Single-Layer Halide Perovskite Light-Emitting Diodes with Sub-Band

Sep 30, 2016 - Charge-carrier injection into an emissive semiconductor thin film can result in electroluminescence and is generally achieved by using ...
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Single-Layer Halide Perovskite Light-Emitting Diodes with Sub-Band Gap Turn-On Voltage and High Brightness Junqiang Li,† Xin Shan,† Sri Ganesh R. Bade,† Thomas Geske,†,‡ Qinglong Jiang,† Xin Yang,† and Zhibin Yu*,†,‡ †

Department of Industrial and Manufacturing Engineering, High-Performance Materials Institute, Florida State University, Tallahassee, Florida 32310, United States ‡ Materials Science and Engineering, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Charge-carrier injection into an emissive semiconductor thin film can result in electroluminescence and is generally achieved by using a multilayer device structure, which requires an electron-injection layer (EIL) between the cathode and the emissive layer and a hole-injection layer (HIL) between the anode and the emissive layer. The recent advancement of halide perovskite semiconductors opens up a new path to electroluminescent devices with a greatly simplified device structure. We report cesium lead tribromide light-emitting diodes (LEDs) without the aid of an EIL or HIL. These so-called single-layer LEDs have exhibited a sub-band gap turn-on voltage. The devices obtained a brightness of 591 197 cd m−2 at 4.8 V, with an external quantum efficiency of 5.7% and a power efficiency of 14.1 lm W−1. Such an advancement demonstrates that very high efficiency of electron and hole injection can be obtained in perovskite LEDs even without using an EIL or HIL.

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to be much higher than the band gap (Eg)/e of the perovskite emitters. For instance, a turn-on voltage of more than 3.0 V was commonly reported among green pero-LEDs that emitted photons with an energy of about 2.3 eV.10,12−22 The only exception was observed by Wang et al. in their green peroLEDs which had a turn-on voltage of 2.1 V and a device structure of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine)/molybdenum oxide as the hole-injection layer (HIL) and polyethylenimine-modified zinc oxide as the electron-injection layer (EIL).11 Nonetheless, the perovskite emissive layer in such a device had discontinuous coverage, and current leakage prevented it from reaching a high luminance intensity. Here we report bright pero-LEDs with a composite emissive layer consisting of cesium lead tribromide (CsPbBr3), poly(ethylene oxide) (PEO), and poly(vinylpyrrolidone) (PVP). The LEDs were constructed with an indium tin oxide (ITO) anode, CsPbBr3−PEO−PVP composite emissive layer, and indium−gallium eutectic (In−Ga) cathode without intentionally employing an EIL or HIL. Such single-layer devices start emitting green light at 1.9 V and reach a maximum luminance of 593 178 cd m−2 at 4.9 V. Photocurrent measurements suggest a p-i-n junction is formed in situ in the perovskite emissive layer when an external bias is applied. The junction formation increases charge-carrier injection efficiencies, leading to the observed sub-band gap turn-on and a high luminance intensity in our devices. Noticeably, the maximum luminance

n recent years, light-emitting diodes (LEDs) have evolved as important commercial products to replace traditional incandescent and fluorescent light bulbs for display and lighting applications. The need for larger device size and lower fabrication cost has motivated the exploration of novel LED technologies including organic LEDs (OLEDs) based on organic small molecular and polymeric semiconductors, quantum-dot LEDs, and more recently halide perovskite LEDs (pero-LEDs). Pero-LEDs are made from a group of ABX3 halide perovskite semiconductors, in which A is a cesium (Cs+) or an aliphatic ammonium (RNH3+) cation, B a divalent Pb2+ or Sn2+ cation, and X a halide anion such as Cl−, Br−, or I−.1−3 Halide perovskites have shown the advantages of lowtemperature and cost-effective processing and have also manifested exceptional electronic and optical properties that are desired for efficient electroluminescent devices.4−9 Pero-LEDs can be based on methylammonium lead halides (CH3NH3PbX3, hereafter denoted as MA-pero) or cesium lead halides (CsPbX3, hereafter Cs-pero).10−23 Until now, the best MA-pero LEDs that emit green light (peak wavelength at 520− 540 nm) had a maximum luminance of about 70 000 cd m−2,17 and the best Cs-pero-based green LEDs had a maximum luminance of 53 525 cd m−2,23 both of which are still lower than the state of the art in OLEDs, quantum-dot LEDs, and gallium nitride-based LEDs.24−27 The relatively low luminance in pero-LEDs can be attributed to inefficient electron and hole injection from the cathode and anode, respectively, into the halide perovskite emitters. The majority of reported pero-LEDs used a multilayer device structure to enhance both electron and hole injection, and their turn-on voltages (defined at 1 cd m−2 luminance) were found © 2016 American Chemical Society

Received: August 26, 2016 Accepted: September 30, 2016 Published: September 30, 2016 4059

DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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The Journal of Physical Chemistry Letters

Figure 1. Top-view SEM images of samples with (a) CsPbBr3:PEO = 100:50, (c) CsPbBr3:PEO:PVP = 100:50:5, (e) 100:50:15, and (f) 100:50:30. Tilted-view SEM images of (b) 100:50 and (d) 100:50:5 samples.

Figure 2. (a) XRD patterns of CsPbBr3:PEO (100:50) and CsPbBr3:PEO:PVP (100:50:5) composite thin films. (b) Photoluminescence (green) and absorbance (black) spectra of the 100:50:5 sample.

cross-sectional SEM image in Figure 1b. Tuning the PEO content in the composite did not improve film continuity, as shown in Figure S1; lower PEO content led to larger grains and therefore an even rougher surface morphology, while higher PEO content caused the CsPbBr3 crystals to become further separated, reducing their total surface coverage to about 50%. In contrast, we found the CsPbBr3 can be well dispersed by PVP and that the CsPbBr3 grain size of CsPbBr3/PVP composite films was much smaller than that of CsPbBr3/PEO composite films. A composite film with CsPbBr3:PVP = 100:5 had a nearly continuous morphology except for some scattered voids of about 20 nm diameter (Figure S2a,b). With even smaller and more uniform grain size, the film became fully continuous and pinhole free at CsPbBr3:PVP = 100:30 (Figure S2c,d). The chemical structure of PVP is shown in Figure S3. We assume the pyrrolidone group in PVP exhibits a high polarity that aids dispersing perovskite in the polymer. Other than CsPbBr3, the PVP polymer can also well disperse other perovskite materials including MAPbI 3, MAPbBr3, and MAPbI3−xClx, which is in consistent with literature reports.30−32 Inspired by such an observation, PVP was then added to the CsPbBr3/PEO composite to engineer the surface morphology of the films. As shown in Figure 1c,d from a film

intensity is about 9 times the previous record in MA-pero LEDs and 11 times that of Cs-pero LEDs.17,23 Such an advancement demonstrates for the first time that very high efficiency of electron and hole injection can be obtained in pero-LEDs even without using an EIL or HIL. In addition, our single-layer devices exhibit fast turn-on (∼10 ms), indicating the lightemitting p-i-n junction can be formed within a 10 ms time scale. The emissive perovskite composite thin films in our singlelayer pero-LEDs were obtained by spin coating a mixture solution containing the CsPbBr3 precursors, PEO, and PVP with a desired weight ratio. It has been shown that direct spincoating of the MA-pero or Cs-pero precursor solution onto an ITO surface usually leads to a discontinuous film.13,16,18,28 In our previous work, we improved the film morphology of MApero by blending it with an ionic conducting polymer, PEO. Such a composite film has been successfully applied as the emissive layer in pero-LEDs.28,29 However, mixing the CsPbBr3 with PEO did not produce a continuous film as shown in the scanning electron microscopy (SEM) image in Figure 1a. The film with CsPbBr3:PEO = 100:50 ratio (by weight and hereafter) consisted of isolated crystals of about 0.5−2 μm in size. The overall surface coverage of the film was about 60%, leaving the remaining area nearly uncovered as seen from the 4060

DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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Figure 3. (a) Current density and (b) luminance versus voltage characteristics of the single-layer Cs-pero LEDs with different PVP compositions in the emissive layers. The inset in panel a shows the LED device structure as “ITO anode/CsPbBr3-polymer composite/In−Ga cathode”. (c) Current efficiency/EQE versus voltage characteristics of the single-layer Cs-pero LEDs. (d) Electroluminescence spectra collected at various luminance intensities from a device with CsPbBr3:PEO:PVP = 100:50:5 in the emissive layer. The insets are photos of lit devices operating at 2 V bias in dark (left) and at 4 V bias at an indoor lighting environment (right).

showed a current density about 2 orders of magnitude higher before 1.9 V when compared to the 100:50:5 and 100:50:15 devices. This can be attributed to electrical leakage due to the discontinuous nature of the binary composite film, as shown in Figure 1a,b. The current densities started to rapidly increase after 1.9 V for all the devices, reaching peak values of 2226 mA cm−2 at 4.3 V for the 100:50 composite, 2787 mA cm−2 at 4.9 V for the 100:50:5 composite, and 1226 mA cm−2 at 4.7 V for the 100:50:15 composite. It is worth mentioning that the In−Ga cathode showed no sign of reaction with the perovskite, and the perovskite still manifested strong photoluminescence under ultraviolet (UV) light after the LED testing. As illustrated by the L−V characteristics in Figure 3b, the 100:50:5 device turned-on at 1.9 V and the luminance increased rapidly with voltage, reaching 100 000 cd m−2 at 3.7 V and a maximum of 593 178 cd m−2 at 4.9 V. The other devices both had a turn-on voltage at 2.2 V, increasing to a maximum of 54 930 cd m−2 at 4.5 V for the 100:50 device and 17 543 cd m−2 at 4.4 V for the 100:50:15 device. Figure 3c shows the efficiency versus voltage characteristics of LEDs with 100:50, 100:50:5, and 100:50:15 composite emissive layers. The current efficiency achieved a maximum of 21.5 cd A−1 at 4.8 V, corresponding to an external quantum efficiency (EQE) of 5.7% for the 100:50:5 device; 2.5 cd A−1 at 4.5 V corresponding to an EQE of 0.7% for the 100:50 device; and 1.6 cd A−1 at 4.3 V corresponding to an EQE of 0.4% for the 100:50:15 device. All devices emitted green light, and Figure 3d shows the EL spectra of the 100:50:5 device at various luminance intensities. All spectra had peak positions located at ∼522 nm with a fwhm of 21 nm, correlating quite well to the PL spectrum in Figure 2b. The insets in Figure 3d show two optical images of a lit LED device with 100:50:5 composite emissive film at 2 V (left) and 4 V

with CsPbBr3:PEO:PVP = 100:50:5, all the pinholes previously seen in the CsPbBr3/PEO composite had been removed and a dense composite film was formed. At higher PVP ratios (CsPbBr3:PEO:PVP = 100:50:15 and 100:50:30), the films remained continuous; however, the density of the CsPbBr3 crystals became reduced (Figure 1e,f). The crystallinity of the composite films was characterized by X-ray diffraction (XRD). The XRD patterns shown in Figure 2a indicated both the binary and the ternary composite films consisted of polycrystalline CsPbBr 3 belonging to an orthorhombic phase.33 The lattice parameters of the CsPbBr3 were calculated to be a = 0.821 nm, b = 0.826 nm, and c = 1.176 nm, which are in agreement with literature values.33 Absorbance and photoluminescence (PL) spectra were also collected to evaluate the optical properties of the composite thin films. All films with various PVP ratios behaved nearly the same, and the measurement results of a CsPbBr3:PEO:PVP = 100:50:5 film are presented in Figure 2b. A sharp transition is observed at around ∼525 nm in the absorbance spectrum that agrees well with the peak position in the PL spectrum, both corresponding to a band gap of 2.36 eV in CsPbBr3.8,9,18−22 The CsPbBr3:PEO:PVP = 100:50:5 film exhibits a photoluminescence quantum yield of about 21.8%. LEDs were constructed using an ITO anode/CsPbBr3polymer composite/In−Ga (eutectic) cathode, as illustrated in the inset of Figure 3a. The active area of the devices was about 2 mm diameter as defined by the area of the In−Ga electrode (Experimental Section). Current density−voltage (J−V) and luminance−voltage (L−V) characteristics were collected for devices with CsPbBr3:PEO = 100:50 and CsPbBr3:PEO:PVP = 100:50:5 and 100:50:15 in the emissive layers (Figure 3a,b). Noticeably, the device without PVP (CsPbBr3:PEO = 100:50) 4061

DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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The Journal of Physical Chemistry Letters (right) applied bias. Real-time videos were also recorded for devices lit at 4 and 2 V as shown in the Supporting Information. To evaluate the reproducibility of the best performance, eight devices were fabricated in a single batch with the 100:50:5 composite emissive layer. The device performances are summarized in Table S1. The turn-on voltage varied from 1.8 to 2.1 V, maximum current efficiency from 16.3 to 25.6 cd A−1, EQE from 4.3% to 6.8%, maximum power efficiency from 9.6 to 14.9 lm W−1, and maximum brightness from 416 744 to 804 719 cd m−2. All devices exhibit very high luminance intensities and high power efficiencies. It is worth noting that the combination of high luminous efficiency and high luminance of our pero-LEDs has greatly outperformed all reported devices using MA-pero or Cs-pero as the emissive layer (Figure S4). For instance, Cho et al. recently obtained a power efficiency of 14.9 lm W−1, and a maximum luminance of ∼20 000 cd m−2 based on CH3NH3PbBr3 polycrystalline thin films.16 Chih et al. reported a power efficiency of 5.9 lm W−1, and a maximum luminance of 70 000 cd m−2 based on CH3NH3PbBr3 by introducing a NiOx electrode interlayer.17 Yantara et al. reported ∼0.02 lm W−1 and 407 cd m−2 using CsPbBr3 polycrystalline thin films.18 Ling et al. recently achieved 12.3 lm W−1 and 53 525 cd m−2 using a CsPbBr3/ PEO composite emitter and a multilayered device structure.23 Remarkably, the turn-on voltage in our devices was 0.26− 0.56 V lower than the Eg/e of the perovskite emitter. Such an efficient turn-on is usually found in commercial inorganic LEDs that emit infrared, red, or green light based on small band gap III−V semiconductors such as GaAs and AlGaInP.34,35 In those devices, both p- and n-type doping can be readily achieved, and the employment of a p-i-n device structure effectively removes the charge injection barriers between the electrodes and the emissive semiconductor layer. Given the extremely simplified device structure in our work, we hypothesize that a p-i-n junction may have formed in situ within the CsPbBr3−PEO− PVP composite film when an external bias was applied. It is worth mentioning ion migration in halide perovskites has been supported by theoretical calculations and experimental observations.36−43 For instance, Yuan et al. reported that the methylammonium cations in MAPbI3 could migrate toward the cathode at a relatively low electrical field (≈ 0.3 V μm−1).39 Shao et al. observed iodide anions could migrate toward the anode at 0.5 V μm−1.40 Therefore, we assume the ionic species in the CsPbBr3−PEO−PVP composites can respond in a similar way to an external electric field and develop net charges at the electrode/perovskite interfaces (Figure 4a). The accumulation of ions can create a p-type equivalent region along the anode/perovskite interface and an n-type equivalent region along the cathode/perovskite interface, thus form an analogy of a p-i-n junction in the perovskite layer.39−42 To verify the ion migration and accumulation hypothesis, time-dependent discharging current (Idis) was measured as shown in Figure 4b by short-circuiting a prebiased LED device. The device had an emissive layer thickness of 2 μm and was exposed to 20 V for 10 s prior the discharging test. The Idis started at 240 nA and decayed exponentially with time. Such a behavior is in agreement with the neutralization process of the accumulated ions through ionic diffusion during the discharging test. Junction formation in the CsPbBr3−PEO−PVP composite film is further supported by photocurrent response (Figure 4b insets) of a prebiased device. A photocurrent of 150 nA was measured under a light source irradiation within the first minute of discharging, showing the separation of photoinduced

Figure 4. (a) Schematic illustration of junction formation in the CsPbBr3−polymer composite film under an external electrical field. Cations and/or anions migrate toward and accumulate at the perovskite/cathode and perovskite/anode interfaces, resulting in formation of a p-i-n junction. (b) Discharging current evolution with time by short-circuiting a prebiased LED device. The insets are enlarged portions showing the photo response of the device using a commercial white LED light source (∼10 mW cm−2). (c) Band diagram schematic at the anode/CsPbBr3 interface for the single-layer LED device with (solid) and without (dot-dashed) doping effect. e is elementary charge, and V represents the voltage drop across the anode/CsPbBr3 interface; EF is the quasi Fermi level of CsPbBr3 in the composite film.

carriers was quite efficient because of the p-i-n junction formation. In contrast, the photocurrent was ∼3 nA for a freshly prepared unbiased device (Figure S5), which is typical for a device without a p-i-n junction. After 15−16 min of discharging, the photocurrent reduced to about 20 nA, suggesting the degradation of the junction during the discharging process. As a result of large density ion accumulation and heavy doping, a sharp tunneling barrier evolves for electron and hole injection at the perovskite/ electrode interfaces and greatly reduces contact resistance compared to a conceptual device that does not form such a junction. Figure 4c and Figure S6 manifest the narrowing of 4062

DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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Figure 5. (a) Current density and (b) luminance versus voltage characteristics of the single-layer Cs-pero LEDs at different voltage scanning rates (0.1 and 0.5 V s−1). Transient light emission response under a 0−4 V pulse train with 50% duty cycle at 1 Hz (c) and 10 Hz (d) frequency.

concentrations both about 1 ppm. As shown in Figure S7a, when the applied bias was 3.5 V, the EL intensity obtained a peak value of 33 680 cd m−2 in 3 s; it then decayed to about 13 000 cd m−2 in 1 min and remained relatively stable for more than 6 min. When 2.7 V was used, the device required about 5 min to reach its peak luminance (∼400 cd m−2), as shown in Figure S7b, decaying to 50% of its peak value after about 50 min. In addition, the devices were illuminated in ambient air (30 °C and 60% relative humidity) and showed a high brightness that was clearly seen under sunlight (Figure S8). Multiple cycles of J−V and L−V tests were also carried out, as shown in Figures S9 and S10. When the voltage limit was set at 4 V, the device degradation was insignificant, and the luminance intensity could retain ∼95% of the original value after 10 scanning cycles (Figure S9b). However, once the device had passed its maximum luminance intensity (V > 5 V) for the first time, severe damage was observed that significantly increased the current densities and lowered the luminance intensities of the LEDs in subsequent scanning cycles (Figure S10a,b). The device could still be lit at 3 V with uniform emission over the whole device area (Figure S10c). Likely the above damage was not caused by heat burning, which usually causes dark spots in the devices. We realize the stability of pero-LEDs have seldom been reported in the literature,20,28 but our devices from this work performed the best among those reported. Nonetheless, continuing research to improve the device operation stability remains an important task on the path to application. We are currently investigating the ionic migration dynamics and the electro-optic properties of the doped perovskites with the hope of improving the stability of those devices. In summary, we developed pinhole-free CsPbBr3−PEO− PVP ternary composite thin films using a one-step solution process. Single-layer LEDs were fabricated with a device

barrier width after ion migration and accumulation, which is equivalent to lowering the effective barrier height. The in situ junction formation has resulted in very efficient electron and hole injection into the CsPbBr3, which contributes to the subband gap turn-on voltage and the ultrahigh luminance intensity in our pero-LEDs. The in situ junction formation mechanism was originally found in polymer light-emitting electrochemical cells (LECs).43 In typical polymer LECs, light emission can be delayed from minutes to several seconds after applying the voltage, accompanying severe hysteresis in the J−V and L−V measurements.44−46 Aygüler et al. have reported MAPbBr3- and FAPbBr3-based LECs, where the emission layer comprised perovskite nanoparticles and an electrolyte mixture (LiCF3SO3 dissolved in trimethylolpropane ethoxylate). Such perovskitebased LECs started to illuminate after 3 min when a constant voltage of 14 V was applied.47 A shorter responsive time of 15 s was reported by Zhang et al. in their MAPbI3-based LECs without adding an electrolyte.48 In contrast, the single-layer pero-LED in this work exhibits much faster turn-on and has no obvious hysteresis: as shown in Figure 5a,b both the J−V and I−V characteristics are nearly the same at different voltage scanning rates (0.1 and 0.5 V s−1). Transient light emission response was also tested under a 0−4 V pulse train with 50% duty cycle at 1 or 10 Hz frequency, as shown in panels c and d of Figure 5, respectively. Nearly instantaneous turn-on was achieved with a response time of about 10 ms to reach their maximum light intensities. Such a fast turn-on is comparable to conventional LEDs and close to the resistance−capacitance time constant of the pero-LEDs. The stability of our devices was first evaluated under continuous operation at a constant voltage at room temperature inside a nitrogen-filled glovebox with oxygen and moisture 4063

DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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structure of “ITO/CsPbBr3−PEO−PVP composite thin film/ In−Ga”. The LEDs exhibit a sub-band gap turn-on voltage of 1.9 V and an ultrahigh luminance of 593 178 cd m−2 with a maximum power efficiency of 14.1 lm W−1. The low turn-on voltage and high luminance are both attributed to an in situ junction formation in the perovskite composite thin film under an external bias. The simplified device structure and high performance of our devices may stimulate more studies in applying solution-processed single-layer pero-LEDs for largearea illumination objects.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01942. Device lit at 4 V (AVI) Device lit at 2 V (AVI) Supplemental figures and table (PDF)

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AUTHOR INFORMATION

Corresponding Author

EXPERIMENTAL SECTION Materials. Lead(II) bromide (99.999%), cesium bromide (99.999%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, anhydrous, 99.9%), poly(ethylene oxide) (average Mw ∼ 5 000 000), poly(vinylpyrrolidone) (average Mw ∼ 1 300 000), and indium− gallium eutectic (99.99%) were purchased from Sigma-Aldrich. All materials were used as received. Film Preparation and Characterization. The Cs-pero precursor solution was prepared by dissolving PbBr2 and CsBr in a 1:1.5 molar ratio in anhydrous DMSO to give a concentration of 120 mg mL−1. PEO and PVP were dissolved in DMF with a concentration of 10 mg mL−1 and 50 mg mL−1, respectively. The pero precursor, PEO, and PVP solutions were then mixed with desired ratio. All the solutions were stirred at 120 °C for 30 min before use. The ITO/glass substrates (20 ohm sq−1) were cleaned subsequently with detergent water, deionized water, acetone, and isopropanol for 5 min with sonication, then blow dried with nitrogen, and treated with oxygen plasma at 100 W power for 2 min. The mixture solution was spun-coated onto the ITO/glass at 1500 rpm for 1 min. The films were then annealed at 200 °C for 30 s. Solution and film preparation and the following device testing were carried out inside a nitrogenfilled glovebox with oxygen and moisture level both at ∼1 ppm. Commercial tools of field emission SEM (Zeiss 1540 EsB) and an ultraviolet−visible−near-infrared spectrometer (Varian Cary 5000) were used to characterize the composite thin films. PL spectra were collected at room temperature on a Horiba Jobin Yvon FluoroMax-4 Fluorometer. The excitation wavelength was fixed at 460 nm. The emission spectra from 480 to 780 nm were collected with an integration time of 0.1 s. Photoluminescence quantum yield of the perovskite thin film was measured using one commercial light-emitting polymer (super yellow, EMD Performance Materials Corp., PDY-132) as the reference. A solid PDY-132 thin film has a PLQY of about 68%.49 LED Measurement. Current density−voltage and luminance− voltage characteristics were measured with a Keithley 2410 source meter and a silicon photodiode. The silicon photodiode was calibrated by a Photo Research PR-655 spectroradiometer with details described in the Supporting Information. To obtain the active area of the LEDs, −10 V was applied (the direction of current flow was from In−Ga to ITO) for a few seconds for all our LEDs after the J−V−L measurements. This method turned the active area to a dark color, as shown in Figure S11. The dark region clearly defines the active area of our LEDs. We further used ImageJ to process the picture and obtained an accurate estimation of the active area of the LEDs. EQE was estimated by assuming a Lambertian emission profile. The EL spectra were collected by the PR-655 with neutral density filters providing attenuation down to 3% within the visible wavelength region.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support from National Science Foundation under Award ECCS-1609032 (program manager Dr. Nadia El-Masry) and the support from Air Force Office of Scientific Research under Award FA955016-1-0124 (program manager Dr. Charles Lee). We also thank Dr. Peng Xiong at FSU for valuable discussions, Dr. Fangchao Zhao and Dr. Qibing Pei at UCLA for providing the PR-655 instrument, Mr. Dong Li and Dr. Jianming Cao at FSU for providing the neutral density filters, and Dr. Bert van de Burgt for assisting with PL measurements at the Materials Characterization Laboratory of FSU.

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REFERENCES

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DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066

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The Journal of Physical Chemistry Letters (49) Ullah, M.; Tandy, K.; Yambem, S. D.; Aljada, M.; Burn, P. L.; Meredith, P.; Namdas, E. B. Simultaneous Enhancement of Brightness, Efficiency, and Switching in RGB Organic Light Emitting Transistors. Adv. Mater. 2013, 25, 6213−6218.

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DOI: 10.1021/acs.jpclett.6b01942 J. Phys. Chem. Lett. 2016, 7, 4059−4066