OPEN (2018) 7, 17168; Officil journl of the CIOMP 2047-7538/18 www.nture.com/ls REVIEW ARTICLE Liquid crystl disply nd orgnic light-emitting diode disply: present sttus nd future perspectives Hi-Wei Chen 1, Jiun-Hw Lee 2, Bo-Yen Lin 2, Stnley Chen 3 nd Shin-Tson Wu 1 Recently, Liquid crystl disply (LCD) vs. orgnic light-emitting diode (OLED) disply: who wins? hs ecome topic of heted dete. In this review, we perform systemtic nd comprtive study of these two flt pnel disply technologies. First, we review recent dvnces in LCDs nd OLEDs, including mteril development, device configurtion nd system integrtion. Next we nlyze nd compre their performnces y six key disply metrics: response time, contrst rtio, color gmut, lifetime, power efficiency, nd pnel flexiility. In this section, we focus on two key prmeters: motion picture response time (MPRT) nd mient contrst rtio (ACR), which drmticlly ffect imge qulity in prcticl ppliction scenrios. MPRT determines the imge lur of moving picture, nd ACR governs the perceived imge contrst under mient lighting conditions. It is intriguing tht LCD cn chieve comprle or even slightly etter MPRT nd ACR thn OLED, lthough its response time nd contrst rtio re generlly perceived to e much inferior to those of OLED. Finlly, three future trends re highlighted, including high dynmic rnge, virtul relity/ugmented relity nd smrt displys with verstile functions. (2018) 7, 17168; ; pulished online 23 Mrch 2018 Keywords: mient contrst rtio; liquid crystl displys; motion picture response time; orgnic light-emitting diode INTRODUCTION Disply technology hs grdully ut profoundly shped the lifestyle of humn eings, which is widely recognized s n indispensle prt of the modern world 1. Presently, liquid crystl displys (LCDs) re the dominnt technology, with pplictions spnning smrtphones, tlets, computer monitors, televisions (TVs), to dt projectors 2 5. However, in recent yers, the mrket for orgnic light-emitting diode (OLED) displys hs grown rpidly nd hs strted to chllenge LCDs in ll pplictions, especilly in the smll-sized disply mrket 6 8.Ltely, LCD vs. OLED: who wins? hs ecome topic of heted dete 9. LCDs re non-emissive, nd their invention cn e trced ck to the 1960s nd erly 1970s 10 15. With extensive mteril reserch nd development, device innovtion nd hevy investment on dvnced mnufcturing technologies, thin-film trnsistor (TFT) LCD technology hs grdully mtured in ll spects; some key hurdles, such s the viewing ngle, response time nd color gmut, hve een overcome 5. Compred with OLEDs, LCDs hve dvntges in lifetime, cost, resolution density nd pek rightness 16. On the other hnd, OLEDs re emissive; their inherent dvntges re ovious, such s true lck stte, fst response time nd n ultr-thin profile, which enles flexile displys 8,9. As for color performnce, OLEDs hve wider color gmut over LCDs employing white light-emitting diode (WLED) s cklight. Nevertheless, LCD with quntum dot (QD) cklight hs een developed nd promoted 17 20. The full width t hlf mximum (FWHM) of green nd red QDs is only 25 nm. As result, QD-enhnced LCD hs wider color gmut thn n OLED. Generlly speking, oth technologies hve their own pros nd cons. The competition is getting fierce; therefore, n ojective systemtic nlysis nd comprison on these two super technologies is in gret demnd. In this review pper, we present recent progress on LCDs nd OLEDs regrding mterils, device structures to finl pnel performnces. First, in Section II, we riefly descrie the device configurtions nd opertion principles of these two technologies. Then, in Section III, we choose six key metrics: response time, contrst rtio, color gmut, lifetime, power efficiency, nd pnel flexiility, to evlute LCDs nd OLEDs. Their future perspectives re discussed in Section IV, including high dynmic rnge (HDR), virtul relity/ ugmented relity (VR/AR) nd smrt displys with verstile functions. DEVICE CONFIGURATIONS AND OPERATION PRINCIPLES Liquid crystl displys Liquid crystl (LC) mterils do not emit light; therefore, cklight unit is usully needed (except in reflective displys) to illuminte the disply pnel. Figure 1 depicts n edge-lit TFT-LCD. The incident LED psses through the light-guide plte nd multiple films nd is then modulted y the LC lyer sndwiched etween two crossed 1 College of Optics nd Photonics, University of Centrl Florid, Orlndo, FL 32816, USA; 2 Grdute Institute of Photonics nd Optoelectronics nd Deprtment of Electricl Engineering, Tiwn University nd 3 Nichem Fine Technology Co. Ltd. Correspondence: S-T Wu, Emil: swu@creol.ucf.edu Received 14 Septemer 2017; revised 29 Novemer 2017; ccepted 29 Novemer 2017; ccepted rticle preview online 1 Decemer 2017 The ccepted rticle preview ws ville with the detils: (2018) 7, e17168; doi: 10.1038/ls.2017.168
HW Chen et l 2 polrizers 5. In generl, four populr LCD opertion modes re used depending on the moleculr lignments nd electrode configurtions: (1) twisted nemtic (TN) mode, (2) verticl lignment (VA) mode, (3) in-plne switching (IPS) mode, nd (4) fringe-field switching (FFS) mode 13 15,21. Below, we will riefly discuss ech opertion mode. TN mode. The 90 TN mode ws first pulished in 1971 y Schdt nd Helfrich 13. In the voltge-off stte, the LC director twists 90 continully from the top to the ottom sustrtes (Figure 2), introducing so-clled polriztion rottion effect. As the voltge exceeds threshold (V th ), the LC directors strt to unwind nd the polriztion rottion effect grdully diminishes, leding to decresed trnsmittnce. This TN mode hs high trnsmittnce nd low opertion voltge (~5 V rms ), ut its viewing ngle is somewht limited 22. To improve the viewing ngle nd extend its pplictions to desktop computers nd TVs, some specilly designed compenstion films, such s discotic film or Fuji film, re commonly used 23,24. Recently, Shrp developed specil micro-tue film to further widen the viewing ngle nd mient contrst rtio (ACR) for TN LCDs 25. z x Anlyzer Color filters LC TFT rry Polrizer DBEF BEF LGP BLU Figure 1 Schemtic digrm of n LCD. BEF, rightness enhncement film; BLU, cklight unit; DBEF, dul rightness enhncement film; LGP, light guide plte. VA mode. VA ws first invented in 1971 y Schiekel nd Fhrenschon 14 ut did not receive widespred ttention until the lte 1990s, when multi-domin VA (MVA) mode ws proposed to solve the viewing ngle prolem 26 28. In the VA mode, n LC with negtive Δεo0 is used nd the electric field is in the longitudinl direction. In the initil stte (V = 0), the LC directors re ligned in the verticl direction (Figure 2). As the voltge exceeds threshold, the LC directors re grdully tilted so tht the incident light trnsmits through the crossed polrizers. Film-compensted MVA mode hs high on-xis contrst rtio (CR; 45000:1), wide viewing ngle nd firly fst response time (5 ms). Thus it is widely used in lrge TVs 29,30. Recently, curved MVA LCD TVs hve ecome populr ecuse VA mode enles the smllest ending curvture compred with other LCDs 31,32. IPS mode. IPS mode ws first proposed in 1973 y Soref 15 ut remined scientific curiosity until the mid-1990s owing to the demnd of touch pnels 33,34. In n IPS cell, the LC directors re homogeneously ligned nd the electric fields re in the lterl direction (Figure 2c). As the voltge increses, the strong in-plne fringing electric fields etween the interdigitl electrodes reorient the LC directors. Such unique mechnism mkes IPS fvorle cndidte for touch pnels ecuse no ripple effect occurs upon touching the pnel. However, the pek trnsmittnce of IPS is reltively low (~75%) ecuse the LC molecules ove the electrodes cnnot e effectively reoriented. This low trnsmittnce region is clled ded zone 5. FFS mode. FFS mode ws proposed in 1998 y three Koren scientists: SH Lee, SL Lee, nd HY Kim 21. Soon fter its invention, FFS ecme populr LCD mode due to its outstnding fetures, including high trnsmittnce, wide viewing ngle, wek color shift, uilt-in storge cpcitnce, nd roustness to touch pressure 35 37. Bsiclly, FFS shres similr working principle with IPS, ut the pixel nd common electrodes re seprted y thin pssivtion lyer (Figure 2d). As result, the electrode width nd gp re le to e OFF ON OFF ON Anlyzer Top sustrte E E Pixel electrode Common electrode Bottom sustrte Polrizer c OFF ON d OFF ON E E Pixel electrode Pssivtion lyer Common electrode Figure 2 Schemtic digrm of the () TNmode,() VAmode,(c) IPS mode nd (d) FFS mode. The LC director orienttions re shown in the voltge-off (left) nd voltge-on (right) sttes.
H-W Chen et l 3 Tle 1 Performnce comprisons of four populr LCD modes TN mode MVA mode IPS mode FFS mode Trnsmittnce (normlized to TN) 100% 70% 80% 70% 80% 88% 98% On-xis contrst rtio ~ 1000:1 ~ 5000:1 ~ 2000:1 ~ 2000:1 LC mixture +Δε Δε +Δε or Δε +Δε or Δε Viewing ngle Fir Good Excellent Excellent Response time ~ 5 ms ~ 5 ms ~ 10 ms ~ 10 ms Touch pnel No No Yes Yes Applictions Wristwtches, signge, lptop computers TV, desktop computers Desktop computers, pds Smrtphones, pds, noteook computers Arevitions: FFS, fringe-field switching; IPS, in-plne switching; LCD, liquid crystl disply; MVA, multi-domin verticl lignment; TN, twisted nemtic; TV, television. much smller thn those of IPS, leding to much stronger fringe fields covering oth the electrode nd gp regions. Thus the ded zone res re reduced. In generl, oth positive (p-ffs) nd negtive (n-ffs) Δε LCs cn e used in the FFS mode 38,39. Currently, n-ffs is preferred for moile pplictions ecuse its trnsmittnce is higher thn tht of p-ffs (98 vs. 88%) 40. As summrized in Tle 1, these four LCD modes hve their own unique fetures nd re used for different pplictions. For exmple, TN hs the dvntges of low cost nd high opticl efficiency; thus, it is mostly used in wristwtches, signge nd lptop computers, for which wide view is not solutely necessry. MVA mode is prticulrly ttrctive for lrge TVs ecuse fst response time, high CR nd wide viewing ngle re required to disply motion pictures. On the other hnd, IPS nd FFS modes re used in moile displys, where low power consumption for long ttery life nd pressure resistnce for touch screens re criticl. Orgnic light-emitting diode The sic structure of n OLED disply, proposed y Tng nd VnSlyke 41 in 1987, consists of orgnic stcks sndwiched etween node nd cthode, s shown in Figure 3. Electrons nd holes re injected from electrodes to orgnic lyers for recomintion nd light emission; hence, n OLED disply is n emissive disply, unlike n LCD. Currently, multi-lyer structures in OLEDs with different functionl mterils re commonly used, s shown in Figure 3. The emitting lyer (EML), which is used for light emission, consists of dopnt nd host mterils with high quntum efficiency nd high crrier moility. Hole-trnsporting lyer (HTL) nd electrontrnsporting lyer (ETL) etween the EML nd electrodes ring crriers into the EML for recomintion. Hole- nd electroninjection lyers (HIL nd EIL) re inserted etween the electrodes nd the HTL nd ETL interfce to fcilitte crrier injection from the conductors to the orgnic lyers. When pplying voltge to the OLED, electrons nd holes supplied from the cthode nd node, respectively, trnsport to the EML for recomintion to give light. Generlly, ech lyer in n OLED is quite thin, nd the totl thickness of the whole device is o1 μm (sustrtes re not included). Thus the OLED is perfect cndidte for flexile displys. For n intrinsic orgnic mteril, its crrier moility (o0.1 cm 2 Vs 1 )nd free crrier concentrtion (10 10 cm 3 ) re firly low, limiting the device efficiency. Thus doping technology is commonly used 42,43. Additionlly, to generte white light, two configurtions cn e considered: (1) ptterned red, green nd lue (RGB) OLEDs; nd (2) white OLED with RGB color filters (CFs). Both hve pros nd cons. In generl, RGB OLEDs re mostly used for smll-sized moile displys, while white OLEDs with CFs re used for lrge TVs. Cthode EML HTL Anode Cthode The EML is the core of n OLED. Bsed on the emitters inside, OLED devices cn e ctegorized into four types: fluorescence, triplettriplet fluorescence (TTF), phosphorescence, nd thermlly ctivted delyed fluorescence (TADF) 41,44 47. Fluorescent OLED. First, upon electricl excittion, 25% singlets nd 75% triplets re formed with higher nd lower energy, respectively. In fluorescent OLED, only singlets decy rditively through fluorescence with n ~ ns exciton lifetime, which sets the theoreticl limit of the internl quntum efficiency (IQE) to 25%, s shown in Figure 4. Triplet-triplet fluorescent OLED. Two triplet excitons my fuse to form one singlet exciton through the so-clled triplet fusion process, s shown in Figure 4, nd relxes to the energy from the singlet stte, clled TTF, which improves the theoreticl limit of the IQE to 62.5%. Phosphorescent OLED. With the introduction of hevy metl toms (such s Ir nd Pt) into the emitters, strong spin-oritl coupling gretly reduces the triplet lifetime to ~ μs, which results in efficient phosphorescent emission. The singlet exciton experiences intersystem crossing to the triplet stte for light emission, chieving 100% IQE, s shown in Figure 4c. Owing to the long rditive lifetime (~ μs) in phosphorescent OLED, the triplet my interct with nother triplet nd polron (triplet-triplet nnihiltion nd triplet-polron nnihiltion, respectively), which results in efficiency roll-off under high current driving 48. Such processes my crete hot excitons nd hot polrons to shorten the opertion lifetime, especilly for lue-emitting devices, s will e discussed in the next section 49. Thermlly ctivted delyed fluorescent OLED. The energy etween the singlet nd triplet cn e reduced (o0.1 ev) y minimizing the EIL ETL EML HTL HIL Anode Figure 3 Schemtic digrm of n OLED. () Bsic structure proposed y Tng nd VnSlyke in 1987. () Multi-lyer structure employed in current OLED products. EIL, electron-injection lyer; ETL, electron-trnsporting lyer; EML, emitting lyer; HTL, hole-trnsporting lyer; HIL, hole-injection lyer.
HW Chen et l 4 c S 1 S 0 S 1 S 0 25% 25% Fluorescence ISC Phosphorescence 75% T 1 75% T 1 exchnge energy 50 ; thus the triplet cn jump ck to the singlet stte y mens of therml energy (reverse intersystem crossing) for fluorescence emission, which is clled TADF, s shown in Figure 4d. Achieving 100% IQE is possile for TADF emission without hevy tom in the orgnic mteril, which reduces the mteril cost nd is more flexile for orgnic moleculr design. In prcticl pplictions, red nd green phosphorescent emitters re the minstrem for ctive mtrix (AM) OLEDs due to their high IQE. While, for lue emitters, TTF is mostly used ecuse of its longer opertion lifetime 51. However, recently, TADF mterils hve een rpidly emerging nd re expected to hve widespred pplictions in the ner future. It is worth mentioning tht, lthough IQE could e s high s 100% in theory, due to the refrctive index difference the emission generted inside the OLED experiences totl internl reflection, which reduces the extrction efficiency. Tking ottom emission OLED with glss sustrte (n ~1.5) nd n indium-tin-oxide node (n ~1.8) s n exmple, the finl extrction efficiency is only ~ 20% 52. DISPLAY METRICS To evlute the performnce of disply devices, severl metrics re commonly used, such s response time, CR, color gmut, pnel flexiility, viewing ngle, resolution density, pek rightness, lifetime, mong others. Here we compre LCD nd OLED devices sed on these metrics one y one. Response time nd motion picture response time A fst response time helps to mitigte motion imge lur nd oost the opticl efficiency, ut this sttement is only qulittively correct. When quntifying the visul performnce of moving oject, motion picture response time (MPRT) is more representtive, nd the following eqution should e used 53 58 : MPRT ¼ 25% 25% qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi t 2 þð0:8t f Þ 2 where T f is the frme time (e.g., T f = 16.67 ms for 60 fps). Using this eqution, we cn esily otin n MPRT s long s the LC response d S 1 S 0 S 1 S 0 TF TTF RISC <0.1 ev TADF 75% T 1 T 1 75% Figure 4 Illustrtion of the emission mechnisms of OLEDs: () fluorescence, () TTF, (c) phosphorescence, nd (d) TADF. ISC, intersystem crossing; RISC, reverse intersystem crossing; TF, triplet fusion. ð1þ time nd TFT frme rte re known. The results re plotted in Figure 5. From Figure 5, we cn gin severl importnt physicl insights: (1) Incresing the frme rte is simple pproch to suppress imge motion lur, ut its improvement grdully sturtes. For exmple, if the LC response time is 10 ms, then incresing the frme rte from 30 to 60 fps would significntly reduce the MPRT. However, s the TFT frme rte continues to increse to 120 nd 240 fps, then the improvement grdully sturtes. (2) At given frme rte, sy 120 fps, s the LC response time decreses, the MPRT decreses lmost linerly nd then sturtes. This mens tht the MPRT is minly determined y the TFT frme rte once the LC response time is fst enough, i.e., τ5t f. Under such conditions, Eqution (1) is reduced to MPRT 0.8T f. (3) When the LC response is o2ms, its MPRT is comprle to tht of n OLED t the sme frme rte, e.g., 120 fps. Here we ssume the OLED s responsetimeis0. The lst finding is somehow counter to the intuition tht LCD should hve more severe motion picture imge lur, s its response time is pproximtely 1000 slower thn tht of n OLED (ms vs. μs). To vlidte this prediction, Chen et l. 58 performed n experiment using n ultr-low viscosity LC mixture in commercil VA test cell. The mesured verge gry-to-gry response time is 1.29 ms y pplying commonly used overdrive nd undershoot voltge method. The corresponding verge MPRT t 120 fps is 6.88 ms, while tht of n OLED is 6.66 ms. These two results re indeed comprle. If the frme rte is douled to 240 fps, oth LCDs nd OLEDs show much fster ut still similr MPRT vlues (3.71 vs. 3.34 ms). Thus the ove finding is confirmed experimentlly. If we wnt to further suppress imge lur to n unnoticele level (MPRTo2 ms), decresing the duty rtio (for LCDs, this is the ontime rtio of the cklight, clled scnning cklight or linking cklight) is mostly dopted 59 61. However, the trdeoff is reduced rightness. To compenste for the decresed rightness due to the lower duty rtio, we cn oost the LED cklight rightness. For OLEDs, we cn increse the driving current, ut the penlties re shortened lifetime nd efficiency roll-off 62 64. CR nd ACR High CR is criticl requirement for chieving supreme imge qulity. OLEDs re emissive, so, in theory, their CR could pproch infinity to one. However, this is true only under drk mient conditions. In most cses, mient light is inevitle. Therefore, for prcticl pplictions, more meningful prmeter, clled the ACR, should e considered 65 68 : ACR ¼ T on þ A ð2þ T off þ A where T on (T off ) represents the on-stte (off-stte) rightness of n LCD or OLED nd A is the intensity of reflected light y the disply device. As Figure 6 depicts, there re two types of surfce reflections. The first one is from direct light source, i.e., the sun or light ul, denoted s A1. Its reflection is firly speculr, nd in prctice, we cn void this reflection (i.e., strong glre from direct sun) y simply djusting the disply position or viewing direction. However, the second reflection, denoted s A2, is quite difficult to void. It comes from n extended ckground light source, such s cler sky or scttered ceiling light. In our nlysis, we minly focus on the second reflection (A2). To investigte the ACR, we hve to clrify the reflectnce first. A lrge TV is often operted y remote control, so touchscreen
H-W Chen et l 5 functionlity is not required. As result, n nti-reflection coting is commonly dopted. Let us ssume tht the reflectnce is 1.2% for oth LCD nd OLED TVs. For the pek rightness nd CR, different TV mkers hve their own specifictions. Here, without losing generlity, let us use the following rnds s exmples for comprison: MPRT (ms) 40 35 30 25 20 15 10 5 30 fps 60 fps 120 fps 240 fps 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 LCD (or OLED) response time (ms) Figure 5 Clculted MPRT s function of the LC (or OLED) response time t different frme rtes. LCD pek rightness = 1200 nits, LCD CR = 5000:1 (Sony 75 X940E LCD TV); OLED pek rightness = 600 nits, nd OLED CR = infinity (Sony 77 A1E OLED TV). The otined ACR for oth LCD nd OLED TVs is plotted in Figure 7. As expected, OLEDs hve much higher ACR in the low illuminnce region (drk room) ut drop shrply s mient light gets righter. At 63 lux, OLEDs hve the sme ACR s LCDs. Beyond 63 lux, LCDs tke over. In mny countries, 60 lux is the typicl lighting condition in fmily living room. This implies tht LCDs hve higher ACR when the mient light is righter thn 60 lux, such s in office lighting (320 500 lux) nd living room with the window shdes or curtin open. Plese note tht, in our simultion, we used the rel pek rightness of LCDs (1200 nits) nd OLEDs (600 nits). In most cses, the displyed contents could vry from lck to white. If we consider typicl 50% verge picture level (i.e., 600 nits for LCDs vs. 300 nits for OLEDs), then the crossover point drops to 31 lux (not shown here), nd LCDs re even more fvorle. This is ecuse the on-stte rightness plys n importnt role to the ACR, s Eqution (2) shows. Recently, n LCD pnel with n in-cell polrizer ws proposed to decouple the depolriztion effect of the LC lyer nd color filters 69. Thus the light lekge ws le to e suppressed sustntilly, leding to significntly enhnced CR. It hs een reported tht the CR of VA LCD could e oosted to 20 000:1. Then we reclculted the ACR, Direct source (sun) Bckground (sky) A1 A2 T Front surfce of LCD LCD cklight Figure 6 Schemtic digrm of two types of reflections for n LCD (or OLED). 14 000 12 000 LCD OLED 14 000 12 000 LCD OLED Amient contrst rtio 10 000 8000 6000 4000 Amient contrst rtio 10 000 8000 6000 4000 2000 2000 0 0 0 50 100 150 200 250 0 50 100 150 200 250 Amient light (lux) Amient light (lux) Figure 7 Clculted ACR s function of different mient light conditions for LCD nd OLED TVs. Here we ssume tht the LCD pek rightness is 1200 nits nd OLED pek rightness is 600 nits, with surfce reflectnce of 1.2% for oth the LCD nd OLED. () LCD CR: 5000:1, OLED CR: infinity; () LCD CR: 20 000:1, OLED CR: infinity.
HW Chen et l 6 2500 2500 2000 LCD OLED 2000 LCD OLED Amient contrst rtio 1500 1000 500 Amient contrst rtio 1500 1000 500 0 0 100 200 300 Amient light (lux) 0 400 500 0 100 200 300 Amient light (lux) 400 500 Figure 8 Clculted ACR s function of different mient light conditions for LCD nd OLED smrtphones. Reflectnce is ssumed to e 4.4% for oth LCD nd OLED. () LCD CR: 2000:1, OLED CR: infinity; () LCD CR: 3000:1, OLED CR: infinity. (LCD pek rightness: 600 nits; OLED pek rightness: 500 nits). nd the results re shown in Figure 7. Now, the crossover point tkes plce t 16 lux, which continues to fvor LCDs. For moile displys, such s smrtphones, touch functionlity is required. Thus the outer surfce is often suject to fingerprints, grese nd other contminnts. Therefore, only simple grde AR coting is used, nd the totl surfce reflectnce mounts to ~ 4.4%. Let us use the FFS LCD s n exmple for comprison with n OLED. The following prmeters re used in our simultions: the LCD pek rightness is 600 nits nd CR is 2000:1, while the OLED pek rightness is 500 nits nd CR is infinity. Figure 8 depicts the clculted results, where the intersection occurs t 107 lux, which corresponds to very drk overcst dy. If the newly proposed structure with n in-cell polrizer is used, the FFS LCD could ttin 3000:1 CR 69. In tht cse, the intersection is decresed to 72 lux (Figure 8), corresponding to n office uilding hllwy or restroom lighting. For reference, typicl office light is in the rnge of 320 500 lux 70. As Figure 8 depicts, OLEDs hve superior ACR under drk mient conditions, ut this dvntge grdully diminishes s the mient light increses. This ws indeed experimentlly confirmed y LG Disply 71. Disply rightness nd surfce reflection hve key roles in the sunlight redility of disply device. Color gmut Vivid color is nother criticl requirement of ll disply devices 72. Until now, severl color stndrds hve een proposed to evlute color performnce, including srgb, NTSC, DCI-P3 nd Rec. 2020 73 76. It is elieved tht Rec. 2020 is the ultimte gol, nd its coverge re in color spce is the lrgest, nerly twice s wide s tht of srgb. However, t the present time, only RGB lsers cn chieve this gol. For conventionl LCDs employing WLED cklight, the yellow spectrum generted y YAG (yttrium luminum grnet) phosphor is too rod to ecome highly sturted RGB primry colors, s shown in Figure 9 77. As result, the color gmut is only ~ 50% Rec. 2020. To improve the color gmut, more dvnced cklight units hve een developed, s summrized in Tle 2. The first choice is the RGphosphor-converted WLED 78,79. From Figure 9, the red nd green emission spectr re well seprted; still, the green spectrum (generted y β-silon:eu 2+ phosphor) is firly rod nd red spectrum (generted y K 2 SiF 6 :Mn 4+ (potssium silicofluoride, KSF) phosphor) is not deep enough, leding to 70% 80% Rec. 2020, depending on the color filters used. A QD-enhnced cklight (e.g., quntum dot enhncement film, QDEF) offers nother option for wide color gmut 20,80,81. QDs exhiit much nrrower ndwidth (FWHM ~ 20 30 nm) (Figure 9c), so tht high purity RGB colors cn e relized nd color gmut of ~ 90% Rec. 2020 cn e chieved. One sfety concern is tht some high-performnce QDs contin the hevy metl Cd. To e comptile with the restriction of hzrdous sustnces, the mximum cdmium content should e under 100 ppm in ny consumer electronic product 82. Some hevy-metl-free QDs, such s InP, hve een developed nd used in commercil products 83 85. Recently, new LED technology, clled the Vivid Color LED, ws demonstrted 86. Its FWHM is only 10 nm (Figure 9d), which leds to n unprecedented color gmut (~98% Rec. 2020) together with specilly designed color filters. Such color gmut is comprle to tht of lser-lit displys ut without lser speckles. Moreover, the Vivid Color LED is hevy-metl free nd shows good therml stility. If the efficiency nd cost cn e further improved, it would e perfect cndidte for n LCD cklight. The color performnce of RGB OLED is minly governed y the three independent RGB EMLs. Currently, oth deep lue fluorescent OLEDs 87 nd deep red phosphorescent OLEDs 88 hve een developed. The corresponding color gmut is 490% Rec. 2020. Aprt from mteril development 89, the color gmut of OLEDs could lso e enhnced y device optimiztion. For exmple, strong cvity could e formed etween semitrnsprent nd reflective lyer. This selects certin emission wvelengths nd hence reduces the FWHM 90. However, the trdeoff is incresed color shift t lrge viewing ngles 91. A color filter rry is nother effective pproch to enhnce the color gmut of n OLED. For exmple, in 2017, AUO demonstrted 5-inch top-emission OLED pnel with 95% Rec. 2020. In this design, so-clled symmetric pnel stcking with color filter is employed to generte purer RGB primry colors 92. Similrly, SEL developed tndem white top-emitting OLED with color filters to chieve high color gmut (96% Rec. 2020) nd high resolution density (664 pixels per inch (ppi) simultneously 93. Lifetime As mentioned erlier, TFT LCDs re firly mture technology. They cn e operted for 410 yers without noticele performnce degrdtion. However, OLEDs re more sensitive to moisture nd oxygen thn LCDs. Thus their lifetime, especilly for lue OLEDs, is
H-W Chen et l 7 1.0 1.0 Intensity (.u.) 0.8 0.6 0.4 R G B YAG WLED Intensity (.u.) 0.8 0.6 0.4 R G B KSF WLED 0.2 0.2 c 0.0 1.0 0.0 400 450 500 550 600 650 700 750 400 450 500 550 600 650 700 750 Wvelength (nm) Wvelength (nm) d 1.0 Intensity (.u.) 0.8 0.6 0.4 R G B QDEF Intensity (.u.) 0.8 0.6 0.4 R G B Vivid color LED 0.2 0.2 0.0 400 450 500 550 600 650 700 750 Wvelength (nm) 0.0 400 450 500 550 600 650 700 750 Wvelength (nm) Figure 9 Trnsmission spectr of color filters nd emission spectr of () YAG WLED, () KSF WLED, (c) QDEF nd (d) Vivid Color LED. KSF, potssium silicofluoride; QDEF, quntum dot enhncement film; WLED, white light-emitting diode; YAG, yttrium luminum grnet. Tle 2 Comprison of different light sources in LCD cklights YAG WLED KSF WLED QDEF LED Vivid Color FWHM 4100 nm 55 nm for green 2 nm for red (5 peks) 20 30 nm 10 nm Tunility No No Yes Yes Color gmut ~ 50% Rec. 2020 70% 80% Rec. 2020 80% 90% Rec. 2020 490% Rec. 2020 Efficiency High High Moderte Low Cost Low Moderte High High Stility Excellent Good Good Excellent RoHS Yes Yes Cd-sed Yes Arevitions: FWHM, full width t hlf mximum; KSF, potssium silicofluoride; LED, lightemitting diode; QDEF, quntum dot enhncement film; RoHS, restriction of hzrdous sustnces; WLED, white light-emitting diode; YAG, yttrium luminum grnet. Here we only consider Cd-sed quntum-dots (QDs). For hevy-metl-free QDs, e.g., InP QD, the FWHM is roder (40 50 nm) nd color gmut is 70 80%. Their opticl efficiency is slightly lower thn tht of Cd-sed QDs. still n issue. For moile displys, this is not criticl issue ecuse the expected usge of smrtphone is pproximtely 2 3 yers. However, for lrge TVs, lifetime of 430 000 h (410 yers) hs ecome the norml expecttion for consumers. Here we focus on two types of lifetime: storge nd opertionl. To enle 10-yer storge lifetime, ccording to the nlysis 94, the wter vpor permetion rte nd oxygen trnsmission rte for n OLED disply should e o1 10 6 g(m 2 -dy) 1 nd 1 10 5 cm 3 (m 2 -dy) 1, respectively. To chieve these vlues, orgnic nd/or inorgnic thin films hve een developed to effectively protect the OLED nd lengthen its storge lifetime. Menwhile, it is comptile to flexile sustrtes nd fvors thinner disply profile 95 97. The next type of lifetime is opertionl lifetime. Owing to mteril degrdtion, OLED luminnce will decrese nd voltge will increse fter long-term driving 98. For red, yellow nd green phosphorescent OLEDs, their lifetime vlues t 50% luminnce decrese (T 50 )cne s long s 480 000 h with 1000 cd m 2 luminnce 99 101.Nevertheless, the opertionl lifetime of the lue phosphor is fr from stisfctory. Owing to the long exciton lifetime (~ μs) nd wide ndgp ( ~3 ev), triplet-polron nnihiltion occurs in the lue phosphorescent OLED, which genertes hot polrons (~6 ev; this energy is higher thn some ond energies, e.g., 3.04 ev for the C-N single ond), leding to short lifetime. To improve its lifetime, severl pproches hve een proposed, such s designing suitle device structure to roden the recomintion zone, stcking two or three OLEDs or introducing n exciton quenching lyer. The opertion lifetime of lue phosphorescent OLED cn e improved to 3700 h (T 50, hlf lifetime) with n initil luminnce of 1000 nits. However, this is still ~ 20 shorter thn tht of red nd green phosphorescent OLEDs 101 103. To further enhnce the lifetime of the lue OLED, the NTU group hs developed new ETL nd TTF-EML mterils together with n optimized lyer structure nd doule EML structure 104.Figure10
HW Chen et l 8 100 000 Normlized luminnce 1.0 0.8 0.6 0.4 5000 nits 10 000 nits 15 000 nits T 50 (h) 10 000 1000 0.2 0.0 0 100 200 300 Time (h) 400 500 100 1000 10 000 Initil luminnce (nits) Figure 10 () Luminnce decy curves for the lue OLED with different initil luminnce vlues. () EstimtedT 50 under different initil luminnce vlues. shows the luminnce decy curves of such lue OLED under different initil luminnce vlues (5000, 10 000, nd 15 000 nits). From Figure 10, the estimted T 50 t 1000 nits of this lue OLED is ~56 000h (~6 7 yers) 104,105. As new mterils nd novel device structures continue to dvnce, the lifetime of OLEDs will e grdully improved. Power efficiency Power consumption is eqully importnt s other metrics. For LCDs, power consumption consists of two prts: the cklight nd driving electronics. The rtio etween these two depends on the disply size nd resolution density. For 55 4K LCD TV, the cklight occupies pproximtely 90% of the totl power consumption. To mke full use of the cklight, dul rightness enhncement film is commonly emedded to recycle mismtched polrized light 106. The totl efficiency could e improved y ~ 60%. The power efficiency of n OLED is generlly limited y the extrction efficiency (η ext ~ 20%). To improve the power efficiency, multiple pproches cn e used, such s microlens rry, corrugted structure with high refrctive index sustrte 107,replcing the metl electrode (such s the Al cthode) with trnsprent metl oxide 108, incresing the distnce from the emission dipole to the metl electrode 109 or incresing the crrier concentrtion y mteril optimiztions 110. Experimentlly, externl quntum efficiencies s high s 63% hve een demonstrted 107,108. Note tht sometimes the light-extrction techniques result in hze nd imge lur, which deteriorte the ACR nd disply shrpness 111 113.Additionlly,friction complexity nd production yield re two dditionl concerns. Figure 11 shows the power efficiencies of white, green, red nd lue phosphorescent s well s lue fluorescent/ttf OLEDs over time. For OLEDs with fluorescent emitters in the 1980s nd 1990s, the power efficiency ws limited y the IQE, typiclly o10 lm W 1 (Refs. 41,114 118). With the incorportion of phosphorescent emitters in the ~ 2000 s, the power efficiency ws significntly improved owing to the mterils nd device engineering 45,119 125.Thephotonic design of OLEDs with regrd to the light extrction efficiency ws tken into considertion for further enchntment of the power efficiency 126 130. For green OLED, power efficiency of 290 lm W 1 ws demonstrted in 2011 (Ref. 127), which showed 4100 improvement compred with tht of the sic two-lyer device proposed in 1987 (1.5 lm W 1 in Ref. 41). A white OLED with power efficiency 4100 lm W 1 ws lso demonstrted, which ws comprle to the power efficiency of LCD cklight. For red nd Power efficiency (Im W 1 ) 100 10 1 W R G B (Ph) B (F/TTF) 0.1 1985 1990 1995 2000 2005 2010 2015 2020 Yer Figure 11 Power efficiency of white, red, green nd phosphorescent lue nd fluorescent/ttf lue OLEDs over time. Dt re compiled from Refs. 41,45,114 133. lue OLEDs, their power efficiencies re generlly lower thn tht of the green OLED due to their lower photopic sensitivity function, nd there is trdeoff etween color sturtion nd power efficiency. Note, we seprted the performnces of lue phosphorescent nd fluorescent/ttf OLEDs. For the lue phosphorescent OLEDs, lthough the power efficiency cn e s high s ~ 80 lm W 1, the opertion lifetime is short nd color is sky-lue. For disply pplictions, the lue TTF OLED is the fvored choice, with n cceptle lifetime nd color ut much lower power efficiency (16 lm W 1 ) thn its phosphorescent counterprt 131,132. Overll, over the pst three decdes (1987 2017), the power efficiency of OLEDs hs improved drmticlly, s Figure 11 shows. To compre the power consumption of LCDs nd OLEDs with the sme resolution density, the displyed contents should e considered s well. In generl, OLEDs re more efficient thn LCDs for displying drk imges ecuse lck pixels consume little power for n emissive disply, while LCDs re more efficient thn OLEDs t displying right imges. Currently, ~ 65% verge picture level is the intersection point etween RGB OLEDs nd LCDs 134. For colorfilter-sed white OLED TVs, this intersection point drops to ~ 30%. As oth technologies continue to dvnce, the crossover point will undoutedly chnge with time. Pnel flexiility Flexile displys hve long history nd hve een ttempted y mny compnies, ut this technology hs only recently egun to see
H-W Chen et l 9 commercil implementtions for consumer electronics 135. A good exmple is Smsung s flgship smrtphone, the Glxy S series, which hs n OLED disply pnel tht covers the edge of the phone. However, strictly speking, it is curved disply rther thn flexile disply. One step forwrd, foldle AM-OLED hs een demonstrted with the curvture rdius of 2 mm for 100 000 repeted folds 136. Owing to the superior flexiility of the orgnic mterils, rollle AM-OLED disply driven y n orgnic TFT ws fricted 137. By replcing the rittle indium-tin-oxide with flexile Ag nnowire s the node, stretchle OLED for up to 120% strin ws demonstrted 138. LCDs hve limited flexiility. A curved TV is prcticl ut going eyond tht is rther difficult with rigid nd thick glss sustrtes 139. Fortuntely, this ostcle hs een removed with the implementtion of thin plstic sustrte 140 142. In 2017, 12.1 rollle LCD using orgnic TFT, clled OLCD, ws demonstrted, nd its rdius of curvture is 60 mm 143. To mintin uniform cell gp, polymer wll ws formed within the LC lyer 144. Additionlly, it is reported tht LCDs could e foldle with segmented cklight. This is good choice, ut until now, no demo or rel device hs een demonstrted. Comining two ezel-less LCDs together is nother solution to enle foldle disply, ut this technology is still under development 145. Others In ddition to the forementioned six disply metrics, other prmeters re eqully importnt. For exmple, high-resolution density hs ecome stndrd for ll high-end disply devices. Currently, LCD is tking the led in consumer electronic products. Eight-hundred ppi or even 41000 ppi LCDs hve lredy een demonstrted nd commercilized, such s in the Sony 5.5 4k Smrtphone Xperi Z5 Premium. The resolution of RGB OLEDs is limited y the physicl dimension of the fine-pitch shdow msk. To compete with LCDs, most OLED displys use the PenTile RGB supixel mtrix scheme 146. The effective resolution density of n RGB OLED moile disply is ~ 500 ppi. In the PenTile configurtion, the lue supixel hs lrger size thn the green nd red supixels. Thus lower current is needed to chieve the required rightness, which is helpful for improving the lifetime of the lue OLED. On the other hnd, owing to the lower efficiency of the lue TTF OLED compred with the red nd green phosphorescent ones, this results in higher power consumption. To further enhnce the resolution density, multiple pproches hve een developed, s will e discussed lter. The viewing ngle is nother importnt property tht defines the viewing experience t lrge olique ngles, which is quite criticl for multi-viewer pplictions. OLEDs re self-emissive nd hve n ngulr distriution tht is much roder thn tht of LCDs. For instnce, t 30 viewing ngle, the OLED rightness only decreses y 30%, wheres the LCD rightness decrese exceeds 50%. To widen n LCD s viewing ngle, three options cn e used. (1) Remove the rightness-enhncement film in the cklight system. The trdeoff is decresed on-xis rightness 147. (2) Use directionl cklight with front diffuser 148,149. Such configurtion enles excellent imge qulity regrdless of viewing ngle; however, imge lur induced y strong diffuser should e crefully treted. (3) Use QD rrys s the color filters 20,150 152. This design produces n isotropic viewing cone nd high-purity RGB colors. However, preventing mient light excittion of QDs remins technicl chllenge 20. In ddition to rightness, color, gryscle nd the CR lso vry with the viewing ngle, known s color shift nd gmm shift. In these spects, LCDs nd OLEDs hve different mechnisms. For LCDs, they re induced y the nisotropic property of the LC mteril, which could e compensted for with unixil or ixil films 5.ForOLEDs, they re cused y the cvity effect nd color-mixing effect 153,154.With extensive efforts nd development, oth technologies hve firly mture solutions; currently, color shift nd gmm shift hve een minimized t lrge olique ngles. Cost is nother key fctor for consumers. LCDs hve een the topic of extensive investigtion nd investment, wheres OLED technology is emerging nd its friction yield nd cpility re still fr ehind LCDs. As result, the price of OLEDs is out twice s high s tht of LCDs, especilly for lrge displys. As more investment is mde in OLEDs nd more dvnced friction technology is developed, such s ink-jet printing 155 157, their price should decrese noticely in the ner future. FUTURE PERSPECTIVES Currently, oth LCDs nd OLEDs re commercilized nd compete with ech other in lmost every disply segment. They re siclly two different technologies (non-emissive vs. emissive), ut s disply, they shre quite similr perspectives in the ner future. Here we will focus on three spects: HDR, VR/AR nd smrt displys with verstile functions. High dynmic rnge HDR is n emerging technology tht cn significntly improve picture qulity 158 160. However, strictly speking, HDR is not single metric; insted, it is more like technicl stndrd or formt (e.g., HDR10, Doly Vision, etc.), unifying the forementioned metrics. In generl, HDR requires higher CR (CR 100 000:1), deeper drk stte, higher pek rightness, richer gryscle ( 10 its) nd more vivid color. Both LCD nd OLED re HDR-comptile. Currently, the est HDR LCDs cn produce righter highlights thn OLEDs, ut OLEDs hve etter overll CRs thnks to their superior lck level. To enhnce n LCD s CR, locl dimming cklight is commonly used, ut its dimming ccurcy is limited y the numer of LED segmenttions 161 163. Recently, dul-pnel LCD system ws proposed for pixel-y-pixel locl dimming 164,165. In n experiment, n exceedingly high CR (41 000 000:1) nd high it-depth (414 its) were relized t merely 5 volts. In 2017, such dul-pnel LCD ws demonstrted y Pnsonic, iming t medicl nd vehiculr pplictions. At 2018 consumer electronics show, Innolux demonstrted 10.1 LCD with n ctive mtrix mini-led cklight. The size of ech mini-led is 1 mm nd pitch length is 2 mm. In totl, there re 6720 locl dimming zones. Such mini-led sed LCD offers severl ttrctive fetures: CR41 000 000:1, pek rightness = 1500 nits, HDR: 10-it mini-led nd 8-it LCD, nd thin profile. Also worth mentioning here is ultr-high rightness. Mostly, people py more ttention to the required high CR (CR4100 000:1) of HDR ut fil to notice tht CR is jointly determined y the drk stte nd pek rightness. For exmple, 12-it Perceptul Quntizer curve is generted for rnge up to 10 000 nits, which is fr eyond wht current displys cn provide 166,167. The pek rightness of LCDs could e oosted to 2000 nits or even higher y simply using high-power cklight. OLEDs re selfemissive, so their pek rightness would trde with lifetime. As result, more dvnced OLED mterils nd novel structurl designs re highly desirle in the future. Another reson to oost pek rightness is to increse sunlight redility. Especilly for some outdoor pplictions, such s pulic displys, pek rightness is criticl to ensure good redility under strong mient light. As discussed in the section of CR nd ACR, high rightness leds to high ACR, except tht the power consumption will increse.
HW Chen et l 10 Virtul relity nd ugmented relity Immersive VR/AR re two emerging werle disply technologies with gret potentil in entertinment, eduction, trining, design, dvertisement nd medicl dignostics. However, new opportunities rise long with new chllenges. VR hed-mounted displys require resolution density s high s 42000 ppi to eliminte the so-clled screen door effect nd generte more relistic immersive experiences. An LCD s resolution density is determined y the TFTs nd color filter rrys. In SID 2017, Smsung demonstrted n LCD pnel with resolution of 2250 ppi for VR pplictions. The pitches of the supixel nd pixel re 3.76 nd 11.28 μm, respectively. Menwhile, field sequentil color provides nother promising option to triple the LCD resolution density 168,169. However, more dvnced LC mixtures nd fst response LCD modes re needed to suppress the color rekup issue 170 179. For OLED microdisplys, emgin proposed novel direct ptterning pproch to enle 2645 ppi RGB orgnic emitters on CMOS ckplne 180. Similr performnce hs een otined y Sony. They developed 0.5-inch AM-OLED pnel with 3200 ppi using wellcontrolled color filter rrys 181. As for AR pplictions, lightweight, low power nd high rightness re minly determined y the disply components. LC on silicon cn generte high rightness 182,utitsprofile is too ulky nd hevy with the implementtion of polriztion em splitter. Removing the polriztion em splitter with front light guide would e the pproprite solution 183. However, integrting RGB LEDs with this light guide remins significnt chllenge. Additionlly, RGB LEDs, especilly green LEDs, re not efficient enough. OLEDs hve thin profiles, ut their pek rightness nd power efficiency re still fr from stisfctory, especilly for such AR devices, s they re mostly used outdoors, mening high rightness is commonly required to increse the ACR of displyed imges. Smrt displys with verstile functions Currently, displys re no longer limited to trditionl usges, such s TVs, pds or smrtphones. Insted, they hve ecome more diversified nd re used in smrt windows, smrt mirrors, smrt fridges, smrt vending mchines nd so on. They hve entered ll spects of our dily lives. As these new pplictions re emerging, LCDs nd OLEDs hve new opportunities s well s new chllenges. Let us tke vehicle disply s n exmple: high rightness, good sunlight redility, nd wide working temperture rnge re required 184.Tofollowthistrend,LC mixtures with n ultr-high clering temperture (4140 C) hve een recently developed, ensuring tht the LCD works properly even t some extreme tempertures 185. OLEDs hve n ttrctive form fctor for vehicle displys, ut their performnce needs to qulify under the ovementioned hrsh working conditions. Similrly, for trnsprent displys or mirror displys, LCDs nd OLEDs hve their own merits nd demerits 186 189. They should im t verstile functions sed on their own strengths. CONCLUSION We hve riefly reviewed the recent progress of LCD nd OLED technologies. Ech technology hs its own pros nd cons. For exmple, LCDs re leding in lifetime, cost, resolution density nd pek rightness; re comprle to OLEDs in ACR, viewing ngle, power consumption nd color gmut (with QD-sed cklights); nd re inferior to OLED in lck stte, pnel flexiility nd response time. Two concepts re elucidted in detil: the motion picture response time nd ACR. It hs een demonstrted tht LCDs cn chieve comprle imge motion lur to OLEDs, lthough their response time is 1000 slower thn tht of OLEDs (ms vs. μs). In terms of the ACR, our study shows tht LCDs hve comprle or even etter ACR thn OLEDs if the mient illuminnce is 450 lux, even if its sttic CR is only 5000:1. The min reson is the higher rightness of LCDs. 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