http://pubs.acs.org/journal/aelccp
Opportunities and Challenges in PerovskiteBased Display Technologies: A Conversation with Andrey Rogach and Haibo Zeng
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cocorresponding author Prof. Haibo Zeng and an independent expert in the field, Prof. Andrey Rogach of City University Hong Kong (a recent recipient of the Croucher Senior Fellowship on perovskite-enabled devices), to weigh in on the role perovskites could play in the field and to provide some more detail about technical aspects that first need to be addressed.
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eveloping the next brightest, most robust, costeffective, wide-color gamut device display is an active area of materials science research. This is with great reason considering the ubiquity of these displays in our day-today lives and the number of applications to which they have become crucial (Figure 1). Organic light-emitting diodes
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CONVERSATION WITH ANDREY ROGACH For those new to the field, why the interest in perovskites for the production of LED devices? What advantages do they bring as compared to conventional LED materials or OLEDS, for example? In the contemporary information society, a display technology belongs to the core technologies able to increase convenience and quality of the human’s life. One of the major components of displays are light-emitting diodes (LEDs), which are targeted to offer high color purity, high resolution, and high efficiency; most important is the combination of all three parameters (not to mention long-term stability, which is a prerequisite for successful display marketing, by definition). Organic LEDs (OLEDs) offer high efficiency, and have been widely applied in the market, such as in OLED TVs from LG, mobile phones from Huawei, and smartwatches from Apple, but they also have intrinsic disadvantages of narrower color gamut and lower color purity, as the emission spectra of organic fluorophores are rather broad. Moreover, OLEDs are typically fabricated via vacuum-based thermal evaporation, which is not well-suited for cost-effective, large-area processing. Quantum dot LEDs (QLEDs) based on colloidal semiconductor nanocrystals (quantum dots, QD) offer better color purity (so-called highly saturated colors), and they have been used as down-converters to improve color gamut in liquid-crystal displays, such as Samsung QD TVs, in the consumer market. Both organic fluorophores and QDs, on one hand, could be used for much simpler device manufacturing which employs solution processing technologies, but at the same time, the fabrication of high-quality (highly efficient and stable) QDs of a stable core−multiple-shell structure requires rather complex high-temperature synthesis and expensive precursor materials. Metal halide perovskites, which have recently emerged as luminescent materials, offer several advantages for their use as components of LEDs and displays. They are direct-bandgap semiconductor materials and possess high photoluminescence quantum yield (PL QY) reaching 100%; they have narrow and
Figure 1. LED-based technologies are widely used for device displays.
(OLEDs) and quantum dot light-emitting diodes (QLEDs) have advanced for use in displays for commercial products ranging from TVs to cellphones and smartwatches; however, there remain aspects inherent to their physical and photonic properties that are not ideal. In recent years, researchers in the field have refined an exciting addition to the toolkit of materials available for producing LED displays, namely, perovskite nanocrystals. While perovskite nanocrystals are produced from cost-effective starting materials, require relatively facile reaction conditions, and possess tunable optical properties, they present roadblocks to widespread commercial adoption. Blue-emitting perovskites in particular, which are critical to the production of wide color-gamut displays, require further optimization in order to become suitable for incorporation into perovskite light-emitting diodes (PeLEDs) and subsequent application to commerical devices. In their recent ACS Energy Letters Perspective (DOI: 10.1021/acsenergylett.8b02100), Ye Wu, Xiaoming Li, and Haibo Zeng of the MIIT Key Laboratory of Advanced Display Materials and Devices at Nanjing University of Science and Technology survey the status of the field and provide an overview of the materials science limitations and approaches that have been advanced for the optimization of both blueemitting perovskite nanocrystals and the external quantum efficiencies of blue PeLEDs into which they are incorporated. To gain further insight into the opportunities and challenges presented by perovskite-based display technologies, we invited © XXXX American Chemical Society
Received: March 25, 2019 Accepted: March 25, 2019
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DOI: 10.1021/acsenergylett.9b00647 ACS Energy Lett. 2019, 4, 977−979
Energy Focus
Cite This: ACS Energy Lett. 2019, 4, 977−979
Energy Focus
ACS Energy Letters
Cs2SnCl6 perovskites with emission at 455 nm and PL QY of 80%; PL QY of 96% was realized by Yong et al. through doping Ni into CsPbCl3 QDs. However, these highly efficient blue perovskite materials were not yet applied for the fabrication of PeLEDs. Zeng and co-workers synthesized blue emissive CsPbBr3 nanoplatelets with near 100% PL QY, but their respective PeLEDs only showed maximum EQE of 0.1%, which illustrates that just using highly efficient emitters is not enough to achieve high-performance PeLEDs. The choice of optimum charge transfer layers is yet another crucial requirement, while the wide bandgap of blue perovskite emitters limits selection of available materials for those. An inefficient electron/hole blocking would lead to the excess carrier accumulation and thereby result in a large leakage current and low EQE. Thus, to achieve highly efficient blue PeLEDs, first, the phase segregation for the mixed halide perovskites should be prevented by suitable means; second, the conductivity should be improved for the blue-emitting 2D layered perovskite materials; and finally, suitable transport materials should be designed according to the energy band structure of blue perovskite emitters to guarantee balanced charge injection and transport in PeLEDs. What are your insights on the current outlook for widespread adoption of perovskite nanocrystal-based LED devices? Even though PeLEDs have undergone an impressive development in recent years, there is still a gap between them as compared with OLEDs and QLEDs. For instance, the best device efficiency has exceeded 40% for both OLEDs and QLEDs, but the PeLEDs only reached 21% to date. Especially, the device performance of blue PeLEDs urgently needs to be improved. In addition, the operational stability of PeLEDs is still a matter of significant improvement,moreover, eventual elimination of toxic lead through the development of alternative perovskites needs to be addressed. A tandem device structure that has two or more units connected electrically in series using intermediate connectors within the device is rather common in both OLEDs and QLEDs and may be employed in the future for PeLEDS as well in order to improve their performance. Stability issues of LED devices can be resolved via proper encapsulation, but it is the general instability of metal halide perovskite materials which holds top priority to be solved. Despite all of these challenges, the wide color gamut, high color purity, and low cost of PeLEDs are very attractive for displays. When red, green, and blue PeLEDs with high efficiency and operational stability will be realized, their commercial application will inevitably follow. To make largearea flexible displays, developing PeLED fabrication technologies such as ink printing and transfer printing will come in focus. Considering the massive efforts of researchers worldwide which led to the immense improvements of PeLEDs over a rather short span of time, I am optimistic that the nextgeneration display technology based on PeLEDs will soon be created.
Figure 2. Dr. Andrey Rogach, Chair Professor of Photonic Materials, and Founding Director of the Centre for Functional Photonics (CFP) in the Department of Materials Science and Engineering at City University of Hong Kong (http://personal. cityu.edu.hk/~arogatch/Prof%20Andrey%20Rogach.htm).
symmetric PL peaks (full width at half-maximum about 15−30 nm, which is even better than for the traditional QDs) which are tunable all over the visible spectral range through straightforward halide composition adjustment, so that the related wide color gamut (≈140%) is broader than the National Television System Committee (NTSC) standard on a CIE chromaticity diagram. Perovskites in the form of thin films or colloidal nanocrystals can easily be synthesized and processed at room temperature, or vapor-processed at rather low temperatures (typically
