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Tunable Full-Color Electroluminescence from All-Organic Optical Upconversion Devices by Near-Infrared Sensing Hiroki Tachibana, Naoya Aizawa, Yu Hidaka, and Takuma Yasuda ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00964 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017
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Tunable Full-Color Electroluminescence from All-Organic Optical Upconversion Devices by Near-Infrared Sensing Hiroki Tachibana,†,‡ Naoya Aizawa,† Yu Hidaka,†,‡ and Takuma Yasuda*,†,‡ †
INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡
ABSTRACT: Full-color all-organic optical upconversion devices that can directly convert incident near-infrared (NIR) light into tunable visible light were developed by integrating an organic light-emitting diode (OLED) on an NIR-sensitizing organic photodetector. Thermally activated delayed fluorescence (TADF) emitters were utilized for the first time in the upconversion devices for achieving high electroluminescence (EL) efficiency in the OLED unit and high overall upconversion efficiency. The emission color of upconversion EL can be varied across the entire visible region ranging from blue to red and white by judicious selection of the incorporating TADF emitters. These all-organic optical upconversion devices have a great potential for lowcost, large-area, pixelless NIR imaging applications. KEYWORDS: upconversion, near infrared, organic light -emitting diode, thermally activated delayed fluorescence, imaging
Near-infrared (NIR) imaging devices have attracted immense research interest owing to their potential applicability in night vision, security, process monitoring, three-dimensional display, and biomedical imaging technologies. 1–5 Currently, III-V compound semiconductor sensors interconnected with Si-based readout integrated circuits are u sed for commercial NIR imaging applications. However, the integration process for inorganic NIR sensing materials such as InGaAs requires complex manufacturing techniques as well as a high cost. An alternative strategy for NIR imaging is the optical upconversion of incoming lower-energy NIR light into higher-energy visible light that can be effectively read out by naked eyes or conventional charge-coupled devices (CCD). Over the last two decades, various types of NIRto-visible upconversion devices based on inorganic, 6–8 hybrid organic/inorganic,9–13 and organic semiconductors14–17 have been developed. Liu and coworkers reported hybrid organic/inorganic upconversion devices by direct tandem integration of an organic light-emitting diode (OLED) and an InGaAs/InP photodetector with sensitivity up to 1.5 m.9,10 Typically, OLEDs consisting of very thin, stacked layers of organic semiconductors sandwiched between an anode and a cathode can be readily constructed by thermal evaporation techniques.18 This makes it possible to avoid the fundamental limit of lattice matching required for the fabrication of inorganic LEDs. Thus, substituting OLEDs for inorganic LEDs can simplify the device fabrication processes . However, the incorporated inorganic photodetectors with multilayer structures still require complex epitaxial-growth manufacturing of inorganic semiconductor materials.
For large-area pixelless NIR imaging applications, all-organic optical upconversion systems combining an OLED and an organic photodetector19 (or organic photovoltaics) into a single device are particularly attractive because of their great advantages such as the diversity of organic semiconductor materials, easy processability, cost effectiveness, and the possibility of producing flexible thin -film devices. Early organic upconversion devices were proposed as photoresponsive OLEDs, 14,15,20,21 in which an NIR-sensitive or visible-sensitive organic material was simply inserted as a charge-generation layer (CGL) in a conventional OLED architecture. However, these devices showed rather low overall upconversion efficiencies, primarily originating from the inherent low internal electroluminescence (EL) quantum efficiencies (typically less than 25%) of fluorescent OLEDs. In this respect, So and coworkers have recently improved the performance of organic upconversion devices 16 by adopting an efficient phosphorescent OLED based on fac-tris(2phenylpyridinato)iridium(III) (Ir(ppy) 3) as a phosphorescent emitter. In OLEDs, electrically injected holes and electrons recombine to generate singlet and triplet excitons with a probability of 25% and 75%, respectively. 22 Thus, phosphorescent OLEDs containing such precious-metal complexes could lead to a high internal EL quantum efficiency up to ~100% by harvesting both singlet and triplet excitons for phosphorescence emission. 23 Recent studies by Adachi and coworkers have paved a promising way for producing high-efficiency precious-metal-free OLEDs by utilizing the mechanism of thermally activated delayed fluorescence (TADF). 24,25 In the TADF mechanism, purely organic emitters with a very small singlet–triplet energy gap
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Figure 1. (a) Schematic cross-sectional view of organic NIR-to-visible upconversion devices based on TADF-OLEDs and chemical structures of the representative organic semiconductor materials used in the devices. (b) Operational principle of the upconversion devices in the dark (turn-off) and under NIR illumination (turn-on). (E ST) enable efficient triplet-to-singlet reverse intersystem crossing (RISC) and delayed fluorescence emission, leading to an internal EL quantum efficiency as high as 100%, by harvesting all electrogenerated excitons. This prompted us to exploit unprecedented NIR-to-visible upconversion systems featuring a high-efficiency TADF emission mechanism. Here, we demonstrate all-organic full-color upconversion devices integrating a TADF-OLED and an NIR-sensitive bulkheterojunction (BHJ) CGL that can directly respond with visible light emission to incoming NIR illumination, without using any interconnected electronics or additional external displays. As shown in Figure 1a, our optical upconversion devices had the following basic configuration: indium-tin-oxide (ITO, 100 nm)/ZnO (30 nm)/CGL (30 nm)/hole-transporting layer (HTL, 35 nm)/TADF emission layer (EML, 15 nm)/electron -transporting layer (ETL, 65 nm)/8-hydroxyquinoline lithium (Liq, 1 nm)/Al (100 nm). The detailed experimental procedures are described in the Methods section. For the OLED part of these upconversion devices, we focused on three representative TADF materials, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN, 1), 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN, 2), and 1,4dicyano-2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)benzene (4CzTPN-Ph, 3), as green, blue, and red emitters, respectively. 24 It is noted that the EML based on 1–3 (6 wt%) doped in a 4,4'bis(carbazol-9-yl)-1,1'-biphenyl (CBP for 1 and 3) or 1,3 bis(carbazol-9-yl)benzene (mCP for 2) host matrix displayed strong green, blue, and red TADF emissions with photoluminescence quantum yields of 85%, 82%, and 37%, respectively, at room temperature (Supporting Information). To complete the OLED part, 4,4'-bis[N-(1-naphthyl)-Nphenylamino]-1,1'-biphenyl (α-NPD) and 1,3,5 -tris(N-
phenylbanzimidazol-2 -yl)benzene (TPBi) were inserted as an adjacent HTL and ETL, respectively. For NIR-sensitive CGL, we employed a BHJ blend (1:1, w/w) of a new isoindigo/diketopyrrolopyrrole-based narrow-bandgap material ING-T-DPP (4) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61BM). A BHJ blend film of 4:PC 61BM showed strong photoabsorption in the visible and NIR region s, extending to approximately 1.0 m (Supporting Information). Figure 1b represents the operational principle of the NIR -tovisible optical upconversion. In order to suppress hole injection from the ITO anode under forward bias, and hence, to keep the device in an off-state in the absence of NIR illumination, a thin ZnO layer, which can function as a hole-blocking layer (HBL), was deposited between the ITO anode and the CGL using the sol– gel process.26 In a dark environment (without NIR illumination), holes are effectively blocked at the anode/HBL interface because of a large injection barrier resulting from a large energy offset between the work function of the ITO anode (5.0 eV) and the valence-band edge of ZnO (7.8 eV) (Figure 1b, left). Under NIR illumination, the CGL can absorb the incoming NIR light and generate holes and electrons, which are successively transported to the respective electrodes along the external applied bias. The photogenerated holes are then injected into the HTL and they recombine with electrons injected from the Al cathode within the EML, thus leading to emission of upconverted visible light (Figure 1b, right). The NIR-sensitive 4 in the CGL has the highest occupied molecular orbital (HOMO) level of −5.4 eV, which exhibits a favorable energy level matching that of α-NPD (−5.5 eV), enabling effective injection and transport of holes across the CGL/HTL interface.
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To examine the pixelless NIR-to-visible imaging capability, we first fabricated and evaluated an upconversion device containing 1 as a green TADF emitter with a large active area of 400 mm 2. In this experiment, a patterned mask with the shape of Mount Fuji was inserted between the NIR light source and the upconversion device. The incoming NIR light from the laser passed through the shadow mask and then reached the top of the device. Figure 2 shows an image of the NIR-to-green upconversion device containing 1 with 810-nm-NIR illumination at a driving voltage of 10 V. The on/off photoresponse from the device can be clearly observed by naked eyes and captured by a digital camera. (A short video, demonstrating NIR -to-green upconversion, can also be found in the Supporting Information.)
Figure 2. A photograph of an NIR-to-green upconversion device containing 1 with 810-nm-NIR illumination through a shadow mask at a driving voltage of 10 V.
It should be noted here that the emission color of the upconverted EL can be tuned over the entire visible range by using different TADF emitters. Figure 3a–f shows the current density–voltage (J–V) and luminance –voltage (L–V) characteristics of the upconversion devices exhibiting green (1), blue (2), and red (3) EL responses. In the dark environment (without NIR illumination), the current density slightly increases with an increasing applied bias voltage; however, it is lower than 10 −3 mA cm −2 even at 10 V. In this case, no light emission could be detected from these devices at applied voltages up to 10 V. These low, monotonically increasing dark currents are mainly attributable to electrons injected from the cathode because the ZnO HBL effectively prevents hole injection from the ITO anode (Figure 1b). This electrical behavior is crucial for efficient optical upconversion as the dark current affects noise in the device. In contrast, upon illumination with 810-nm-NIR light, these devices steeply turned on at voltages of 2.2–2.5 V along with onsets of visible light emission, indicating effective hole injection from the CGL into the OLED part. For these upconversion devices in the turn-on state, the output luminance also gradually increased as the power density of NIR illumination increased up to 100 mW cm −2. This is because the number of photogenerated holes depends on the power density of the incident NIR light. To further demonstrate the facile tunability of the emission color, we fabricated an NIR-to-white upconversion device by adopting two separate EMLs containing 2 and 3 (Figure 3g, h). To this end, a very thin red EML (5 nm) was inserted between the HTL and blue EML to avoid complete exothermic energy transfer from 2 to 3, and thus, to attain dichromatic upconverted EL from both the TADF emitters. The NIR-to-white upconversion device containing 2 and 3 indeed exhibited broad EL spectra covering most of visible region from 400 to 700 nm, originating from their blue and red TADF emissions.
Figure 3. (Top panels) current density–voltage characteristics and (bottom panels) luminance–voltage characteristics as a function of NIR power intensity for the optical upconversion devices with an active area of 4 mm 2. The insets of the bottom panels show the photographs and EL spectra of these upconversion devices measured at 0.1 mA cm −2. Device configurations: ITO/ZnO (30 nm)/4:PC 61BM (1:1, w/w; 30 nm)/α-NPD (35 nm)/EML/TPBi (65 nm)/Liq (1 nm)/Al (100 nm); EML = 1:CBP (6:94, w/w; 15 nm) for the green upconversion device (a,b), mCP (5 nm)/2:mCP (6:94, w/w; 15 nm) for the blue upconversion device (c,d), 3:CBP (6:94, w/w; 15 nm) for the red upconversion device (e,f), and 3:mCBP (4:96, w/w; 5 nm)/2:PPT (6:94, w/w; 10 nm) for the white upconversion device (g,h). Note that for the white upconversion device (g,h) , TPBi is replaced with 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (PPT, 50 nm) as ETL and 3,3'-bis(carbazol-9-yl)-1,1'-biphenyl (mCBP) and PPT are used as hosts in EML.
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Table 1. Performance of the NIR-to-visible upconversion devices based on TADF-OLEDs under NIR illumination a EL emission external EL CGL OLED optical external CIE b peak quantum responsivity power efficiency upconversion (x, y) (nm) efficiency (%) (A W−1) (W A−1) efficiency (%) −3 Green 512 (0.30, 0.57) 12 4.0 × 10 2.6 × 10−1 0.11 1 Blue 480 (0.19, 0.33) 4.6 4.0 × 10−3 1.1 × 10−1 0.044 2 Red 584 (0.54, 0.44) 9.8 3.5 × 10−3 2.0 × 10−1 0.070 3 White 2/3 480, 577 (0.31, 0.36) 6.2 6.3 × 10−3 1.4 × 10−1 0.088 a Device performance was evaluated at a driving voltage of 10 V with an NIR (810 nm) power density of 100, 102, 91, and 71 mW cm −2 for green, blue, red, and white upconversion devices, respectively. bCommission Internationale de l'Éclairage (CIE) color coordinates. device
emitter
The performance of these full-color upconversion devices are summarized in Table 1. At a driving voltage of 10 V, the photodetection responsivities of the devices were measured to be 3.5 × 10 −3 to 6.3 × 10 −3 A W −1 (see the Supporting Information for details). Therefore, the overall external upconversion efficiency (W/W), i.e., the conversion of the irradiated NIR-light power to emitted visible -light power, can be calculated by multiplying the responsivity of the CGL (A W−1) by the optical power efficiency of the OLED (W A−1). As listed in Table 1, the NIR-to-green upconversion device containing 1 showed among the highest external upconversion efficiency of 0.11%. At the NIR power density of 100 mW cm −2, the luminance of this device reached over 150 cd m −2, resulting in a high external EL quantum efficiency of 12%. This value is much higher than the theoretical limit of 5% for conventional fluorescent OLEDs, indicating that both electro-generated singlet and triplet excitons indeed contribute to efficient upconverted EL emission. Although the external upconversion efficiencies are still not sufficiently high compared to state-of-the-art phosphorescent Ir(ppy)3 -based upconversion devices, 17 the overall performance is primarily limited by the low responsivity of the CGL. The absorption coefficient of the current CGL (a BHJ blend film of 4:PC 61BM) at 810 nm is 5 × 10 4 cm −1; therefore, the penetration depth of the incident NIR light is estimated to be approximately 200 nm. This implies that the 30-nm-thick forefront CGL cannot fully absorb the incoming NIR photons. Therefore, we can expect that there is still great potential for further improvement of the responsivity of the CGL as well as the overall upconversion efficiency by utilizing organic NIR photosensitizers with much higher photoabsorptivity in this region. In summary, we demonstrated novel all-organic upconversion devices with NIR sensitivity up to 810 nm by integrating TADFOLEDs with an organic BHJ CGL. These devices can upconvert NIR light into tunable full-color visible emissions including white, under NIR illumination, by selecting appropriate TADF emitters. Unlike conventional fluorescent OLEDs, the TADF-OLEDs that are capable of harvesting both electrogenerated singlet and triplet excitons for EL emission demonstrated a great potential for application in optical upconversion systems. We believe that these results can offer a promising way to further develop highefficiency, all-organic full-color upconversion devices for low cost, large-area, pixelless NIR imaging applications.
METHODS Materials. TADF emitters 1–3 were prepared according to the procedures reported in the literature, 24 and were purified before use, by temperature-gradient sublimation under vacuum. Other OLED materials were purchased from e -Ray Optoelectronics Technology Co., Ltd. and were used without further purification. PC 61BM was purchased from Frontier Carbon Corporation. The synthetic procedures and characterization data for 4 are described in the Supporting Information.
Device Fabrication and Measurements. ITO-coated glass substrates were cleaned by sonicating them in detergent solution, deionized water, acetone, and isopropanol for 10 min each, and then by subjecting them to UV/ozone treatment for 30 min. A thin layer (ca. 30 nm) of ZnO was deposited onto t he substrate by spin-coating from a precursor solution of zinc acetate (1.00 g) and PEIE (polyethyleneimine 80% ethoxylated aqueous solution; 0.12 g) in 2-methoxyethanol (10 mL), followed by baking at 200 °C for 10 min in ambient air. A charge-generation layer (CGL) was then deposited by spin-coating from a chloroform solution of 4 and PC 61BM (1:1, w/w; 6.0 mg mL −1). The remaining organic layers were thermally evaporated on the CGL under vacuum (
