Perovskite Crystals for Tunable White Light Emission - Chemistry of

Nov 4, 2015 - Abstract Image. A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of effic...
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Perovskite Crystals for Tunable White Light Emission Sandeep Pathak,† Nobuya Sakai,† Florencia Wisnivesky Rocca Rivarola,§ Samuel D. Stranks,†,⊥ Jiewei Liu,† Giles E. Eperon,† Caterina Ducati,§ Konrad Wojciechowski,† James T. Griffiths,§ Amir Abbas Haghighirad,† Alba Pellaroque,† Richard H. Friend,‡ and Henry J. Snaith*,† †

Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom Cavendish Laboratory, Department of Physics, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom § Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom ‡

S Supporting Information *

ABSTRACT: A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via costeffective means. Here, we synthesize organic−inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5−50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.

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INTRODUCTION Approximately 25% of the global electricity demand is used for lighting, which accounts for 1.9 GT of CO2 emissions.1−3 To reduce the energy demand for lighting, white light-emitting diodes (LEDs) are becoming prevalent, which are much more efficient than incandescent bulbs and fluorescent tube lighting.4−6 If all conventional white-light sources in the world were converted to energy-efficient LED light sources, energy consumption could be reduced by approximately 1,000 TW h yr−1. This would be the equivalent of ∼230 typical 500 MW coal plants, reducing greenhouse gas emission by ∼200 million tons.7 A standard white light LED is typically composed of a blue GaN LED (440−470 nm emission) with a coating of either a yellow−green phosphor or a combination of red and green phosphors (multiphosphor approach).4,8,9 The role of the phosphor is to absorb a certain fraction of the blue light, downconvert, and reemit this light across the visible spectrum. Most commercially available single-phosphor pc-LEDs are based on Ce 3 + -d o ped yt t rium alum in um g arnet (YAG:Ce: Y3−xGdxAl5−yGayO12:Ce) as the yellow-emitting phosphor material.7,10 Depending on the application, the requirements for the precise spectrum of the white light can change. The © 2015 American Chemical Society

design for an indoor light source requires successful color rendering, good spectral overlap with human eye sensitivity functions, and a warm white shade. Importantly, gaps in the spectrum should be minimized, otherwise some colors will appear unnaturally dark, and any light emitted beyond the tail of the sensitivity of the human eye should be avoided, as it represents wasted energy. In addition, the optimal spectrum differs for each application depending on the “mood” desired and may significantly deviate from the solar spectrum11 By employing semiconductor quantum dots (QDs) in LEDs, it is possible to accomplish successful color rendition of illuminated objects together with a good spectral overlap between the responsivity of the eye and the emission spectrum of the LED. The light emission can also be tuned to achieve a warm white color or indeed any desired hue.3,11−16 Recently, metal halide perovskite materials, such as CH3NH3PbI3, have created great excitement among the photovoltaics scientific community17,18 owing to their remarkable performance in solar cells.19,20 As expected for ideal solar Received: September 24, 2015 Revised: November 4, 2015 Published: November 4, 2015 8066

DOI: 10.1021/acs.chemmater.5b03769 Chem. Mater. 2015, 27, 8066−8075

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Chemistry of Materials

and PbBr2, typically 0.0112 g of MABr and 0.0367 g of PbBr2, were dissolved in 7 mL of anhydrous dimmethylformamide (DMF). Two hundred microliters of as-prepared precursor solution was injected in 10 mL of anhydrous toluene under vigorous stirring with 10 μL steps at 5 min intervals. As soon as the precursor solution was injected into the toluene, MAPbBr3 precipitated out as crystals. To synthesize MAPbCl3, we followed a similar method as that for MAPbBr3. MAPbI3 perovskite crystals were prepared by a similar method except for the solvent used to prepare the precursor solution, where a DMF and ACN solution mixture was used instead of pure DMF. Equimolar amounts of MAI (0.0159 g) and PbI2 (0.0416 g) were first dissolved in 200 μL of DMF followed by the addition of 6.8 mL of ACN. The precursor solution was injected into the toluene as described previously. To improve the dispersion of the colloidal crystals, we added oleic acid (40 mg) to 10 mL of toluene before injecting the precursor solution. CH3(C8H17)NH3PbX3 (MA(OA)PbX3, where X = Br, I, and Cl). Here, we demonstrate the synthesis of mixed alkyl lead halide perovskite, i.e., MA(OA)PbX3. The entire process of crystal synthesis was performed as described earlier for MAPbX3 except for the addition of a longer chain length alkyl group into the precursor solution. While maintaining constant total molar concentration of alkyl ammonium halide equimolar with respect to the lead halide salt, the molar ratios of methylammonium halide (MAX) and octyl ammonium (OAX) were varied to 1:0, 0.7:0.3, 0.4:0.6, and 0.1:0.9. Furthermore, to synthesize mixed halide crystals, we fixed the MA+/OA+ ratio to 0.7/0.3 and mixed the precursor solution containing different halide groups (I, Br, and Cl) into varying molar ratios. Perovskite Crystal/Polymer Thin Film Fabrication. The assynthesized perovskite crystals (in toluene) were centrifuged for 1 hour at 7000 rpm (Fisher Scientific, AccuSpin 400) and blended with a desired amount of polymer (e.g., polystyrene, PS; poly-methyl methacrylate, PMMA). The crystal and polymer concentration can be varied to achieve a certain level of absorbance and PLQE providing absolute flexibility in tuning the desired spectral region and intensity of the final PL emission. Characterization of Perovskite Crystals. Photophysical properties (e.g., UV−vis absorption, PL emission and quantum yield) of the perovskite crystals/toluene were performed using cuvettes. X-ray diffraction of the crystal film was measured using an XPERT-PRO powder diffractometer system with Cu Kα (1.54060 Å) with an X-ray tube operated at 40 kV and 40 mA. Bright-field TEM images were acquired using an FEI Tecnai F20 operating at 200 kV. High resolution TEM (HRTEM) images were obtained using a JEOL 4000EX operating at 400 kV. UV−vis absorption was measured on perovskite crystal/toluene dispersions in air using a commercial spectrophotometer (Varian Cary 300 UV−Vis, USA). The steady state PL was collected using a high-resolution monochromator and hybrid photomultiplier detector assembly (PMA Hybrid 40, PicoQuant GmbH). Time-resolved PL measurements were acquired using a timecorrelated single photon counting (TCSPC) setup (FluoTime 300, PicoQuant GmbH). A perovskite crystal/toluene dispersion was photoexcited using 405 nm (for Br/I-based perovskite and Cl-based perovskite, respectively) laser head (LDH-P-C-510, PicoQuant GmbH) pulsed at frequencies of 1 MHz with a pulse duration of 117 ps and fluence of ∼300 nJ/cm2. Photoluminescence quantum efficiency (PLQE) measurements were carried out using a 405 nm CW (continuous wave) laser excitation source (Suwtech LDC-800) to illuminate a diluted crystal colloidal sample in an integrating sphere (Oriel Instruments 70682NS), and the laser scatter and PL were collected using a fiber-coupled detector (Ocean Optics MayaPro. The spectral response of the fiber-coupled detector setup was calibrated using a spectral irradiance standard (Oriel Instruments 63358). Excitation intensities were adjusted using optical density filters. PLQE calculations were carried out using established techniques described elsewhere.30

cell materials, it has transpired that these perovskites are also highly emissive with promising demonstrations of LED and laser applications. 19,21,22 Importantly, thin films of CH3NH3PbX3 have also been shown to exhibit tunable emission wavelengths from 780 nm (X = I) to 530 nm (X = Br) to 410 nm (X = Cl).22,23 Several reports on perovskite crystals have been published, indicating the broad tunability of this family of materials.22,24−26 Here, we investigate organic−inorganic metal halide perovskites that can be synthesized in the form of colloidal crystals with sizes down to the 5−10-nm length scale.27,28 We demonstrate that, as we tune the synthesis to reduce the size of the crystals, the luminescence efficiency increases. However, the occurrence of 2-dimensional platelets appears to be disadvantageous for high luminescence yield. We further exploited the ability to mix halides to deliver emission wavelengths spanning the entire range of the visible spectrum from 410 to 775 nm with high quantum efficiency. We show that mixed crystal solutions of different halide compositions can exhibit compositional instability, as has been observed in thin films,29 which is disadvantageous for generating stable white light LEDs. However, by blending the perovskite crystals into an insulating and transparent polymer matrix, we observe that the cast films sustain their spectral emission profile. We show that perovskite crystals with a range of emission wavelengths can be blended together in a common polymer host to create a film with white light emission with a tunable hue. We demonstrate that that multiple layers containing different perovskite crystals can also be stacked on top of each other to realize white light emission. Finally, we show an operational solid-state white LED by illuminating a film of mixed perovskites crystals with a commercial blue LED.

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EXPERIMENTAL SECTION

Precursor Materials. The lead halide compounds with 99% purity (i.e., PbCl2, PbBr2, and PbI2) and hydrogen halide acids (HCl, HBr, and HI) were purchased from Sigma-Aldrich. Methylamine (CH3NH2, 41% in water) and octylamine (C8H17NH2) were also purchased from Sigma-Aldrich and used as received. Synthesis of Alkyl-Ammonium-Halide. Alkyl ammonium halide was synthesized in the lab by reacting alkyl amine (R = CH3NH2 and/ or C8H17NH2) and HX (i.e., HI, HBr, and HCl). Methylammonium iodide was synthesized by typically reacting 24 mL of methylamine at 33 wt % in ethanol with 10 mL of hydriiodic acid (57% in H2O) in 100 mL of C2H5OH. The reaction was carried out with vigorous stirring under ambient conditions for 30−60 min. The reaction mixture was then subjected to rotary evaporator at 60 °C to remove the solvent, leading to the precipitation of a white/yellowish-colored methylammonium iodide (MAI) powder. The resulting product was then washed with diethyl ether several times to remove impurity phases. The as-synthesized compound was then recrystallized with either C2H5OH or C2H5OH/CH3OCH3 solvent and dried in a vacuum furnace at 60 °C for 4 h to obtain a purified white-colored CH3NH3I (MAI) powder. To synthesize methylammonium bromide (MABr) and methylammonium chloride (MACl), we followed a similar procedure except, for the source of halide anions where HBr and HCl were used, respectively, octylammoniumiodide (OAI), octylammonium bromide (OABr), and octylammonium chloride (OACl) were synthesized by reacting the respective alkyl amine (i.e., octylamine) and hydrogen halide acid (i.e., HI, HBr, and HCl) in a similar method as described for MAI. Perovskite Crystal Synthesis. CH3NH3PbX3 (MAPbX3, where X = Br, I, and Cl). Methylammonium lead halide (MAPbX3, where X = Cl, Br, and I) perovskite crystals were synthesized using methylammonium-halides (MACl, MABr, and MAI) and the lead halide (PbCl2, PbBr2, and PbI2) precursor. To synthesize MAPbBr3, equimolar MABr 8067

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Figure 1. TEM and HRTEM images of (OA:MA)PbBr3 perovskite crystals with varying OA+:MA+ molar ratios. (a) With neat MA+, mesoscopic 3D crystals are formed with sizes varying from 50 to 100 nm. (b) Addition of OA+ (i.e., 0.3 OA+:0.7 MA+) results into the formation of nanoscopic spherical 3D crystals with sizes varying from 5 to 10 nm. (c) Increasing the OA+ content to 0.6 OA+:0.4 MA+, it forms micrometer-sized platelets along with the 3D crystals (5−10 nm), as we show in the inset. (d) Further increasing the OA+ content to 0.9O A+:0.1 MA+ results predominantly in the formation of platelets. In some regions, we also see elongated 3D crystals with ∼5 nm width, as shown in the inset. (e) Image showing 3D crystal endowing on the thin platelets (e, f) HRTEM images of a 3D crystal (from 0.6 OA+:0.4 MA+). The inset shows the fast Fourier transform of the respective crystal, indicating the formation of cubic structure.

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RESULTS AND DISCUSSIONS The Impact of Octylamonium on Crystal Size and Shape. We synthesized CH3NH3PbX3 (X = Cl, Br, and I) perovskite crystals by the controlled precipitation of CH3NH3+, Pb2+, and X− in a noncoordinating solvent (e.g., toluene) containing oleic acid as we describe in detail in the Experimental section.28 To confirm the structure of assynthesized perovskite crystals, we precipitated them from solution by centrifugation and performed X-ray diffraction on the powder (Figure S1). The X-ray diffraction pattern confirms the cubic structure with Pm3m (no. 221) space group for MAPbCl3 and MAPbBr3 and tetragonal crystal structure with I4/mcm (No. 140) for MAPbI 3 . Our crystallographic observations agree well with those from previous literature reports.31−34 Typically in nanocrystal synthesis, long chain organic ligands are added to control the growth rates and resultant sizes of the nanocrystals. For perovskite nanocrystals, both carboxylic acidbased ligands, such as oleic acid as we have used here and longer chain ammonium molecules, have been added.27 Here, instead of simply adding additional alkyl ammonium ligands to the synthesis, we have substituted a certain fraction of the methylammonium cation (MA+) with octylammonium cation (OA+) in the form of octlyammonium halide salt. We synthesized (OA:MA)PbBr 3 crystals with varying (i.e., OA+:MA+) molar ratios of 0.0:1.0, 0.3:0.7, 0.6:0.4, and 0.9:0.1.

The TEM images, which we show in Figure 1, highlight the strong impact of the OA+ substitution on the shape and dimensions of the as-synthesized crystals. In the case of neat MA+ (i.e., MAPbBr3), we observe mesoscopic 3D cubic crystals with sizes ranging from 50 to 100 nm (Figure 1a). With the addition of OA+, we observe a reduction in the average crystal size, which ranges from 5 to 10 nm (e.g., 0.3 OA+:0.7 MA+). By increasing the OA+ content to 0.6 OA+:0.4 MA+, we observe the formation of 2D “platelets” mixed together with smaller 3D spherical (5−10 nm) crystals. With a further increase in the OA+ content to 0.9 OA+:0.1 MA+, we predominantly observe the platelets (Figure 1d) in the colloidal solution along with a fraction of elongated (width of ∼5 nm) crystals. We show an HRTEM image of the 3D nanoscopic crystals imaged on the [0 0 1] zone axis in Figure 1f. The FFT image suggests cubic structure belonging to the space group Pm3m (221), as is corroborated by the X-ray data, which we will discuss in the following section. Unfortunately, we could not capture any reliable HRTEM image of the platelets because they degenerate during the measurement at 400 kV beam (Figure S2). The average width and length of platelets are in the range of a micrometer scale (Figure 1c and d, inset). We could not estimate the precise thickness of the platelets. However, on the basis of the TEM contrast (Figure 1c and d), these platelets appear to be much thinner than the thickness of 3D spherical crystals. We analyzed the bright-field TEM images of crystals 8068

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Figure 2. (OA:MA)PbBr3 perovskite crystals with varying MA+ and OA+ ratios. (a) The PL (dotted line) emission (λ exc = 405 nm) and UV−vis absorption (solid line) of perovskite crystals with varying OA+:MA+ molar ratios. (b) XRD of perovskite crystal from MA+ shows a cubic crystal structure with space group Pm3m at room temperature. Pure OA+-based crystals form 2D layered (OA)2PbBr4. Crystals with a 0.6 OA+:0.4 MA+ molar ratio show a single phase X-ray diffraction with cubic crystal structure (Pm3m) and with a similar lattice parameter as that of MAPbBr3. Excessive addition of OA+ (e.g., 0.9 OA+:0.1 MA+) forms a mixture of several phases. We also show the X-ray patterns of OABr and PbBr2. (c) PLQE measurements of the perovskite crystals when excited at 405 nm. (d) Images of (MA/OA)PbBr3 crystals in toluene under a UV lamp (365 nm).

TEM images. The substitution of MA+ with OA+ leads to multiple absorption peaks at higher energy (Figure 2a). There is one peak close to 525 nm, consistent with the bulk 3D MAPbBr3 crystal, and further peaks at 471, 451, and 433 nm. Papavassiliou et al.,35 and more recently Tisdale et al.,26 have attributed these high energy absorption peaks at 433, 451, 471, and 525 nm to 2D platelets with unit cell thicknesses (n) of 1, 3, 4, and ∞, respectively.26,35,36 In contrast to the clear variation in the absorption spectra, we observe in Figure 2a that the steady-state PL emission spectra are dominated by the single emission peak of the 3D bulk crystals (close to 525 nm).26 These peaks show monotonic blue shifts in their positions as compared with those of the neat MAPbBr3 crystals, which are consistent with the beginnings of 3D quantum confinement, as we discuss in more detail in the Supporting Information. For the crystals synthesized with 0.6 OA+:0.4 MA+, we do not observe strong PL emission peaks for the 2D platelets, which are clear in the absorption in the spectra. This is likely due to charge transfer from the platelets to the surfaceadsorbed 3D crystals, the presence of which are evident in the TEM images we show in Figure 1c and e. In contrast, for the material synthesized with the 0.9 OA+:0.1 MA+, the PL is

and platelets in terms of the variation of intensity through them to give an indication of their relative thicknesses, which we present in Figure S3. This puts an approximation of the thickness as being less than 5 to 10 nm. Recently, Tisdale et al.26 also observed the formation of platelets along with the 3D spherical crystals with a similar synthesis route. In the perovskite crystal structure, methylammonium cations are embedded in the center of a simple cubic cell (Pm3m) inside a set of corner-sharing [PbX6]4- octahedral symmetry. When longer alkyl chain cations are introduced in the crystal, the methylammonium cations can only fit in the periphery of the octahedral26,27 or adsorb at the outer surface of the perovskite crystal with the longer alkyl chains dangling outside. This longer alkyl chain would then limit the 3-dimensional growth of the crystal and thus induce a particle-size quantum confinement effect,26,27 leading to blue-shifted emission. Our observation is in agreement with the observation made by Tisdale et al.26 In Figure 2, we show the UV−vis absorption and PL emission spectra for the different crystal colloidal solutions. In the case of neat MA+-based crystals (MAPbBr3), we observe a single absorption maximum at 525 nm, similar to the bulk crystal, as we expect from the mesoscopic crystal apparent in 8069

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Figure 3. Structural characterization of (0.3 OA:0.7 MA)PbX3 crystals. (a) Powder X-ray diffraction pattern of mixed halide crystals. The reflections (101), (040), and (141) are compared for neat and mixed halide crystals. The shift toward the lower 2θ position of the mentioned reflections is an indication of the substitution of halide ions with larger ionic radii, e.g., Br− in the (OA:MA)PbCl3 crystal and I− in the (OA:MA)PbBr3 crystals. (b and c) Representative tetragonal and cubic unit cell of (OA:MA)OAPbBr3 and (OA:MA)PbCl3 are shown, respectively.

useful for high PLQE, and indeed, suppressing the formation of platelets may be important for optimizing the perovskite crystals for luminescence. Therefore, for the remainder of the article, we employ 0.3 OA+:0.7 MA+ in the precursor solutions to synthesize the perovskite crystals. Tuning the PL Emission with Mixed Halide Perovskite Crystals. To investigate the potential spectral bandwidth and resolution achievable, we synthesized a series of neat and mixed halide perovskite crystals varying from MAPbCl3−xBrx through to MAPbBr3−xIx with x ranging from 0 to 3. For this work, we kept the OA+:MA+ cation ratio fixed at 0.7:0.3. In Figure 3a, we show the X-ray diffraction patterns of the single and mixed halide perovskite crystals. As the doping concentration of the Br− anion in the neat chloride-based perovskite crystals is increased, a gradual shift toward lower 2θ is observed for the (101), (040), and (141) reflections highlighted. A similar trend is observed when I− anions are added to the neat bromidebased perovskite. The shift in the 2θ position and the absence of a secondary phase confirms the formation of single phase crystals with the mixed halide lattice.23,28,40 At room temperature, the neat iodide-based perovskites crystallize in a tetragonal crystal structure, whereas neat chloride- and bromide-based perovskites crystallize in a cubic crystal structure, which we illustrate in Figure 3b and c. In Figure 4a, we show that the emission spectra of colloidal mixed halide (OA:MA)PbX3 (X = Cl−, Br−, and I−) crystals can be tuned over the entire visible spectral region by adjusting their halide composition. The PL emission peaks for (OA:MA)PbI3, (OA:MA)Br3, and (OA:MA)PbCl3 are at 770, 522, and 385 nm, respectively. Intermediate PL emission can be achieved by mixing I−/Br− and Br−/Cl− halides in the perovskite crystals. The observed trend in the PL emission spectra agrees well with the X-ray diffraction data of the mixed halide perovskite crystals. As expected, we found that mixing the I−/Cl− halide did not form stable perovskite crystals due to their large difference in ionic radii.28 In Figure 4b, we show the band gap,

considerably broadened and consists of at least 3 distinct peaks at higher energy. This is consistent with emission from the multiple species of perovskite platelets present in the solution.26 We also show the PL emission and absorbance of crystals synthesized with neat OA+ in Figure 2a, which is in agreement with the 2D layered perovskite, i.e., (OA)2PbBr4, reported previously.37,38 From the XRD spectrum (Figure 2b), we confirm that the crystals with neat MA+ (i.e., MAPbBr3) crystallize in cubic crystal structure and neat OA+-based perovskite crystals form 2D layered perovskite (i.e., (OA)2PbBr4).37,38 Interestingly, with the mixed OA+:MA+ composition of up to 0.6 OA+:0.4 MA+ molar ratio, we do not observe any identifiable reflections assigned to the (OA)2PbBr4 compound in the XRD patterns, and we predominantly observe a single-phase cubic crystal structure with similar lattice parameters as for the neat MAPbBr3. Upon further increasing the OA+ molar ratio to 0.9 OA+:0.1 MA+, we observe what appears to be mixed phases or a mixture of different families of crystals, consisting of (OA)2PbBr4 along with crystals of the MAPbBr3 family. Considering both the XRD data and the TEM analysis, it appears that the OA+ cations act as ligands, restricting the size of the 3D crystals. Up to a fraction of 0.6 OA+, the OA+ acts as a ligand bound to the surface of the platelets. With a further increase in OA+, in addition to the platelets, there is the formation of crystals of layered perovskite (OA)2PbBr4. Concerning the PL quantum yield (PLQE), with the substitution of MA+ with OA+, the PLQE gradually increases up to a 0.6 OA+:0.4 MA+ molar ratio. However, further increasing the molar ratio to 0.9 OA+:0.1 MA+ leads to a substantial decrease in the PLQE28,39 with the average values measuring 20, 30, 36, and 18% for MA+, 0.3 OA+:0.7 MA+, 0.6 OA+:0.4 MA+, and 0.9 OA+:0.1 MA+ crystals, respectively (Figure 2c). We observe a similar trend in the PL lifetime of the diluted colloidal solutions, which we show in Figure S4. Therefore, although the platelets are interesting from a fundamental viewpoint, the 3D crystals appear to be most 8070

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Figure 4. Emission from perovskite (0.3 OA:0.7 MA)PbX3 (X = I, Br, and Cl) crystals in toluene. (a) PL emission (λ exc = 360 nm) of (0.3 OA:0.6 MA)PbX3 (X = I, Br, and Cl) crystals. Intermediate PL emissions were achieved by mixing halide ions (i.e., I, Br, and Cl) in the crystals. (b) Band gap (extracted from the PL peak position) vs stoichiometric ratio of halide ions in the mixed halide perovskite crystals. Dotted line guides through the tetragonal to cubic phase. (c) Images of mixed halide (0.3 OA:0.7 MA)PbX3 crystals under UV light (365 nm).

0.4:0.6 volume-to-volume ratio, respectively. The volumetric ratio was chosen so as to achieve a reasonably similar PL intensity from the two peaks. The individual colloidal solutions of neat (0.3 OA:0.7 MA)PbBr3 and (0.3 OA:0.7 MA)PbI3 have individual PL emission peaks at 520 and 753 nm, respectively (shown with dotted line). However, by mixing these two solutions together, they lose these individual distinct PL emission peak positions, and instead, the mixed solution shows PL peak position maxima at 551 and 704 nm (scan_1). We kept the solution mixture under continuous illumination and measured the PL spectra every 2 min over the course of 400 min. The fluctuation in the PL peak position throughout the measurement indicates the composition instability of crystals when they are mixed in colloidal form. On the basis of the measurements of the PL emission from individual solutions of mixed halide crystals, which are shown in Figure 4, we can estimate that the Br/I ratio for the crystals emitting at 551 and 704 nm are approximately 0.8:0.2 and 0.2:0.8 Br:I, respectively. In addition to the shift in PL peaks for the mixed solutions, the intensity of the 551 nm peak diminishes rapidly over time. This

which we estimated from the PL emission peak for each starting solution stoichiometry. Apart from a little bump at the tetragonal to cubic phase transition range, the bandgap varies linearly with composition, indicative of a solid solution of the mixed halides formed within the perovskite crystals over the entire range. As the anion is varied, the crystal lattice parameter, the octahedral tilting angle, and the halide orbitals contributing to the energy band are altered, which results in the variation observed in the photophysical properties.41 In addition to the data shown here, we give the full experimental methods and show further synthetic variations in Figures S5−8. Perovskite Crystal/Polymer Composite Thin Film Fabrication for White Light Emission. The colloidal solution of the perovskite crystals shows compositional instability when they are mixed with other crystals with different halide anions. In Figure 5 (top panel), we show the PL emission spectra of the neat (0.3 OA:0.7 MA)PbBr3 and (0.3 OA:0.7 MA)PbI3 perovskite crystals in solution, in addition to the PL emission from a blend of the two (i.e., (0.3 OA:0.7 MA)PbBr3 and (0.3 OA:0.7 MA)PbI3) at an approximate 8071

DOI: 10.1021/acs.chemmater.5b03769 Chem. Mater. 2015, 27, 8066−8075

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Chemistry of Materials

the crystals in a polymer matrix inhibits the anion exchange and thus stabilizes the emission wavelength at a desired wavelength. However, we do observe a slow reduction in the emission intensity of the (0.3 OA:0.7 MA)PbI3 peak at 750 nm, which indicates a degradation of some form other than ion exchange. Hence, embedding the crystals in an insulating matrix appears to solve the problem of ion exchange, although further developments of the crystals and matrix may be required to achieve total stability of emission for long-term light output. We can realize selective and broad emission either by fabricating a film of mixed crystals/polymer composite or by fabricating separate films of different crystals each with their own selective emission and then laminating them together. In Figure 6a, we demonstrate a broad PL emission covering the entire visible spectral region from 410 to 770 nm from a single film, which we fabricated by mixing blue, green, red, and intermediate emitting crystals into the polystyrene matrix. This wide PL emission is a perfect precursor to generate efficient and tunable white light emission from a perovskite crystal/polymer composite film. In the background of Figure 6a, we show the air mass (AM) 1.5 solar spectrum (http://rredc.nrel.gov/solar/ spectra/am1.5/). In addition, we fabricated individual blue, green, and red emitting films by employing perovskite crystals with Cl−, Br−, and I− halide groups, respectively; we show images of these crystal/polymer composite films illuminated by a UV lamp (365 nm) in Figure 6b. The PL emission, UV−vis absorption spectra, and PL decay spectra are shown in Figure S9a and b. The PLQEs of the MAPbI3, MAPbBr3, and MAPbCl3 films were 15, 25, and 10%, respectively. The ease with which these flexible films can be synthesized provides enough opportunity to widen their applicability. As we described in the Introduction, commercial white LEDs for solid-state lighting are typically based on GaN blue LEDs with yellow-emitting Ce3+-doped Y3Al5O12 (YAG:Ce) phosphors.14 Here, we demonstrate that the conventional phosphor can be replaced with the blue/green/red-emitting perovskite crystals embedded in a polymer matrix. In Figure 6c, we show separate strategies to generate bluegreen, blue-red, and blue-near IR emissions by separately placing (0.3 OA:0.7 MA)PbBr3 (emission at 520 nm), (0.3 OA:0.7 MA)Pb(I0.66Br0.34)3 (678 nm), and (0.3 OA:0.7 MA)PbI3 (775 nm) perovskite crystal thin films in front of the GaN blue LED. When these crystal films are stacked on top of the blue LED in decreasing band gap order, we observe white light emission, the spectra of which we show in Figure 6d. We positioned the green and red emitting crystal/polymer stack in front of the commercial blue GaN LED with the higher band gap (green) emitting perovskite film in the stack illuminated first followed by the lower band gap (red)-emitting perovskite. We ensured that the optical density of the perovskite crystal thin film was low enough to enable part of the blue light from the GaN LED to be transmitted through the films giving a blue emission (450 nm), whereas the blue light absorbed by the crystals was downconverted and reemitted in the green and red spectral regions, giving rise to the desired white light emission. We have made a selective demonstration of white light emission from a combination of perovskite crystals with different PL emission. We show the Chromaticity color coordinates (CIE) plotted from the corresponding perovskite crystals emission in Figure 7. The white light generated from the perovskite crystal is located at the center of the CIE chart covering cool white, natural white and warm white light. We

Figure 5. Compositional stability of the (0.3 OA:0.7 MA)PbX3 crystals. The top panel shows the PL emission (λexc = 425 nm) in colloidal solution of OA0.3MA0.7PbI3 (neat I3 sol., 753 nm) and OA0.3MA0.7PbBr3 (neat Br3 sol., 525 nm) crystals (dotted line) and their respective mixed solution (mixed sol., 551 and 704 nm). The excitation wavelength (λ exc) was 425 nm, and the solutions were continuously illuminated for several hours. The time between each scan was approximately 2 min. The bottom panel shows the emission spectra of the crystal/polymer film with OA0.3MA0.7PbI3 (neat I3 crystal/PS film) and OA0.3MA0.7PbBr3 (neat Br3 crystal/PS film) polymer film emitting at 753 and 525 nm, respectively (dotted lines), and the film made from the mixed solution (mixed crystal/PS film) emitting at 753 and 525 nm. The excitation wavelength (λexc) was 425 nm, and the films were continuously illuminated for several hours. The time between each scan was approximately 2 min.

indicates a general lack of stability of the green emitting crystals in a mixed solution. This halide exchange and instability has previously been observed to be a problem in polycrystalline thin films,29 and this anion exchange has in fact very recently been exploited to synthesize a range of perovskite crystals over the Cl, Br, and I phase space.42−44 However, this type of ion exchange would be detrimental to prospects of stable and controlled white light emission from mixed perovskite crystals. To address this, we blend neat (0.3 OA:0.7 MA)PbBr3 and neat (0.3 OA:0.7 MA)PbI3 crystals in an insulating and transparent polymer (i.e., polystyrene/PS) matrix and deposited the mixture as a solidstate film (i.e., mixed-crystal/PS). The polystyrene beads (Acros Organics 178890250) were added directly to each individual crystal solution at a concentration of 220 mg/mL, and the two solutions were then mixed together. Immediately, the films were cast by depositing a small volume of the mixed solution onto a glass substrate and spin-coating it to form a dry film. Encouragingly, there is no identifiable shift in wavelength position of either the 525 nm peak or the 750 nm peak (scan_1) when compared with the neat (0.3 OA:0.7 MA)PbBr3/PS and (0.3 OA:0.7 MA)PbI3/PS film (dotted line), respectively. Even after 400 min of continuous illumination of the mixed-crystal/PS film, we do not observe any shift in the emission peak positions. This strongly indicates that embedding 8072

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Figure 6. Emission from perovskite crystal/polymer composite films. (a) Broad spectrum PL emission covering the entire UV−vis spectrum of from a single film fabricated by mixing blue, green, red and intermediate emitting crystals into the polystyrene matrix. We also show theoretical combined PL emission (dotted line) of the individual PL emissions of crystals matching with the background AM 1.5 solar spectrum (http://rredc.nrel.gov/ solar/spectra/am1.5/). (b) Image of the perovskite crystal/polymer composite films emitting blue ((0.3 OA:0.7 MA)PbCl3), green ((0.3 OA:0.7 MA)PbBr3), and red ((0.3 OA:0.7 MA)PbI3) light under a UV lamp (365 nm). (c) Stacking various crystal/polymer films of selective emission wavelengths on a commercial blue LED (450 nm emission), where the optical density of the crystal/polymer was kept low enough to transmit part of the blue light from the commercial LED, and the absorbed blue light is then down-converted to green, red, or brown light from the crystal/ polymer film. (d) Stacking the films from (c) together and tuning the OD to achieve white light with a desired color rendering index.

showing the CIE coordinates of the individual crystal emissions from all of the PL spectra, which we have shown in Figure 4. Notably, a combination of an appropriate fraction of these crystals into a single film or layered stack of films could position the CIE anywhere on the chart with broad spectral coverage. As a final illustration, we have calculated the compositional blend of crystals required to closely match the AM1.5 solar spectrum. We show this “semi-theoretical” combined emission spectrum as a dotted line in Figure 6a and list the precise composition in Figure S10.

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SUMMARY

We have demonstrated that varying the ratio of an octlyammonium and methylammonium in the synthesis of MAPbX3 crystals can control the size and advantageously enhance the PLQE. However, an overtly excessive fraction of OA results in the formation of platelets, which exhibit lower PLQE. Changing the halide (X−) anions and mixing them in different ratios provides a flexibility to achieve a desired PL emission across the UV−vis spectrum. However, by mixing multiple crystals of differing halide compositions in a common solution, we observe evidence of anion exchange and a shift in the PL emission wavelength, which is disadvantageous for stable emission color. Nevertheless, we observe that embedding the crystals in a polymer matrix arrests this instability and gives relatively stable mixtures of perovskite crystals, which maintain the predetermined emission spectra. Finally, we also demonstrate the emission of white light (and other desired color mixtures) when exciting perovskite crystal polymer composite films with a commercial blue LED, demonstrating their application as a complete solution as a phosphor replacement for lighting.

Figure 7. Chromaticity color coordinates (CIE) plotted from the corresponding perovskite crystal emissions. The white light generated from the perovskite crystal is located at the center of the CIE chart covering cool white, natural white, and warm white light marked as 1, 2, and 3, respectively.45 We have marked a dotted circle for the x−y coordinates of commercial incandescent white light.

have also calculated the color-rendering index (CRI) and colorcorrelated temperature (CCT) of the white light generated by the perovskite crystals. The CRI and CCT were calculated using the ColorCalculator software available from Osram Sylvania Incorporated according to equations defined by the CIE (International Commission on Illumination). The CRI and CCT of the white light is 86 and 5229 K, respectively, making the perovskite crystals a potential candidate for future lighting technology.11 In principle, it is possible to obtain any hue, color, or spectral composition from these perovskite crystals. We illustrate this by 8073

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03769. Quantum confinement methodology, XRD data, TEM images, time-resolved PL decay graphs, PL emission and absorbance graphs, and a theoretical combined spectrum (PDF)

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AUTHOR INFORMATION

Present Address ⊥

S.D.S.: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the ERC under the Hyper (DKRUZG00) and Meso projects (DKR000z0). F.W.R.R. and C.D. acknowledge funding from the ERC under Grant 259619 PHOTO-EM and CNPq under Grant 246050/2012-8.

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