Unraveling the Origin of an Unusual Shift in the Electroluminescence of 1D CsCuI Light-Emitting Diodes.

Lead-free low-dimensional copper-based metal halides are promising luminescent materials for broadband LEDs owing to their broad self-trapped exciton (STE) emission. However, recently, in 1D CsCuI, a discrepancy between their electroluminescence (EL) and photoluminescence (PL) has been observed. As a result, the overall output color from LEDs is significantly different than the anticipated emission. To unveil the origin of this discrepancy, here, we provide comprehensive analyses and show that the shift in the EL is caused neither by any structural/optical interactions between CsCuI and electron transport layers (ETL) nor by the degradation of 1D CsCuI. Instead, it depends on the carrier imbalance on CsCuI, mainly due to the difference in the electron mobility of the ETLs and the electron density on the CsCuI layer. By varying the ETLs, different colored 1D CsCuI LEDs with peaks at 556, 590, and 647 nm are fabricated, and a maximum luminance of over 2000 cd/m is achieved for a 556 nm LED. Further, by limiting the electron mobility and injection to 1D CsCuI using an insulating LiF layer at the CsCuI/ETL interface, more red-shifted LEDs are achieved confirming the critical role of electron density on the EL characteristics of 1D CsCuI.