A team of researchers headed by Gyumin Jang of the Yonsei University, Department of Materials Science and Engineering (Seoul, Republic of Korea) are developing means to produce a high-quality, deep-blue light-emitting perovskite-based LED (PeLED).
First, a few words of background information.
Solution processed PeLEDs are a promising candidate for the next generation of full-color displays. One problem preventing the realization of this promise is the fact that the external quantum efficiency and operational stability of deep-blue (<460 nm) PeLEDs are not nearly as good as needed, certainly not as good as red and green PeLEDs. One potential means to address this problem is believed to be found in a class of materials called 2D Ruddlesden-Popper perovskites (2D-RPPs).
(Note drawn from Wikipedia: Ruddlesden-Popper phases are a type of perovskite structure that consists of two dimensional perovskite like slabs interleaved with cations. The general formula of a Ruddlesden-Popper phase is An+1BnX3n+1, where A and B are cations, X is an anion (e.g., oxygen), and n is the number of octahedral layers in the perovskite like stack. Generally, it has a phase structure that results from the intergrowth of perovskite type and NaCl type structures.)
The promise offered by 2D-RPPs has yet to be realized due to technical problems.
One problem relates to the hot casting – antisolvent dripping processes that is typically used to fabricate 2D-RPP films. (Note: an antisolvent is a solvent in which the product is insoluble.) This process is found to induce spatial segregation of chemical species in the film during crystallization of the film. The segregation, in turn, is found to induce smaller bandgaps in the light emitted from such a perovskite film. More specifically, the smaller bandgaps hinder emission in the deep-blue portion of the spectrum. Thus, a means to precisely control the phase evolution of 2D-RPPs during crystallization is needed to achieve deep-blue LEDs.
A separate problem related to the production of conventional deep-blue LEDs is financial in nature. It relates to the need to use of indium gallium nitride in the fabrication process. This is an expensive material.
Addressing these problems were the focus of the research undertaken by the team.
A recent article by the team on this topic is entitled “Rapid crystallization-driven high-efficiency phase-pure deep-blue Ruddlesden–Popper perovskite light-emitting diodes.” It was published in Advanced Photonics, Vol. 5, Issue 1, 016001 (January 2023). A copy of the article is available on line and can be found here.
In this article, the team explains the rational for their new approach to the fabrication of phase-pure 2D-RPPs. The idea is that a rapid crystallization method can be used to manipulate 2D perovskite phase evolution by controlling the crystallization kinetics. Some of the details by which this was accomplished are as follows.
When the as-spin-coated precursor wet film is submerged in a hot bath of diethyl ether at 40°C, immediate crystallization occurs. This is due to the rapid extraction of precursor solvent by the diethyl ether antisolvent. More specifically, by promoting immediate removal of the precursor solvent from the wet perovskite films, development of quasi two dimensional 2D-RPP crystals with n values >3 is completely stopped. Put more simply, extremely fast crystallization kinetics allowed all the chemical species to be randomly distributed throughout the film. The result: “highly phase-pure 2D-RPP crystals.”
Owing to enhanced charge transfer and transport induced by the random orientations of the 2D perovskite crystals, the external quantum efficiency of the deep-blue emissive perovskite LEDs reached 0.63% with an emission wavelength centered at 437 nm. The color coordinates were (0.165, 0.044), which match well with the Rec.2020 standard blue gamut. (Note that Rec.2020 refers to various specifications related to aspects of video broadcasts, including a color gamut. Rec. 2020 is geared for ultra-high definition televisions. Ultra-high refers to 4K and 8K televisions.)
The left hand portion of the figure below presents the chromatic coordinates on a CIE 1931 graph. The right hand portion of the figure below is a photograph of an operating device.
An additional result reported by the team was the extended stability of unencapsulated PeLEDs. The researchers report negligible change in the electroluminescent spectra and that the position and full width half maximum of the electroluminescent peak were maintained even after five minutes of device operation. This result is described as “highly comparable to those of state-of-the-art devices.”
The researchers conclude their article with the assessment that “This work provides a novel approach to realize high performance and spectrally stable deep-blue perovskite LEDs. Our research suggests that the control of the crystallization kinetic is the key for the preparation of phase-pure 2D-RPP crystals, exhibiting great promise for addressing current challenges.”
Reference
Gyumin Jang, Hyowon Han, Sunihl Ma, Junwoo Lee, Chan Uk Lee, Wooyong Jeong, Jaehyun Son, Dongki Cho, Ji-Hee Kim, Cheolmin Park, Jooho Moon, “Rapid crystallization-driven high-efficiency phase-pure deep-blue Ruddlesden–Popper perovskite light-emitting diodes,” Adv. Photon. 5(1) 016001 (4 January 2023) https://doi.org/10.1117/1.AP.5.1.016001
