Status and Challenges of Blue OLEDs: A Review

Abstract

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

Keywords: blue and deep-blue OLEDs; chemical structure; high-efficiency; lifetime; organic light-emitting diode (OLED).

Figure 8

Blue OLEDs were seemingly quite challenging to devise since there was only one patent filed in 1998 after the first patent appeared in 1995. There were no blue OLED patents issued in 1999 or 2000. After that, blue OLED patents were continuously granted, peaking to 23 in the year 2017. The overall increasing trend over the past 20 and some years indicates that blue OLEDs are critically crucial. Keyword: Blue organic light-emitting diodes. Source: patentscope.wipo.int [71].

Figure 1

Global revenues of LCD- and OLED-based displays from 2016 to 2024. Keyword: Global revenue OLED display 2020. Source: DSCC’s Quarterly Display Capex and Equipment Market Share Report [15].

Figure 2

Global LED Lighting Revenues and Forecast by Statista Research Development. Keyword: Global revenue LED lighting. Source: https://www.statista.com/statistics/753939/global-led-luminaire-market-size [16].

Figure 3

Global OLED Lighting Revenues and Forecast by ElectroniCast. Keyword: Global revenue OLED lighting. Source: https://www.oled-info.com/electronicast-sees-fast-growing-oled-lighting-market-starting-2015 [17].

Figure 4

Research papers on blue fluorescent OLEDs per year since 1990. Keywords: Fluorescent blue organic light-emitting device. Source: Web of science.

Figure 5

Research papers on blue phosphorescent OLEDs per year; the first paper appeared in 1999 and rapidly peaked to 31 papers in 2009. Keywords: Phosphorescent blue organic light-emitting device. Source: Web of science.

Figure 6

Research papers on blue TADF OLEDs per year since 2015; the first paper appeared in 2012 and rapidly peaked to 71 papers in 8 years. Keywords: TADF blue organic light-emitting device. Source: Web of science.

Figure 7

An overall chart depicting the rise of blue OLED papers using fluorescent (Gen-1), phosphorescent (Gen-2), and TADF (Gen-3) emitters. The yearly paper amount for Gen-2 emitter-composed devices surpassed that of Gen-1 counterparts in 2008 (after the first paper was published in 2001), while the yearly paper amount for Gen-3-based devices surpassed that of Gen-2 in 2020 (after the first paper was published in 2012). Keywords: Blue organic light-emitting device. Source: Web of science.

Figure 9

EQE performance vs. CIE_y of fluorescent blue OLEDs with dry and wet processes. The horizontal black dashed lines represent the theoretical 5% upper limit. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively. Some deep-blue devices were reported with a greater than 5% EQE at 1000 cd/m² with a CIE_y less than 0.1 by using the dry process.

Figure 10

Power efficacy (PE) performance vs. CIE_y of fluorescent blue OLEDs with dry and wet processes. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively. None of the deep-blue devices shows a greater than 10 lm/W power efficacy at any reported luminance, regardless of whether dry or wet processes were used.

Figure 11

Current efficacy (CE) vs. CIE_y for dry- and wet-processed fluorescent blue OLEDs. From display application perspective, commercially viable ones are those with a CIE_y lying on or above the dashed line with a CE equal to 70*CIE_y. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 12

Lifetime and current efficacy vs. CIE_y for fluorescent blue OLEDs. The shaded area represents a CE ≥ 70*CIE_y [30,37,41,42,43]. The different shades of blue denote the emission-color at CIE_y.

Figure 13

EQE performance vs. CIE_y of phosphorescent blue OLEDs with dry and wet processes. Some deep-blue devices were reported with a greater than 20% EQE at a maximum luminance dimmer than 100 nits with a CIE_y less than 0.1 by using dry processes. The horizontal black dashed lines represent the theoretical upper limit. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 14

Power efficacy performance vs. CIE_y of phosphorescent blue OLEDs with dry and wet processes. No deep-blue devices show a power efficacy greater than 10 lumens per watt at 100 or 1000 nits. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 15

Current efficacy (CE) performance vs. CIE_y for dry- and wet-processed phosphorescent blue OLEDs. Many dry-processed devices exhibit a CE much greater than 70*CIE= in the typical blue region, while only a few exhibits deep-blue emission. The black dashed lines represent the minimum CE requirement for display. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 16

Lifetime and current efficacy vs. CIE_y for phosphorescent blue OLEDs. Some phosphorescent OLEDs show relatively high current efficacy and comparatively long lifetime in the sky-blue region. Those that fall into the grey area are plausibly commercially viable. The shaded area represents a CE ≥ 70*CIE_y [30,37,41,42,43]. The different shades of blue denote the emission-color at CIE_y.

Figure 17

EQE performance vs. CIE_y of TADF blue OLEDs with dry and wet processes. Some deep-blue devices were successfully fabricated with an EQE greater than 20% via dry-process. The shaded area represents the upper and lower limits expected for a TADF-based device. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 18

Power efficacy performance vs. CIE_y of TADF blue OLEDs with dry and wet processes. One device shows an around 20 lm/W power efficacy at 100 nits with deep blue emission. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 19

Current efficacy (CE) performance vs. CIE_y for dry- and wet-processed TADF blue OLEDs. One dry-processed device exhibits a near 25 cd/A maximum current efficacy with relatively deep-blue emission. Greater than 20 cd/A is also reported at 100 nits. The black dashed lines represent the minimum CE requirement for display. The vertical dark and light blue dashed lines represent the region for deep blue and greenish blue, respectively.

Figure 20

Lifetime and current efficacy vs. CIEy for TADF blue OLED. A couple TADF-based OLEDs show a fair lifetime with high current efficacy in the sky-blue region. The shaded area represents a CE ≥ 70*CIEy [80].

Figure 21

Numerous series of fluorescent blue emitters based on different core structure units.

Figure 22

Chemical structures of some typical blue emitters, OXD-8, PPP, and SA-PC, for OLED devices.

Figure 23

Molecular structures of typical DSA derivative-based blue emitters with electron donor groups.

Figure 24

Molecular structures of non-planar (a) DSA diphenyl and (b) IDB phenyl, diphenyl, and triphenyl derivative-based blue emitters.

Figure 25

Molecular structures of pyrene-based blue emitters.

Figure 26

Molecular structures of anthracene-based blue emitters.

Figure 27

Molecular structures of fluorene-based blue emitters.

Figure 28

Molecular structures of biaryl-based blue emitters.

Figure 29

Molecular structures of spiro-based blue emitters.

Figure 30

Molecular structures of silane-based blue emitters.

Figure 31

Molecular structures of carbazole-based blue emitters.

Figure 32

Molecular structures of oxadiazole-based blue emitters.

Figure 33

Molecular structures of Ir-complex-based blue emitters.

Figure 34

Molecular structure of platinum complex-based blue emitters.

Figure 35

Molecular structure of Zn(Lc)2 blue emitter.

Figure 36

Types of aromatic TADF blue emitters based on different donor–acceptor pairs. All abbreviations are given as follows: PA (phenylamine), DPS (diphenylsulfone), ACR (acridine), Cz (carbazole), PN (phthalonitrile), PXB (phenoxaborin), PXZ (phenoxazines), TAZ (triazole), and TRZ (triazine).

Figure 37

Molecular structures of (a) electron donor moities and (b) electron acceptor moities, frequently used in aromatic-based blue TADF emitters.

Figure 38

Molecular structures of DPA-DPS-based blue emitters.

Figure 39

Molecular structures of ACR-DPS-based blue emitters.

Figure 40

Molecular structures of Cz-DPS-based blue emitters.

Figure 41

Molecular structures of Cz-TRZ-based blue emitters.

Figure 42

Molecular structures of Cz-PN-based blue emitters.

Figure 43

Molecular structures of Cz-PXB-based blue emitters.

Figure 44

Molecular structures of PXZ-TAZ-based blue emitters.

Figure 45

Molecular structures of ACR-TRZ-based blue emitters.

Figure 46

Types of non-precious metal-based TADF blue emitters.

Figure 47

Molecular structures of copper (Cu)-based blue emitters.

Figure 48

Molecular structures of zinc (Zn)-based blue emitters.

Figure 49

Molecular structures of Lithium (Li) and Magnesium (Mg)-based blue emitters.

Figure 50

Jablonski diagram of a postulated TTA mechanism. The abbreviations used are S₀: ground state, S₁ and S₂: excited singlet state, T₁: triplet state, ISC: intersystem crossing, TTA: triplet–triplet annihilation, and hv: emitted photon.