(X=Cl, Br, I) Colloidal Nanocrystals

Letter pubs.acs.org/NanoLett

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals Ievgen Levchuk,*,†,‡ Andres Osvet,† Xiaofeng Tang,†,⊥ Marco Brandl,∥ José Darío Perea,† Florian Hoegl,† Gebhard J. Matt,† Rainer Hock,∥ Miroslaw Batentschuk,† and Christoph J. Brabec†,‡,§,⊥ †

Friedrich-Alexander-Universitat Erlangen-Nurnberg, Materials for Electronics and Energy Technology (i-MEET), Martensstrasse 7, 91058 Erlangen, Germany ‡ Energy Campus Nürnberg (EnCN), Fürther Str. 250, 90429 Nürnberg, Germany § ZAE Bayern, Renewable Energies, Haberstr. 2a, 91058 Erlangen, Germany ∥ Chair for Crystallography and Structural Physics, Friedrich-Alexander-University Erlangen-Nürnberg, Staudtstrasse 3, 91058 Erlangen, Germany ⊥ Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Str.6, 91052 Erlangen, Germany S Supporting Information *

ABSTRACT: In the past few years, hybrid organic−inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15−25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415−740 nm, full width at half-maximum (fwhm) of 20− 44 nm, and radiative lifetimes of 5−166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics. KEYWORDS: Formamidinium, perovskite nanocrystals, optoelectronics, photoluminescence, ligand-assisted reprecipitation

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version of lead halide nanocrystals to perovskite by methylammonium halide incorporation.25 Compared to the organic−inorganic MA or all-inorganic Cs perovskite analogues, the pristine formamidinium perovskites have a couple of attractive features like higher thermal, moisture, and chemical stability.5,11,20,26−29 Nonetheless, compared to the MA perovskite analogues, the preparation technology of thin films,26 single crystals,30,31 and microcrystalline powders2 is not well developed due to the following features: (i) FAPbI3 thin films or crystallites typically crystallize in a “yellow” nonperovskite phase26,30 after a few hours of storage; (ii) differences in the ionic radii (2.17 Å for MA+ and 2.53 for FA+) may affect the growth kinetics.32 The unique properties of the pristine bulk material inspired us to synthesize and investigate the properties of corresponding colloidal nanocrystals and to establish a new protocol for the synthesis

ormamidinium lead halide perovskites (CH(NH2)2PbX3 or FAPbX3, X = Cl, Br, I) are an advanced class of direct bandgap semiconductors for optoelectronic devices, and they have established themselves as a promising alternative for the thermodynamically less stable methylammonium (MA, CH3NH3+) perovskites.1−5 In addition, the hybrid organic− inorganic and all-inorganic metal halide perovskite (ABX3, where A = Cs+, CH(NH2)2+, CH3NH3+; B = Sn2+, Pb2+; X = Cl−, Br−, I−) thin films and their colloidal nanocrystals (NCs) offer a wide variability and multiple opportunities for finetuning optoelectronic applications.6−12 Ease of size control and compositional mixing, band gap, and emission tuning inspired researchers to successfully utilize this material class for efficient solar cells,13 sensitive photodetectors,14−18 low threshold lasers,19,20 laser diodes,21 advanced photonics,21 and lightemitting diodes,9,21,22 as well as for chemical reaction monitoring.23 Various methods and approaches have been proposed for the synthesis of organic and inorganic metal halide perovskite colloidal nanocrystals (CsPb(Sn)X 3 , MAPbX3, X = Cl−, Br−, I−) including hot-injection,6,10,24 ligand-assisted reprecipitation (LARP),8 or structural con© XXXX American Chemical Society

Received: November 15, 2016 Revised: February 28, 2017 Published: April 7, 2017 A

DOI: 10.1021/acs.nanolett.6b04781 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Representative perovskite FAPbX3 (X = Cl, Br, I) unit cell with FA+ cation in the center of eight [PbX6]4− octahedra. (b) HRTEM of single FAPbI3 NC. (c) TEM micrograph of the monolayer FAPbI3 NCs on a grid; σ = size distribution. (d) XRD curves for single and mix-halide FAPbX3 NCs.

week in glovebox under nitrogen atmosphere. This is a significant improvement compared to the MAPbI3 NC dispersions.8,24,33 Not surprisingly, FAPbBr3 NCs display even higher colloidal and chemical stability, being stable for several weeks under ambient conditions, while FAPbCl3 NCs precipitate within few hours. Structural Properties. As-formed colloidal FAPbI3 NCs were 14.4 ± 3.4 nm in size with nearly cubic shape (Figure 1c), while FAPbBr3 (Figure S1c) and FAPbCl3 (Figure S2a-b) crystallized as platelets with lateral sizes of 21.5 ± 4 and 22 ± 3 nm, respectively. High-resolution transmission electron microscopy (HRTEM) displays high crystallinity of a single FAPbI3 NC as confirmed by the characteristic lattice spacing of 0.32 nm (Figure 1b) related to the (112) plane of the cubic perovskite structure. Selected area electron diffraction (SAED) data confirm the crystalline nature of all the FAPbX3 NCs measured during the TEM session (Figure S3a−c). The measured X-ray diffraction (XRD) patterns display pure cubic perovskite phase for all three FAPbX3 (X = Cl, Br, I) NCs and their mixed halide FAPbCl1.5Br1.5 and FAPbBr1.5I1.5 NCs samples (Figure 1d). Interestingly, FAPbI3 NCs crystallize in cubic structure with trigonal symmetry, which is also known as black α-phase.30,31 It is well-known that the black α-FAPbI3 polycrystalline films as well as single crystals slowly transform to nonperovskite hexagonal yellow δ-phase at room temperature.26,28−32 Further temperature annealing over 154 °C (phase transformation temperature) promotes a reverse transformation from the yellow to the black phase.2,30,31 However, in FAPbI3 NCs colloidal solution investigated in this study we did not observe a transition from the black to the yellow phase after 150 days of storage (Figure S4). Even after 30 days the drop-casted NC film remained in the black phase upon storage under ambient conditions and further temperature treatment from −195 to 160 °C (Figure S5). A similar phenomenon of reduced-size stabilization of the cubic polymorph has been observed for CsPbI3 perovskite nanocrystals.6,13 It is well-known that nanosized materials can display different phase transition behavior as compared to bulk crystals due to the surface energy difference between the polymorphs.37,38 As the size decreases, the surface-to-volume ratio increases, thus resulting in a lower transition temperature for the nanosized materials. We found that our results are in excellent agreement with the

of highly luminescent and stable colloidal perovskite nanocrystals with tunable optical as well as structural properties. In this study, we present a facile and fast strategy to synthesize high quality FAPbX3 (X = Cl, Br, I) colloidal nanocrystals by a ligand-assisted reprecipitation reaction at room temperature. The brightly emitting FAPbX3 NCs are uniform in size, have a high quantum yield of up to 85%, and their optical properties can be tuned by band gap engineering via simple halide mixing or size control, resulting in luminescence in the visible range between 415 and 740 nm. Additional NC surface modification allows protecting the material against water and storing in solution as well as in the form of powder for a long time. We also demonstrate FAPbI3 based photodetectors. Result and Discussion. Nanocrystal Synthesis. Synthesis of colloidal FAPbX3 (X = Cl, Br, I) nanocrystals was carried out by LARP technique similar to the one for MAPbX3 (X = Br, I) NCs published by our group elsewhere.33 As the antisolvent media we utilized chloroform in all our experiments. The choice of the antisolvent plays a decisive role in the LARP method. Toluene, which is commonly used for perovskite NC LARP synthesis,8,34 did not result in the formation of FAPbI3 NCs, while FAPbBr3 or FAPbCl3 immediately formed a turbid suspension of large particles and agglomerates. In this work, we rapidly injected a DMF solution of PbX2 and MAX (X = Cl, Br, I) precursors, containing oleic acid (OA) and oleylamine (OAm) as ligands, into vigorously stirred chloroform at room temperature and observed the immediate formation of NCs. Due to the ionic nature of the metathesis, nucleation and growth kinetics are very fast, and the NCs appear within seconds (Video S1). Blending appropriate FAPbX3 precursor solutions with different aspect ratios is the usual strategy to produce mixedhalide FAPb(Cl/Br)3 and FAPb(Br/I)3 nanocrystals. However, we could not obtain NCs with FAPb(Cl/I)3 composition, mainly due to the large difference in the ionic radii.6,35,36 The resulting OAm/OA ligand shell passivated FAPbX3 NCs were subsequently washed by precipitation/redissolving with an acetonitrile/toluene solvent composition. Afterward, NCs could be easily redissolved in nonpolar solvents such as chloroform, toluene, and hexane. The washed FAPbI3 NCs, redispersed in toluene, formed a colloidal solution that was stable for at least 1 B


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Figure 2. Optical properties of the colloidal solution of FAPbX3 nanocrystals in toluene. (a) Representative optical absorption and PL spectra. (b) PL decay dynamics of the pure and mixed-halide NCs. (c) Compositional PL tuning diagram. (d) Digital picture of colloidal solution in toluene taken under UV-light (λ = 365 nm). (e) Compositional tuning of the PL spectra of the mixed halide NCs.

reports by Li et. al39 on MAPbI3 system, where the temperature of tetragonal (330 K) to orthorhombic (161 K) phase transition strongly depended on the thickness of the microplates. Furthermore, purification of those NCs has a strong impact on the black α-phase stabilization. Unpurified or badly washed by high-speed centrifugation α-FAPbI3 NCs agglomerate within a few days causing a partial conversion to the yellow phase (Figure S6). On the contrary, washing with acetonitrile/ toluene mixtures isolates NCs without considerable ligand removal. A similar effect has been reported for size-confined CsPbI3 quantum dots (QDs).13 However, and to underline the main findings of this investigation, we are not aware of any previous reports on the synthesis and size-reduced stabilization of α-FAPbI3 NCs perovskites. Optical Properties. The optical UV−vis absorption and photoluminescence (PL) spectra of the obtained NCs with varying halide composition display a single peak at the first excitonic transition and PL emission (Figure 2a). Compared to bulk materials (Figure S7), optical absorption edge and the emission bands of the single halide FAPbX3 (X = Cl, Br, I) NCs are blue-shifted due to the quantum size effect, which will be discussed further. Purified and redispersed in toluene, FAPbI3 and FAPbBr3 nanocrystals exhibit a high PLQY of 55% and 84%, respectively, in contrast to FAPbCl3, which has a PLQY of less than 1%. The mixed halide FAPbCl1.5Br1.5 or FAPbBr1.5I1.5 NC samples have 21% and 23% efficiency, respectively. Lower PLQY of mixed halide compositions and that of FAPbCl3 is an intrinsic property of the halide perovskites.36,40−42 Also, Figure S14a−d shows that the PL from colloidal FAPbBr3 NC film is stable under prolonged exposure to blue (445 nm, 7.5 W/cm2) and UV (375 nm, 3 W/cm2) light. In case of blue light, PL peak and FHWM remains the same, while under UV light, 5 nm redshift was observed (Figure S14d), indicating partial NCs sintering at irradiated point.43 This observation suggests that blue light is more applicable for these NCs in luminescent application. Precise mixing of various halide starting solutions results in a series FAPb(Cl/Br)3 and FAPb(Br/I)3 NC samples with a seamlessly tuned PL peak wavelength covering (Figure 2c) the

visible spectral range from 415 to 740 nm (2.98−1.68 eV). Moreover, relatively narrow FHWMs ranging from 20 to 44 nm are observed (Figure 2d,e). The bandgap undergoes a linear shift for different Br/I and Br/Cl ratio samples following the Vegard’s law for lattice constants of alloys (Figure S8).44−47 Time-resolved PL spectroscopy of FAPbX3 (Cl, Br, I) NCs shows nonexponential decay traces with average lifetimes of τaver = 15−116 ns with a faster emission for wider band gap NC samples (Figures 2b and S9). While the PL decay time in the mixed Br/I samples increases gradually from 20 to 116 ns with the increase of the content of I ions, FAPbCl1.5Br1.5 sample display a slower decay dynamics, compared to pure FAPbBr3 (24 ns vs 20 ns) and FAPbCl3. This observation may be discussed in context with observations that the addition of Cl improves charge carrier lifetime in FAPbCl1.5Br1.5 NCs.48 Similar results were reported for CsPb(Cl/Br)3 perovskites.49 It is worth noticing that we have observed phase segregation in the PL spectra of the mix-halide NCs upon laser irradiation. Such effect is well described by Hoke et al.50 in microcrystalline perovskite MAPb(Br/I)3 and FAPb(Br/I)3 thin films. We have tested four NCs films systems: FAPbBr1.2I 1.8 (I-rich), FAPbBr 1.8 I 1.2 (Br-rich), FAPbCl 1.2 Br 1.8 (Br-rich), and FAPbCl1.8Br1.2 (Cl-rich). The Cl/Br samples were irradiated by a 375 nm continuous wave (CW) diode laser and the Br/I samples by a 445 nm laser at intensities up to 20W/cm2, monitoring continuously the PL spectra (Figure S11). In all cases we observed phase segregation also by XRD measurements (Figure S12). Interestingly, the FAPbBr3 phase PL emission was always predominant. Similar to the results of Hoke et al.,50 this process is reversible (Figure S13). Additional study of phase segregation under laser irradiation in the nanocrystalline mixed halide perovskite systems is required to understand its mechanism. Quantum Size Effect. As we have shown above (Figure 2a), the luminescence bands of single halide nanocrystalline samples are blueshifted compared to the bulk (Figure S7) due to quantum size effect. In colloidal semiconductor NCs (e.g., CdSe) this phenomenon occurs if the size of the crystallites becomes comparable to or smaller than the exciton Bohr radius C


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Figure 3. Comprehensive characterization of purified FAPbBr3 NPLs toluene solutions with different thicknesses. (a) Band gap tuning by changing the OAm/OA ratio. (b) PLQY for NPLs with different thicknesses. (c−e) Bright-field TEM characterization of the vertically stacked FAPbBr3 NPLs synthesized with different OAm/OA volume ratio: (c) 200 μL/150 μL blue (439 nm) emitting NPLs with lateral size 36.6 ± 9 nm and thickness 1.4 ± 0.1 nm; (d) 200 μL/80 μL cyan (486 nm) emitting NPLs with lateral size 22.7 ± 3 nm and thickness 2 ± 0.1 nm; (e) 200 μL/40 μL green (533 nm) emitting NPLs with lateral size 21.5 ± 4 nm and thickness 2.6 ± 0.2 nm; σ = thicknesses distribution. (f) Theoretical (EMA) versus experimental band gap of the FAPbBr3 NPLs as a function of the thicknesses (d). Inset shows unit cell number (n) for single NPL with different thickness. (g) PL decay.

excellent agreement with the calculated thickness of NPLs with n = 2 (1.2 nm), n = 3 (1.8 nm), and n = 4 (2.4 nm), where n is the number of FAPbBr3 monolayers with the unit cell thickness of 5.9944 Å. The average lateral size of blue, cyan, and green emitting NPLs is 36.6 ± 9, 22.7 ± 3, and 21.5 ± 4 nm, respectively (Figure S1a−c). The absolute PLQY increases from 21% (NPLs with 438 nm PL peak wavelength) to a remarkably high value of 84% (500 nm peak wavelength) (Figure 3b). For the thickest, more bulky NPL (2.6 nm), the PLQY drops to 74%. Previous studies discussed similar findings in the context of reduced exciton stabilization.8,58 Identical trends were also observed for the structurally similar MAPbBr3 micro- or single crystals, which exhibit significantly lower PLQY (90%).33,59 Our PLQY data for size-confined FAPbBr3 NPLs is in excellent agreement with the PLQY values of cube-like FAPbBr3 NCs.11 To estimate the Bohr radius in FA-perovskites, we used the values of reduced effective mass (μ) and effective dielectric constant (εeff) calculated by Galkowski et al.60 for bulk FAPbBr3 and FAPbI3 perovskites. Within the effective mass approximation (EMA),61 we calculated a Bohr radius of 3.5 nm for FAPbBr3 (exciton binding energy 24 meV60) and 6.35 nm for FAPbI3 (exciton binding energy 10 meV60 (or 8 meV62); Supporting Information, eqs 3 and 4). The dependence of the band gap of the FAPbBr3 NPLs on their thickness is shown in Figure 3f. For comparison, we have calculated the size-

in the bulk material. Decreasing the size of NCs down to the size of the exciton Bohr radius leads to a blueshift of their absorption edge and the PL peak. Size tuning in hybrid organic−inorganic or all-inorganic perovskite NCs51−56 was recently reported by several groups. In our previous work33 we demonstrated the thicknesses tailoring in MAPbX3 (X = Br, I) nanoplatelets (NPLs) by OAm/OA ligand ratio, which leads to a one-dimensional quantum confinement. Since FAPbBr3 NCs crystallize as nanoplatelets, we applied the same technique here. The ligand ratio control allowed to tailor the thickness of the platelet-like FAPbBr3 NCs, leading to shift of the narrow PL band (Figure 3a) and the band−band absorption edge. The measured PL spectra are blueshifted from 533 to 438 nm with the increasing OAm content. Such a gradual blueshift indicates quantum confinement in FAPbBr3 NPLs, which is supported by TEM image analysis (Figures 3c− e S1d). Similar features are observed for FAPbI3 and FAPbCl3 (Figure S10a,b). The observed NPL emission peak at 438 nm and its PL shape are in excellent agreement with data of Weidman et al.57 for two-layer FAPbBr3 perovskite NPLs. Furthermore, PLQY value presented in our work for these NPLs (21%) is almost similar to the data of Weidman et al. (22%). From the evaluation of the TEM images we found that the thickness of blue (438 nm), cyan (486 nm), and green (533 nm) emitting FAPbBr3 NPLs is 1.4 ± 0.1, 2 ± 0.1, and 2.6 ± 0.2 nm, respectively (Figures 3c−e and S1d). This data is in D


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dependent bandgap of quantum dots with the same characteristic size (solid line in Figure 3f), using the EMA, and the literature values for the effective masses and dielectric constant (Supporting Information, eqs 1 and 2). The calculated bandgap follows the trend in experimental values of the bandgap, although overestimates it especially in the case of smaller particles. As expected, the PL decay time gradually increases with the FAPbBr3 NPLs thickness from 5 to 23 ns (Figure 3g). This is in excellent agreement with the PLQY enhancement with increasing thickness. The faster and nonexponential PL decay curves and lower PLQY in thinner NPLs may be explained by additional nonradiative relaxation caused by Brrich surface causing the quenching of the luminescence.58 Similar results were observed for MAPbBr3 NPLs in our previous report.33 Such a high PLQY of FAPbX3 NCs in the visible spectral region and the remarkable stability of the FA-perovskites compared to other perovskites (MA or Cs) make this material an ideal candidate for optoelectronic devices or for application as a phosphor material. However, moisture resistance is still a problem for organo-lead halide perovskites.63 Only special surface coatings or the incorporation in a polymer matrix may protect the material from dissolution in water.64−66 To increase the stability of the NPLs, we have applied a postsynthesis surface modification by polyhedral oligomeric silsesquioxane, previously proposed by Rogach et al.64 This allows to protect the brightly luminescent nanocrystalline FAPbX 3 from decomposition by water. The NC colloids and nanocrystalline powders with various halide compositions are stable in toluene solution as well as in water for at least 2 months without losing the brightness of their emission (Figure S15a−d). Additionally, we have examined the photoconductivity of FAPbI3 perovskite NCs in the context of using the QDs in photovoltaics (see details in SI). The calculated light/dark ratio of the as-synthesized FAPbI3 NCs are ca. ∼2, which makes these NCs promising candidates for QD photovoltaics and quantum dot light-emitting diode (QLED) applications (Figure S16a,b). Conclusion. In summary, we presented the room temperature synthesis of highly luminescent FAPbX3 (X = Cl, Br, I) colloidal NCs. Band gap engineering by halide alloying gives tunable photoluminescence bands across the visible region from 415 to 740 nm with a remarkably high quantum yield of up to 85%. Furthermore, we proved the quantum size controlled luminescence shifting in FAPbBr3 nanoplatelets with their thicknesses varying from 2 to 4 unit cells. Finally we demonstrated a FAPbI3 based photodetector, potentially applicable in perovskite quantum dot photovoltaics.

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Ievgen Levchuk: 0000-0003-0644-2283 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was performed in the framework of the Energie Campus Nürnberg (EnCN) and supported by funding through the “Bavaria on the Move” initiative of the state of Bavaria and the project 1006-11 of the Bavaria Research Foundation (BFS). We gratefully acknowledge the financial support from the Cluster of Excellence “Engineering of Advanced Materials” at the University Erlangen-Nürnberg and from the Federal Ministry for Economic Affairs and Energy (MNPQ program BMWI 11/12). The authors also gratefully acknowledge financial support from the Joint Project Helmholtz-Institute Erlangen Nürnberg (HI-ERN) under the project number DBF01253. J.D.P. is funded by a doctoral fellowship grant of the Colombian Agency COLCIENCIAS. I.L. highly acknowledges Patrick Herre for TEM and HRTEM measurements, ́ Quiroz for help with video recording, and César Omar Ramirez Anatolii Polovitsyn for scientific discussion. I.L. also strongly acknowledges financial support by the DFG through the research training group 1896 “In situ microscopy with electrons, X-rays and scanning probes”. Electron microscopy resources have been kindly provided by the Center for Nanoanalysis and Electron Microscopy (CENEM).

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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.nanolett.6b04781.

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REFERENCES

Synthesis details, calculations, photodetector characterization, and additional figures(PDF) Process of FAPbBr3 and FAPbI3 NCs synthesis (MOV)

AUTHOR INFORMATION

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Letter

Nano Letters

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