA outcomes indicate the transfer of hot electrons from CdSe to Cs₂AgBiBr₆to be more favorable out of the bidirectional pathways. Employing further lower pump energies (480 nm) when only CdSe NCs are capable of being excited, the transfer efficiency of the electrons from CdSe to Cs₂AgBiBr₆is noticed to be fairly low. Besides this, when the pump wavelength is tuned to 530 nm i.e. quite close to the CdSe band edge, no electron transfer is noticeable despite the anticipation from thermodynamic feasibility. Thus, as reflected by the TA kinetics, electron transfer is discerned to be more efficient from the hot states rather than the band edges. Most advantageously, charge separation is successfully achieved in this never explored composite architecture which eases the carrier extraction and minimizes the otherwise prevalent fast recombination processes.