Lead-free white light-emitting thin film achieves full range of light

Despite the advances of white LEDs, challenges remain, including low photoluminescence quantum yield (PLQY), use of toxic or rare phosphor materials and complicated synthesis processes that result in high production costs.

To address these challenges, Jun et al. focused on the material dimensions of two cesium copper iodide (CsCuI)-based phosphor compounds to develop a white light-emitting thin film using a one-step spin coating process.

Previously, the researchers fabricated Cs₃Cu₂I₅ into broadband blue light-emitting single crystals with a high PLQY. They mixed the lead-free, zero-dimensional (0D) particles with a conventional copper-iodine phosphor to create a white photoluminescent film, but the phosphor’s yellow light emission wasn’t sufficient to produce quality white light.

In this latest research, they found that using another CsCuI compound – CsCu₂I₃ – as a one-dimensional (1D) material exhibited much stronger yellow light emission with a large Stokes shift. They then fabricated the white light-emitting thin films on glass substrates, incorporating the 0D and 1D materials in a one-step spin-coating process to achieve full coverage in the visible range of the spectrum.

Lead-free copper halide systems have been developed in recent years. But the significance here, researchers say, is that their CsCuI hybrid material can be fabricated into white light films using a single-step process to enable the different phase of each CsCuI compound to co-exist without the defects that can occur in conventional multiple-step processes.

The one-step process was developed based on the finding that optical properties of CsCuI thin films originate from controlling the material dimensionality of the photo-active sites. The cesium positive ions took an active role in this process, resulting in the high PLQY in both the 1D yellow phosphor and the 0D blue phosphor.

Source: “One-step solution synthesis of white-light-emitting films via dimensionality control of the Cs-Cu-I system,” by Taehwan Jun, Taketo Handa, Kihyung Sim, Soshi Iimura, Masato Sasase, Junghwan Kim, Yoshihiko Kanemitsu, and Hideo Hosono, APL Materials (2019). The article can be accessed at https://doi.org/10.1063/1.5127300 .