1. Introduction

Numerous medical studies reported that modern electric lighting and displays, especially the blue emission therein, may cause some hazards, such as irreversible retinal damage, discoloring the famous paintings of Van Gogh and others, physiological disorders, and increasing risk of various cancers resulting from the suppression of melatonin secretion [1–3]. In 2014, Stevens et al revealed electric light at night to be a critical factor explaining at least in part the burden to breast cancer [4]. Later in the same year, International Energy Agency formally released a report confirming the potential hazards that blue light emitting diode (LED) and cold-white LED may cause damage to human eyes [5]. All these explain why physicians have long been calling for the development of low color temperature (CT) or blue hazard free lighting sources to safeguard human health.

Ironically, electric light sources, both cold-white and warm-white lights, contain intensive blue emission and have been increasingly adopted for general lighting since the invention of incandescent bulb 150 years ago. Contrarily, candles emit less blue emission and have been used since 5,000 years ago [6,7]. Although the low CT candles may be used as a physiologically-friendly lighting measure, they are very energy-inefficient, not to mention their potential fire hazard problems, flickering nature, and unpleasant smoke due to incomplete burning. Hence, it is no less urgent at all to devise a blue-hazard free, low CT than an energy efficient, high CT lighting source. Earlier, few efforts had been attempted to realize energy-efficient, blue-hazard free lighting sources, though having the promising solid state lighting technologies, such as LEDs and organic light emitting diodes (OLEDs). In 2012, Jou’s group reported a physiologically-friendly low color-temperature (1,773 K) OLED with an efficacy of 11.9 lm W⁻¹ [8]. The device provided rational visual comfort with a color rendering index (CRI) of 87. In early 2013, Jou's group reported the world’s first electricity-driven candle-light style OLED [9]. The device showed an efficacy of about 300 times that of typical candles. In same year, another OLED device with chromaticity tunable between that of dusk-hue and candle-light was reported [10]. It exhibited a maximum 21 lm W⁻¹ efficacy and a maximum 91 CRI with CT tunable between 1,580 K and 2,600 K. In 2014, Hu et al. reported a hybrid OLED device with a CT of 1,910 K and highest efficacy of 54.6 lm W⁻¹ [11]. Recently, Shen et al. reported a candlelight-like LED by employing a nano composites core-shell of zinc oxide (ZnO) and Mn⁺² doped zinc sulfide (ZnS) quantum dots [12]. Unlike typical candles, the resultant warm white LED exhibited a CT of 3,113 K and chromaticity coordinates of (0.430, 0.404).

To the moment, there shows barely any blue-hazard free lighting source that can reach the fluorescent tube efficacy. It is hence worthy to design a new device architecture to realize high-efficiency for the blue-hazard free OLEDs. We demonstrate here how to enable a high quality, blue hazard free general lighting on basis of low color-temperature, candle light-style OLED with a high natural light spectrum resemblance index, (SRI), and with an efficacy at least 800 times that of candles and over five times of incandescent bulbs. The resultant candle-light OLED is approaching the fluorescent tube efficacy without the use of any light out-coupling enhancement.

2. Experimental

2.1. Device fabrication

Figure 1 shows all the studied OLED device structures and their corresponding energy level diagrams. As shown Fig. 1(a), the device is composed of a 125 nm indium tin oxide (ITO) anode layer, following a 35 nm poly(3,4-ethylene- dioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) electron injection layer, a 20 nm di-[4-(N,N-ditolylamino)-phenyl]cyclohexane (TAPC) hole transporting/electron-confining layer (HTL/ECL), an 5-nm-short wavelength emissive-layer (EML), a 15-nm-long wavelength EML, a 32-nm 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) electron transporting layer (ETL), a 0.8-nm LiF layer and a150 nm aluminum (Al) layer; short wavelength EML is 4,4’,4”-tri(N-carbazolyl)triphenylamine (TCTA) doped with 20 wt% bis[3,5-difluoro-2-(2-pyridyl)phenyl]-(2-carboxypyridyl)iridium(III) (FIrpic). The long wavelength EML consisted of a TPBi host doped with 12.5% tris(2-phenyl-pyridine) iridium (Ir(ppy)₃) green dyes, 3 wt% iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C 2’)acetylacetonate (PO-01) yellow dye, and 1 wt% bis(1-phenylisoquinolinolato-C2,N) iridium (acetylacetonate) (Ir(piq)₂(acac)) deep-red dye. In order to investigate the carrier regulation function, we have also employed a 2 nm mix carrier modulation layer (CML) of TCTA and TPBi in the ratio of 3: 1 between the EMLs. The fabrication of the blend CML, short and long wavelength EMLs involved vapor-deposition, and the sources were prepared via solution premixing method [13].

 

Fig. 1 Schematic diagrams of the energy-levels of the candle light-style OLED devices containing (a) four black bod radiation complementary emitters, namely, blue, red, green, and yellow, and (b) three emitters, namely, red, green, and yellow, dispersed in two EMLs.

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Figure 1(b) shows an OLED device, which is fabricated by the sequential thermal evaporation of both organic and inorganic materials. Each device consisted a 5 nm 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) charge generation layer (GCL), a 25 nm TAPC as HTL, a x nm (2.5 and 3.5 nm) EML-1 and y nm (9 and 8 nm) EML-2, then a 36 nm 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB) as ETL, a 1 nm LiF layer, and a 100 nm Al cathode layer. The EML-1 is comprised a TCTA host doped with 2.5 wt% tris(2-phenylquinoline) iridium (III) (Ir(2-phq)₃) orange-red emitter. The EML-2 consisted of a TPBi host doped with 10 wt% Ir(ppy)₃ and 5 wt% PO-01 emitters.

2.2. Device measurement

The current-voltage-luminance (I-V-L) characteristics of resulting devices were measured using a Keithley 2400 electrometer together with a Minolta CS-100 luminance meter. The CIE chromatic coordinates, and electroluminescent spectrum of the resultant OELDs were measured by using Photo Research PR-655 spectrascan. The emission area of the device was 9 mm², and only the luminance in the forward direction was measured to devices. The EQE and power efficiency were estimated from luminance measured at a normal direction with angular profile of Lambertian emission assumed.

3. Results and discussion

As shown in Fig. 1, Device I to III and Device VI are composed of four electroactive emitters with red (624 nm), yellow (558 nm), green (513 nm), and sky-blue (472 nm) emission. These four emitters are dispersed in properly engineered hosting materials, and then deposited into two different emissive layers with a total thickness of 20 nm. On the other hand, Device IV and V are made of three highly efficient dopants with orange-red (578 nm), yellow (558 nm) and green (513 nm) emission. In order to realize an effective carrier injection balance, orange-red emitter is dispersed in TCTA host, while yellow and green emitters are doped in TPBi host, and then deposited into two emissive layers with total thickness of 11.5 nm. To attain high device efficiency, the emissive layers are sandwiched by two thin layers of light emitting-auxiliary materials to facilitate the transport of carriers (holes and electrons), and which are further sandwiched by two additional carrier injection layers to minimize the interfacial barrier in between the organic molecules and electrodes. A thin layer of aluminum is deposited as the cathode, and a transparent conductive oxide, ITO, is used as the anode.

Table 1 summarizes the CT, SRI, power efficiency, and external quantum efficiency (EQE) of the devices studied. In order to realize high efficiency candle light-style OLEDs, the doping concentration of the red, green, sky-blue and yellow emitters, emissive layer thickness, EML composition, and effect of nano CML is optimized. The employed four emitters enabled the generation of the desirable candle light-style chromaticity with a CT ranging from 1,850 K to 2,100 K, at 100 to 10,000 cd m⁻², respectively. The device efficacy can be further enhanced to 30.3 lm W⁻¹, as first EML thickness reduced from 10 nm to 5 nm and second EML thickness increased from 10 to 15 nm.

Table 1. Effects of emissive layer (EML) thickness and nano carrier modulation layer (CML) composition on the color temperature (CT), spectrum resemblance index (SRI), power efficiency (PE), and external quantum efficiency (EQE) of the candle light-style OLEDs.

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As shown in Fig. 2(a) and (b), the efficiency of the OLEDs decreased considerably at high luminance. The efficiency roll-off may be attributed to numerous factors, including concentration quenching [14,15], exciton quenching [15], imbalance carrier injection [16], and poor carrier confinement [17]. To overcome this problem, several approaches can be employed, such as using multiple emission layers with stepwise energy-levels to extend exciton generation zone [18], and incorporating effective carrier (holes and electrons) transporting and confining layers to realize a balance carrier injection [19]. Amongst, CML is considered to be a favorable approach because of its carrier regulation ability. By incorporating a 2 nm CML between the two EMLs, the power efficiency is further increased to 21.5 lm W⁻¹ at 1,000 cd m⁻², as shown in Table 1. The bipolar nature of mixed CML effectively distribute the charge carriers in a wide recombination zone, and hence obtain a relatively higher device efficiency than that of Device I and Device II [20]. Besides, high CRI (90) may be achieved by the deposition a nano CML between a short-wavelength and a long-wavelength EMLs doped with four complementary candle light style emitters, because nano CML can successfully regulate the injection of carriers [9, 21].

 

Fig. 2 Effect of EML thickness and nano CML on (b) power efficiency and (b) current efficiency of the PEDOT:PSS based devices, (c) effect of EML thickness on the power efficiency and current efficiency of the HAT-CN based candle light-style OLEDs.

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The resultant Device V with three complementary emitters in two EMLs emit blue hazard free emission. The efficacy of candle-light style OLED device reaches that of conventional compact fluorescent lamps (CFLs) or fluorescent tubes. The Device V shows an operation voltage of 2.7V, 2.9V, and 3.8V, respectively, at 100 cd m⁻², 1,000 cd m⁻² and 10,000 cd m⁻², which is much lower than that of Device VI (3.2V, 4.1V, and 6.2V). The lower operation voltage may arises because of thin emissive layers thickness and a −0.1 eV trap between TPBi host and high mobility BmPyPB ETL for injection of minor carriers, electrons. The electron-trapping character would subsequently enable the effective injection of electrons, leading to a desirable balanced carrier injection on emissive layer. As shown in Fig. 2(c), at 1,000 cd m⁻² for example, Device V exhibits a power efficiency of 76 lm W⁻¹, current efficiency of 70 cd A⁻¹ and EQE of 26.4% with a CT of 2,284 K and a SRI of 77. The reason why Device V shows highest efficiencies may be attributed to four important factors in the device architecture [22, 23], which are (i) energy trap for effective electron injection, (ii) effective host-to-guest energy transfer, (iii) balanced charge carrier injection, (iv) effective charge carriers, hole and electron, confining function. Besides, effective device architecture approaches, an assortment of highly EL-active orange-red, green and yellow dopants facilitate almost 100% internal quantum efficiency, and high mobility electron transporting material BmPyPB [23] enables charge carrier injection balance in EMLs. The resultant device also shows an efficacy of 52.4 lm W⁻¹ (62.4 cd A⁻¹ and 24.1%) at 10,000 cd m².

The resultant devices (Device I, Device II and Device III) without incorporating a nano CML exhibit CT between 1,851 K and 3,200 K. By inserting a 2 nm CML in between the two emissive layers, the SRI is further increased to 89 with the CT of 2,160 K. Figure 3(a) shows the significant change in EL-spectra as the short-wavelength EML thickness decreased from 15 to 5 nm and long-wavelength EML thickness increased from 5 to 15 nm. As much as the thickness of short-wavelength EML decreased to 5 nm, blue emission band almost disappear from the EL-spectra. Hence, the carrier recombination zone shifted towards electron transporting layer and most excitons are formed in long-wavelength EML. As shown in Fig. 3(b), a blend nano CML is used to control the flow of holes, so that some holes can be regulated in blue EML to yield a little amount of low wavelength emission needed to realizing a high CRI [9]. In contrast, three emitter components based Device IV and Device V resulted a relatively lower CRI from 30 to 35. The lower value of CRI may result due to the lack of low wavelength emission in EL-spectra, as illustrated in Fig. 3(c). The resultant EL-spectrum of Device IV shows a narrow emission peak in yellow wavelength with a weaker orange red emission tail, this explains why Device IV exhibits lowest CRI and SRI value with relatively much higher CT of 2,624 K. On the other side, Device V shows a wide emission peak in yellow and orange wavelength as the EML-1 thickness increases from 2.5 nm to 3.5 nm and EML-2 thickness decreases from 9 nm to 8 nm. A weak shoulder is appeared in the EL-spectrum of Device V at 500 nm, and realized a relatively higher SRI of 77 and CRI of 35 with CT of 2,279 K. A relatively high light quality and low CT may be attributed due to a slight shift of recombination zone towards HTL. However, both efficiency and light quality of Device V can be further increased by introducing a nano CML between the long-wavelength and short-wavelength emissive layers [9]. This additional layer may also play crucial role to realize a much lower CT and high SRI.

 

Fig. 3 Effect of (a) EML thickness and (b) nano CML on the electroluminescence (EL) spectra of PEDOT:PSS based candle light-style OLEDs, and (c) effect of EML thickness on EL-spectra of HAT-CN based OLED devices.

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4. Conclusions

We demonstrated here a high performance candle-light style OLED based lighting source. The resultant device approaches an efficacy of CFLs or fluorescent tubes without the use of any light out-coupling enhancement. The resulting three-emitter component based OLED device shows, at 100 cd m⁻², an efficacy of 85.4 lm W⁻¹ and EQE of 27.4% with a respective SRI of 79 and color temperature of 2,279 K. The low color temperature OLED is free from Hg/flickering/glare/UV/IR, and contains little blue emission. The candlelight style light source may serve as the desired illumination to safeguard human eyes and health, or be used in museums to minimize blue emission-caused discoloring.