Unraveling the Origin of an Unusual Shift in the Electroluminescence of 1D CsCu₂I₃ 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 CsCu₂I₃, 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 CsCu₂I₃ and electron transport layers (ETL) nor by the degradation of 1D CsCu₂I₃. Instead, it depends on the carrier imbalance on CsCu₂I₃, mainly due to the difference in the electron mobility of the ETLs and the electron density on the CsCu₂I₃ layer. By varying the ETLs, different colored 1D CsCu₂I₃ LEDs with peaks at 556, 590, and 647 nm are fabricated, and a maximum luminance of over 2000 cd/m² is achieved for a 556 nm LED. Further, by limiting the electron mobility and injection to 1D CsCu₂I₃ using an insulating LiF layer at the CsCu₂I₃/ETL interface, more red-shifted LEDs are achieved confirming the critical role of electron density on the EL characteristics of 1D CsCu₂I₃.