Junction Propagation in Organometal Halide PerovskiteâPolymer

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Letter

Junction Propagation in Organometal Halide Perovskite-Polymer Composite Thin Films Xin Shan, Junqiang Li, Mingming Chen, Thomas Geske, Sri Ganesh R. Bade, and Zhibin Yu J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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The Journal of Physical Chemistry Letters

Junction Propagation in Organometal Halide Perovskite-Polymer Composite Thin Films Xin Shan1, Junqiang Li1, Mingming Chen1, Thomas Geske1,2, Sri Ganesh R. Bade1, and 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 *

Email: [email protected]

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Abstract With the emergence of organometal halide perovskite semiconductors, it has been discovered that a p-i-n junction can be formed in situ due to the migration of ionic species in the perovskite when a bias is applied. In this work, we investigated the junction formation dynamics in methylammonium lead tribromide (MAPbBr3)-polymer composite thin films. It was concluded that the p- and n- doped regions propagated into the intrinsic region with an increasing bias, leading to a reduced intrinsic perovskite layer thickness and the formation of an effective light-emitting junction regardless of perovskite layer thicknesses (300 nm to 30 µm). The junction propagation also played a major role in deteriorating the LED operation lifetime. Stable perovskite LEDs can be achieved by restricting the junction propagation after its formation.

Table of contents graphic Turn-on Voltage (V)

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3

2 3.6 um 600 nm 450 nm 300 nm

Emissive Layer Thickness


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Organometal halide perovskites have recently emerged as new types of semiconducting materials.1–4 The power conversion efficiencies of perovskite solar cells have increased rapidly from 3.8 % to more than 22 % within the past few years, approaching the state-of-the-art efficiency of silicon based solar cells.5–10 A number of research groups have also reported high brightness, high efficiency perovskite LEDs.11–22 A majority of those achievements were made possible by employing a multilayer device structure with an intrinsic halide perovskite layer as the light absorber or emitter, and layers of dissimilar materials to enhance both electron (electron transportation layers, ETLs) and hole (hole transportation layers, HTLs) transportation. In contrast, perovskite solar cells and LEDs with a single-layer device structure have recently been demonstrated. It was discovered that a homogeneous p-i-n junction could be developed when an external bias was applied on the perovskite thin film. The pristine perovskite layer becomes p-doped next to the anode and n-doped at the cathode side.23,24 This new approach of junction formation has enabled very efficient charge carrier collection or injection without use of additional HTLs or ETLs.25–27 Such a single-layer device architecture is beneficial for the scalable manufacturing of large area perovskite electronics using solution-based printing processes.28 Nonetheless, reported single-layer perovskite LEDs and solar cells thus far have exhibited lower efficiencies compared to the best performing multilayered devices.25,28 For instance, 42.9 cd A-1 was obtained by Cho et al. in their perovskite LEDs with a self-organized conducting polymer as the anode and HTL, a MAPbBr3 thin film as the emissive layer, and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI)/LiF as the ETLs.12 15.9 cd A−1 was achieved by Chih et al. using a device structure of NiOx/ MAPbBr3/TPBI/LiF.29 However, only 4.9 cd A-1 was reported in single-layer LEDs based on MAPbBr3.28 Such an



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efficiency gap may be largely due to limited understanding of the in situ junction formation process in the single-layered devices. In this work, the junction evolution process in MAPbBr3/polymer composite thin films were investigated using short-circuit current (with and without light irradiation) and AC impedance measurements. It was discovered the doped regions propagated into the intrinsic region upon applying an external electric field, leading to a reduced thickness of the intrinsic perovskite layer. Such a doping propagation phenomenon was further supported by observing single-layer perovskite LEDs with nearly the same low turn-on voltages regardless of perovskite layer thicknesses (300 nm to 30 µm). Ion migration has been reported in both polycrystalline and single-crystalline halide perovskite thin films.30–39 Recently, it has been discovered that mixing a polymer with the halide perovskite to form a composite can largely improve the continuity of the perovskite thin film.26– 28

To confirm the ion migration process in the MAPbBr3/polymer composite film, a test device

was fabricated by depositing a MAPbBr3/polymer composite film onto an indium tin oxide (ITO)/glass substrate. A eutectic indium-gallium (In-Ga) was used as the top electrode. The thickness of the MAPbBr3/polymer layer was about 30 µm. A bias of 4 V was applied to the device for one minute with the ITO connected as the anode and In-Ga as the cathode. The discharge current after short-circuiting the device was then monitored over time with and without a light source irradiation. The results are shown in Figure 1a. The device exhibited a noticeable discharge current and the photocurrent was superimposed on the discharging current once the light source was turned on. The photocurrent was about 10 µA after removing the 4 V bias, exponentially decreased to ~ 2.4 µA after 75 seconds. In contrast, a freshly prepared ITO/MAPbBr3 and polymer composite/In-Ga device had a photocurrent of ~1 µA under the


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Figure 1. Short-circuit current over time with and without a light source irradiation from a device with ITO/MAPbBr3 and polymer composite/In-Ga. a) the device was pre-biased at 4 V for one minute, and b) from a pristine device without biasing history. c) Nyquist plot of impedance measurements for the pristine device, and after biased it at 5 V and 10 V, respectively. d) A simplified Randles circuit model for simulating the arc regions of the impedance spectra in Figure 1c. Experimental data in Figure 1c are shown as discrete squares, and the dotted lines represent simulation results using the Randles model for the arc regions. e) Summary of the contact resistance (Rct), series resistance (Rs), and capacitance (c) following the Randles circuit model for the pristine device, and after 5 V and 10 V biasing. same test condition, and no time dependent decay was observed (Figure 1b). The observed discharge current and enhancement of photocurrent in the pre-biased device agree well with the ion migration and junction formation hypothesis in the perovskite thin film under an applied bias,24 and their time dependent decay correlates to the disappearing of the junction after



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removing the bias. It is worth mentioning that the decay of the discharge current and photocurrent in Figure 1a were not caused by the degradation of the perovskite. As shown in Figure S1, the charge-discharge process was repeated for four times and most the photocurrent could be resumed at the start of a new discharge cycle. We also believe charge trapping and detrapping mechanism may not play a significant role for the exponential decay of photocurrent due to the relative fast photocurrent responses to light on/off in both the poled and pristine devices. The normalized discharge current at 25 °C is shown in Figure S2. The curves can be well fitted using an exponential decay equation with two decay constants. The occurrence of two decay constants possibly indicates either two ionic species or two migration pathways had contributed to the ion migration and junction formation processes. We further investigated the junction formation process at different applied voltages with AC impedance spectroscopy. The device was first biased for one minute at an indicated DC bias, then switched to an AC bias (100 mV) with a frequency range from 1 MHz to 100 Hz for impedance measurements. As shown in Figure 1c, the Nyquist plots exhibited a straight line at the high frequency region and another straight line at the low frequency region, both of which pointed towards the 45° diagonal direction. The two straight lines were caused by diffusion of ionic species in the film.40,41 An arc region could be identified within intermediate frequencies that corresponds to the charge transfer processes between the electrodes and the perovskite film. The arc portion of the impedance spectra can be simulated using a simplified Randles circuit as shown in Figure 1d. The R represents a series resistance of the device, R is the total contact resistance quantifying the charge transfer process between the electrodes and the perovskite film, and C is the capacitance. Thus, the following equations 1 and 2 can be used to simulate the experimental data in Figure 1c, where is frequency.


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= + ()

(1)

| | =

()

(2)

The dotted lines in Figure 1c show the simulation results using the R , R , and C values in Figure 1e. It is clear that the R exhibits a decreasing trend when going from the pristine, to the 5 V biased and then the 10 V biased device. Such results indicate the charge transfer process had been greatly improved after the 5 V or 10 V biasing. This observation is in support of the junction formation hypothesis. It is noted the capacitance of the perovskite film increased by 100 % after biased at 5 V and 156 % after 10 V, respectively, compared to the pristine device. The solid-state ITO/perovskite composite/In-Ga device can be approximately viewed as a parallel plate capacitor. From the capacitance data, it is estimated the thickness of the intrinsic perovskite layer shrunk by 50% in the 5 V biased device and by 61 % in the 10 V biased device due to propagation of the n-doped and p-doped regions. In our experiments, there was about two-minute delay time from the end of the DC biasing to the start of impedance measurements. Thus, the above propagation depths were underestimated due to fast relaxation of the doped regions after removing the bias as evident by the discharge current and photocurrent responses in Figure 1a. To this end, it is clear the ion migration process in the MAPbBr3/polymer composites follows a scheme as shown in Figure 2a-c. The cations and anions are uniformly distributed at their equilibrium positions at zero bias (Figure 2a), which start to migrate toward the cathode and the anode, respectively, when a bias is applied. The perovskite becomes anion rich and p-type doped near the perovskite/anode interface. At the same time, n-doped perovskite is formed close to the perovskite/cathode, and a layer of intrinsic perovskite is preserved in the center of the film (Figure 2b). The p-doped and n-doped regions can propagate towards the intrinsic region with an 7 ACS Paragon Plus Environment


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Figure 2. a) Schematically shows the uniform distribution of cations and anions in a pristine perovskite film; b) migration of cations/anions under an external electrical field. The perovskite became p-type doped near the perovskite/anode interface and n-type near the perovskite/cathode, leading to a reduced intrinsic layer thickness; c) relaxation of the junction after removing the bias. increasing electrical field to reduce the portion of the intrinsic layer. Nonetheless, the accumulated ions will diffuse back and tend to recover their original equilibrium locations upon removing the bias, and the p-i-n junction will gradually disappear (Figure 2c). The in situ junction formation in halide perovskites under an external electrical field has shown to improve charge transportation and reduce effective thickness of the intrinsic layer in the perovskite film. Based on such knowledge, it is conjectured that halide perovskite LEDs may


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tolerate the use of a thick perovskite emissive layer without increasing turn-on voltages or sacrificing device efficiencies as compared to devices with a thin emissive layer. To verify such a hypothesis, we prepared perovskite/polymer composite films with different thicknesses and examined their electroluminescent characteristics in perovskite LEDs. Figure 3 a-d show the

Figure 3. a)-d) SEM images of the 300 nm, 450 nm, 600 nm, and 3.6 µm MAPbBr3/PEO composite films, respectively; e) the XRD patterns, and f) absorbance and PL spectra of the 450 nm MAPbBr3/PEO composite film. cross sectional scanning electron microscope (SEM) images of the composite films. From top to bottom, the film thickness varies from 300 nm, 450 nm, 600 nm to 3.6 µm. It should be noted the 300 nm, 450 nm and 600 nm films were processed by a spin-coating method at different spinning speeds, and the 3.6 µm film was prepared via drop-casting. The fabrication details can be found in the experimental section. The 300 nm, 450 nm and 600 nm films appeared to have similar 9 ACS Paragon Plus Environment


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morphologies consisting of one layer of perovskite crystals that are about a micron to a few micrometers in the lateral dimension. On the other hand, the drop-casted film shows a stack of perovskite crystals along thickness direction. The crystals have about the same size range as in the spin-coated films, but submicron gaps can be seen between many of the crystals. The crystallinity of the above films was characterized by X-ray diffraction (XRD) as shown in Figure 3e. All the films exhibited identical diffraction peaks corresponding to the cubic phase MAPbBr3 crystals.42 Figure 3f shows the absorbance and PL spectra of the 450-nm film. Both agree well with a direct bandgap of ~2.3 eV in MAPbBr3. The absorbance and PL spectra for the 300 nm, 600 nm and 3.6 µm films are similar to the 450 nm film except differing in absolute intensities. LEDs were constructed using the above perovskite/polymer composite films as the emitters. The inset in Figure 4a shows a schematic of the device structure: In-Ga was used as a cathode and ITO as an anode. Current density-voltage (J-V) and luminance-voltage (L-V) characteristics were collected as shown in Figure 4a and 4b for a set of LEDs with different film thicknesses. For each device, the J-V curve had a localized peak value at 1.4-1.5 V, then entered a region of negative differential resistance, and started to rise again after 2.0-2.2 V and obtained a maximum current density at 4.0-4.7 V. The L-V plots in Figure 4b indicate the onset of lightemission occurred at 2.4-2.6 V (defined at 1 cd m-2 luminance) for all devices, and peak light intensities were achieved at 4.0-4.6 V. The maximum luminance observed was 104,954 cd m-2 in the device with a 450-nm emissive layer. Such a high luminance places our device among one of the brightest perovskite LEDs based on MAPbBr3.29 The current efficiency-voltage (E-V) relationship is shown in Figure 4c. Maximum efficiencies of 1.9 cd A-1 to 3.8 cd A-1 were obtained, corresponding to external quantum efficiencies (EQEs) of 0.45 % to 0.9 %. Electroluminescence spectra (Figure 4d) of the LEDs had a peak intensity at 538 nm and a full


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Figure 4. a) Current density-voltage, (b) luminance-voltage, and (c) current efficiency-voltage characteristics of a set of MAPbBr3 LEDs with different film thicknesses. d) Electroluminescence spectrum and a photograph (inset) of a LED with 450 nm MAPbBr3 at 4 V. width at half maximum of 18 nm. The inset in Figure 4d shows a photograph manifesting the intense green light emission from an LED at 4 V. A device with a 30-µm thick emissive layer was also prepared. A cross-sectional SEM image of the emissive layer is shown in Figure S3 confirming the large thickness of the film. It is worth noting that the device was still functional as evident by the L-V and E-V characteristics shown in Figure S4. The device could be turned on at 3 V with a peak luminance of ~ 450 cd m-2 (at ~5 V) and a peak efficiency of 0.7 - 0.8 cd A-1 (at 4 - 4.5 V).



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To further assess the statistical differences of the LEDs with various emissive layer thicknesses, six to nine devices were measured from each sample group, and four performance metrics were compared as shown in Figure 5a-d. The turn-on voltages demonstrate a slight

Figure 5. Statistical summaries of (a) Turn-on voltage; (b) Voltage at maximum luminance; (c) Maximum efficiency; and (d) Maximum luminance for LEDs with various emissive layer thicknesses. Six to nine devices were measured for each sample group. decreasing trend with reduced emissive layer thicknesses. Nonetheless, the average turn-on voltages differed by only 0.2 V despite a thickness variation of 12 times (2.5 V in the 300 nm device vs. 2.7 V in the 3.6 µm device). Meanwhile, the average voltage to obtain maximum luminance intensities were also nearly the same for the 450 nm, 600 nm and 3.6 µm devices, which were only 0.3 V higher than the 300 nm device. The maximum current efficiency and 12 ACS Paragon Plus Environment

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luminance comparisons are presented in Figure 5c and 5d. While the 450 nm device showed the best performance, the other devices exhibited comparable efficiencies and luminance intensities. Our experimental observation of a high tolerance to thickness variations of the light-emissive layer in the perovskite LEDs once again supports the doping propagation hypothesis. Considering the junction propagation, it is speculated the original intrinsic layer can eventually disappear, and the p- and/or n- doped regions may even protrude into the opposite region(s) at a high bias or with a long operating time. Such a hypothesis explains the relatively short lifetime of reported organometal perovskite LEDs regardless the use of a single-layer or multilayer device structure.15,26 To verify this hypothesis, we investigated the temperature dependence of the ion migration process and its impact on LED lifetime. The relative currentvoltage, and relative luminance-voltage characteristics of the MAPbBr3 LEDs were measured at different temperatures as shown in Figure 6 a, b. All the devices had no poling history before the measurements. It was seen that the devices exhibited much lower current and no light emission up to a 7 V bias when the temperature was at -100 °C or -193 °C. Such findings suggest the ions became immobilized and no p-i-n junction was formed at or below -100 °C. The operation stability of the LEDs at different temperatures was also compared. For such measurements, the LEDs were first poled at 4 V at ambient temperature, cooled down to a desired temperature with the poling voltage, and light-emitting intensities were monitored with time. As shown in Figure 6c, the devices appeared very unstable at both -10 °C and -50 °C. However, a substantial improvement was observed at -100 °C. Such temperature dependence of device stability coincides with the temperature dependence of ion migration. Thus, we believe the ion migration and junction propagation is the dominant failure mechanism in perovskite LEDs. Figure 6d shows the lifetime comparison for LEDs at a liquid nitrogen temperature and at ambient



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Figure 6. a) Relative current-Voltage, and b) Relative luminance-Voltage of pristine MAPbBr3 LEDs measured at different temperatures. The devices had no poling history before the measurements. c) Luminance vs. time studies of MAPbBr3 LEDs at -10 °C, -50 °C, and -100 °C. d) Relative luminance vs. time studies of MAPbBr3 LEDs at ambient and liquid nitrogen temperatures. temperature, respectively. The luminance intensity dropped from 100 cd m-2 to 84 cd m-2 after 200 mins continuous operation at liquid nitrogen temperature, and to 65 cd m-2 after 630 mins. In comparison, the device suffered with a much faster degradation at ambient temperature, and the luminance dropped from 100 cd m-2 to 10 cd m-2 in 15 mins. The results again support the junction propagation as one of the key failure mechanisms in perovskite LEDs. Therefore, it is critical to properly manipulate the junction propagation dynamics towards stable and more practical perovskite LEDs in the future. 14 ACS Paragon Plus Environment

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In summary, we have presented experimental observations supporting the in situ junction formation and junction propagation in organometal halide perovskite-polymer composite thin films under an applied bias. Such a junction facilitated charge carrier injection in single-layer perovskite LEDs, and enabled a high tolerance to thickness variations of the emissive layer in such LEDs. Future work should be focused on optimizing the propagation dynamics with a goal to improve the device efficiencies and stability, while maintaining the benefits such a junction provides such as: a simplified device structure that is more compatible to scalable printing processes. Experimental Section Materials: Lead(II) bromide (99.999%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, anhydrous, 99.9%), poly(ethylene oxide) (PEO, average Mw ~5,000,000), poly(vinylpyrrolidone) (PVP, average Mw ~1,300,000), and indium-gallium eutectic (99.99%) were purchased from Sigma-Aldrich. Methylammonium bromide (MABr) was purchased from “1-Material Inc”. All materials were used as received. Film preparation and characterizations: The MAPbBr3 precursor solution was prepared by dissolving PbBr2 and MABr in a 1:1.5 molar ratio in anhydrous DMSO to give a concentration of 500 mg mL-1. PEO was dissolved in DMF with a concentration of 10 mg mL-1. PVP was dissolved in DMSO with a concentration of 50 mg mL-1. The MAPbBr3 precursor, PEO and PVP solutions were then mixed with 100:50:0.25 weight ratio. All the solutions were stirred at room temperature for 5 mins before use. The ITO/glass substrates (20 ohms sq-1) were cleaned subsequently with detergent water, deionized water, acetone and isopropanol for 10 mins with sonication, then blow dried with nitrogen and treated with oxygen plasma at 100 W power for 3 mins. For obtaining the 300 nm, 450 nm and 600 nm films, 20 µL mixture solution was dropped



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onto the ITO/glass and spun at 2000 rpm, 1500 rpm and 1000 rpm, respectively. The films were annealed on a hotplate at 95 °C for 3 minutes under a glass petri dish cover. For the 3.6 µm film, 20 µL solution was dropped onto the ITO/glass substrate and spread uniformly with a micropipette, then dried at 95 oC for 5 minutes. Preparation of the 30 µm film for the shortcircuit current and impedance measurements followed a similar procedure except that 180 µL solution and 15 minutes annealing time were used. All solution and film preparation were carried out in ambient (~50% relative humidity, 25°C). Commercial tools of field emission SEM (Zeiss 1540 EsB) and UV-Vis-NIR spectrometer (Varian Cary 5000) were used to characterize the composite thin films. Powder XRD patterns were recorded using X’PERT Pro MPD equipped with a Cu kα radiation source. PL was measured with a 488 nm excitation laser using a confocal Raman system (Renishaw). AC impedance was taken on a VersaSTAT 3 Potentiostat Galvanostat (Princeton Applied Research) using a 100 mV AC signal with frequency scanned from 1 MHz to 100 Hz. LED measurement: Device testing was carried out in a nitrogen filled glove box with oxygen and moisture level both at ~1 ppm. Current density-voltage and luminance-voltage characteristics were measured with a Keithley 2410 source meter and a silicon photodiode. The silicon photodiode was then calibrated by a Photo Research PR-655 spectroradiometer. 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. ImageJ was used to process a picture of the dark area and obtain the active area of the LEDs. EQEs were estimated by assuming a Lambertian emission profile. The EL spectrum was collected with the above confocal Raman system. Acknowledgements


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The authors are thankful for the financial support from National Science Foundation under Award ECCS-1609032 and the support from Air Force Office of Scientific Research under Award FA9550-16-1-0124. We also thank Dr. Qinglong Jiang for valuable technical discussions. Dr. Mingming Chen thanks the support from China Scholarship Council (CSC) under Grant 201508320105. Supporting Information Supplemental figures (Figure S1-4) References (1)

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