A Comparative Study of Light-Emitting Diodes Based onAll-Inorganic Perovskite Nanoparticles (CsPbBr3)Synthesized at Room Temperature and by a Hot-InjectionMethodBruno Clasen Hames,[a] Rafael S�nchez S�nchez,[b] Azhar Fakharuddin,[a, c]

and Iv�n Mora-Ser�*[a]

Introduction

All-inorganic perovskite nanoparticles (PeNPs) with the generalformula CsPbX3 (X = Cl� , Br� , I�) have gained attention in thescientific community, owing to their outstanding optoelectron-ic properties, such as extremely high photoluminescence quan-tum yield (PLQY; as high as 90 %); narrow emission spectra(full-width at half-maximum (FWMH)�12–42 nm); tunableband gap, depending on particle size and composition; lowpreparation cost; and abundance of precursor materials.[1–4]

Since their emergence in 2014, PeNPs have been consideredan excellent candidate as primary semiconductors in photode-tectors,[5] light-emitting electrochemical cells,[6] photochemicalconversion,[7] solar cells,[8, 9] lasers,[10] and light-emitting diodes(LEDs).[11–16] The particularly interesting optoelectronic charac-teristics of PeNPs make this family of materials an exceptionalalternative for the development of unprecedented high-qualityfull-color displays.

In just two years of research, LEDs based on PeNPs synthe-sized by the hot-injection (HI) method have reached externalquantum efficiencies (EQEs) beyond 8 %,[17] which is an out-standing achievement compared with other technologies.Nowadays, cadmium-based quantum dot (Cd-QD) LEDs, whichhave been extensively investigated during the last two de-cades, have a state of the art EQE�20.5 % for red LEDs.[18, 19]

Nonetheless, there are some limiting factors that hinder indus-trial applications for quotidian purposes. Cd-QDs employ haz-ardous elements, such as cadmium, in their synthesis and re-quire relatively high temperature; controlled atmosphere toavoid undesired oxidation reactions; the use of expensive rawmaterials; and, most importantly, sophisticated core/shell struc-tures to reach high PLQYs.

Soon after the first report of organic–inorganic hybrid PeNPsby P�rez-Prieto and co-workers,[20] Kovalenko and co-workerssynthesized,[1] for the first time, all-inorganic PeNPs through aHI method; a method that requires high temperature. Howev-er, a more recent low-temperature compatible synthesis wasreported in 2016.[21] The new methodology exploits a supersa-turated recrystallization method, which transfers Cs+ , Pb2+ ,and X� ions dissolved in N,N-dimethylformamide (DMF) to anonpolar solvent (toluene) at room temperature (RT). Surpris-ingly, these PeNPs also presented PLQY values above 90 %.This new method significantly simplifies the synthesis of semi-conductor nanocrystals without the need for high tempera-tures or a controlled atmosphere. Despite the relative simplici-ty of the synthetic method and outstanding optical propertiesof the nanocrystals, their application in an optoelectronicdevice has not yet been reported.

A crucial parameter to be taken into account towards thepreparation of high-efficiency LEDs based on PeNPs is the care-

Perovskite nanoparticles (PeNPs) have been extensively studiedfor optoelectronic applications, owing to their extremely highphotoluminescence quantum yield, tunable band gap, and ex-ceptionally narrow emission spectra. Therefore, PeNPs are con-sidered excellent candidates for the development of high-effi-ciency, low-cost, wide-gamut, and high-purity color displays.However, their synthesis typically involves multistep cumber-

some processes that might hinder commercial development.Herein, green light-emitting diodes (LEDs) prepared by usingall-inorganic PeNPs CsPbBr3 synthesized at room temperature(RT) are reported and their performance compared with thoseprepared by a traditional hot-injection method. Insights intothe morphology and optoelectronic properties of RT PeNPs areprovided through AFM and TEM and employing them in LEDs.

[a] B. Clasen Hames, Dr. A. Fakharuddin, Dr. I. Mora-Ser�Institute of Advanced Materials (INAM), Universitat Jaume I12071 Castell� (Spain)E-mail : sero@uji.es

[b] Dr. R. S�nchez S�nchezDepartment of ChemistryUniversity of LiverpoolCrown St. , L69 3BX Liverpool (UK)

[c] Dr. A. FakharuddinDepartment of PhysicsUniversity of Konstanz78457 Konstanz (Germany)

Supporting information and the ORCID identification number(s) for theauthor(s) of this article can be found under https://doi.org/10.1002/cplu.201800014.

This article is part of the “Optoelectronics” Special Issue. A link to theissue will appear here once it is compiled.

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ful removal of excess organic solvents and surfactants, whichare essential for proper nucleation and controlled crystalliza-tion, present in the crude solution. Therefore, great attentionmust be paid to the purification process of the nanoparticles(NPs), which would otherwise decrease the PLQY or even leadto PeNP aggregation and subsequent precipitation. Swarnkarand co-workers recently developed a purification method byusing MeOAc; an antisolvent to remove excess nonvolatile sol-vents and reagents without inducing agglomeration.[8] Li andco-workers demonstrated the preparation of LEDs with high ef-ficiency from solution-processed CsPbBr3 NPs through balanc-ing surface passivation and carrier injection by means of liganddensity control with a mixture of hexane/ethyl acetate as a sol-vent.[15] Despite growing research progress in PeNPs, the stabil-ity of colloidal solutions and stable device lifetime are yet tobe demonstrated.

Herein, we report, for the first time, the preparation of greenLEDs by using CsPbBr3 PeNPs synthesized at RT as a light-emit-ting material ; this could be an important step forward from anindustrial point of view, upon taking into account the simplici-ty of the synthetic method. We note that the RT PeNPs dem-onstrate inferior performance, owing to poorer size distribu-tion, and perhaps the fact that no further purification was ap-plied, compared with the HI PeNPs, for which solvent purifica-tion led to improved NP morphology.

Results and Discussion

Green light-emitting CsPbBr3 PeNPs have been preparedthrough two different methods (see the Experimental Sectionfor further details). We use the HI method, previously reportedin the literature to yield suitable PeNPs for the preparation ofLEDs for comparison with the optoelectronic properties of RTPeNPs. Both methodologies produced CsPbBr3 NPs with cubicstructures, as determined by XRD (Figure S1 in the SupportingInformation). Figure 1 shows the absorbance and photolumi-nescence (PL) spectra measured for the PeNPs preparedthrough the two different approaches. The RT NPs exhibitPLQYs of 49 (solution) and 15 % (film), the emission maximum(lmax

em ) of which is centered at l= 509 nm and the PL spectrumshows a FWHM of 29 nm, whereas the HI PeNPs show PLQYsof 60 (solution) and 20 % (film) and the emission band is cen-

tered at 514 nm (FWHM 28 nm). In both cases, the PL spectraare situated within the green spectral region, with a slightbathochromic shift of 5 nm for the PeNPs synthesized throughthe HI method. This redshift between the PL emission bands ofthe two PeNPs is due to the purification performed on HIPeNPs, as proposed by Li et al.[15] We note a similar redshift forthe HI PeNPs before and after purification (see Figure S2 in theSupporting Information).

The Stokes shift observed in Figure 1 a and b for both typesof PeNPs is also very similar: approximately 105 (21) and118 meV (24 nm) for the RT and HI PeNPs, respectively. CdSe/CdS, CdSe/CdPbS, and CdSe/CdZnS quantum dots (QDs) typi-cally show Stoke shift values of approximately 400 meV, whichare significantly larger than those observed for PeNPs.[22] Thesmaller Stokes shifts observed herein imply that the PL of thePeNPs arises from a direct exciton recombination process,which is in good agreement with previous reports.[23, 24] The ab-sorbance onset of the PeNPs prepared at RT starts at l=

525 nm, whereas that observed for the HI PeNPs starts at l=

531 nm. From the absorbance spectrum, it is possible to esti-mate the optical band gap of the two types of PeNPs from aTauc plot, (Figure S3 in the Supporting Information). The calcu-lated band gaps match closely: 2.40 and 2.37 eV for the RT andHI methods, respectively (Figure S3 in the Supporting Informa-tion).[25] These results are in good agreement with previous re-ports that have been used as a reference for this study.[11, 21]

Figure 1 c shows the transient PL decay for both types ofPeNPs (in solution). Although a distinction between slow andfast decay, which is typically ascribed to nonradiative and radi-ative processes, respectively, is hard to make, quantitative anal-ysis of the two shows a larger nonradiative lifetime for the HIPeNPs. As recently reported by Tress, a longer nonradiative life-time (or a higher radiative yield) leads to higher internal quan-tum efficiency (IQE).[26]

Figure 2 a and b shows the high-resolution (HR) TEM imagesof the RT and HI PeNPs, respectively. Both PeNPs show size dis-tributions with values below 20 nm and well-defined crystallineplanes. However, we clearly observe that the geometry for thetwo types of PeNPs studied is significantly different; it is morerectangular for the RT nanocrystals and more square for thenanocrystals synthesized through the HI method. The PL spec-tra of the PeNPs were transformed into the corresponding

Figure 1. Absorbance (dashed line) and PL (solid line) spectra of green light-emitting CsPbBr3 PeNPs synthesized a) at RT and b) by the HI method. The PLspectra were measured at an excitation wavelength of lexc = 440 nm for both NPs. The insets show the solutions under white light and under UV light.c) Transient PL decay curves for the RT (red) and HI (blue) PeNPs.

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chromaticity indexes and plotted in the CIE 1931 color spacechromaticity diagram. As shown in Figure 2 c, the red dot(0.057,0.642) and the blue dot (0.068,0.696) represent thosechromaticity indexes corresponding to the PL spectra of the RTand HI PeNPs, respectively; the proximity of the CIE values tothe curve edge indicates the pure color nature of the emittedlight.

Figure 2 d shows the PL (in solution) and EL spectra of acomplete device (8 V) for the PeNPs synthesized at RT and bymeans of the HI method. A small redshift between the PL ofNPs in solution and EL from PeNPs layers is evident for bothkinds of NPs. The redshift in the spectra (for EL) could be ex-plained by the Stark effect, for example, the redshift in thepresence of a high electric field. Also, because recombinationof hot electrons, which are present owing to the applied bias,is not the same as that in the case of PL radiative recombina-tion is in a steady state, and may lead to such a spectralshift.[27]

To determine the surface coverage and surface roughness ofthe films, we recorded the topography of as-deposited filmsby means of AFM on a relatively large area (10 mm � 10 mm).The root-mean-square (r.m.s.) roughness calculated forpoly(3,4-ethylenedioxythiophene) (PEDOT) was 2.8 nm, whichreduced to 1.2 nm if a thin layer of poly[N,N’-bis(4-butylphen-yl)-N,N’-bisphenylbenzidine] (Poly-TPD) was deposited on top,which suggested the formation of a smoother surface (Fig-ure 3 b). The HI PeNPs showed a smaller r.m.s. roughness value(5.3 nm) than that of RT PeNPs (7.3 nm), which suggested thatthe former led to a smooth film formation, probably owing toa narrow size distribution and smaller particle size. A lower sur-

face roughness and narrow size distribution are prerequisitesto high-performing LEDs. Furthermore, the RT method leads tothe formation of small clusters in the film, which might hinderthe performance of the device.

We employed both kinds of nanocrystals as absorber materi-als for LEDs; see the Experimental Section. Figure 4 a depictsthe energy-level diagram of the materials employed for thecomplete devices, whereas Figure 4 b and c shows a photo-graph of the green LED at 8 V prepared at RT and by means ofHI, respectively. EL spectra at different applied voltages are de-picted in Figure S4 in the Supporting Information. All of thedevices were prepared by using ITO as the substrate, a thinlayer of PEDOT:PSS as a hole transport material on top of theITO, then a poly-TPD layer as an electron blocking layer, fol-lowed by the deposition of the PeNPs and a TPBi layer as anelectron transport material. To finally complete the devices, thethermal evaporation of aluminum electrodes was performed.

The performances of the LEDs prepared with both PeNPswas studied. Figure 5 shows the electro-optical response of theLEDs prepared with the RT and HI PeNPs. Figure 5 a shows thecurrent density of both kinds of PeNPs. Devices prepared withPeNPs synthesized through the HI method show lower currentat low applied voltages. At voltages between 8 and 9 V, bothdevices exhibit similar current densities. However, at voltagesbeyond 9 V, the HI particles show higher current densityvalues. These variations probably arise from the different syn-thetic procedures (the HI involves high-temperature synthesisand subsequent purification that yields high-quality PeNPs) ;see the Experimental Section. Figure 5 b reveals significant var-iations of the current efficiency values, depending on the

Figure 2. HR-TEM images of the a) RT and b) HI PeNPs nanocrystals. The white rectangle in b) highlights defective NPs. c) Chromaticity indexes correspondingto the PL spectra for the RT (red) and HI (blue) PeNPs plotted in the CIE 1931 diagram; and d) PL in solution (solid lines) with hexane as solvent and the elec-troluminescence (EL) spectra (dashed lines) of the PeNPs synthesized at RT and by means of the HI method.

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PeNPs employed. Higher PLQYs and lower currents contributeto the observed higher efficiency for HI LEDs. The maximumcurrent efficiency value observed was 0.7 Cd A�1, which corre-

sponded to the devices prepared from the HI particles. Thedata plotted in Figure 5 c show that the maximum luminancevalues range from 303.2 Cd m�2 for HI PeNPs and 2.66 Cd m�2

for RT PeNPs. Consequently, the maximum EQE values rangefrom 0.31 % for the HI PeNPs and 0.01 % for the RT PeNPs. It isworth highlighting that all devices show turn-on voltages ofapproximately 4 V, regardless of the type of PeNPs employed,and all devices were measured from 4 to 10 V.

The clear difference in EQE of both kinds of NPs (Figure 5 d),of which HI PeNPs exhibit superior performance, can be attrib-uted to several factors. The first concerns the synthetic pro-cess; the PeNPs synthesized by the HI method provide PeNPsof higher quality than those prepared at RT. Different factorscould contribute to this result ; on one hand, HI PeNPs present-ed a narrower size distribution (Figure 2 a and b), to preventsmall variations in the band gap acting as traps and reducingthe performance of the LEDs. On the other hand, the rectangu-lar shape of PeNPs prepared at RT probably arises from themerging of different NPs and defects at the boundary betweenthe original particles ; see, for example, the NPs highlighted bya white rectangle in Figure 2 b. The higher radiative yield ofthe HI PeNPs (a ratio between nonradiative and radiative re-combination) is another reason for their high performance (seeFigure 1 c). The lower PLQY or particles synthesized at RT com-pared with those prepared through the HI method also indi-cates a higher degree of lattice defects for the RT PeNPs. More-over, the purification method also introduces differences be-tween both systems; this is the second factor that affects thelower performance of devices prepared with RT NPs. In asingle synthesis exploiting the HI method, it is possible to pre-pare a larger amount of material and apply some purificationprocesses to remove excess organic solvents, which is not pos-sible with the synthesis at RT because the amount of materialobtained after each synthesis is relatively small. Finally, thelower roughness of HI PeNPs also favors the higher per-formance of this kind of NPs.

It must be noted that the stability of the devices is still inquestion because both LEDs start to degrade at an appliedbias of 8 V (Figure S5 in the Supporting Information), as alsoevident from a sudden drop in EQE (Figure 5 d). However, thestability of these devices is in the scope of our future studiesand will be explained in detail in our upcoming reports.

Conclusion

We have synthesized PeNPs through two different procedures:HI and RT syntheses. LEDs prepared from the two types ofPeNPs show superior performance for the HI PeNPs. Morpholo-gy and optoelectronic investigations revealed that the HIPeNPs were characterized by a lower thin-film surface rough-ness, narrow size distribution, and a higher radiative yield thatis responsible for the higher performance. Although the LEDEQE of the RT PeNPs is lower, their facile and large-scale syn-thetic compatibility offer potential for mass production. A fur-ther improvement in the performance with RT PeNPs could bemade through control of the particle size distribution and theirfurther purification.

Figure 3. AFM images of PEDOT:polystyrene sulfonate (PSS; a), PEDOT:PSSwith a thin layer of Poly-TPD on top (b), HI PeNPs (c), and RT PeNPs (d)deposited on top of a PEDOT/Poly-TPD layer. The r.m.s. roughness is markedat the top left of each image. The scan area is 10 mm � 10 mm.

Figure 4. a) Energy diagram of the materials employed for the preparationof the PeNPs-LEDs. ITO = indium tin oxide, TPBi = 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1 H-benzimidazole). Bright-green LEDs driven at 8 V, with PeNPsprepared at b) RT and c) by means of the HI method.

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Experimental Section

General

All of the materials employed herein were used as received fromcommercial suppliers without further purification. Those precursorsand solvents employed for the PeNP synthesis were purchasedfrom Sigma Aldrich. Oleic acid (OA; 90 %), oleylamine (OLA;�98 %), 1-octadecene (ODE; 90 %), hexane (�97 %), toluene(99.8 %), MeOAc (99.5 %), DMF (99.8 %), cesium bromide (99.999 %),cesium carbonate (99.995 %), and lead(II) bromide were purchasedfrom TCI. The prepatterned ITO substrates (20 � 20 mm) were pur-chased from thin-film devices (TFD), PEDOT:PSS (Al4083) from Her-aeus Clevios, and Poly-TPD (Mw = 100 000–150 000) from Ossila, andTPBi was acquired from Sigma Aldrich.

HI method

Preparation of Cs-oleate : Cs2CO3 (0.814 g) was loaded into a100 mL three-necked flask, along with ODE (40 mL; Sigma–Aldrich,90 %) and OA (2.5 mL; Sigma–Aldrich, 90 %); dried for 1 h at120 8C; and then heated under N2 to 150 8C until all Cs2CO3 reactedwith OA. Because Cs-oleate precipitated out of ODE at RT, it had tobe preheated to 100 8C before injection.

Synthesis of CsPbBr3 nanocrystals (NCs): ODE (10 mL) and PbBr2

(0.138 g) were loaded into a 50 mL three-necked flask and driedunder vacuum for 1 h at 120 8C. Dried OLA (1.0 mL) and dried OA(1.0 mL) were injected at 120 8C under N2. After complete solubili-zation of PbBr2 salt, the temperature was raised to 180 8C and a so-lution of Cs-oleate (0.8 mL, 0.125 m in ODE, prepared as describedabove) was quickly injected and, 5 s later, the reaction mixture wascooled on an ice–water bath.[1]

Purification of CsPbBr3 NCs : The crude solution was cooled on awater bath and aggregated NCs were separated by centrifugingand as much solvent as possible was removed. Thereafter, after dis-carding the supernatant, the PeNPs were dispersed in hexane toapply the purification process with MeOAc in the ratio of 1:3. ThePeNPs were centrifuged for 10 min at 4700 rpm, dispersed again inhexane, and stored at low temperature for 48 h to precipitate Pb-oleate and Cs-oleate; these products solidified and precipitated atlow temperatures. After this period, the PeNPs were separatedfrom the precipitated material. The solutions, stored in the fridge,were stable for months.

RT method

To synthesize CsPbBr3, PbBr2 (0.4 mmol) and CsBr (0.4 mmol) weredissolved in DMF (10 mL). Upon complete dissolution of the pre-cursors, OA (1.0 mL) and OLA (0.5 mL) were added to stabilize theprecursor solution. Then, the precursor solution (1.0 mL) wasadded dropwise into toluene (10 mL) under vigorous stirring.Strong green emission was observed immediately after injection.[21]

The solutions, stored in the fridge, were stable for months.

Purification of CsPbX3 NCs

After the synthesis of the PeNPs, each solution was centrifuged for10 min at 4700 rpm. The supernatant was discarded and the solidwas dispersed in hexane. All of the material was then placed in thesame flask, which was kept at low temperature for 48 h. The pre-cipitated material was removed and the final solution was concen-trated to 10 mg mL�1.

Figure 5. Analysis of the QD-LED performance: a) current density curves (J/V), b) current efficiency, c) luminance, and d) EQE.

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Preparation of the NP-based LEDs

The ITO substrates were introduced in a soap solution and sonicat-ed for 30 min. Then the substrates were first rinsed with Mili-Qwater and then with ethanol. Next, the ITO substrates were intro-duced into a mixture of solvents consisting of isopropanol/acetone(1:1 v/v) and sonicated for 30 min. the substrates were rinsed withethanol and dried with compressed air. Then, the substrates wereintroduced into a UV–O3 cleaner for 30 min and the solution of PE-DOT:PSS was spun-cast at 3000 rpm for 60 s and treated at 150 8Cfor 30 min in air, to yield a thin layer (20 nm). Next, a Poly-TPDlayer (20 nm) was deposited by spin-casting (10 mg mL�1 in chloro-benzene) at 3000 rpm for 60 s, and subsequently annealed at150 8C for 30 min in air. The solutions of PeNPs (10 mg mL�1) inhexane were spun-cast at 2000 rpm for 20 s. Finally, a 20 nm layerof TPBi and a 100 nm thick aluminum top electrode were thermallyevaporated at rates of 0.5 and 1.5–2 � s�1, respectively; the activeareas were encapsulated with adhesive tape and then with a UVphotocurable epoxy resin from Lighting Enterprises (ELC4908-30)and a cover glass.

Characterization

The absorbance spectra were recorded with a UV/Vis Varian Cary300 BIO spectrophotometer and the PL spectra were recorded byusing a fluorometer from Horiba Fluorolog 3-11. The LED perform-ances (J–V curves, EL spectra, current efficiency, luminance, andEQE) were quantified with an EQE measurement system (C9920-12)from Hamamatsu, based on an integrating sphere connected to aPMA-12 photonic multichannel detector through an optical fiberand by using a Keithley 2400 instrument as a current/voltagesource meter. The PLQYs were measured with an absolute PLQYmeasurement system with a monochromatic light source (C9920-02, -03 from Hamamatsu) connected to an integrating sphere. TheHR-TEM images of the QDs were recorded with a JEOL 2100 micro-scope. Time-resolved emission measurements were performed bymeans of the time-correlated single-photon-counting technique inan IBH-5000U apparatus. Samples were excited with a l= 464 nmnano-LED with a FWHM of 1.4 ns and a repetition rate of 100 kHz.AFM (Concept Scientific Instrument) was employed to probe thesurface roughness of the deposited films.

Acknowledgements

The study was supported by MINECO of Spain (project MAT2016-76892-C3-1-R). B.C.H. is grateful for the support of the NationalCouncil of Technological and Scientific Development (CNPq),Brazil, through the Science without Borders program. A.F. ac-knowledges the Alexander von Humboldt Foundation for a post-doctoral fellowship.

Conflict of interest

The authors declare no conflict of interest.

Keywords: light-emitting diodes · nanoparticles ·perovskite phases · photochemistry · synthesis design

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Manuscript received: January 13, 2018Revised manuscript received: February 5, 2018Accepted manuscript online: February 6, 2018Version of record online: && &&, 0000

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FULL PAPERS

B. Clasen Hames, R. S�nchez S�nchez,A. Fakharuddin, I. Mora-Ser�*

&& –&&

A Comparative Study of Light-Emitting Diodes Based on All-Inorganic Perovskite Nanoparticles(CsPbBr3) Synthesized at RoomTemperature and by a Hot-InjectionMethodHot or not? Perovskite nanoparticles

(PeNPs) are prepared through two dif-ferent synthetic procedures: hot injec-tion (HI) and room-temperature (RT)synthesis. Light-emitting diodes (LEDs)are prepared from the two types ofPeNPs (see figure). Superior per-

formance is exhibited by the HI PeNPs.Morphology and optoelectronic investi-gations reveal that HI PeNPs are charac-terized by a lower thin-film surfaceroughness, narrow size distribution, andhigher radiative yield that is responsiblefor higher performance.

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