*3.3.1.3 Aggregation-induced red and NIR OLEDs*

The emission of Pt(II) complexes in aggregation forms is dramatically redshifted from that of monomers. Because of the increased metal character in the excited states (e.g., MMLCT) leading to the enhanced radiative decay rates, the emission lifetimes of aggregated Pt(II) emitters are usually short, in the range of 0.1–1 μs, which is fundamentally important for addressing the efficiency roll-off and the operational lifetime issues of phosphorescent OLEDs. In addition, this aggregation emission can be manipulated by tuning the doping concentration; this is particularly useful for the design of high-performance red and NIR OLEDs.

Recently, two series of platinum [O^N^C^N] complexes (**Figures 6** and **7**), i.e., type-I (**Pt-21** and **Pt-22**) and type-II (**Pt-15**, **Pt-16** and **Pt-23**) [39, 40], which are prone to excited-state aggregation, were employed as emitting material in both doped and non-doped deep red and NIR devices; these complexes exhibited high EQE and low efficiency roll-off. For devices with neat complexes, high emission quantum yields were only realized with type-I complexes. For instance, when using a neat **Pt-21** film as the EML, the device demonstrated NIR emission with λmax exceeding 700 nm and with an EQE of 15.84%. In addition, the EQE remained at 11.19% even at a high current density of 100 mA cm<sup>−</sup><sup>2</sup> . Of the doped devices, the device based on **Pt-16** (26 wt%) exhibited a deep red emission with λmax of 661 nm, CIE coordinates of (0.63, 0.37), and an EQE value of 21.75% at a luminance of 1000 cd m<sup>−</sup><sup>2</sup> . The operational lifetimes at 90% initial luminance (LT90, L0 = 100 cd m<sup>−</sup><sup>2</sup> ) with 10 and 30 wt% **Pt-23** as the dopant were 59 and 374 h, respectively, demonstrating that aggregation-based devices would have longer lifetimes.
