White light-emitting diodes (WLEDs) based on copper iodides have attracted tremendous interest due to their broad emission and environmentally friendly nature. Cesium copper iodide is usually regarded as the emissive center in this type of LED. Here, we reveal that it is the in situ formed copper iodide complex, generated during the deposition of the electron transporting layer on cesium copper iodide films, that dominates the electroluminescence. It is also found that the narrow bandgap and widely distributed band-edge states of the complex facilitate the hole injection, resulting in low-voltage driving. Combining a modified hole injection layer to suppress the interfacial exciton quenching, an efficient broadband LED with a turn-on voltage of 2.3 V and a peak EQE of 4.6% was achieved. The driving voltages at 100 and 1000 cd m-2are 2.9 and 4.9 V, respectively, both of which are the lowest among all reported Cu(I) complex-based broadband WLEDs.
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