Blue-Hazard-Free Organic Light-Emitting Diode with a Lifetime Greater than 200,000 h

Abstract

Blue-hazard-free lighting is in urgent need in order to protect human eyes and physiology. A candlelight-style organic light emitting diode (OLED) is so far the safest measure for its blue-hazard-free, low-color-temperature illumination. However, lifetime is still the most critical reliability issue in OLEDs, especially in lighting applications that are closely associated with high current density operation. Here, we present a novel approach to develop blue-hazard-free OLEDs with a lifetime exceeding 200,000 h at 1000 cd/m² by employing a tandem structure with ultra-high purity emitter. The resultant device shows a current efficacy of 37.4 cd/A, an external quantum efficiency (EQE) of 22.7%, an 80 CRI, and a 78 SRI with a 223,500 h lifetime (t₅₀). Additionally, the resultant device also exhibits a long retina exposure duration of 12,300 s (3.42 h) and suppresses very less melatonin generation (1.7%), demonstrating that the fabricated OLED device is a highly prospective general lighting source to safeguard human health.

1. Introduction

1.1. Applications and Disruptive Advantages of OLED

OLEDs have attracted enormous attention due to their disruptive features, such as being energy-saving [1], being environmentally [2], physiologically, and psychologically friendly [3], color-temperature tunability [4], dimmability [5], thin and light weight [6], compatibility for roll-to-roll production [7], and flexibility [8] with unlimited design possibilities, etc. Their inherent diffused emission with continuous and smooth spectrum provides visual comfort [9]. Moreover, some other superlative characteristics, e.g., full color [9], high color contrast [10], wide viewing angle [11], and very-high color rendering index (CRI) or spectrum resemblance index (SRI) [9], etc., make OLEDs superior components for the fabrication of high-quality displays for TVs and smartphones and panels for solid-state lighting. Furthermore, OLEDs enable the design of cold white and warm white as well as blue-hazard-free display and lighting luminaires.

1.2. Challenges of OLEDs for Display and Lighting

In general, high efficiency, low cost, and long lifetime are the three basic requirements for any display or lighting device to meet consumer’s desire. Although noteworthy advancements in developing high efficiency devices have already been accomplished, OLEDs possess some intrinsic shortcomings which affect their operational lifetime. Developing devices with a considerably long lifetime would hence play a significant role for OLEDs to disrupt the existing lighting and display markets.

1.3. Blue Hazards

Besides lifetime, one also needs to consider public health issues from modern lighting. Numerous medical reports showed health hazards to be caused by blue light-enriched white light, which can harm the human retina and can cause blindness [12,13]. It was also reported that blue light or intense white light causes disruption of circadian rhythm [14], sleeping disorders [15], and breast cancer [16] resulting from the suppression of melatonin, an oncostatic hormone. The International Dark Sky Association reported that light at night can cause light pollution and interrupt the life sustaining behaviors of nocturnal animals, including amphibians, insects, and birds, causing disruption of ecosystems [17]. Utilization of blue-hazard-free light sources can significantly minimize such health and environmental issues. In response to the need for such human and eco-friendly light, the development of low color temperature (CT) light sources is crucial.

1.4. Prior Studies in Blue-Hazard-Free OLEDs

Despite the urgent need, only a few lighting devices reported are healthy, i.e., of low color temperature. In 2011, Jou et al. reported a white OLED with a CT of 1880 K and an efficacy of 36 lm/W [18]. In 2012, they reported a physiologically friendly, single emissive layer dry-processed OLED with a CT of 1773 K and an efficacy of 11.9 lm/W [3]. The device showed a much lower CT as compared with the incandescent bulb with a CT of 2300 K; however, it was not energy-efficient. In 2013, Jou et al. reported a dry-processed OLED device with a CT of 1970 K and a power efficacy of 24 lm/W [19]. In 2014, Hu et al. reported a hybrid OLED with an interlayer using dry-process, which showed a 54.6 lm/W power efficacy and a 1910 K CT [20]. In 2016, Li et al. reported a white OLED with a CT of 2200 K and an efficacy of 42 lm/W [21]. In 2017, Sun’s group developed a hybrid white OLED with 1945 K and a maximum power efficacy of 20.6 lm/W [22]. So far, nearly all approaches have been used to develop high-efficiency, low-CT blue-hazard-free OLED devices. However, none have yet reported the lifetime of low-color-temperature OLED devices.

1.5. Prior Studies in Long Lifetime OLEDs

Due to some inevitable limitations, it is very difficult to achieve a long lifetime in OLED devices. However, a few research groups and industries have managed OLED devices with considerably high operational lifetime. In 2013, Panasonic developed OLED panels with over 100,000 h in a lifetime (LT₅₀) at 1000 cd/m² [23]. In 2014, Liu’s group obtained an extremely long lifetime of 30,920 h at 1000 cd/m² [24]. Acuity brands had developed OLED lighting panels with a variety of CT including 3000 K, 3500 K, and 4000 K with lifetime (LT₇₀) of 54,000–72,000 h at 2000 cd/m² [25]. Recently, LG Chemical reported an extremely thin (0.88 mm) OLED lighting panel with a lifetime (LT₇₀) of 40,000 h at 3000 cd/m² [26]. OLEDWorks announced a commercially available OLED lighting panel with a lifetime (LT₇₀) of 10,000 h at 300 cd/m² [27]. In 2017, Yeolight Technology reported a new Amber OLED panel [28]. The panel’s color temperature is 2000–2600 K which exhibits a lifetime of over 20,000 h at 2000 cd/m². Although many OLED devices with high lifetime had been reported, most of them were enriched with health-hazardous blue light emission.

1.6. Summary of Present Study

We demonstrate here a novel approach to develop blue-hazard-free OLEDs with a lifetime more than 200,000 h at 1000 cd/m² by employing a tandem structure with ultra-high purity emitters. For comparison, we have changed the purity of a green emitter from 99.9% (3N) to 99.99% (4N). The resultant device shows a maximum permissible exposure limit (MPE) of 11,025 s and melatonin suppression sensitivity (MSS) of only 1.8% of that of the reference blue light (480 nm). The resultant device shows an efficacy of 7.5 lm/W, a current efficacy of 37.4 cd/A, an external quantum efficiency (EQE) of 22.7%, and an 80 CRI and a 78 SRI. For comparison, we have considered electroluminescent properties and lifetime of a typical blue hazard free OLED.

2. Theoretical

2.1. Melatonin Suppression Sensitivity (MSS)

Melatonin suppression sensitivity of any given light source can be calculated by using the action spectrum of melatonin suppression per photon quanta, S_PQ, which was first proposed by Jou’s group [29]. The resultant formula can be expressed as follows.

$${\mathrm{S}}_{\mathrm{PQ}}\left(\mathsf{\lambda }\right)={10}^{\frac{\left({\lambda }_{\mathrm{r}}-\mathsf{\lambda }\right)}{\mathrm{C}}}$$

where S_{PQ (λ)} is defined as the suppression power per quanta of a monochromatic light, λ, relative to that of the reference light, λ_r, and C is a fitting constant. The chosen reference light is 480 nm blue light in this study.

To make practically meaningful, the above formula is converted into lux, with a unit of lx, which is shown below.

$${\mathrm{S}}_{\mathrm{L}}\left(\mathsf{\lambda }\right)=\frac{{{\displaystyle \int }}^{\text{}}{\mathsf{\lambda }\mathrm{S}}_{\mathrm{PQ}}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }}{\mathrm{V}\left(\mathsf{\lambda }\right)}$$

where S_L (λ) is the action spectrum of melatonin suppression sensitivity per lux, and V(λ) is the photopic luminosity function.

For polychromatic light sources, the correlated suppression power per lux, S_LC(λ), can be expressed as follows.

$${\mathrm{S}}_{\mathrm{LC}}\left(\mathsf{\lambda }\right)=\frac{{{\displaystyle \int }}^{\text{}}{\mathsf{\lambda }\mathrm{S}}_{\mathrm{PQ}}\left(\mathsf{\lambda }\right){\mathrm{S}}_{\mathrm{I}}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }}{{{\displaystyle \int }}^{\text{}}\mathrm{V}\left(\mathsf{\lambda }\right){\mathrm{S}}_{\mathrm{I}}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }}$$

where S_I (λ) is measured spectrum of a given light source.

2.2. Maximum Permissible Retina Exposure Limit (MPE)

According to the International Commission on Non-Radiation Protection Council (ICNIRP), the maximum permissible retinal exposure limit (MPE) with a unit of second (s) can be calculated as follows [30]:

$$\mathrm{t}=\frac{100}{{\mathrm{E}}_{\mathrm{B}}}$$

where E_B is blue light weighted radiation, with a unit of Wm⁻², and the value can be obtained by the following formula:

$${\mathrm{E}}_{\mathrm{B}}={\displaystyle \sum }_{300}^{700}{\mathrm{E}}_{\mathsf{\lambda }}·\mathrm{B}\left(\mathsf{\lambda }\right)$$

where E_λ represents the luminance (cd/m²); B(λ) represents the blue light hazard function (Wm⁻²sr⁻¹); and λ represents the light wavelength (nm).

2.3. Spectrum Resemblance Index (SRI)

The light quality of a given light source can be quantified in terms of the natural light spectrum resemblance index, SRI, which was first presented by Jou’s group [31]. The SRI index provides a direct comparison of the luminance spectrum of a given light source with its blackbody-radiation counterpart at the same correlated color temperature and luminance, which can be defined as follows.

$$\mathrm{SRI}=\frac{{{\displaystyle \int }}^{\text{}}\mathrm{L}\left(\mathsf{\lambda },\mathrm{T}\right)\mathrm{d}\mathsf{\lambda }}{{{\displaystyle \int }}^{\text{}}{\mathrm{L}}_{\mathrm{BR}}\left(\mathsf{\lambda },\mathrm{T}\right)\mathrm{d}\mathsf{\lambda }}$$

where L_BR($\mathsf{\lambda }$,T) is the luminance spectrum of the blackbody-radiation. L($\mathsf{\lambda }$,T) is the overlapping area between the luminance spectra of a studied light and blackbody-radiation. Moreover, the value of the SRI can be ranged from 0 to 100 for any given lighting source, meaning a total dissimilarity and similarity, respectively.

2.4. Color Rendering Index (CRI)

Color rendering index is utilized to determine light quality which indicates how accurate a given light source is at rendering color when compared to a reference light source. The CRI can be calculated via the following steps.

1. Measure the chromaticity coordinates of the given light source on the CIE-1960 color space.

2. From the color coordinates, find the closest point on the blackbody’s radiation path to determine its correlated color temperature (CCT).

3. Selection of reference light source: If the correlated color temperature is less than 5000 K, then assume blackbody-radiation as reference light source. If the correlated color temperature is greater than 5000 K, then CIE standard light source (daylight) can act as a reference light source.

4. To irradiate the standard test pieces: The first eight specimens of the reference light source and the given light source were irradiated to find the light color coordinates of the test pieces on CIE-1960.

5. Calculate CRI: Calculate the average light color difference according to the eight light color coordinates measured above. If eight light color coordinates reflected from the given light source are reflected with the eight lights from the reference light source, the light color coordinates are the same, and the average light color difference is zero, then the CRI would be 100.

2.5. Lifetime

The lifetime (t_1/2) of OLEDs is the time required for the brightness to decay to half of its initial value [32], which can be calculated as below:

L_oⁿ × t_½ = constant

where L_o represents the initial brightness; t_½ is the time for brightness to decay to half of its initial value; and n is the accelerating factor.

In the aspect of lifetime measurement, we usually adopt an accelerating experiment for time saving. From this experiment, we can obtain a brightness decay curve of the device, which combines an initial decay and normal decay. The resultant decay curve can be expressed as follows.

Total decay = initial decay + normal decay

3. Experimental Work

3.1. Device structures and Fabrication of Typical and Tandem OLEDs

Figure 1 shows schematic illustrations of the blue-hazard-free OLEDs with typical and tandem structures. Figure 2 shows energy level diagram of typical tandem structure. The typical device is composed of a 150 nm indium tin oxide (ITO) layer, a 30 nm WHI-001 hole-injection layer (HIL), a 70 nm WHT-215 hole transporting layer (HTL), a 30 nm emissive layer (EML), a 10 nm WET-603 electron transporting layer (ETL), a 20 nm WET-603:WDN-651 N-doped ETL, a 1 nm lithium fluoride (LiF) layer, and a 150 nm aluminum layer. The emissive layer consists of a WPH-401 host with a 33 wt% WPH-501 co-host doped with a 10 wt% WPGD-832 green emitter and 0.8 wt% PER 53 red emitter. While the tandem device is composed of two emitting units which are connected by charge generation layers, each emitting unit contains a 70 nm WHT-215 HTL, a 30 nm EML, and a 10 nm WET-603 ETL, and the charge generation layers are composed of a 20 nm N-doped ETL, a 2 nm WDN-651 EIL, a 1.5 nm aluminum layer, and a 30 nm WHI-001 HIL. All of the organic and metal layers were deposited under base pressure of 10⁻⁶ Torr onto pre-cleaned and oxygen plasma pre-treated ITO glass by using thermal evaporation.

3.2. Encapsulation

After all the layers were deposited, the device was encapsulated to prevent moisture and oxygen from deteriorating the sensitive organic layers. The encapsulation was performed by generating a continuous epoxy lining along the periphery of the emissive area, and then a glass was capped lid on. The whole process was done under low moisture and oxygen condition. The encapsulated device was thereafter treated with UV-light for curing.

3.3. Device Characterization

The current-voltage (I–V) characteristics of the resultant devices were measured using a Keithley 2400 electrometer with Minolta CS-100A luminance-meter. The spectrum, luminance, and CIE chromatic coordinates of all the devices were measured by using a Photo Research PR-655 spectrascan spectroradiometer. All the fabricated devices have an emission area of 9 mm². All the measurements were done in the forward direction.

3.4. Lifetime Measurement

From the CS-100A, applied current for desired initial luminance is found. Then, the device is kept in the base of a Chroma lifetime test system where the current is set to attain the desired initial brightness. Then, the device is allowed to run under the constant current where the brightness of the device will decrease gradually. The t₅₀ lifetime is measured when the brightness becomes half of its initial value.

4. Results and Discussion

4.1. Device Characteristics

4.1.1. Typical vs. Tandem OLED

The corresponding electroluminescence performances of both the blue-hazard-free OLED devices are shown in

Table 1. Effect of device architecture on the operation voltage (OV), power efficacy (PE), current efficacy (CE), external quantum efficiency (EQE), CIE color coordinates, maximum luminance, and lifetime of the studied blue hazard free OLEDs.

Device Architecture	Doping Concentration (wt%)	OV [V]	PE [lm/W]	CE [cd/A]	EQE [%]	CIE (x, y) Chromatic Coordinates	Max Luminance (cd/m2)	Lifetime (h) @ 1000 cd/m2
		@ 100/1000/10,000 cd/m2
Typical	1 wt%	2.6/ 3.2/ 4.9	26.1/ 20.5/ 9.6	21.6/ 20.7/ 14.9	14.7/ 14.9/ 10.5	(0.60, 0.38)/ (0.62, 0.38)/ (0.61, 0.38)/	61,000	10,000
Tandem	1 wt%	6.3/ 7.9/ 11.3	22.3/ 17.0/ 9.3	44.6/ 42.8/ 33.5	28.9/ 28.6/ 22.4	(0.61, 0.39)/ (0.61, 0.38)/ (0.61, 0.38)/	72,500	23,382

. Figure 3 shows the comparison of luminance, current density, power efficacy, and current efficacy of the double stacked tandem OLED with that of a typical single electroluminescent unit OLED. Remarkably, the fabricated tandem OLED displayed higher brightness and driving voltage than that of the typical OLED counterpart. For example, maximum brightness of the designed tandem OLED is 1.2 time higher than the typical OLED. Similarly, the driving voltage of double-stack OLED at 100, 1000, and 10,000 cd/m² is 2.4, 2.4, and 2.3 times higher than that of the single-stack OLED. The reason behind this may be that the much thicker device architecture and designed tandem OLEDs are formed by connecting two single unit OLEDs in series, so the driving voltage and luminance of tandem OLEDs should be equal to the sum of that of individual single unit OLEDs under a certain voltage, as expected in most ideal situation. However, the observed results deviate from the ideal situation, which may be attributed to the produced extra resistance in the designed device structure due to the inefficient electron injection and transportation of in the charge generation layer under bias voltage.

It is also interesting to note that the current efficacy of tandem OLED is more than double in extent than that of the typical device at varying luminance from 100 to 10,000 cd/m². For example, at 100 cd/m², the tandem OLED exhibited a current efficacy of 44.6 cd/A, while typical device showed 21.6 cd/A. However, the power efficacy of tandem OLED is slightly lower than the typical device. For example, at 100 cd/m², the tandem OLED exhibited a power efficacy of 22.3 lm/W; however, 26 lm/W is observed from the typical OLED. Three major reasons may contribute to this. First, tandem device structure reduced the plasmon quenching effect from electrodes because it occurs only at the nearby electrode since the other electrode is too far away. Firstly, the plasmon quenching effect from electrodes is weakened by two single units as plasmon quenching occurs only at the nearby electrode since the other electrode is too far away. Secondly, a high-quality charge carrier unit exists between both the light emitting unit offering equal amounts of holes and electrons under forward bias to improve balance by preventing an excess of charges that would be lost during OLED operation. Hence, the overall carrier recombination balance can be realized leading to higher CE and EQE. Thirdly, the electric-field-induced quenching effect, remarkable in phosphorescent OLEDs, can be restrained. Triplet-triplet annihilation (TTA), triplet-polaron quenching (TPQ), and field-induced quenching are possible quenching processes commonly in PhOLEDs. In our designed OLED with UEML, although the emitting layer is very thin, the relatively high roll-off at the high current density could be TTA due to the short distances among the emitting molecules in the non-doped layer.

It may be because of interface resistance between the stacked organic layers in the tandem architecture. Additionally, the tandem device showed higher driving voltages as compared to the typical OLED because of its much thicker tandem architecture.

The electroluminescent spectra of both the OLED devices are nearly similar with a major peaking at 610 nm, as shown in Figure 4.

The tandem OLED showed a current efficacy of 33.5 cd/A and an EQE of 22.4% at 10,000 cd/m² with a maximum luminance of 72,500 cd/m². It may be because of more balanced charge carrier injection to the stacked electroluminescent units than those in the typical device.

4.1.2. Doping Concentration Effect

Further, the effect of doping concentration of the red emitter has been studied in the tandem OLED, as shown in

Table 2. Effect of doping concentration, emissive layer thickness and purity of green dye on the OV, PE, CE, EQE, CIE coordinates, CRI, SRI, maximum permissible exposure-limit (MPE), melatonin suppression sensitivity (MSS), and lifetime (t₅₀) of the blue-hazard-free tandem OLEDs studied.

Doping Concentration (wt%)	Emissive Layer Thickness (nm)	Purity of Green Dye	OV [V]	PE [lm/W]	CE [cd/A]	EQE [%]	CIE (x, y) Chromatic Coordinates	CRI	SRI	MPE (s)	MSS (%)	Lifetime (h) @ 1000 cd/m2
			@ 100/1000/10,000 cd/m2
1	25	3N	6.3/ 7.9/ 11.3	22.3/ 17.0/ 9.3	44.6/ 42.8/ 33.5	28.9/ 28.6/ 22.4	(0.61, 0.39)/ (0.61, 0.38)/ (0.61, 0.39)/	85/ 84/ 85	76/ 75/ 77	14,969/ 15,403/ 14,862	1.6/ 1.5/ 1.5	23,382
0.8	25	3N	5.6/ 7.2/ 10.5	21.3/ 16.7/ 8.8	37.6/ 38.1/ 29.5	22.0/ 22.7/ 17.6	(0.58, 0.40)/ (0.59, 0.40)/ (0.58, 0.40)/	75/ 75/ 76	77/ 76/ 77	8,783/ 9,595/ 8,817	2.1/ 2.0/ 2.1	16,037
0.8	30	3N	9.4/ 14.3/ 20.5	11.8/ 7.5/ 3.7	35.7/ 34.0/ 23.9	21.2/ 20.9/ 14.8	(0.59, 0.40)/ (0.59, 0.40)/ (0.58, 0.41)/	78/ 77/ 76	77/ 78/ 79	10,802/ 10,106/ 7,685	1.9/ 2.0/ 2.3	223,500
0.8	30	4N	8.5/ 15.6/ 24.7	16.8/ 7.5/ 3.2	45.5/ 37.4/ 24.8	27.6/ 22.7/ 14.9	(0.60, 0.39)/ (0.59, 0.40)/ (0.57, 0.41)/	80/ 80/ 80	77/ 78/ 78	12,306/ 11,025/ 10,708	1.7/ 1.8/ 1.8	>223,500

. On decreasing the doping concentration of the red dopant from 1 wt.% to 0.8 wt.%, the resulting device showed slightly decreased efficiencies. For example, at a luminance of 100 cd/m², the resultant device exhibited a power efficacy of 21.3 lm/W, a current efficacy of 37.6 cd/A, and an EQE of 22%. At high brightness, for example, 10,000 cd/m², the device exhibited an 8.8 lm/W power efficacy, a 29.5 cd/A current efficacy, and a 17.6% EQE. Additionally, the tandem OLED device with 0.8 wt% red emitter exhibited a slight rise in color temperature from 1500 K to 1600 K with a CIE (0.58, 0.40). It should be noted that the light quality of the device still lies in high light-quality range, i.e., CRI and SRI > 75.

In addition to light quality, emission spectrum of the device has also been characterized in terms of MPE and MSS to ensure the human-eye and -physiology friendliness of the emission. The respective data of both the devices are shown in Table 2. There is no such noticeable variation in MSS values, and both devices suppress 2% melatonin secretion with respect to the reference blue light of 480 nm. It can be explained by considering the electroluminescent (EL) spectra of both the OLED devices, as shown in Figure 5.

As can be seen in Figure 5, the device with a 0.8 wt.% red emitter exhibited a slightly lower intensity emission in long wavelength side (620–720 nm) as compared to the device with 1 wt.% counterpart. Additionally, a significant difference in the MPE values has been observed. At an illuminance of 100 lx, the device with 1 wt.% red emitter showed a 15,403 s (4.3 h) MPE which further decreases to 9595 s (2.6 h) upon decreasing the doping concentration to 0.8 wt.%. It shows that the fabricated blue-hazard-free tandem OLED devices exhibited human eye-friendly lighting which can be used for continuous source of light from 2–4 h without any damage to retina cells.

4.1.3. Emissive Layer Thickness Effect

Further, the initial thickness of the emissive layers was increased from 25 nm to 30 nm in the tandem architecture with 0.8 wt.% red dopant. The device performance decreased slightly in terms of power efficacy, current efficacy, and EQE, as shown in Table 2. However, color coordinates, light-quality, i.e., CRI and SRI >75, MPE, and MSS values remain unchanged. It shows the stability of emission spectrum of the fabricated devices.

4.1.4. OLED Material Purity Effect

Organic electronic devices are highly sensitive to the purity of the organic materials used. Hence, we have studied the effect of purity level of an emitter on the device characteristics, as shown in Figure 6.

The higher-purity, emitter-based OLED showed a slightly lower operation voltage. The tandem OLED with a green dye of 4N purity exhibited higher efficiencies than those of the 3N purity counterpart. For example, at 100 cd/m², a power efficacy of 16.8 lm/W, a current efficacy of 45.4 cd/A, and an EQE of 27.6% are observed in the 4N purity-based tandem OLED. On the other hand, the 3N purity-based device exhibited a power efficacy of 11.8 lm/W, a current efficacy of 35.7 cd/A, and an EQE of 21.2%. As shown in Table 2, the tandem OLED with the 4N purity green emitter showed a CRI of 80 and an SRI of 78 at 1000 cd/m². Regardless, the emitter purity of both blue-hazard-free OLED devices suppresses melatonin secretion by around 1.7 ± 0.2% which is much lower than that of the high-color-temperature, cold-white lighting sources. In other words, the cold-white compact fluorescent bulb (CT 5921 K, MSS 29%), cold-white light emitting diode (CT 5501 K, MSS 20%), and cold-white OLED (CT 5000 K, MSS 12%) are 16.1-, 10.8-, and 6.1-fold more hazardous to the generation of melatonin than the two fabricated blue-hazard-free OLED counterparts.

4.2. Device Lifetime

The device operational stability in terms of normalized luminance against operative time of the fabricated OLEDs is shown in Figure 7. A typical single stack OLED device exhibited a lifetime of 10,000 h under the application of a constant current with an initial luminance of 10,000 cd/m². On the other hand, the fabricated double stacked tandem OLED showed a lifetime of 23,382 h. It is more than double in extent with that of the typical OLED at 50% decay of the initial luminance, as shown in Figure 7a. A tandem OLED with a reduced doping concentration of red emitter exhibited a lifetime of 16,037 h. As shown in Figure 7b, the plot of the 70% decay of the initial luminance of 10,000 cd/m² is extrapolated up to 50% of its decay. Further, by increasing the thickness of the emissive layer from 25 to 30 nm, the resulting OLED showed a drastic change in the device lifetime, as can be seen in Figure 7c. By extrapolating the plot of lifetime up to 50% decrement in the initial brightness of 10,000 cd/m², the OLED device exhibited a long lifetime of 223,500 h. A slightly thick emissive layer structure facilitates the carrier recombination at high current density without degradation in the organic layers and results in substantial improvement in the lifetime of the device. The purity of organic materials is crucial for the device stability. It controls self-heating of organic films caused by joule heating at high luminance and consequently inhibits the degradation mechanisms inside the device. As shown in Figure 7d, the device with the high purity 4N emitter showed a sharp fall in lifetime until the 95% decrement of initial luminance of 10,000 cd/m² which later decreases steadily. However, the device with 3N purity exhibited a sharp fall until 80% decay of initial luminance, and the luminance further decreases unsteadily till 70% of its decay, representing accelerated degradation mechanisms at high luminance. The fabricated tandem OLED with 4N purity emitter showed a lifetime (t₅₀) longer than 223,500 h, which is highest among all the reported long lifetime tandem OLEDs to date.

5. Conclusions

In conclusion, we have fabricated a long-lifetime, blue-hazard-free OLED by using tandem device architecture. The resultant OLED exhibited a long operation time, i.e., more than 223,500 h at 1000 cd/m². The high-purity, organic emitter provides enough stability to achieve a long device lifetime. Notably, the OLED device showed a color temperature (1600 K) much lower than that of candles (1800 K) and also exhibited high light-quality emission with a CRI of 80 and an SRI of 78. This high light-quality, blue-hazard-free OLED can be recommended as the safest lighting measure for indoor lighting after dusk. At an applied illuminance of 100 lx, the general brightness for indoor lighting, the resultant device showed a longer eye exposure duration of 12,306 s without any damage to retina cells. Additionally, it suppresses only 1.7% of melatonin secretion, lower than that of candlelight. The fabricated OLED fulfills all the needed criteria of human-friendly lighting for commercialization with a sufficiently long lifetime.