In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly

Mar 9, 2017 - Tel: +82-31-299-4716., *E-mail (D. Kim): [email protected]. ... drying process was developed to preserve nanocrystals in the film, whi...
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In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient LightEmitting Diode Jin-Wook Lee,†,¶,□ Yung Ji Choi,‡,¶ June-Mo Yang,† Sujin Ham,‡ Sang Kyu Jeon,† Jun Yeob Lee,*,† Young-Hyun Song,§ Eun Kyung Ji,∥ Dae-Ho Yoon,§,∥ Seongrok Seo,⊥ Hyunjung Shin,⊥ Gil Sang Han,# Hyun Suk Jung,§ Dongho Kim,*,‡ and Nam-Gyu Park*,† †

School of Chemical Engineering, §Department of Material Science and Engineering, ∥SKKU Advanced Institute of Nanotechnology (SAINT), and ⊥Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 440-746, Korea ‡ Department of Chemistry, Yonsei University, Seoul 120-749, Korea # Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States S Supporting Information *

ABSTRACT: Excellent color purity with a tunable band gap renders organic−inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated. KEYWORDS: perovskite, light-emitting diode, high efficiency, nanocrystal, type I band alignment

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Unique optoelectronic properties of organic−inorganic halide perovskites have been extended to various optoelectronic applications.12−16 Halide perovskites are considered to be good candidates for light-emitting diodes (LEDs) because of their high photoluminescence quantum yield (PLQY) together with easily tunable band gap and narrow PL emission characteristics.17−25 However, the current efficiencies (CEs) of perovskite LEDs (PrLEDs) were reported to be mostly below 1 cd/ A, with the maximum external quantum efficiency (EQE) as low as ∼1%,13,17−21,24,25 which was due to an inappropriate manipulation of charge carriers in the perovskite.

eports on long-term-stable solid-state perovskite solar cells in 2012,1,2 following the attempts to use organic− inorganic halide perovskites as a light harvester in a dye-sensitized solar cell structure,3,4 changed the paradigm in photovoltaics and triggered research activities on perovskite photovoltaics. Power conversion efficiencies (PCEs) of perovskite solar cells increased from 9.7%1 to 22.1%5 as of September 2016. The extremely high PCE is attributed to superb optoelectronic properties of halide perovskites. Absorption coefficients of both CH3NH3PbI3 (methylammonium lead iodide = MAPbI3) and HC(NH2)2PbI3 (formamidinium lead iodide = FAPbI3) perovskites exceed 105 cm−1 in the visible region, and their charge carrier diffusion lengths are almost balanced and exceed 100 μm.1,6−9 Moreover, strong Pb lonepair s orbital and I p orbital antibonding coupling along with high ionicity enables unusual defect-tolerant characteristics.9−11 © 2017 American Chemical Society

Received: January 26, 2017 Accepted: March 9, 2017 Published: March 9, 2017 3311

DOI: 10.1021/acsnano.7b00608 ACS Nano 2017, 11, 3311−3319

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Figure 1. Effect of excess MABr on the morphology of MAPbBr3 crystal. (a) Schematic illustration of adduct approach for as-spun MAPbBr3 films formed from precursor solutions with xMABr:PbBr2 (x = 1, 2, and 3). (b−d) Transmission electron microscopic (TEM) images and photos (inset) of the films for x = 1, 2, and 3. Samples for TEM images were prepared by peeling off the as-spun films (without heat treatment) and dispersing them with toluene.

formation of adducts from the shift of the SO stretching vibration of DMSO from 1045 to 1010 cm−1 for PbBr2·DMSO and to 950 cm−1 for MABr·PbBr2·DMSO.33 Only a small trace of pure DMSO was identified in the adducts, indicating that most of the DMSO takes part in complexation with the Lewis acid DMSO. The adduct approach was compared with a simple one-step spin-coating process (method A in Figure S2a) and an antisolvent (diethyl ether) crystallization method in the absence of DMSO (method B in Figure S2a). The adduct approach was found to be more efficient in forming the highly uniform MAPbBr3 film reproducibly compared to the simple one-step or the antisolvent process (Figure S2b−g). MAPbBr3 islands were formed by method A. More uniform morphology can be obtained by method B but with poor adhesion on the substrate. The highly crystalline MAPbBr3 was formed by method C (adduct approach) using a stoichiometric precursor (MABr:PbBr2 = 1:1) as measured by X-ray diffraction (XRD) (Figure S3a). The strongest and reproducible absorbance is observed for the MAPbBr3 film formed by method C (Figure S3b). Nevertheless, the PL intensity of 2.3 × 105 of MAPbBr3 made by method C was relatively low (Figure S3c), which could be attributed to charge carrier trapping or nonradiative recombination processes of long-lived charge carriers in the bulk crystal.20,22,23 Since the morphology and luminescence intensity of perovskites were found to be significantly affected by the composition of the adduct solution,34 we tried to control the ratio between MABr and PbBr2 in the adduct solution (Figure 1a). Hereafter, we denote the ratio between MABr and PbBr2 as the moles of MABr with respect to 1 mol of PbBr2 (x in Figure 1a). Interestingly, changing the stoichiometry from x = 1 to x = 3 results in significant alternation in color from yellowbrown to green, as shown in the inset of Figure 1b, c, and d. Since this color change might be associated with local structures of MAPbBr3, the local morphology of the film was investigated by transmission electron microscopic (TEM) images in Figure 1b, c, and d. For the stoichiometric sample (x = 1), large MAPbBr3 crystals exceeding 200 nm in size were already formed before thermal annealing. As nonstoichiometric

An extremely high PLQY exceeding 90% was reported from the surface-passivated perovskite nanocrystals (NCs) with an average diameter of 7.57 nm formed in organic solvent.26,27 The NCs passivated with organic surfactant were reported to strongly confine the charge carriers, leading to a fast recombination rate (τav) of 1−4 ns.28−30 However, PLQY was significantly degraded to less than 20% upon formation in the form of a thin film due to aggregation of NCs.27 Therefore, it is quite challenging to realize highly efficient LEDs based on the thin film with perovskite NCs or the relevant nanocrystalline films that retain high PLQY. A dramatic improvement in the CE and EQE was recently reported by elimination of metallic Pb and formation of nanograins, which led to a reduction of the nonradiative recombination pathway and an acceleration of radiative recombination of excitons.20 However, the nanograins were still ca. 100 nm in average size (from SEM image) and interconnected, which resulted in rather long radiative recombination lifetimes (τ) in the range of 40−60 ns and a calculated diffusion length as short as 67 nm, implying there is further room for improvement.20 We report here a method to prepare highly luminescent nanocrystalline MAPbBr3 self-formed in an MABr matrix based on Lewis acid−base adduct and solvent-vacuum drying approaches. Addition of excess MABr to the adduct solution led to the formation of ca. 12-nm-sized MAPbBr3 nanocrystals after spin-coating, where the nanocrystals were stabilized in an unreacted MABr matrix. The scaffolding MABr played critical roles in passivating surface traps and confining charge carriers in the nanocrystalline MAPbBr3 film. The highly luminescent nanoscrystalline MAPbBr3 film enabled a highly efficient green PrLED with a CE of 34.46 cd/A and an EQE of 8.21%.

RESULTS AND DISCUSSION MAPbBr3 thin films were prepared via a Lewis acid−base adduct,31,32 where MABr, PbBr2, and dimethyl sulfoxide (DMSO) were dissolved in dimethylformamide (DMF). The molar ratio DMSO:PbBr2 was fixed to 2:1, but the MABr:PbBr2 ratio was varied from 1:1 to 2:1, 3:1, and 4:1. The Fourier transform infrared (FT-IR) spectrum in Figure S1 confirms the 3312

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Figure 2. Crystallinity, optical absorption and emission, and carrier recombination mechanism depending on x in xMABr:PbBr2. (a) (100) XRD peak of MAPbBr3 films prepared using a precursor solution with different MABr ratios in xMABr:PbBr2, x = 1, 2, 3, and 4, in which empty circles are measured data and solid lines are fit results based on the Gaussian distribution function. (b) Absorption, (c) steady-state photoluminescence (PL), and (d) time-resolved PL spectra of the MAPbBr3 films prepared on a glass substrate using precursor solutions with different x. Empty circles represent the measured data, while solid lines are fit results based on biexponential decay.

Grancini et al. previously reported that a reduction in the size of the perovskite crystal has an effect on the orientational and electrostatic disorder of the organic cations, which lowers Eb.22,23,37 In this regard, we concluded that smaller crystallite sizes manifested in our nonstoichiometric samples (Figures 1 and 2a) resulted in reduced excitonic features and a slightly larger band gap. The film for x = 4 shows significantly diminished absorbance in the whole wavelength region, which will be discussed later. In Figure 2c, the PL spectra of the MAPbBr3 films are demonstrated. The PL intensity increases significantly from 2.02 × 105 for x = 1 to 6.08 × 105 (3-fold increment) for x = 2 and to 5.66 × 106 (28-fold increment) for x = 3. However, the PL intensity decreases to 1.36 × 106 as x increases further from 3 to 4, which is associated with the decreased absorbance as shown in Figure 2b. In the normalized PL spectra (inset of Figure 2c), the PL peak showed a blue shift with peak broadening (fwhm increased from 28.6 to 29.4, 31.6, and 35.0 nm as x increases from 1 to 2, 3, and 4, respectively) as MABr is increased, which also can be attributed to a lowered exciton binding energy.22,23,38 The film for x = 3 shows the brightest green emission under 365 nm ultraviolet light (Figure S5), which coincides well with PL data. As shown in Figure 2d, the photoluminescence decay profiles of the MAPbBr3 film with x = 1, 2, and 3 are obtained at 545, 542, and 538 nm, respectively, by a time-correlated single photon counting (TCSPC) technique to investigate their charge carrier relaxation dynamics. In order to protect the samples from air and moisture, the samples were wrapped with poly(methyl methacrylate) (PMMA), and our measurements were carefully conducted in an Ar-purged atmosphere. The decay profile of the stoichiometric sample (x = 1) exhibits biexponential decay even at low excitation power, ∼1 nJ, with a fast initial decay (τ1 ≈ 700 ps, inset of Figure 2d) and a slower decay arising from the well-known bimolecular free carrier recombination in bulk perovskites (τbulk > 20 ns);30,34,39−42 the detailed parameters are listed in Table S1. The fast decay component (τtrap) is known as a trapping to surface and/or bulk

precursors are used (x = 2 or 3), however, MAPbBr3 crystals become much smaller. For x = 2, shapeless MAPbBr3 crystals with a size of ca. 100−200 nm were created. For excess MABr of x = 3, spherical shaped MAPbBr3 nanocrystals were formed with a size of about 12 nm in diameter, where nanocrystals (yellow arrow in Figure 1d) were separated and embedded in an MABr matrix (black arrow in Figure 1d). The spin-coated adduct films were heat-treated at 65 °C for 1 min followed by a heat treatment at 100 °C for 2 min to eliminate the residual DMSO for further characterization. The (100) XRD peaks of thermally annealed MAPbBr3 films with different MABr:PbBr2 ratios are shown in Figure 2a. The entire XRD spectra are presented in Figure S6a. As the amount of MABr increased, the (100) peak intensity decreased and the peaks corresponding to MABr appeared (Figure S4). The unreacted excess MABr is separately precipitated out as a matrix in the resulting film. The full-width at half-maximum (fwhm) of the (100) peak was calculated by fitting the peak with a Gaussian distribution function as shown in the inset of Figure 2a. The fwhm increases from 0.16° to 0.17°, 0.20°, and 0.23° as the ratio of MABr to PbBr2 increases from 1 to 2, 3, and 4, respectively, which is indicative of decreased crystallite size from 50 nm to 47, 40, and 34 nm. As the amount of MABr increased, the size of the stoichiometric MAPbBr3 crystallites also decreased, as observed in as-spun films. After annealing at 100 °C, the crystallites seem to become a little larger. Figure 2b and c show the absorption and PL spectra of thermally annealed MAPbBr3 films. The absorption onset of the film is around 540 nm, which correlates with the reported band gap of MAPbBr3 (2.28 eV).35,36 A closer inspection reveals that an excitonic feature, a distinct peak near the absorption edge, is clearly diminished along with a slight blue shift as the MABr ratio (x) increases. The decreased excitonic feature in Figure 2b indicates that the exciton fraction with respect to free carriers decreases, which can be attributed to the lower exciton binding energy (Eb), an energy gap between exciton states and the free carrier continuum, thereby slightly increasing the band gaps. 3313

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Figure 3. (a) Schematic representation showing crystallization and carrier recombination mechanisms for x = 1 and 3. NP represents nanoparticle. For the excess MABr case, a type I structure was formed with larger band gap MABr (4.0 eV) and a smaller one for MAPbBr3 (∼2.2 eV). (b) Ultraviolet photoelectron spectroscopy (UPS) spectra of MAPbBr3 films prepared using precursor solutions with different MABr ratios in xMABr:PbBr2 (x = 1, 2, 3, and 4) and MABr film. (c) Magnified UPS spectra in the low photon energy region. The MABr film was prepared by spin-coating the precursor solution containing 2 M MABr + 1 M DMSO in DMF. All the films were formed on ITO glass.

i). In the case of a nonstoichiometric solution, excess MABr acts like a surfactant to isolate small nuclei of MABr·PbBr2· 2DMSO adducts, leading to the formation of nanosized perovskite crystals in an MABr matrix, as confirmed by TEM in Figure 1d,44 which is followed by crystal growth induced by evaporation of residual solvent during thermal annealing at 100 °C (Figure S6b−d and f−h). It is clearly seen that MAPbBr3 becomes isolated by MABr as x increases (Figure S6k and i). However, with x = 4, some parts of the film showed complete phase separation of the MABr and MAPbBr3 (Figure S6d and h), which is responsible for decreased absorbance and PL intensity with x = 4 in Figure 2. As depicted in Figure 2e, the nanocrystalline MAPbBr3 probably can confine the electron and hole pairs to accelerate the radiative recombination pathway.28 Ultraviolet photoelectron spectroscopy (UPS) was measured in Figure 3a and b to investigate the local band alignment in the microstructure. The valence band maximum (VBM) and conduction band minimum (CBM) of MAPbBr3 films (vs vacuum) were measured to be 5.75−5.86 eV and 3.51−3.62 eV, respectively, while the CBM and VBM of MABr film were measured to be 2.64 and 6.64 eV,45 respectively. As a result, the passivated excess MABr for x = 3 can form type I band alignment with MAPbBr3 (lower panel in Figure 3a), which more effectively confines the charge carriers of MAPbBr3 nanocrystals, and consequently, radiative recombination is accelerated as confirmed by time-resolved PL (tr-PL) in Figure 2d.46 Although the thermal annealing of the adduct film is inevitable to eliminate DMSO in the adduct and the residual solvent, it is hard to preserve the size of the 12-nm-sized nanocrystalline MAPbBr3 for x = 3 after thermal annealing due to aggregation of nanocrystals by heat treatment, as confirmed

defects (such as vacancies, interstitials, and antisites) usually observed in picosecond to femtosecond time scales.14 On the other hand, nonstoichiometric samples of x = 2 and 3, with considerably improved PL intensities (Figure 2c), show a negligible portion of the fast decay component (τtrap), which implies nonradiative trapping processes are effectively suppressed by remnant MABr passivating defects.34 As we compared the average charge carrier decay (τav) of each sample, the film with x = 3 exhibits a faster τav of 9.2 ns than those of x = 2 (12.7 ns) or x = 1 (17.0 ns), although the former, as mentioned above, shows much brighter PL emission than the others, which is indicative of the accelerated radiative recombination with smaller crystallite size.22 Especially, the decay components (τcorrelated) of 6.1 ns were observed only in samples with x = 3, implying the presence of a different charge recombination pathway with the nonstoichiometric precursor solution; the detailed trace fit data are listed in Table S1. This additional decay pathway is probably attributed to the weak confinement effect of 12-nm-sized nanocrystals existing in the nanocrystalline structure of these samples (Figures 1 and 2).43 In this confinement regime, correlated carrier species in perovskite nanocrystals may play a role in a more efficient radiative recombination pathway and consequently enhance the PL emission.28,43 In Figure 3a, the correlations between microstructures and recombination processes are schematically represented for the MAPbBr3 crystals formed from stoichiometric (x = 1) and nonstoichiometric solution (x = 3). For stoichiometric solution, bulk MAPbBr3 crystals (>200 nm) were formed from the MABr·PbBr2·2DMSO adducts after removal of weakly bound DMSO.33 As a result, a uniform film with the large grains of MAPbBr3 was formed as observed by SEM (Figure S6a, e, and 3314

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Figure 4. (a) Schematics showing post-treatment processes to eliminate the DMSO from the MAPbBr3 adduct film. The film was prepared with x = 3 and thermally annealed (TA sample) or treated with chloroform dripping and vacuum (SVD sample). (b, c) Fluorescence images and (d, e) local photoluminescence (PL) measurement showing localized PL decay, in which the measured spots are marked in (b) and (c), respectively. Insets of (d) and (e) show photos of corresponding film under visible and UV light.

Figure 5. (a) Voltage−current efficiency and (b) luminescence−external quantum efficiency (EQE) graph of the device comprising an MAPbBr3 film prepared by thermal annealing (TA) or solvent-vacuum drying (SVD) treatment. Electroluminescence (EL) spectra of the devices are presented in the inset of (a).

by scanning electron microscopic (SEM) (Figure S6c, g, and k). The aggregation of nanocrystals reduces the passivation by MABr and increases the grain boundaries, which might impede the effective radiative recombination. To eliminate DMSO without thermal annealing (TA), nonpolar chloroform that is miscible with DMSO was dropped before the end of spincoating to wash out DMSO, which was followed by vacuum treatment to eliminate DMSO completely (Figure 4a). We define this process as “solvent-vacuum drying (SVD)”. Surface SEM images of the MAPbBr3 film (x = 3) prepared by the SVD method are presented in Figure S7. As can be seen in Figure S7,

the aggregated MAPbBr3 crystals, which were observed in the film prepared by TA (Figure S6c), are hardly observed in the film prepared by the SVD method, and the observed smooth surface is probably MABr, passivating MAPbBr3 nanoscrystals. Confocal PL microscopy images of thermal annealed and SVD samples are shown in Figure 4b and c, respectively. In the TA sample, bright fluorescence regions were widespread due to aggregated nanocrystals, while bright regions were focused on the separated spheres and brightness was enhanced in the SVD sample, which indicates that the formed MAPbBr3 nanocrystals were less aggregated with the SVD method. In Figure 4d and e, 3315

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CONCLUSIONS In this study, we developed highly luminescent nanocrystalline perovskite MAPbBr3 thin films based on a nonstoichiometric method with excess MABr and a nonthermal solvent-vacuum drying approach. An increase in MABr content in the precursor solution generally reduced the crystal size and accelerated the carrier recombination kinetics. MABr:PbBr2 = 3:1 and DMSO:PbBr2 = 2:1 compositions dissolved in DMF led to the highest PL intensity and fastest carrier recombination among the studied nonstoichiometric compositions. The unreacted MABr played an important role in forming the type I structure. Nanocrystalline MAPbBr3 induced by a 3 mol excess of MABr was further stabilized in the film by the SVD method, which accelerated the photoluminescence decay time by more than 2 times compared to the thermal annealing method due to preservation of nanocrystals in the film. Highly efficient green LED performance with a maximum EQE of 8.21% and CE of 34.46 cd/A was demonstrated using the nanocrystalline MAPbBr3 film prepared by the SVD approach. It is worth emphasizing that nanosized perovskite was successfully formed in situ in the form of a film, not via a colloidal QD approach, and realized highly efficient LED performances. Our approach will provide important insight into perovskite LEDs and other optoelectronic applications.

the tr-PL decay profiles were obtained at the representative region marked as red and blue circles in Figure 4b and c, respectively; the tr-PL decay profiles at other regions are shown in Figures S8 and S9. The tr-PL decay profiles of the TA and SVD samples have been fitted with a bi- and single-exponential decay model, respectively; the detailed trace fit data are listed in Table S2. A much faster average PL lifetime is detected for the SVD sample (τav = 3.9 ns) than the TA sample (τav = 8.3 ns), as calculated by using the weighted average PL lifetimes, which suggests the radiative recombination rates were further increased in the SVD samples compared to the TA samples. Photos of the TA and SVD samples under visible and UV light are presented in the insets of Figure 4d and e, respectively, in which brighter PL was observed with the SVD sample. Thus, the observed local tr-PL decays of the TA and SVD films could be direct evidence that fast recombination decay, as a result of correlated electron and hole pairs in the nanocrystals, is responsible for much stronger PL in the SVD sample. We have prepared LED devices based on the MAPbBr3 film with x = 3 prepared by the SVD method. It is noteworthy from conductive atomic force microscopy measurements as shown in Figure S10 that the MABr thin film coated ITO (indium-doped tin oxide) glass (Figure S10b and d) is highly conductive compared to the bare ITO glass (Figure S10a and c), which means that excess MABr present in the film is conductive enough to transport the charge carrier to be injected to the MAPbBr3 nanocrystals. The current density−voltage−luminescence (J−V−L) curves of the devices are shown in Figure S11, while CE−voltage (Figure 5a) and EQE−luminance (Figure 5b) are compared between the TA and SVD films. Electroluminescence (EL) spectra of the devices are presented in the inset of Figure 5a, and the measured parameters are summarized in Table 1. The LED device comprising the

EXPERIMENTAL METHODS Synthesis of CH3NH3Br. CH3NH3Br (MABr) was synthesized by reacting 30 mL of CH3NH2 (TCI, 40 wt % in methanol) with 50 mL of HBr (Aldrich, 48 wt % in H2O). The CH3NH2 solution was placed in a round-bottom flask with magnetic stirring, to which a HBr solution was added dropwise. The reaction proceeded in an ice bath for 2 h. The white precipitate was collected by evaporation using a rotary evaporator at 60 °C. The collected white precipitate was washed with diethyl ether three times and recrystallized using ethanol. The white solid of MABr was collected by filtration and dried for 24 h at 60 °C under vacuum before use. Fabrication of MAPbBr3 Films. The perovskite layer was formed via the adduct of xMABr·PbBr2·2DMSO.31,32 The precursor solutions for the adduct were prepared by dissolving 1 mmol of PbBr2 (Alfa Aesar, 99.999%), x mmol (112 mg for x = 1, 224 mg for x = 2, 336 mg for x = 3, and 448 mg for x = 4) of MABr, and 2 mmol of DMSO (156 mg, Sigma, >99.9%) in 494 mg of DMF (Sigma-Aldrich, 99.8%). The solution was fully dissolved at room temperature and filtered using a 0.45 μm-pore-sized syringe filter before use. The glass substrates were cleaned with detergent and ultrasonicated in an ethanol bath, which were treated with UV-ozone for 15 min before spin-coating of the precursor solutions. The solution was spin-coated on the cleaned glass substrate at 4000 rpm for 30 s (acceleration = 1000 rpm/s), to which 0.4 mL of diethyl ether was dropped after 10 s of spinning. As-spun films were investigated by TEM. The spin-coated adduct film was dried at 65 °C for 1 min and 100 °C for 2 min, which is designated as the thermal annealing sample (TA sample). A nonthermal annealing method was developed, where chloroform was dropped on the spincoated adduct film and then the film was dried under vacuum for 10 min. This process is designated as the solvent-vacuum drying process (SVD sample). For spectroscopic measurements and some fundamental studies, PMMA (Alfar Aesar) in chlorobenzene (0.1 g/mL) was spin-coated on top of the perovskite layer. Characterization. The morphology of the thin films was investigated by using a transmission electron microscope (JEOL Ltd., JEM-2010) operating at 200 kV with a CCD camera. After drying briefly in ambient conditions, preannealed films were carefully scraped with a razor blade and dispersed in toluene. Then, this solution was disposed on a Cu grid and dried for TEM measurement. SEM images were measured by JSM7500F (JEOL), in which the distribution of the Pb element was analyzed by element distribution (EDS) measurement.

Table 1. Performance of Light-Emitting Diodes Depending on the Annealing Processa LED parameter

TA

SVD

CEmax (cd/A) EQEmax (%) luminancemax (cd/m2)

4.35 1.03 6.38 × 102

34.46 8.21 6.95 × 103

a

Maximum current efficiency (CEmax), external quantum efficiency (EQEmax), and luminance (Luminancemax) of LED devices employing MAPbBr3 film (x = 3) prepared by thermal annealing (TA) and solvent-vacuum drying (SVD) processes.

SVD sample shows the smaller fwhm (23 nm) in EL than that of the TA sample (29 nm), along with a blue shift due to a size effect. Furthermore, the maximum CE is significantly enhanced from 4.35 cd/A (TA sample) to 34.46 cd/A (SVD sample). The device with the SVD film shows a maximum EQE of 8.21% at luminance = 5651.5 cd/m2, which is 7-fold higher than that of the device with the TA film (1.03%). Such a high EQE is attributed to the nanocrystalline MAPbBr3 film stabilized by the SVD process and the excess MABr passivating the surfaces of nanocrystals. We measured time-dependent luminance of the MAPbBr3 LED devices prepared by the TA and SVD method. As can be seen in Figure S12, the luminance of both devices decayed to 20% of the initial value in 150 s. However, such a fast degradation under the operating conditions, which was also observed in previous studies,21,47 has not been unveiled yet and will need further investigation. 3316

DOI: 10.1021/acsnano.7b00608 ACS Nano 2017, 11, 3311−3319

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ACS Nano The band structure of the MABr and MAPbBr3 was analyzed by ultraviolet photoelectron spectroscopy using He I (21.2 eV) as a light source. X-ray diffraction patterns were collected by an X-ray diffractometer (D8 Advance, Bruker Corporation), in which Cu Kα radiation was used with a scan rate of 4 °/min. Current and topological local microscopic images were measured by an atomic force microscope (SPA-400, SII, Japan) using Pt/Ir-coated Si tips (CONTPt-W, Nanoworld, Inc.) with a typical resonant frequency of 13 kHz and a spring constant of 0.2 N/m. All images were acquired with a bias voltage of 30 mV at a scan rate of 0.5 Hz. Positive biases were applied to the substrate, while the tips were grounded. The current and topographic images were taken simultaneously in the ambient air. Ensemble Spectroscopic Measurement. Absorption spectra were measured by UV−vis spectrometer (PerkinElmer, Lamda35) equipped with an integrating sphere. The light was incident to the glass substrate side. Steady-state photoluminescence spectra were measured by a compact fluorescence lifetime spectrometer (Quantaurus-Tau C11367-12, Hamamatsu). The films were photoexcited with a 464 nm laser (PLP-10, Hamamatsu) pulsed at a frequency of 2 MHz. Time-resolved photoluminescence was detected using a TCSPC technique to measure spontaneous photoluminescence decay. The excitation light source was a mode-locked Ti:sapphire laser (MaiTai BB, Spectra-Physics, Santa Clara, CA, USA), which provides ultrashort pulses (80 fs at fwhm) with a high repetition rate (80 MHz). This high repetition rate may be slowed to 1 MHz to 800 kHz using a homemade pulse picker. The pulse-picked output pulse was frequencydoubled by a 1-mm-thick BBO crystal (EKSMA, Vilnius, Lithuania). The photoluminescence was collected by a microchannel plate photomultiplier (R3809U-51, Hamamatsu) with a thermoelectric cooler (C4878, Hamamatsu) connected to a TCSPC board (SPC-130, Becker&Hickl GmbH, Berlin, Germany). The overall instrumental response function was ∼25 ps (fwhm). A pump pulse vertically polarized by a Glan-laser polarizer was used to irradiate the samples, and a sheet polarizer, set at an angle complementary to the magic angle (54.7°), was placed in the photoluminescence collection path to obtain polarization-independent photoluminescence decays. Spatially Resolved Photoluminescence Images and Lifetime Measurements. Detection of PL was performed using an inverted microscope (TE2000-U) equipped with a sample scanning stage (XE120, Park Systems). Picosecond pulsed excitation light at 420 nm with a repetition rate of 250 kHz (LDH-D-C-420, Picoquant) was circularly polarized using a Berek compensator (5540, New Focus), directed into the microscope, using a laser line filter (FF01-420/10-25, Semrock) and collimating lens, and then focused on the sample via an oil immersion objective (Plan Fluor, 1.4 NA, 100×, Nikon). The lateral resolution of our measurements is determined by the optical diffraction limit (∼350 nm), making the imaging system suitable for direct visualization of perovskite films. The fluorescence signals were collected using the same objective, passed through a dichroic mirror (T425lpxr, Chroma Technology), and spectrally filtered using a notch filter (HNPF-420.0-1.0, Kaiser Optical Systems) and a band-pass filter (FF-01-430/LP-25, Semrock). The fluorescence signals detected with the APD were registered by a PC card operated in first-in-first-out (FIFO) mode; for each detected photon, the system recorded the arrival time after the beginning of acquisition (with 20 ns resolution) and the time lag with respect to the excitation pulse (with 6 ps resolution). The data were processed using the BIFL data analyzer (Scientific Software Technologies Center) to reproduce the fluorescence intensity trajectories with a user-defined binning time. By using photons belonging to a user-defined region in the intensity trajectories, we were able to reproduce the fluorescence decays. LED Device Fabrication and Characterization. Device architecture of the green perovskite light emitting devices was ITO (120 nm)/PEDOT:PSS (60 nm)/MAPbBr3 (880 nm)/TPBi (50 nm)/LiF (1 nm)/Al (200 nm). ITO was indium tin oxide, PEDOT:PSS was poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonate) (Aldrich Product No. 560596), and TBPi was 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene. The patterned ITO glass substrate was cleaned using distilled water and 2-propanol for 30

min under sonication. The PEDOT:PSS solution was coated on the ITO substrate by spin-coating followed by baking at 140 °C for 10 min. The perovskite emitting layer was formed by an adduct approach (x = 3) as described above. TPBi, LiF, and Al were vacuum deposited at a base pressure of 1.0 × 10−7 Torr. Electrical and optical measurements of the green perovskite light-emitting device were performed using a CS2000 spectroradiometer and a Keithley 2635 source measurement unit. Lambertian distribution of light emission was assumed in the calculation of EQE.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00608. Time-resolved PL data for different MABr ratios and annealing methods, FT-IR spectra, SEM images, XRD patterns, EDS element mapping, UV−vis spectra, photos of luminescent films, fluorescence images, conductive AFM, current density−voltage−luminance curves, and time-dependent luminance in Tables S1 and S2 and Figures S1−S12 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (J. Y. Lee): [email protected]. Tel: +82-31-299-4716. *E-mail (D. Kim): [email protected]. Tel: +82-2-21232652. *E-mail (N.-G. Park): [email protected]. Tel: +82-31-290-7241. ORCID

Jun Yeob Lee: 0000-0002-7677-0605 Dongho Kim: 0000-0001-8668-2644 Nam-Gyu Park: 0000-0003-2368-6300 Present Address □

Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States.

Author Contributions ¶

J.-W. Lee and Y. J. Choi contributed equally to this work.

Author Contributions

N.G.P. and J.W.L. conceived of the concept and experiments, performed data analysis, and prepared the manuscript. J.W.L. and J.M.Y. prepared materials, fabricated devices, and performed characterization. D.K., Y.J.C., and S.H. performed time-resolved PL and TEM measurement and wrote the relevant part. J.Y.L. and S.K.J. fabricated and measured LED and wrote the relevant part. D.H.Y., Y.H.S., and E.K.J. measured XRD and commented on LED fabrication. H.S. and S.S. measured AFM and wrote the relevant part. H.S.J. and G.H. commented on fabrication of LED. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF2012M3A6A7054861 and NRF-2014M3A6A7060583 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF-2012M3A7B4049986 (Nano Material Technology Development Program), and NRF-2016M3D1A1027664 (Future Materials Discovery Program). This work was also 3317

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ACS Nano

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supported by Basic Science Research Program through the NRF under contact 2016R1A2B3008845.

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