a Chemical structure of Pt(fprpz)2 and b BTP-eC9 molecules, along with c their energy levels. d Absorption and emission spectra of Pt(fprpz)2 and BTP-eC9, with the overlapping region indicating the radiative energy transfer zone.
We propose that by adhering to three principles, interfacial energy transfer can be realized: (1) The photoluminescence of the energy donor must overlap with the absorption spectrum of the energy acceptor; the former requires a high photoluminescence quantum yield (PLQY), while the latter needs a high absorption coefficient. (2) To ensure effective operation of the donor and acceptor, there must be sufficient energy level differences between them. If they overlap, it would lead to uniform charge distribution rather than local concentration at the interface, resulting in adverse effects. (3) In OLEDs, the device must be optimized so that electron-hole recombination near the interface region is within an effective energy transfer distance, facilitating energy transmission.
This demonstrates an overview of the role of interfacial energy transfer dynamic processes underlying NIR OLED functionality. The solid sky-blue pathway represents the process of interfacial energy transfer, capable of facilitating FRET. Within this framework, the S0 and S1 states denote the ground and excited states, respectively, in the singlet manifold, while the T1 state represents the triplet state. Note that T1 → S1’ FRET is viable because the T1 → S0 transition is virtually allowed for the Pt (II) complex due to its strong spin-orbit coupling. The diagram also outlines alternative pathways with dashed lines, where charge transfer (CT) and charge transfer-triplet (CT-T) states are included, alongside the charge separation (CS) state, which represents subsidiary processes occurring with small probability.
Accordingly, we achieved two successful cases: (1) Using the strongly emissive Pt(fprpz)2 at 740nm, we simultaneously transferred BTP-eC9, which has strong absorption at 740nm and emission at 925nm, increasing the EQE from 0.18% to 2.24% and the radiance from 18.81 to 39.97 W sr−1 m−2. (2) Utilizing the strongly emissive Pt(II) complex at 800nm, paired with BTPV-eC9, which has strong absorption at 800nm and emission at 1022nm, the EQE increased from 0.08% to 0.66%, and the radiance from 9.69 to 18.67 W sr−1 m−2. We believe that interface technology will bring unprecedented breakthroughs to NIR-OLEDs.
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