Organic Upconversion Display with an over 100% Photon-to-photon

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Organic Upconversion Display with an over 100% Photon-to-photon Upconversion Efficiency and a Simple Pixelless Device Structure Qiaogang Song, Tong Lin, Zisheng Su, Bei Chu, Huishan Yang, Wenlian Li, and Chun-Sing Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02738 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Journal of Physical Chemistry Letters

Organic Upconversion Display with an over 100% Photon-to-photon Upconversion Efficiency and a Simple Pixelless Device Structure Qiaogang Songa,b,c, Tong Linb,c, Zisheng Sua,b*, Bei Chub, Huishan Yanga*, Wenlian Lib, Chun-Sing Leed*

a College

of Physics and Information Engineering, Key Laboratory of Information

Functional Material for Fujian Higher Education, Quanzhou Normal University, Quanzhou 362000, P. R. China b

State Key Laboratory of Luminescence and Applications, Changchun Institute of

Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P.R. China c University

of Chinese Academy of Sciences, Beijing 100049, P. R. China

ACS Paragon Plus Environment

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d

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Center of Super-Diamond and Advanced Films (COSDAF) and Department of

Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Zisheng Su); [email protected] (HuishanYang); [email protected] (Chun-Sing Lee)

ABSTRACT Comparing to traditional near infrared (NIR) imaging devices, NIR-to-visible upconversion display which integrated a NIR photodetector with a visible light-emitting diode have merits of simple device structure, low cost, high resolution, and a simple pixelless structure. However, photon-to-photon upconversion efficiencies of these devices are typically much lower than unity. Here we report an all-organic NIR-to-visible upconversion display with a photon-to-photon upconversion efficiency higher than 100% by integrating a photomultiplying organic NIR photodetector with a high-efficiency thermally activated delayed fluorescent organic light-emitting-diode. To the best of our


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knowledge, this is the first report showing a photon-to-photon upconversion efficiency over 100% without using a built-in transistor for current amplification.

TOC GRAPHICS

Near infrared (NIR)-to-visible upconversion devices have important applications in night vision, range findings, biomedical imaging, simiconductor wafer inspections, security, and space science.1 Currently, most NIR-upconverters consist of a photosensitization


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unit directly integrated with a light-emission unit in tandem to allow NIR to visible light conversion without additional component. The photosensitization unit can either be a phototransistor2-4 or a photodetector.5,6 Comparing with photodetectors, phototransistors are inherently advantageous for realizing a photocurrent gain because of their intrinsic current amplification characteristics.7,8 Nevertheless, there is so far only one report on a NIR-to-visible device with photon-to-photon upconversion gain.9 A high photon-tophoton upconversion efficiency (determined by the ratio between the numbers of emitted visible photons and illuminated NIR photons, ηphoton) over 1000% was achieved by stacking a PbS nanocrystal-based vertical phototransistor on a phosphorescent organic light-emitting-diode (OLED). As an alternative, replacing the phototransistor with a photodetector can simplify the device structure and lower the cost. However, this will also remove the transistor function of current amplification and thus lower the upconversion efficiency of the device.

Early

reports

of

such

“photodetector/light-emitting-diode

(LED)”

type

NIR-

upconverters typically compose of a Ⅲ - Ⅴ inorganic semiconductor photodetector


4

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integrated with an LED.10 These inorganic NIR-upconverters are generally prepared via costly epitaxial growth and have to deal with difficulties stem from lattice mismatch especially at the photodetector/LED interface. These issues can be partially addressed by replacing the inorganic LED with an OLED.11-13 On the other hand, image quality and contrast of either the fully inorganic or the inorganic/organic hybrid NIR-upconverters are limited due to lateral current spreading caused by the inorganic semiconductors’ high conductivities. A possible alternative is to adopt an all-organic architecture with an OLED stacked directly on an organic photodetector.14-23 Organic materials are inherently abundant, low-cost, flexible, and suitable for realizing large-area and wearable photoelectric devices.24-26 Such all-organic NIR-upconverters could potentially address the mentioned issues and their emission colors can be easily tuned via using different organic emitters. Thus, all-organic NIR-upconverters are considered to have good potential as an alternative competitor.

All-organic NIR-upconverters were first reported based on fluorescent OLEDs.14,15 This type of devices can only use the singlet excitons, leading to a low internal quantum


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efficiency (IQE) of less than 25%. With this limit, early reported all organic NIRupconverters have an overall ηphoton less than 0.1%.14 This can be significantly improved by incorporation phosphorescent OLEDs to harvest both the singlet and the triplet excitons.16-20 Using such a strategy, Kim et al. obtained a ηphoton of 2.7% at 15 V16 and the ηphoton can be further increased to 11.3% at 12 V via improving the device structure17, while Liu et al. achieved a power-to-power upconversion efficiency (determined by the ratio between the powers of emitted visible light and illuminated NIR light, ηpower) exceeding 6% at 7 V.18 Recently, Yang et al. used tandem phosphorescent OLED as the light-emission unit and achieved a ηphoton up to 29.6%.27 In 2012, Adachi and coworkers have demonstrated a promising mechanism to use non-radiative triplet excitons for light emission by efficient reverse intersystem crossing via thermal activation.28,29 The thermally activated delayed fluorescence (TADF)-OLEDs can achieve the same IQE as phosphorescent OLEDs without using precious metal complexes. As a result, an emission external quantum efficiency (determined by the ratio between the numbers of emitted photons and injected electrons, EQEem) of 20% has been demonstrated in these TADF-OLEDs.30-33 Tachibana et al. firstly utilized


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TADF-OLEDs with EQEem higher than 10% in NIR-upconverters, but the highest ηpower was only about 0.1%, which was attributed to the lower efficiency of the photodetector unit.21 This suggests that both the photosensitization and emission units are important in determining the efficiency of the NIR-upconverters. Up to now the highest EQEem of an OLED is below 70% even with a light out-coupling structure.34 Thus, a photomultiplying detector is required to realize an upconversion gain for the NIR-upconverter. However, high performance organic NIR-photodetectors are still rare due to the lack of high quality NIR absorption materials.35-39 In addition, the response wavelengths of the stateof-the-art organic NIR-upconverters are located mostly shorter than 800 nm, limiting their practical applications.


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Figure 1. (a) Molecular structure of the organic materials. (b) Device structure of the NIR-upconversion device. (c) A schematic energy level diagram of the TADF-OLED.

In this work, a photomultiplying organic NIR photodetector is first demonstrated based on a lead phthalocyanine (PbPc)/C60 planar heterojunction and it is further integrated with a high efficiency TADF-OLED to form a NIR-upconversion display. To minimize energy loss due to self-absorption, materials used in the OLED and the organic photodetector are carefully chosen such that the detector has minimal optical absorption over the emission spectrum of the OLED. Materials and device structures of the all-


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organic upconverter and its constituting TADF-OLED are shown in Figure 1. Here, an inverted structure is used for the photosensitization unit with a pristine C60 layer deposited directly on the indium tin oxide (ITO) substrate. Comparing with the bulk heterojunction photosensitization unit used in reported NIR-upconverter,14,15 this planar structure can reduce hole injection in dark and thus increase the ON/OFF ratio of the device without using any extra hole-blocking layer. Furthermore, a non-doped TADFOLED is also employed, which can reduce the operation voltage as the carriers can be injected directly to the emission material and thus eliminate energy loss due to energy transfer from a high energy host to a low energy dopant.


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Figure 2. (a) Dark current density of the photodetector ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm). (b) EQEdet of the photodetector under illumination of an 808 nm NIR laser with different intensities.

According to the operation principles of an upconversion device (Figure S1), most of the holes should be blocked outside the photosensitization unit and the device shows a low emission even at a high operation voltage. In contrast, holes generated in the photosensitization unit under NIR illumination should be efficiently injected to the lightemission unit and then combined with electrons injected from the outside electric circuit, which forms excitons in the emitting layer (EML) and finally radiates visible light. Figure 2a shows the dark current of a NIR photodetector with a planar structure of ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm). The device presents a low dark current at reverse bias and a high rectification ratio in the orders of 103 at ±5 V, suggesting good selective injection of carriers from the electrodes. The device also has good response covering a wide NIR range with a maximum detection external quantum efficiency (determined by the ratio between the numbers of collected electrons and illuminated photons, EQEdet)


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of 15.7 % at 900 nm, 11.4 % at 808 nm, and even 4.4 % at 1000 nm at zero bias (invert triangle  in Figure 3). To further exploit the EQEdet of the device at higher voltages, photocurrent was measured under illumination of an 808 nm NIR laser at different intensities. The photocurrent gradually increases with negative bias voltage and the EQEdet exceeds 100% at higher voltages (Figure 2b), which demonstrates the photomultiplying characteristics of the photodetector. Under an illumination intensity of 0.052 mW/cm2 the photodetector exhibits an EQEdet of 1.95×104 %, corresponding to a responsivity of 127 A/W at -10 V (Supporting Information). Reynaert et al. observed photomultiplying in a nickel phthalocyanine (NiPc)/C60 bilayer device.40 This is attributed to the energetic disorders presented in both NiPc and C60 layers, which form deep traps for charge carriers. Under illumination, photogenerated excitons interact with and transfer their energy to the trapped carriers. Subsequently these trapped carriers are released from the deep states to shallower states and finally trapped again by the deep states after multiple jumps between shallower states. The time scale of this process can be much longer than the transit time of non-relaxed photoexcited carriers, which results in multiple carrier injection and hence the photomultiplying in the photodetector. Actually,


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energetic disorder is inherent for organic materials. According to the above mechanism, the photomultiplying of our NIR-photodetector should be attributed to the energetic disorders of PbPc and C60 layers. This mechanism is further confirmed by the decreased photomultiplying (i.e. EQEdet) with increasing illumination intensity (Figure 2b), which is attributed to the depletion of the trapped carriers and exciton quenching by the already photoexcited carriers.40

Figure 3. Absorption spectrum () of the PbPc (60 nm)/C60 (65 nm) film, EQEdet spectrum () at zero bias of the photodetector ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm), PL spectrum () of a 2PXZ-OXD (30 nm) film, and EL spectrum () of the TADF-


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OLED ITO/NPB (30 nm)/2PXZ-OXD (30 nm)/Bphen (30 nm)/LiF (1 nm)/Al (100 nm) at 7 V.

Absorption of this photodetector ( in Figure 3) has an apparent valley at about 530 nm. To minimize energy loss due to self-absorption in the upconversion device, 2,5bis(4-(10-phenoxazyl)phenyl)-1,3,4-oxadiazole (2PXZ-OXD) with a photoluminescence (PL) peak at about 509 nm ( in Figure 3) is selected as the emission material. 2PXZOXD

has

been

used

as

an

efficient

TADF

dopant

in

a

host

of

bis[2-

(diphenylphosphino)phenyl]ether oxide (DPEPO).41 2PXZ-OXD has a donor-acceptordonor structure, which may easily form exciplexes with donor or acceptor materials.42 We confirmed that 2PXZ-OXD forms exciplex with neither N,N´-bis-(1-naphthyl)-N,N´diphenyl-1,1´-biphenyl-4,4´-diamine

(NPB)

nor

4,7-diphenyl-1,10-phenanthroline

(Bphen) which are used in the OLED in this work (Figure S2). The PL quantum efficiency of a pristine 2PXZ-OXD film is measured to be 0.75, which is only slightly below that of the doped film in DPEPO (0.87)41, indicating that 2PXZ-OXD can also be used as a non-doped EML. To evaluate the performance of the emission unit, we first


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preparation a non-doped TADF-OLED. The structure of the non-doped TADF-OLED is ITO/NPB(30 nm)/2PXZ-OXD (30 nm)/Bphen (30 nm)/LiF (1 nm)/Al (100 nm) (Figure 1c). The device shows a turn-on voltage (defined as the voltage corresponding to a luminance of 1 cd/m2) of 2.4 V, a luminance of 199 cd/m2 at 3 V, a maximum luminance of 32935 cd/m2 at 9.5 V, a maximum EQEem of 11.72% (Supporting Information), a maximum current efficiency of 36.3 cd/A, and maximum power efficiency of 45.6 lm/W (Figure S3). Compared with the other reported state-of-the-art non-doped TADFOLEDs, this device exhibits the lowest turn-on voltage and the highest luminance at low voltages (Table S1).43-46 Electroluminescence (EL) spectrum of this device is slightly red-shifted from its PL spectrum to a region around the valley of the EQEdet spectrum of the photosensitization unit (Figure 3). This property makes it more favorable to be used as the emission unit in the NIR-upconverters for minimizing energy loss due to selfabsorption. The EL spectra of the TADF-OLED are slightly blue-shifted with the increase of applied voltage (Figure S4), which can be attributed to the recombination zone shift as the increased electron injection at higher voltages.


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Based on the high performance NIR-photodetector and the TADF-OLED, a NIRupconversion device is fabricated by integrating these two units. Without NIR illumination, the current density of the NIR-upconverter is very low because C60 not only acts as an acceptor for the photosensitization unit but also as a hole-blocking layer for the NIR-upconverter (Figure 4a). In contrast, the current density dramatically increases at low voltages and tends to saturate at high voltages when the NIR-upconverter is illuminated with an 808 nm NIR laser. The higher intensity of the NIR laser leads to a larger number of charge carriers generated in the photodetection unit and hence the current density of the NIR-upconverter. The maximum current ON/OFF ratio is 18 under a low illumination intensity of 0.052 mW/cm2 and increases to ~104 at 52.5 mW/cm2. Besides, it can be found that the current ON/OFF ratio decreases at higher voltages. At low voltages, holes in the NIR-upconverter are primarily generated by photoexcitation in the photosensitization unit. At high voltages, the applied electric field can cause hole injection from the ITO/C60 interface even under dark. Thus the NIR-upconverter works at the “light-switching mode’’ at low voltages and “light-enhancement mode” at high voltages.13


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In dark, the turn-on voltage of the NIR-upconverter is about 7.5 V ( in Figure 4b and Table S2). Under NIR illumination, the luminance of the NIR-upconverter traces the same trend of their current curves. The turn-on voltage decreases from 3.1 V at an illumination intensity of 0.052 mW/cm2 to about 2 V at 52.5 mW/cm2. This turn-on voltage is even lower than that of the TADF-OLED (Figure S3). The maximum luminance ON/OFF ratio is ~60 at a low illumination intensity of 0.052 mW/cm2 and boosts to 1.8×104 at 52.5 mW/cm2.

The maximum EQEem of the NIR-upconverter without illumination is 2.7% (Figure 4c). In contrast, a maximum EQEem of about 10% is found under NIR illumination. It also can be found that the NIR-upconverter exhibits efficiency roll-off at high voltages with lower illumination intensity. At low voltages with a lower illumination intensity, both the numbers of photogenerated holes in the photosensitization unit and injected electrons from the Al cathode is low, which is easy to form a charge-balanced state and hence a high EQEem. However, the number of electrons increased dramatically at higher voltages, leading to unbalanced charge carriers injection and exciton quenching in the


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light-emission unit. In contrast, under higher illumination the charges carrier injection is more balanced, which results in a higher EQEem and a decreased efficiency roll-off.

Figure 4. The NIR-to-visible upconversion characteristics of the NIR-upconverter. (a) Current density-voltage, (b) luminance-voltage, (c) emission external quantum efficiency (EQEem), (d)


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detection external quantum efficiency (EQEdet), (e) photon-to-photon upconversion efficiency (ηphoton), and (f) power-to-power upconversion efficiency (ηpower) of the NIR-upconverter under different NIR illumination intensity, and the inset of (f) is an image of the NIR-upconverter under NIR illumination (808 nm, 18.68 mW/cm2) through a shadow mask with “CIOMP” letters at 5 V.

The EQEdet of the NIR-upconverter is shown in Figure 4d. Although the EQEdet is very low at lower voltage, it increases rapidly with voltage within 3 V and then gently beyond 3 V. As a result, the EQEdet in all the illumination intensities used here can reach 100% at high voltages. This indicates a photocurrent gain in the NIR-upconverter. A maximum EQEdet of about 1.5×104% is obtained at 15 V under a low illumination intensity of 0.052 mW/cm2.

The ηphoton of the NIR-upconverter can be calculated with the formula:

 photon 

N vis  EQEem  EQEdet N NIR

(1)

where the Nvis and NNIR are the numbers of emitted visible photons and illuminated NIR photons, respectively. While the relationship between ηphoton and ηpower is:


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 power   photon

hNIR hvis

(2)

where hλNIR and h vis are the NIR photon energy and the averaged output visible photon energy. Due to nearly Gaussian sharp of the EL spectrum of the NIRupconverter, the photon energy at the peak of the EL spectrum of 520 nm is used for estimating h vis . The ηphoton and ηpower of the NIR-upconverter increases with the voltage and more interesting is that the ηphoton exceeds 100% at 15 V under lower illumination intensities (Figure 4e, Table S2), indicating an upconversion gain of the device. A maximum ηphoton of 256%, corresponding to a ηpower of 398% (Figure 4f) is obtained at an illumination intensity of 0.052 mW/cm2. These high upconversion efficiencies should be attributed to the photomultiplying of the photodetection unit. It should be noted that the NIR-upconverter is already turned on at high voltages without illumination. In fact, this is common for NIR-upconverters, for example in the report by Yu et al.9 The ηphoton of the present device is still higher than 100% even after deducting the part of emission due to electric field under dark (Figure S5).


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It can also be noted that even at a low voltage of 3 V, a highest ηphoton of 2.0% can be obtained at an illumination intensity of 4.91 mW/cm2, which originates from the high luminance of the emission unit and the high responsivity of the photosensitization unit at low voltages. The highest ηphoton for all inorganic, organic/inorganic hybrid, and all organic NIR-upconverters with the “photodetector/LED” structure reported so far are 4.8%5, 4.8%12, and 29.6%27, respectively. To the best of our knowledge, this is the first report of an NIR-to-visible upconversion gain without using any transistor structure, although an upconversion gain has been reported in upconverters with a “phototransistor/OLED” structure.9

To examine the NIR-to-visible imaging capability, we fabricated an NIR-upconverter with a large active area of 20×20 mm2. An 808 nm NIR beam is vertical illuminated onto the upconversion device via a “CIOMP” letters patterned shadow mask. As expected a clear image of the “CIOMP” was displayed on the device and could be distinctly seen by naked eyes and captured by digital camera (inset of Figure 4f). The highly ON/OFF photoresponse imaging from the device means that the device can be used in the


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application of image-sensing devices. A short video of this NIR-upconversion display is also shown in Supporting Information. As a NIR laser beam scanned across the device through a line-shaped shadow mask, green stripes can be clearly observed on the device. The response speed (light intensity decreases from its maximum to 10%) of the NIR-upconverter at 3 V is 118 μs (Figure S6), confirming the fast response of the device.

In summary, we firstly demonstrate an upconversion gain in a NIR-to-visible upconversion display with the “photodector/LED” architecture by incorporating a photomultiplying NIR photodetector and a high efficiency TADF-OLED. Other than the high performance of the OLED and the photodetector, the high performance of the upconverter is also attributed to the careful material selection to minimize energy loss due to self-absorption. On the other hand, adoption of a fully non-doped structure not only improve the on-off ratio, it also simplifies the device structure, lowers the fabrication cost, and enhances the liability of the device. This high performance upconversion display has great potential for low-cost, high conversion efficiency, and high-resolution NIR imaging.

EXPERIMENTAL METHODS


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Device fabrication: All devices were fabricated on patterned ITO coated glass substrates with a sheet resistance of 15 Ω/sq. The substrates were routinely cleaned followed by ultraviolet (UV)-ozone treatment for 10 min before loading into a high vacuum deposition chamber. Organic materials were purchased from commercial sources and all materials were used as received without further purification. Organic thin films were deposited at a pressure of 6×10-4 Pa without vacuum break. The deposition rates were 1 and 10 Å/s for organic materials and aluminum, respectively. The TADF-OLED has a structure of ITO/NPB (30 nm)/2PXZ-OXD (30 nm)/Bphen (30 nm)/LiF (1 nm)/Al (100 nm). Here NPB was used as the hole-transporting layer, 2PXZOXD was used as the EML, and Bphen was used as electron-transporting layer. The NIR photodetector has a structure of ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm). Here, PbPc and C60 were used as the donor and acceptor, respectively. For the all-organic NIR-upconverter, it has a structure of ITO/C60 (65 nm)/PbPc (60 nm)/NPB (30 nm)/2PXZ-OXD (30 nm)/Bphen (30 nm)/LiF (1 nm)/Al (100 nm). The active area of the devices is 2×2 mm2.


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Device characterization: All devices were characterized at an ambient environment and without encapsulated. For OLED and NIR-upconverter characterization, luminancevoltage-current characteristics were measured with a source meter (Keithley 2400) and a luminance meter (Minolta LS-160). Steady-state PL and EL spectra were measured with a Hitachi F7000 and an OPT-2000 spectrophotometer, respectively. PL quantum efficiency was obtained by using an integrating sphere with a fluorescence spectrophotometer (FL920). Absorption spectrum was measured with a UV-VIS-NIR spectrophotometer (Shimadzu, UV-3101PC). EQEdet spectrum of the photodetector was obtained with a lock-in amplifier (Stanford SR830) under monochromatic illumination at a chopping frequency of 130 Hz by a chopper (Stanford SR540). The response speed of the NIR-upconverter at 3 V was obtained with a FLS980 fluorescence spectrometer under excitation of an 808 nm laser (Surelite OPO). The NIR laser power density was determined though a laser power meter (Ophir Vega).

ASSOCIATED CONTENT

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61575192 and 61376022) and the Program for New Century Excellent Talents in Fujian Province University.

Supporting Information. The supporting information contains: calculation methods of the responsivity, EQEdet, and EQEem, schematic operation principles of the upconverter, normalized PL spectra, characteristics of the TADF-OLED, comparison of the performance of the TADF-OLEDs, normalized EL spectra, and the performance of the upconverter. “This material is available free of charge via the Internet at http://pubs.acs.org.”


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Figure 1. (a) Molecular structure of the organic materials. (b) Device structure of the NIR-upconversion device. (c) A schematic energy level diagram of the TADF-OLED. 283x174mm (300 x 300 DPI)


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Figure 2. (a) Dark current density of the photodetector ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm). (b) EQEdet of the photodetector under illumination of an 808 nm NIR laser with different intensities.



Figure 3. Absorption spectrum () of the PbPc (60 nm)/C60 (65 nm) film, EQEdet spectrum () at zero bias of the photodetector ITO/PbPc (60 nm)/C60 (65 nm)/Al (100 nm), PL spectrum () of a 2PXZ-OXD (30 nm) film, and EL spectrum () of the TADF-OLED ITO/NPB (30 nm)/2PXZ-OXD (30 nm)/Bphen (30 nm)/LiF (1 nm)/Al (100 nm) at 7 V.


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Figure 4. The NIR-to-visible upconversion characteristics of the NIR-upconverter. (a) Current densityvoltage, (b) luminance-voltage, (c) emission external quantum efficiency (EQEem), (d) detection external quantum efficiency (EQEdet), (e) photon-to-photon upconversion efficiency (ηphoton), and (f) power-topower upconversion efficiency (ηpower) of the NIR-upconverter under different NIR illumination intensity, and the inset of (f) is an image of the NIR-upconverter under NIR illumination (808 nm, 18.68 mW/cm2) through a shadow mask with “CIOMP” letters at 5 V.



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Organic Upconversion Display with an over 100% Photon-to-photon

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