Fig. 5: Multicolor exciton upconversion luminescence from dye-sensitized perovskite nanocrystals.

a A schematic illustration of triplet energy transfer from dye to paired Yb³⁺ dopants in preferred configuration to elicit exciton upconversion. b Simplified energy level diagram illustrating the IR783 molecule, Yb³⁺ ions, and CsPbBr₃ semiconductor bands, along with the upconverting pathways. The diagram includes the involved energy transfer processes (solid wavy yellow arrow), the cooperative sensitization process (dashed wavy yellow arrow), and nonradiative multiphonon relaxations (blue wavy arrow), all contributing to the exciton upconversion luminescence (blue solid arrow). c, d Multicolor exciton upconversion luminescence spectra (solid line) (c) and corresponding upconverting photographic images (d) from IR783-senstized CsPbBr_3-xI_x: Yb³⁺ (x = 0, 1, 1.5, and 2) and non-sensitized CsPbBr₃:Yb³⁺ nanocrystals; the corresponding Stokes luminescence spectra from excitons are also included for reference (under 365 nm excitation). e Microscopic upconversion luminescence images of CsPbBr₃: Yb³⁺ patterns with (left) and without (right) IR783 sensitization; scale bar, 100 µm. Excited at 804 nm; power density of 8.4 Wcm⁻² for c and d, and of 416.7 Wcm⁻² for (e). (ET: energy transfer, S₀: ground state, S₁: excited singlet state, T₁: excited triplet state, ISC: intersystem crossing, TET: triplet energy transfer, CSU: cooperative sensitized upconversion).