Efficient light-emitting diodes based on oriented perovskite nanoplatelets.

Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

INTRODUCTION

Photon emission characteristics in semiconductors are mediated by transition dipole moments (TDMs). The optical TDMs of inorganic nanostructures with reduced dimensions, such as nanoplatelets and nanorods, are highly anisotropic (–). This unique structure-property relationship is of interest for planar light-emitting diodes (LEDs) because the outcoupling efficiency of the devices is fundamentally correlated to the orientation of emissive TDMs (–). In general, TDMs that are horizontally oriented with respect to the electrode interface are favored for light outcoupling, while the vertically oriented TDMs largely contribute to the energy loss (see figs. S1 and S2 for detailed illustrations) ().

Metal halide perovskite is an emerging class of solution-processed semiconductors with intriguing properties, such as high photoluminescence quantum yields (PLQYs) and tunable emission wavelengths (–). Since the first report of the room temperature–operating perovskite LEDs (PeLEDs) in 2014 (), remarkable progress has been made on device efficiency (–). We note that these state-of-the-art planar PeLEDs are based on films with isotropic TDMs (table S1). Enhancing the ratio of horizontally oriented TDMs is expected to further improve light outcoupling and boost the upper limit for the external quantum efficiencies (EQEs) of PeLEDs.

Here, we report efficient LEDs based on in situ grown perovskite films, which simultaneously demonstrate high ratios of horizontal TDMs and high PLQYs. The emitters are oriented perovskite nanoplatelets with a high in-plane TDM ratio of ~84%, leading to LEDs with a light extraction efficiency of ~31%. In contrast, using isotropic emitters (in-plane TDM ratio of 67%) in the same device structure would limit the light extraction efficiency to ~23%. Furthermore, we find that the PLQY of the perovskite film can be boosted to more than 75% by introducing lithium bromide (LiBr) into the precursor solution. These combined efforts enable green PeLEDs with a record EQE of 23.6%, the highest value in planar PeLEDs.

RESULTS

Structural characterization of the in situ formed nanoplatelets

Figure 1A shows a cross-sectional view of our device, including the perovskite layer, analyzed by aberration-corrected scanning transmission electron microscopy (STEM). Our devices consist of multilayers of nickel oxide (NiO; ∼7 nm), poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine)/poly(9-vinylcarbazole) (TFB/PVK, ∼38 nm), perovskite (~9 nm), 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phenyl-1H-benzimidazole) (TPBi; ∼48 nm), lithium fluoride (LiF; ∼1 nm), and aluminum (Al; ∼100 nm) sequentially deposited onto indium tin oxide (ITO)–coated glass substrates (see Materials and Methods for details). The perovskite film was deposited from a precursor solution comprising LiBr, phenylbutylammonium bromide (PBABr), phenylethyl-ammonium bromide (PEABr), cesium bromide (CsBr), and lead bromide (PbBr2) dissolved in dimethyl sulfoxide (DMSO) with a molar ratio of 0.25:0.75:0.25:1.75:1.4.

Fig. 1.

Structural characterizations of the perovskite nanoplatelet films.

(A) A cross-sectional scanning transmission electron microscopy–high-angle annular dark-field (STEM-HAADF) image showing the continuous and pinhole-free perovskite layer. TPBi, 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phenyl-1H-benzimidazole); PVK, poly(9-vinylcarbazole). (B) A zoomed-in STEM-HAADF image showing the fine structure of a perovskite nanoplatelet. Inset: The corresponding fast Fourier transform (FFT) pattern. (C) A typical high-resolution transmission electron microscopy (HRTEM) image of the perovskite nanoplatelets dispersed on a copper grid. Inset: The corresponding FFT pattern. (D) Statistical diagram of the size distribution of the nanoplatelets measured by HRTEM. The average size is 25.8 nm and the corresponding SD is 6.8 nm. The Gaussian fitting is provided as a guide to the eye. (E) Grazing-incidence wide-angle x-ray scattering pattern. The diffraction spots originate from the crystal faces of nanoplatelets. The two diffraction spots at q = 1.065 and q = 1.070 Å−1 correspond to {001} and {010} of β-CsPbBr3, respectively.

The high-angle annular dark-field (HAADF) image indicates a continuous and pinhole-free perovskite film with a thickness of 8.6 ± 1.5 nm (Fig. 1A and fig. S3). Zoom-in observations (Fig. 1B) show an atomic-resolution image with well-resolved atom columns, revealing the high crystallinity of the perovskite nanoplatelets. The crystal structure of the nanoplatelet matches that of tetragonal β-CsPbBr3 (). The perovskite crystal is oriented with the {001} crystal face parallel to the substrate surface. Atomic force microscopy characterizations on a perovskite film show a low root mean square surface roughness of ~1.2 nm (fig. S4), which is in line with the cross-sectional observations. The perovskite nanoplatelets were transferred and dispersed onto a copper grid for high-resolution transmission electron microscopy (HRTEM) analyses (). The results (Fig. 1C and fig. S5) indicate that the perovskite crystals are nanoplatelets with an average lateral size of 25.8 ± 6.8 nm (Fig. 1D). The Grazing-incidence wide-angle x-ray scattering (GIWAXS) measurement (Fig. 1E) of a perovskite film shows discrete diffraction spots. The diffraction spot on q = 1.065 Å−1 corresponds to a real-space distance of 5.89 Å, which can be assigned as the d-spacing of the {001} crystal face of tetragonal β-CsPbBr3. These features suggest that the assemblies of the perovskite nanoplatelets are highly ordered and all nanoplatelets share the same orientation with the {001} crystal face parallel to the substrate. The GIWAXS data, together with the STEM-HAADF and HRTEM observations, suggest that the perovskite films consist of a monolayer of face-on oriented nanoplatelets with an average thickness of 8.6 ± 1.5 nm and a lateral size of 25.8 ± 6.8 nm.

Optical analyses of the in situ formed nanoplatelet film

We expect the electronic and optical properties of the perovskite film to be influenced by the quantum confinement effect because the out-of-plane dimension of the nanoplatelets is comparable to the Bohr diameter of CsPbBr3 (7 nm) (). The photoluminescence (PL) spectrum of the perovskite film shows the symmetric shape and its peak position is at 516 nm (Fig. 2A), corresponding to an optical bandgap of 2.41 eV. This value is larger than the bandgap of bulk β-CsPbBr3, ~2.36 eV (). The excitonic absorption features of quasi–two-dimensional (2D) perovskites in the strong confinement regime, such as n = 2 and n = 3 (n is the number of PbBr4 octahedral layers within a crystallite) layered perovskites, are absent in the ultraviolet-visible absorption spectrum (Fig. 2A). This feature suggests that our film composed of perovskite nanoplatelets is distinctive from the previously reported perovskite films of multiple quantum wells or quantum dots embedded in quasi-2D phase (, , ). The perovskite nanoplatelet film exhibits a high PLQY of ~75% at a low excitation power density of ~0.02 mW/cm2 (Fig. 2B). This result indicates efficient radiative recombination of the photogenerated excitons in the perovskite nanoplatelets.

Fig. 2.

Optical properties of the perovskite nanoplatelet films.

(A) Absorption and PL (excited by a 405-nm laser) spectra. a.u., arbitrary units. (B) Excitation intensity–dependent PLQY. The error bars represent the experimental uncertainties in the PLQY measurements at 0.4 mW/cm2 and the errors in the determination of relative PL intensities and excitation power.

The orientation of TDMs of the perovskite nanoplatelet film is quantified by the ratio of horizontal TDMs, Θ. This parameter is defined as Θ = p∥/ (p∥ + p⊥), where p∥ and p⊥ stand for contributions from horizontal and vertical components of the optical TDMs, respectively. We used the angle-dependent PL technique, which provides an ensemble measurement (photoexcitation spot size: ~1 mm) of the intensity of p-polarized emission against the detection angle (see fig. S6 for schematic illustration) (, , ). The experimental data (Fig. 3A) are fitted to the pattern simulated using the classical dipole radiation model (, , ). The Θ of our perovskite nanoplatelet film is determined to be 84%. This value is substantially higher than that of isotropic emitters (67%). Furthermore, we analyzed the light emission of the perovskite film by using back focal plane (BFP) spectroscopy. BFP spectroscopy (see fig. S7 for schematic illustration) probes a small targeted region of the perovskite nanoplatelet film by using a laser with a spot size of ~750 nm for photoexcitation (–, ). The BFP pattern and the corresponding line-cut data along the p-polarized direction are shown in Fig. 3 (B and C, respectively). An important feature of the horizontally oriented dipoles is that the p-polarized intensity is minimum at k// = k0, where k// is the in-plane wave vector with respect to the substrate and k0 is the wave vector in the vacuum. By fitting the line-cut data in Fig. 3C, Θ is determined to be ~87%. The BFP data of four spots from different regions (fig. S8) demonstrate excellent spatial uniformity of the orientations of TDMs in our film. The results of both measurements unambiguously demonstrate that the emission of our perovskite nanoplatelet film is dominated by horizontal TDMs.

Fig. 3.

Orientations of the TDMs of the perovskite nanoplatelet films.

(A) Angle-dependent PL measurements of the perovskite film on a quartz/TFB/PVK substrate. The experimental data (gray squares) are fitted by the classical electromagnetic dipole model (red line), giving a horizontal TDM ratio of 84 ± 4%. (B) Back focal plane (BFP) image of a perovskite film. (C) p-polarized line cut (gray line) along the dashed line in of the BFP image (B). This line cut is fitted with a horizontal TDM ratio of 87% (red solid line).

We suggest that the anisotropy of local fields in the nanoplatelet, i.e., dielectric confinement, largely contributes to the anisotropy of the dipole orientation (, ). The local electric field normal to the nanoplatelet plane, E, is reduced (screened) due to the dielectric confinement, while the local field in the horizontal directions, E, is less affected. The spontaneous radiative transition probability of the nanoplatelet, which is proportional to the square of local fields, is thus substantially reduced in the normal direction. For our closely packed oriented nanoplatelets, the local field in the horizontal directions is expected to be even less affected owing to the near continuum of the high dielectric constant perovskite material in the lateral directions.

Key factors affecting the perovskite films

Two issues, namely, the concentrations of the bulky organic ammonium cations and the introduction of LiBr in the precursor solution, are critical for the formation of oriented perovskite nanoplatelet film with high PLQY. Without the use of the bulky organic ammonium cations, the resulting perovskite film shows an optical bandgap of ~2.37 eV (fig. S9A) and excitation intensity–dependent PLQY (fig. S9C). These features indicate the formation of 3D CsPbBr3 crystals. Doubling the concentration of the bulky organic ammonium cations leads to the formation of perovskite films with strong excitonic absorption peaks at ~430, ~460, and ~ 475 nm, which corresponds to the n = 2, n = 3, and n = 4 layered perovskites, respectively (fig. S9B). The emission peak of this perovskite film locates at ~501 nm, implying efficient energy transfer from the perovskites with small n values (larger bandgaps) to the emissive centers with smaller bandgaps. The emissive centers are determined to be nanocrystals with a size of ~10 nm (fig. S9D). BFP measurements on the films processed from the precursor solution with various contents of bulky organic ammonium cations indicate that only the oriented perovskite nanoplatelet films have high ratios of horizontal TDMs (fig. S10). Furthermore, control experiments show that other investigated parameters, i.e., the concentration of the precursor solution, the choice of bulky organic ligands, and the underlying substrates, have little impacts on the Θ values of perovskite films (fig. S11). We suggest that the horizontal orientation of the perovskite nanoplatelets on flat substrates may originate from the van der Waals interactions as reported in the literature (, ). Besides, the ultrathin film of ~9 nm would also favor horizontally aligned perovskite nanoplatelets ().

Further experiments show that the introduction of LiBr in the precursor solution is beneficial for improving the PLQY of the perovskite film. GIWAXS, angle-dependent PL, and BFP measurements (fig. S12) on the perovskite films processed from the precursor solution without LiBr indicate the formation of oriented nanoplatelets with anisotropic emission (Θ: 84%). The PLQY of perovskite nanoplatelet films processed from the precursor solution with and without LiBr is ~75% and ~ 50% (fig. S13), respectively. Temperature-dependent PL characterizations (fig. S14) indicate a scenario that the with-Li sample has fewer energy levels below excitonic levels and, thereby, fewer nonradiative losses of photo-generated excitons. We suggest that the introduction of LiBr in the precursor solution may result in perovskite nanoplatelets with better surface passivation.

Characterization of the PeLEDs

The electroluminescence (EL) spectrum of our perovskite nanoplatelet film (Fig. 4A) displays a symmetric peak centered at ~518 nm with a full width at half maximum of 16 nm (74 meV), representing one of the narrowest emission line widths for high-efficiency PeLEDs (, –, –). The ultrapure green emission corresponds to Commission Internationale de l’Eclairage color coordinates of (0.09, 0.78) (fig. S15). The angular emission intensity of our PeLEDs follows the Lambertian profile (Fig. 4B). The EL spectra at different viewing angles are identical (fig. S16). The current density–voltage–luminance curves of a typical device are shown in Fig. 4C. Owing to the pinhole-free morphology of the nanoplatelet film, the device shows negligible leakage current. The current density and luminance increase rapidly once a turn-on voltage of ~3 V is reached. At 7 V, the device shows a brightness of ~3140 cd/m2. The champion device demonstrates a peak EQE of 23.6% (Fig. 4D), which is a record efficiency among PeLEDs (table S1). The statistical diagram of 36 devices (Fig. 4E) indicates an average EQE of 21.3% with a small relative SD of 4.4% demonstrating the excellent reproducibility of our green LEDs. The device shows a typical T50 lifetime of ~15 min at an initial luminance of ~1080 cd/m2 (fig. S17).

Fig. 4.

Device characterizations of the green LEDs based on the perovskite nanoplatelet films.

(A) EL spectrum. Inset: Photograph of an operating green LED (effective area: 3.24 mm2). (B) Angular distribution of the EL intensity follows the Lambertian profile. (C) Current density–luminance–voltage characteristics of a typical device. (D) EQE-voltage relationship of the device with a champion EQE of 23.6%. (E) Histogram of peak EQEs from 36 devices. The Gaussian fits are provided as a guide to the eye. (F) Contour plot of the simulation results of device EQE as a function of PLQY and Θ of the perovskite emissive layer. The device structure shown in (A) is used for the simulation. The refractive indexes of the multilayers are obtained by ellipsometer. For our perovskite nanoplatelet film with a PLQY of ~75% and a Θ of 84%, the optical simulation predicts a maximum EQE of ~23.3%.

We performed optical simulations on the PeLEDs by using the classical dipole model developed for planar microcavities (see fig. S18 for details) (, ). The result suggests a light-outcoupling efficiency of 31.1% for our perovskite devices based on the oriented nanoplatelet film with Θ of 84% (fig. S18B). In contrast, a control device based on isotropic TDMs (Θ: 67%) would have a lower outcoupling efficiency of ~23.4%. Considering that the PLQY of our perovskite nanoplatelet film is ~75%, our optical simulation predicts a maximum EQE of 23.3% (Fig. 4F), which agrees well with the experimental results. Further optimizing the PLQY and enhancing Θ of the perovskite films shall push the upper limit of EQEs to ~40% (Fig. 4F).

We highlight that previous work aiming to control the orientations of TDMs focuses on assemblies of anisotropic colloidal nanostructures (–). High-efficiency EL would require syntheses of anisotropic colloidal nanostructures with high PLQY controlling the orientations of the anisotropic colloidal nanostructures to realize films with high ratios of horizontal TDMs, minimizing energy transfer between the neighboring nanoemitters to maintain high PLQYs and efficient and balanced charge injection into the individual nanoemitters. Fulfilling these stringent requirements is challenging and often imposes dilemmas in material design and assembly. As a result, the EQEs of the LEDs based on colloidal anisotropic perovskite nanoemitters are currently lower than 5% (see table S2 for more information) (). Expanding the scope to all solution-processed planar LEDs using inorganic emitters with oriented TDMs (table S2), the reported highest EQE is 12.1% (). In contrast, our LEDs based on in situ grown perovskite nanoplatelet films demonstrate high EQEs of up to 23.6%.

DISCUSSION

We demonstrate that controlling the orientation of TDMs of the perovskite films overcomes the light-outcoupling limitation of planar LEDs with isotropic emitters, leading to green LEDs with exceptionally high EQEs of up to 23.6%. Considering the chemical versatility of perovskite materials, our facile approach of in situ grown nanoplatelet films is readily extended to the fabrication of differently colored LEDs with high EQEs. Our work represents a simple yet effective approach of exploiting anisotropic optical properties of nanostructures in optoelectronic devices.

MATERIALS AND METHODS

Materials

TFB was purchased from American Dye Source. PBA (>98.0%), PEA (>98.0%), nickel acetate tetrahydrate (>99.0%), and chlorobenzene (99.8%) were purchased from Acros. DMSO (99.9%), CsBr (99.999%), LiF (99.99%), and hydrobromic acid (48 weight % in water) were purchased from Alfa Aesar. PVK (with a molecular weight of 25,000 to 50,000 g mol−1), LiBr (99.999%), ethanolamine (99%), and PbBr2 (99.999%) were purchased from Sigma-Aldrich. TPBi was purchased from Xi’an Polymer Light Technology Corp.

NiOx precursor was prepared by following a literature method (). PBABr and PEABr were synthesized following the instruction of the previous report (). Briefly, hydrobromic acid (2.56 ml) was slowly added into a solution of PEA (3 ml) in methanol (20 ml) at 0°C, and then stirred for 2 hours. Next, the solution was evaporated at 45°C to obtain precipitates, which were washed thrice with a mixed solution of ethyl acetate and diethyl ether with a volume ratio of 1:1. Then, the precipitates were dried at 50°C in the vacuum for 24 hours.

The precursor solution for the perovskite nanoplatelet film was prepared by mixing a CsPbBr3 precursor solution (1 ml), a LiBr precursor solution (0.25 ml), and DMSO (0.5 ml). The CsPbBr3 precursor solution was prepared by mixing PBABr (18.9 mg), PEABr (5.5 mg), CsBr (40.8 mg), and PbBr2 (56.2 mg) in DMSO (1 ml) followed by stirring for 2 hours at 50°C. The LiBr precursor solution was prepared by dissolving LiBr (9.55 mg) in DMSO (1 ml).

Device fabrication

The NiOx precursor was deposited on ITO-coated glass substrates (square resistance of 20 ohms) by a literature method (). The TFB (in chlorobenzene, 8 mg/ml) was spin-coated at 2000 rpm and then annealed at 150°C for 30 min. Then, the TFB film was spin-rinsed by chlorobenzene, leaving an ultrathin layer. Next, the PVK layer was deposited by spin-coating PVK solution (in chlorobenzene, 14 mg/ml) at 2000 rpm, followed by annealing at 150°C for 30 min. The perovskite film was prepared by spin-coating the precursor solution at 4000 rpm for 2 min, followed by annealing at 100°C for 25 min. Next, the TPBi (48 nm), LiF (1 nm), and Al (100 nm) were sequentially deposited by using a thermal evaporating system (Trovato C300) at a high-vacuum environment (under 2 × 10−7 torr). The device area defined by the overlapping area of the ITO and Al electrodes was 3.24 mm2.

Structural and optical characterizations

The cross-sectional samples were prepared by using focused ion beam equipment (Quata 3D FEG). The TEM observations were conducted using a Cs aberration–corrected scanning transmission electron microscope, Titan G2 80-200 ChemiSTEM microscope, operated at 200 kV.

Atomic force microscopy analyses were conducted with a Cypher-S (Asylum Research) atomic force microscope located in a glovebox filled with nitrogen. GIWAXS measurements were performed at Beamline 12-ID-B of Advanced Photon Source (Argonne National Laboratory, USA). A PerkinElmer detector, XRpad 4343F, was applied in the measurements with a sample-to-detector distance of 18 cm. The energy of x-ray radiation was 13.3 keV.

The angle-dependent PL spectra of the perovskite nanoplatelet films (deposited on quartz/TFB/PVK substrate) were collected by using a molecular orientation characteristic measurement system C14234-11 (Hamamatsu Photonics K.K., Japan; fig. S6). The range of angles is 0° to 90°. The excitation wavelength is 365 nm.

The BFP imaging experiments were performed on an optical system as schematically shown in fig. S7. The excitation wavelength is 457 nm and the diameter of the excitation spot is ~750 nm.

The refractive index (n) and extinction coefficient (k) of all layers of the PeLEDs were measured by an ellipsometer (J.A. Woollam, USA). Ultraviolet-visible absorption spectra of the samples were collected by using a Cary 5000 (Agilent) spectrophotometer.

Steady-state PL, transient PL, and temperature-dependent PL spectra were performed on an Edinburgh Instruments (FLS920) spectrometer. Excitation density–dependent PLQYs of the perovskite films were obtained by following the published method (). The angle dependence of emission intensity of the PeLED was measured using a Thorlabs PDA100A detector at a fixed distance of 200 mm from the EL device ().

All LEDs were measured in a glovebox filled with nitrogen at room temperature. A system consisting of an integration sphere (FOIS-1), a Keithley 2400 source meter, and a QE-Pro spectrometer (Ocean Optics) was used for the measurements (). The devices were scanned from zero bias at a rate of 0.1 V s−1. The integral time for each step is 500 ms. The EL characteristics of the LEDs were cross-checked at Zhejiang University (Yizheng Jin group), University of Cambridge (Richard Friend group), and Nanjing Tech University (Jianpu Wang group).