Manipulating Ion Migration for Highly Stable Light-Emitting Diodes

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Manipulating Ion Migration for Highly Stable Light-Emitting Diodes with Single-Crystalline Organometal Halide Perovskite Microplatelets Mingming Chen, Xin Shan, Thomas Geske, Junqiang Li, and Zhibin Yu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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Manipulating Ion Migration for Highly Stable Light-Emitting Diodes with SingleCrystalline Organometal Halide Perovskite Microplatelets Mingming Chen1,3, Xin Shan1, Thomas Geske1,2, Junqiang Li 1, Zhibin Yu1,2* 1. Department of Industrial and Manufacturing Engineering, High-Performance Materials Institute, FAMU-FSU College of Engineering, Florida State University, Tallahassee, FL 32310, USA 2. Materials Science and Engineering, Florida State University, Tallahassee FL 32306, USA 3. Faculty of Science, Jiangsu University, Zhenjiang, Jiangsu 212013, China * E-mail: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT: Ion migration has been commonly observed as a detrimental phenomenon in organometal halide perovskite semiconductors, causing the measurement hysteresis in solar cells and ultra-short operation lifetimes in light-emitting diodes. In this work, ion migration is utilized for the formation of a p-i-n junction at ambient temperature in single-crystalline organometal halide perovskites. The junction is subsequently stabilized by quenching the ionic movement at a low temperature. Such a strategy of manipulating the ion migration has led to efficient singlecrystalline light-emitting diodes that emit 2.3 eV photons starting at 1.8 V and sustain a continuous operation for 54 hours at ~5,000 cd m-2 without degradation of brightness. In addition, a whispering-gallery-mode cavity and exciton-exciton interaction in the perovskite microplatelets have both been observed that can be potentially useful for achieving electricallydriven laser diodes based on single-crystalline organometal halide perovskite semiconductors.

KEYWORDS: halide perovskites, microplatelets, light-emitting diodes, ion migration, laser, single crystal

ToC graphic:


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Organometal halide perovskites (OHPs) have attracted much attention in recent years owing to their remarkable electro-optical properties and good solubility in certain organic solvents, potentially enabling a variety of large area and low cost electronic devices.1–3 The power conversion efficiencies of OHP solar cells have increased rapidly from 3.8% to more than 20% within the past few years.4–7 It has also been observed that OHPs exhibit exceptional photoluminescence efficiency, weak Auger recombination, and high color purity,8–11 making them ideal emitter materials for light-emitting diodes (LEDs) and laser diodes. OHP LEDs were first demonstrated by Tan et al. in 2014,12 and the current state-of-the-art devices have obtained a maximum luminance of 591,197 cd m-2 with a power efficiency of 14.1 lm W-1.13–16 Noticeably, all reported OHP LEDs have employed an emissive layer consisting of polycrystalline or amorphous perovskites with average domain sizes of a few micrometers to a few nanometers.12–25 Thus, coherent light-emission is challenging due to the random orientations of the constituent perovskite domains. In this regard, to explore electroluminescent (EL) devices using a single-crystalline perovskite emitter is an essential step towards achieving electrically driven laser diodes. To date, a number of research groups have reported the synthesis of single-crystalline perovskite microplatelets and nanowires,11,26–28 and have demonstrated optically-pumped laser generation at a low lasing threshold;

11,26,29

however, no EL devices have been realized. One

major challenge is the use of a multi-layered device architecture in most OHP LEDs, which requires at least an electron injection layer (EIL), a hole injection layer (HIL), and an emissive perovskite layer in addition to the cathode and anode.12,19 Such a multi-layered device structure cannot be easily replicated on micro- or nanometer size perovskite single crystals. In this work, we report OHP LEDs using single-crystalline methylammonium lead


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tribromide (MAPbBr3) microplatelets. The LEDs were fabricated without using any EILs or HILs. The device configuration consisted of an indium tin oxide (ITO) anode, a single-crystalline MAPbBr3 microplatelet emissive layer, and a gold (Au) cathode. Despite the large energy barriers between the electrodes and the MAPbBr3, EL was observed at a low applied voltage at ambient temperature. We hypothesize the in situ formation of a p-i-n junction through electrical field induced migration of cations and/or anions within the perovskite microplatelets. Such a junction was found to disappear within a few minutes at ambient temperature when the external voltage was removed. However, by quenching the device to a low temperature with an applied voltage the ion migration was inhibited, and the in situ formed junction was stabilized. This leads to efficient and highly stable OHP LEDs that turned-on at 1.8 V and continuously lit at ~5,000 cd m-2 for 54 hours without degradation.

RESULTS AND DISCUSSION The single-crystalline MAPbBr3 microplatelets were synthesized following a literature procedure.28 Figure 1a and Figure S1 show typical scanning electron microscope (SEM) images of the obtained perovskite crystals. The platelets had square or rectangular shapes with very smooth top surfaces and sharp edges. The lateral dimensions of most platelets fell within 20 µm 100 µm and they had a typical thickness of ~10 µm. Figure 1b shows the x-ray diffraction (XRD) pattern of obtained MAPbBr3 microplatelets. The crystals appeared highly textured with two dominant peaks at 14.92° and 30.12°, corresponding to (100) and (200) of the cubic phase MAPbBr3, respectively. The lattice constant was calculated as 5.98 Å, which is consistent with reported values in literature.30 The ambient temperature photoluminescence (PL) spectra at various excitation intensities are shown in Figure 1c. All the spectra exhibit one main peak


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centering at 543nm, and a shoulder peak at 563 nm wavelength. It is observed that the relative intensity of the shoulder peak is enhanced with increasing excitation power. Such a nonlinear emission behavior agrees well with the recent report by Kunugita et al., attributing the shoulder peak to the exciton-exciton scattering process in the perovskite crystals.31 After preparing the microplatelet crystals, we proceeded to fabricate single-crystalline MAPbBr3 LEDs. The device structure is schematically shown in Figure S2: ITO on glass was used as the anode, a MAPbBr3 microplatelet was used as the active light-emitting media, and a 5 nm Au film was sputtered and used as the cathode. One Au wire of 12.5 µm diameter was mounted onto a micro-manipulator and brought in contact with the crystal top surface. An optical microscope integrated with a digital camera and a spectrometer was used to record the lightemission characteristics of the LEDs. A microscopic optical image of a complete device at 0 V is shown in Figure S3a. Figure 2a shows the current-voltage (I-V) characteristics of one microplatelet LED measured at ambient temperature. The current had a rapid increase at low voltages, followed with a gradual growth after 5 V and turned to saturate after 15 V. Light emission was clearly visible from the LEDs at 3 V (Figure S3b). It is interesting to note that the light emission was not uniform likely caused by a waveguide effect of the perovskite microplatelet: at 3 V bias, a light spot was seen close to one edge of the sample. At 8 V bias, an intense light spot appeared around the center of the microplatelet, accompanied by noticeable light emission from middle of the side surfaces (Figure S3c). At 18 V, the emission intensified in the center region, and light emission from middle of all the four side surfaces became quite bright (Figure S3d). Such a periodic emission pattern can be caused by a whispering-gallerymode (WGM) cavity that is formed by the four side-faces of the microplatelet.32 It is worth mentioning our devices did not employ any EILs or HILs. The relative energy-


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level diagram of ITO, perovskite and Au is schematically drawn in Figure S4. Very large energy offsets are identified between the ITO anode and the valence band maximum of the perovskite (1.0 eV), and between the Au cathode and the perovskite’s conduction band minimum (1.7 eV), respectively.33 It was postulated ionic migration under an external electrical field led to the formation of a p-i-n junction in the single-crystalline perovskite emitter: the perovskite became n-doped next to the cathode, and p-doped next to the anode (Figure 4a).34 The doping facilitates interfacial band bending between the electrodes and the perovskite emissive layer, thus lowering the energy barriers for electron and hole injection.15,35,36 While it still remains unclear what ionic species (the cation, anion and/or complex ions) contribute to such an ion migration process,37–41 recent studies by Shao et al.42 and by Yun et al.43 provided important insights on the migration pathways of such ions: it was discovered the ions predominantly migrated along grain boundaries in polycrystalline perovskites. In another study, Dong et al. observed the formation of a p-i-n junction on the surface of a single-crystalline halide perovskite under a lateral electrical field,44 demonstrating the ions can also migrate along the surfaces of perovskite single crystals. In this work, we obtained light emission at 3 V for our microplatelet OHP LEDs with an emissive layer thickness of ~10 µm. This result suggests ions can also migrate through singlecrystalline perovskites under an external electrical field as low as 0.3 V µm-1 to form a vertical pi-n junction that is demanded for efficient charge carrier injection and EL in our OHP LEDs. The formation of a vertical p-i-n junction in the microplatelet OHP LEDs was further supported by photocurrent response measurements as shown in Figures 2b and c. The I-V characteristics were collected for one pristine LED device by sweeping the voltage from 0 to 1 V while keeping the device in the dark or under a white light (Xenon lamp, 25 mW cm-2) illumination. The photo-generated short-circuit current (Isc) and open-circuit voltage (Voc) were


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both negligible (Figure 2b, black and red curves) due to the large energy barriers along the electrode/perovskite interfaces that diminished charge carrier collection efficiency under photo excitation. The same device was then subjected to a 10 V bias for one minute. I-V characteristics were measured again as shown in Figure 2b (blue and green curves). An Isc of 5.6 nA and a Voc of 0.9 V were obtained under light irradiation, in agreement with the junction formation hypothesis after the 10 V biasing. Isc was also monitored over time with a pulsed light illumination as shown in Figure 2c. After the 10 V biasing, the Isc showed a decreasing tendency with time, indicating the disappearing of the junction likely due to relaxation of the previously accumulated ions through a thermal diffusion process towards their equilibrium positions at a zero bias. The diffusivity of ions in OHPs is expected to decrease with decreasing temperature, therefore the ions can be immobilized below a certain temperature. In one experiment, a pristine microplatelet crystal was cooled down to liquid nitrogen temperature (-193 ºC), and the I-V characteristics were measured as shown in Figure S5. Noticeably, the device exhibited a much smaller current compared with the same measurement at ambient temperature, and no light emission was observed at 6 V. Thus, it is conjectured that the in situ formed junction at ambient temperature can be stabilized at a low temperature by eliminating the thermal movement and relaxation of ions after formation of the junction. Such an approach has been previously applied to realize “frozen junction” polymer light-emitting electrochemical cells with success.45,46 To test such a hypothesis in OHPs, an LED was biased with 6 V at ambient temperature. The bias was maintained while the device was cooled down to -193 ºC at 60 ºC min-1. The bias was then removed after the temperature reached -193 ºC. Photocurrent responses were measured under a pulsed light illumination as shown in Figure 3a. In contrast to the result shown in Figure 2c, a nearly constant Isc was observed over time, proving the junction was preserved even after the


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bias was removed at -193 ºC. A slight decay of photocurrent started to appear when the temperature was elevated to -110 ºC (Figure 3b). More substantial photocurrent decay took place at -50 ºC and -10 ºC as shown in Figure 3c and 3d, respectively. The above results indicate that the junction formed at ambient temperature can be frozen by lowering the temperature below 110 ºC. The I-V characteristics of the perovskite LEDs with a frozen junction were measured at 193 ºC as shown in Figure S6a. Very little hysteresis was observed. In contrast, significant hysteresis was seen when the device was measured at ambient temperature. The photocurrent evolution over time in Figure 2c, 3d, and 3c was reproduced in Figure ∆

S7a-c, respectively. The relative photocurrent decay ( ) can be well fitted using an exponential

equation (1), where and are the photocurrents at t=0 and at a specified time t, respectively; τ is a time constant correlates to the mobile ions relaxation processes back to their equilibrium positions after removing the bias. ∆

=

∝ [1 − exp( )]

(1)

The value of τ was obtained as 46.2, 77.6, and 573.5 s at 25 ºC, -10 ºC and -50 ºC, respectively. According to the Arrhenius formula, equation (2) is valid where ∆E represents the atomistic activation energy for ion migration processes, is the Boltzmann constant and T is the temperature. ∆

∝

(2)

The plot of vs. T-1 is shown in Figure S7d. An activation energy of 0.197 ± 0.042 eV is calculated from the slope of the linear fitting line. It is worth noting that Meloni et al. have experimentally measured an activation energy of 0.168 ± 0.043 eV for ion migration processes in MAPbBr3.41 Moreover, they obtained 0.2-0.28 eV in their theoretical simulation by assuming a mechanism of vacancy-assisted migration of bromide anions. Our measurement here confirms 8 ACS Paragon Plus Environment

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their experimental findings and agrees well with the theoretical simulation that bromide ions dominate the ion migration processes in MAPbBr3. Single-crystalline OHP LEDs with a frozen junction were also characterized. The device structure follows the one used in Figure 2a. The device was biased at 6 V at ambient temperature, then cooled to -193 ºC, and the bias was removed. A microscopic optical image of such a device is shown in Figure 4a inset. I-V characteristics, emission spectra, and representative optical images of the LED at different voltages are shown in Figure 4a, b and c-g, respectively. Light emission was detected at 1.8 V (Figure S8), which had largely shifted towards lower voltages when compared with control devices measured at ambient temperature. The turn-on voltage of ~1.8 V is 0.5 V lower than the bandgap (Eg, 2.3 eV)/e of the MAPbBr3. Such results suggest that the frozen junction at low temperature works very efficiently to assist electron and hole injection into the OHP microplatelet crystals. The measured light-emitting intensity increased with the applied voltage until 3.6 V, but decreased at 4.0 V (Figure 4b) likely due to a reduced efficiency of light collection in our current measurement setup as more and more photons emitted from the side surfaces (Figure 4e-g). Two peaks can be identified in all the EL spectra in Figure 4b, one at 542-544 nm and the 2nd at 548-551 nm, attributed to the same exciton-exciton scattering process as discussed for Figure 1c. The relative intensity of the 548-551 nm peak (Figure S9) was enhanced with increasing voltage after 2.4 V, indicating the exciton-exciton scattering process had become more significant at a high injection current density. Such a tendency correlates well with the PL evolution with different excitation light intensities as shown in Figure 1c. A slight red-shifting trend was also observed for the 2nd emission peak after 2.8 V, due to enhanced exciton-exciton scattering at a high exciton density.47 It is worth mentioning that exciton-exciton scattering has


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been used as a mechanism to realize stimulated light emission at a low excitation threshold in zinc oxide semiconductor thin films.48,49 Therefore, our LED result in this work could be further optimized in the future to achieve electrically-driven excitonic laser based on solution processed OHPs. Finally, the stability of the OHP LEDs with a frozen junction was examined through a stress test with a constant current (1mA) at -193 ºC. The light-emitting intensity was monitored by a silicon photodiode. A starting luminance of ~5,000 cd m-2 was estimated from the measured photocurrent, corresponding to a current efficient of 0.05 cd A-1. These numbers are underestimated since the photons escaping from the side surfaces were not collected by the photodiode. As shown in Figure 5, the luminance decreased to about 92% of the starting value after about two hours. Interestingly, the trend then reversed and the light intensity obtained a maximum of 115% after 40 hrs, and maintained 112% of the starting intensity after 54 hrs, respectively. A microscopic image in Figure 5 inset and a video in the supporting information were taken at 12 hrs after the stress test started, demonstrating the intense and steady green light emission from the LED. In contrast, the LEDs showed flickering emission and each flickering lasted for tens of seconds when they were tested at ambient temperature (supporting video, and Figure S10), suggesting the instability of the in situ formed junction at this temperature due to continuous migration of the ions. Such an observation signifies the importance of immobilizing the ions after the junction formation for achieving stable and long-lasting OHP electroluminescent devices.

CONCLUSION We demonstrated single-crystalline OHP LEDs with an ITO anode, MAPbBr3 microplatelet


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emitter, and Au cathode. It was discovered that ion migration could occur through the OHP microplatelets at ambient temperature upon applying a relatively low electrical field (~ 0.3 V µm1

). A light-emitting junction was formed, which decayed at ambient temperature after removing

the external electrical field. The junction was stabilized by lowering the temperature below -110 ºC to freeze the ionic movement and eliminate their thermally activated relaxation processes. As a result, efficient and stable LEDs have been achieved that turned-on at 1.8 V and lasted for at least 54 hrs with a luminance of ~5,000 cd m-2 without degradation. Although the ultra-long lifetime is achieved at a cryogenic temperature, the work provides insight on manipulating ion migration with more practical approaches for future generation OHP LEDs. In addition, the WGM cavity formed by the smooth outer surfaces and sharp edges and the exciton-exciton interaction in the MAPbBr3 microplatelets can be potentially useful for achieving electricallydriven laser diodes based on OHP semiconductors.

EXPERIMENTAL SECTION Materials. Lead (II) bromide (99.999%), dichloromethane (anhydrous, 99.8%) and N, Ndimethylformamide

(anhydrous,

99.8%)

were

purchased

from

Sigma–Aldrich.

The

methylammonium bromide was purchased from “1-Material Inc.” All materials were used as received. MAPbBr3 microplatelet synthesis and characterizations. The MAPbBr3/DMF precursor solution was prepared by dissolving PbBr2 and CH3NH3Br with a 1:1 molar ratio in anhydrous DMF to give a molar concentration of 0.02 M. The ITO/glass substrates (20 Ω sq-1) were cleaned subsequently with acetone, isopropanol and DI water and then treated with oxygen plasma for two minutes (FEMTO SCIENCE CUTE-MPR, oxygen flow 50 sccm, power 50W). The


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substrates were placed in a Teflon beaker contained 200 µL MAPbBr3/DMF precursor solution. The Teflon was placed in a glass beaker contained 3 mL DCM which was then sealed by a Parafilm and kept for 24 hours at ambient temperature. SEM images of as synthesized microplatelets were acquired using a field emission SEM (JEOL 7401F). The acceleration voltage was set at 10 kV. Powder XRD patterns was recorded using an XRD (X’PERT Pro MPD) equipped with Cu kα radiation source. PL was measured using a 488-nm excitation laser using a confocal Raman system (Renishaw). LED characterizations. Current-voltage characteristic was measured using a Keithley 2410 Source Measure Unit. The LEDs were tested under a nitrogen atmosphere inside a Linkam THMS600 cold/hot stage. The EL spectra were recorded using the same Raman system as PL measurement.

SUPPORTING INFORMATION Supplemental figures (PDF), and videos of LEDs at room temperature (MPG) and at liquid nitrogen temperature (MPG)

ACKNOWLEDGEMENTS The authors are thankful for the financial support from Air Force Office of Scientific Research under Award FA9550-16-1-0124, and the support from National Science Foundation under Award ECCS-1609032. We thank Dr. Jin Gyu Park for assisting the PL and EL spectra collection, thank the Materials Characterization Laboratory of FSU for the XRD measurement, and thank Dr. Qinglong Jiang for technical discussions. M. C. thanks the support from China Scholarship Council (CSC) under Grant 201508320105.


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Figure 1. a) A SEM image, b) XRD, and normalized PL spectra of the MAPbBr3 microplatelets. 18 ACS Paragon Plus Environment

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Figure 2. a) I-V characteristics of one representative MAPbBr3 microplatelet LED at ambient temperature. b) I-V characteristics of one LED measured with (light) and without (dark) light irradiation before and after electrical biasing. The electrical biasing was carried out at 10 V for 19 ACS Paragon Plus Environment

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one minute. c) Photocurrent (at V=0) responses of a pristine and a pre-biased device under a pulsed light illumination.

Figure 3. Photocurrent (at V=0) responses from the MAPbBr3 microplatelet LEDs under a pulsed light illumination at different temperatures, a) -193 ºC, b) -110 ºC, c) -50 ºC, and d) -10 ºC. Before the measurements, the LEDs were pre-biased at 6 V at ambient temperature, cooled to 193 ºC, and the bias was removed.


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Figure 4. a) I-V characteristics of an LED with a frozen junction. The LED was pre-biased at 6 V at ambient temperature, cooled to -193 ºC, and the bias was removed before the measurement. (Inset: a microscopic optical image of the LED at 0 V). b) Electroluminescence spectra of the frozen junction LED at different applied voltages. c)-g) Microscopic optical images of the LEDs at different applied voltages.

Figure 5. Relative light emission intensity vs. time of an LED operated at a constant 1mA current at -193 ºC. The LED had a starting luminance of ~5,000 cd m-2. Inset shows a microscopic image of the LED at t =12 hrs. 21 ACS Paragon Plus Environment

Manipulating Ion Migration for Highly Stable Light-Emitting Diodes

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