Efficient Energy Funnelling by Engineering the Bandgap of a Perovskite: Förster Resonance Energy Transfer or Charge Transfer?

Energy funnelling enables directional carrier transfer along cascaded energy levels, which can be employed to significantly improve energy transfer efficiency and photoelectronic performances. However, the exact mechanism is still under intensive debate on whether Förster resonance energy transfer (FRET) or charge transfer (CT) is playing the dominant role, hindering broad practical device design and applications. Herein, a spectroscopic method is developed to unveil the energy funnelling mechanism by comparing and modeling the photoluminescence (PL) spectra excited by pulsed and continuous-wave (CW) lasers. The applicability of this method is verified in a typical energy funnelling system constructed by engineering the bandgap of a perovskite. Composite hexagonal microplates (MPs) with FAPbBr3, FAPb(BrxI1-x)3, and FAPbI3 (formamidinium = FA) at the surface, middle mezzanine, and bottom layers are synthesized by a two-step chemical vapor deposition (CVD) method, which introduces a directional energy funnelling from wide-bandgap FAPbBr3 to narrow-bandgap FAPbI3. By using the spectroscopic method developed in this work, we reveal that charge transfer is the dominant mechanism for energy funnelling in the FAPbBr3/FAPb(BrxI1-x)3/FAPbI3 sandwich MP. This study not only provides novel insights into the energy funnelling in multiple-bandgap perovskite systems but also develops a widely applicable spectroscopic method to explore the energy funnelling mechanism in other graded bandgap systems.