Advances in Materials for Display Applications
by Ion Bita
At the end of May we will once again welcome the Society for Information Display’s annual Display Week meeting. Among the wide variety of technology topics included in the program, Display Materials and Processes (DMP) was selected as a special topic track for this year. I have been working with SID program committee colleagues to organize this DMP track, so I was glad to learn that the March/April 2017 issue of Information Display was going to include a section on materials. I’d like to take this opportunity to share with you our excitement for the work in materials that is helping to pave the way for future generations of displays.
The number and quality of the papers received in the DMP technical track clearly reaffirmed the role of materials as one of the main pillars in the development of new display technologies and applications. With collaboration across all SID program committee teams, almost 40 presentations were identified and assigned to the DMP track, including papers on emerging electronic and optoelectronic materials, flexible substrates, advanced laser processing, nanomaterials, OLED, LCD, automotive, and touch-sensing applications.
The list above offers an up-to-date perspective of the current trends shaping materials innovation and development across the display industry and in academic research labs. For readers interested in more details, we recommend reviewing the conference program already published on the Display Week website. Most importantly, we recommend attending the conference during May 21–26 at the Los Angeles Convention Center.
In this issue, we include articles on two exciting topics that are currently the subject of intense investigations in the international display community.
In the first article, a team led by Professor Yajie Dong from the Department of Materials Science & Engineering at the University of Central Florida gives an overview of emerging luminescent nanomaterials for applications in displays and lighting devices. Specifically, the authors give an introduction to metal halide perovskite materials and their potential for photoluminescence (PL) and electroluminescence (EL) based display applications. The excitement driving renewed interest in perovskite-based optoelectronics stems from recent progress for photovoltaic applications, coupled with good potential for display applications based on the materials’ narrowband emission spectra, which rivals those produced by semiconductor quantum dots.
Organic-inorganic perovskites (OIPs) of interest in displays have been metal halides of AMX3 composition, where X is a halide (F–, Cl–, Br–, I– anions), and M is a relatively small cation (Pb2+, Sn2+, etc.) with stable octahedral coordination forming an extended network of MX6 octahedra layers intercalated with larger A cations (organic, such as CH3NH3+, or inorganic, such as Cs+). This particular crystal structure leads to very well-defined optoelectronic properties due to the formation of an effective quantum well structure, as the semiconducting inorganic layer is confined between organic layers with larger band gaps. This is one of the main attractive properties of OIP, where efficient and narrow band emission (<20 nm FWHM, with PL quantum yield 70–90%) can be achieved even with low-cost processes. This is due to OIP’s intrinsic tolerance for material processing variations, unlike the traditional case of semiconductor nanoparticles, which require a tight dimensional control.
Due to the large overlap with semiconductor quantum dot (QD) applications, the authors also review the status of QD development and compare the merits of perovskite and QD materials for display applications. Notably, the thesis proposed by Professor Dong is one of finding synergies between these two material systems and pursuing applications that combine them in order to achieve near-term impact in displays and lighting applications.
For more details about these recent exciting developments in perovskite and quantum dot nanomaterials, we recommend this article prepared by Yajie Dong, Hao Chen, Juan He, and Shin-Tson Wu from UCF.
In the second article, Daniel Voltz from CYNORA gives an overview of thermally activated delayed fluorescence (TADF) and the development of highly efficient TADF emitter materials for OLED applications. Similar to the introduction of phosphorescent emitters, which made possible nearly 100% efficient electroluminescence (EL) from small molecule organics, TADF offers an alternative strategy for molecular design that enables efficient organic EL.
The efficiency improvement need addressed by TADF has to do with maximizing the fraction of electrically injected electrons (e–) and holes (h+) that recombine in the emitter molecule in such a way that their energy is converted to light rather than lost to nonradiative processes, e.g. to heat. Due to their typical molecular nature, organic emitters receiving e-h pairs transition into a mix of excited electronic states called singlet (S) and triplet (T) which, importantly, are formed in a 1:3 ratio (S:T). The S excited state has a net spin moment of 0 (i.e. opposite e– and h+ spin orientations) that matches the ground state of the emitter molecule, which is why S excited states can quickly and efficiently relax by emitting a photon. T excited states, on the other hand, have a net spin moment of 1 (i.e. parallel e– and h+ spins), which, by differing from the ground state, are forbidden from relaxing through light emission. Instead, molecules in this unfavorable high-energy state typically lose their energy through nonradiative mechanisms, such as heat generation. TADF represents a mechanism for converting T into S excited states and subsequently enabling efficient EL. This mechanism is possible in specially designed organic molecules that have a smaller energy difference between S and T excited states than the available thermal energy during device operation (which thus allows spontaneous T → S excited state conversion).
The team at CYNORA is one of the pioneers in developing molecules with high-efficiency TADF that are designed for OLED display applications. Please read the article prepared by Dr. Daniel Voltz to learn more about this exciting area.
I hope you enjoy both these articles about exciting new materials. I hope to see you all in Los Angeles in May. •
Ion Bita currently serves as the 2017 Chair of the Display Materials and Processes special topic track for the Society for Information Display’s technical symposium at Display Week, and is a member and past Chair of the SID Display Manufacturing Committee. He can be reached at ion.bita@ gmail.com.