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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4222−4228

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Mechanistic Insight into Surface Defect Control in Perovskite Nanocrystals: Ligands Terminate the Valence Transition from Pb2+ to Metallic Pb0 Artavazd Kirakosyan,† Nguyen Duc Chinh,† Moon Ryul Sihn,† Min-Gi Jeon,† Jong-Ryul Jeong,† Dojin Kim,† Jae Hyuck Jang,*,‡ and Jihoon Choi*,†

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Department of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea ‡ Electron Microscopy Research Center, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea S Supporting Information *

ABSTRACT: Organolead halide perovskite nanocrystals (NCs) have emerged as promising materials for various optoelectronic applications. However, their practical applications have been limited due to low structural integrity and poor luminescence stability associated with fast attachment−detachment dynamics of surface capping molecules during postprocessing. At present, a framework for understanding how the functional additives interact with surface moieties of organolead halide perovskites is not available. Methylammonium lead bromide NCs without surfactants on their surface provide an ideal system to investigate the direct interactions of the perovskite with functional molecules. When the oleic acid is used in a combination with n-octylamine, its contribution to surface passivation is significantly increased by protonating the alkyl amine to the corresponding ammonium ion. Our results demonstrate that the Br vacancies at the nonpassivated surface result in a reduction of Pb2+ to Pb0 by trapping electrons generated from the exciton dissociation, which provides a main pathway for exciton trapping.

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While the adoption of typical organic ligands for OLPs NCs can provide colloidal long-term stability and enhanced photoluminescence quantum yield (PLQY),26 they introduce undesired (i.e., insulating) ligand shells on the OLP surface that hinder efficient charge transport at the interfaces.6,7,27 Furthermore, the excess ligands can induce morphological variation due to exfoliation of layered OLP NCs leading to the incidental quantum confinement effects.28 Because the organic functional groups have a direct influence on charge trapping and transport, underlying surface chemistry associated with the trap-mediated nonradiative recombination is important to understand how the passivation treatment of organic ligands affects the efficient charge injection, exciton formation, and radiative recombination. Whereas recent studies have focused on the enhanced optoelectronic properties of OLP NCs derived by the surface passivation with organic functional groups,26,29−32 they were conducted on the OLP NCs with pre-existing organic ligands, which limits direct observation of subtle changes in chemical state and nanostructures at OLP surfaces, associated with their optoelectronic properties.

rganohalide lead perovskite (OLP) materials hold a promising potential for optoelectronic application in display technology, energy harvesting, environmental detectors, etc. Their superiority has been reported in a number of studies such as photovoltaics with nearly 20% power conversion efficiency,1,2 highly efficient light-emitting diodes with over 20% external quantum efficiency,2−8 high-output piezoelectric generators,9,10 and highly responsive photodetectors.11,12 A wide range of tunability in optoelectronic properties (i.e., large absorption coefficient,13−15 low exciton binding energy,16 direct bandgap,17 and long charge carrier diffusion length,18−20) can be achieved by compositional, dimensional, and morphological variety. The interfacial interaction of the OLP layer with the functional groups of organic molecules has shown to play a crucial role in the efficiency of charge injection and exciton recombination.1−8 Surface passivation of OLP nanocrystals (NCs) with the functional ligands also significantly affects the overall performance of the OLP NC-based optoelectronic systems such as luminescence, charge trapping, recombination, and dynamics.3,21−25 However, despite the progress in the OLP NC-based optoelectronic devices, a framework for understanding how the functional additives interact with the surface moieties of OLPs is still lacking. © XXXX American Chemical Society

Received: June 3, 2019 Accepted: July 10, 2019 Published: July 10, 2019 4222

DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Transmission electron micrograph of the as-synthesized bare MAPbBr3 nanoparticles. Insets show the diffraction pattern and the particle size distribution determined by measuring the size of 50 particles. (b) Optical absorbance and photoluminescence of bare surface (BS, 0 μL) and n-octylamine (12.5−150 μL) capped samples (excitation wavelength; λex. = 365 nm). The inset shows the photo images of corresponding samples under daylight and UV illumination (λex. = 365 nm). Note that the PL intensity of the as-synthesized sample is extremely low and nearly unnoticeable by the naked eye. (c) Relative PL intensity depending on the surfactant type (OAm, OAmBr, OAc) and their amounts. (d) Relative PL intensity depending on the amount of oleic acid in the presence of 12.5 (closed) and 100 (open) μL of OAm (red) and OAmBr (blue). (e) PL decay spectra of BS and surfactant-treated MAPbBr3 samples. The lines represent fitted data using the pre-exponential coefficients presented in Table S1. (f) PLQY of the BS sample as a function of each cycle consisting of surface passivation and surfactant detachment.

recombination.35 The low PLQY (∼0.05%) reveals the presence of a large number of surface defects (Figure S4). To understand how the presence of surfactant (OAm: noctylamine,; OAmBr: n-octylammonium bromide; OAc: oleic acid) influences the PL, surfactant solution was immediately injected after perovskite nanocubes were formed. While the PL of bare MAPbBr3 nanoparticles is nearly unnoticeable by a naked eye (Figure 1b, inset), they start to emit green light after the addition of OAm, and their intensity increases with the concentration (Figure 1b,c). The position and the shape of PL spectra remain identical to those in the initial state. PL increases ∼20 times with the amount of surfactants (i.e., OAm, OAmBr), which indicates that the addition of surfactant successfully improves the radiative recombination of the bare MAPbBr3 NCs by reducing a pathway to nonradiative recombination that originated from their surface defects (Figure 1c). On the other hand, the role of oleic acid as a surfactant is not fully understood despite their common usage in combination with amine derivatives to provide colloidal stability for OLP NCs.29 When only OAc was used as a

Herein, we prepared bare surface cub-shaped methylammonium lead bromide (MAPbBr3) perovskite NCs by the surfactant-free precipitation method (see the Supporting Information for the methods),33,34 which allows us to investigate the direct interaction between organic ligands and the OLP surface. Low solubility of the precursor in 1,2,4trichlorobenzene (TCB) leads to a large amount of nuclei formation. At the same time, the amounts of precursors were carefully controlled to prevent further growth to microscale crystalline. Transmission electron microscopy (TEM) (Figure 1a) and scanning electron microscopy (SEM) (Figures S1 and S2) images show a cubic shape with well-defined edges (70− 120 nm) and smooth surfaces without morphological defects and aggregation despite the absence of surfactants in the synthetic process. The X-ray diffraction pattern (Figure S3) confirms the cubic phase of MAPbBr3 with a space group of Pm-3m and a lattice spacing of a = 5.9897 Å.26,33 UV−vis absorbance exhibits an excitonic band edge at 520 nm, and the PL peaks at 528 nm (Figure 1b).1,22,26 The Stokes shift (∼40 meV) indicates that the PL emission originates from excitonic 4223

DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228

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The Journal of Physical Chemistry Letters

Figure 2. (a) Atomic ratio of elements obtained by EDS mapping, (b) C 1s spectra of BS and OAm samples, and (c) 1H NMR spectra of the nanoparticles with different types of surfactants.

Figure 3. (a) Time-resolved photoresponse of current curves (0−1500 s) to UV (365 nm) light and transient current curves response (1500−3000 s) to O2/N2 (20/80) gas under 365 nm UV illumination at room temperature. (b) O 1s XPS spectra of the BS and OAm samples. (c) O/Pb atomic ratio for three different samples. (d) Electron energy loss spectra (core-loss region) for BS and OAm obtained at the positions near the particle edge, as depicted in TEM images. The scale bar is 50 nm. (e) Magnified core-loss region of EELS spectra highlighting their fine features.

surfactants (OAm and OAmBr) (Figure S5). However, a combination of oleic acid with a minor amount of OAm was able to exhibit comparably enhanced PL, which could be

surfactant, noticeable changes were not observed (Figure 1c, green triangles). Moreover, it does not improve the colloidal stability as well, in contrast to the two above-mentioned 4224

DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228

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The Journal of Physical Chemistry Letters related to efficient protonation of n-octylamine to noctylammonium cation (R-NH3+, Figure 1d). Initially, PL enhancement increases along with the amount of OAc and then is saturated due to the limited amounts of amine groups in the solution. Thus, further acid addition does not increase the PL intensity. A similar trend is also observed for the OAmBr case (will be discussed later). Besides the steady-state PL characterization, the bare MAPbBr3 NCs show a fast PL decay lifetime (τavg.) of 1.13 ns due to their fast nonradiative exciton quenching, whereas it remarkably slowed down for OAm (τavg. = 67 ns) and OAmBr (τavg. = 180 ns) (Figure 1e), revealing that the trapping sites associated with the nonradiative decay process of the exciton are effectively reduced by the amine derivatives. In contrast, the OAc-treated sample exhibits no significant change (τavg. = 1.8 ns). A slight increase of PL intensity could rise from the removal of surface defects by etching of MAPbBr3 NCs via the reaction between oleic acid and Pb ions at the surface. The adsorbed surfactant could be easily removed by washing the NCs with moderately polar solvent such as ethyl acetate, and only a single step of washing was enough to get rid of all organic ligands on the surface.7 Therefore, their reversible PL switching behavior could be achieved during the subsequent cycles of attachment and detachment, where the PL and absorbance were monitored at each step (Figure 1f). The PLQY increased after introducing surfactant (i.e., OAm or OAmBr) and reduced to its initial low value (∼0%) after they were washed away for each cycle. Increased PLQY after a few cycles could arise from the effective removal of surface defects during the detachment step. The chemical composition and states of the MAPbBr3 surface were analyzed to understand the passivation mechanism of exciton trapping sites. The relative increase of carbon (C) and nitrogen (N) over lead (Pb) indicates the presence of surfactant at the surface for OAm and OAm/OAc samples (Figure 2a). However, Br, N, and C contents of the OAc sample are similar to that of the BS sample, denoting that oleic acid barely coordinates at the MAPbBr3 surface. X-ray photoelectron spectroscopy (XPS) spectra of C 1s for the BS and OAm samples show C−C(H) and C−N bonds at 284.8 and 285.7−285.9 eV, respectively (Figure 2b).36−38 The OAm sample shows a relatively strong C−C(H) peak over the C−N peak, confirming the formation of a surface ligand shell consisting of long alkyl chains with amine end groups. Both BS and OAc samples show a dominant C−C(H) peak (Figures 2b and S6). N 1s and Br 3d do not show noticeable change for all samples (Figure S7). The presence of the surfactant on OAm and OAm/OAc samples was also examined by 1H NMR analysis (Figures 2c and S8). The BS sample only exhibited −CH3 and −NH2 groups. An intense signal from −CH2− groups for the OAm sample confirmed that organic ligands are tightly bound to the particle surface, resulting in a ligand shell.39 The OAc sample showed similar peaks, but the characteristic signal of βCH2 at ∼1.6 ppm was not detected, confirming that no acid molecule is present on the surface, as observed in EDS, XPS, and PL data. However, the OAm/OAc sample exhibited signals at 1.56 ppm originating from βCH2 groups, indicating the presence of oleic acid and amine groups together. It suggests that the n-octylamine attracts oleic acid molecules into the ligand shell around the particles. Similarly, the surface defects in MAPbBr3 NCs can be reduced by exposure to an O2 atmosphere, leading to PL enhancement.50−52 The nonradiative recombination of ex-

citons mainly occurs at NC surfaces with the imperfect crystal structure such as uncoordinated Pb ions (metallic Pb clusters) and Br vacancy due to nonstoichiometric reaction during the fast process of MAPbBr3 crystalline formation. Furthermore, the presence of metallic Pb0 is more pronounced for the BS sample compared to OAm and OAm/OAc samples in the XPS spectra of Pb 4f7/2 and Pb 4f5/2 (Figure S9). Thus, the PL enhancement should be related to the effective surface defects reduced by organic ligands or gas molecules. Periodic UV photoresponse to an alternating UV (λ = 365 nm) on/off (violet/gray) state under N2 flow at a bias of 2 V clearly exhibits two distinct states for BS (Figure 3a) and OAc (Figure S10). The photocurrent further increases when exposed to O2 molecules as they effectively coordinate with the surface defects to promote the radiative recombination. In contrast, when passivated by OAm, O2 response is nearly unnoticeable because organic ligands were already coordinated with the active defect sites. Thus, further passivation by O2 molecules is not effective. This can be confirmed by relatively large amounts (∼15 atom %) of O2 molecules for the BS sample compared to the OAm sample (∼2.5 atom %) in Figure 3b. Similar to the BS sample, a relatively high amount of oxygen (12−15 atom %) was observed for OAc (Figure 3c). However, XPS spectra of C 1s, Pb 4f, and O 1s indicate the absence of C−O, CO, and COO−Pb bonds, confirming the physisorbed O 2 molecules. Also, the repeatability of photoresponse confirms that most of the O2 molecules are physisorbed rather than chemically bonded to the surface moieties of MAPbBr3 crystals. In addition, when BS and OAm powders are exposed to oxygen under open air, the PL intensity of BS increases over time, while no further enhancement is observed for the OAm sample (Figure S11). Thus, in this context, the reduction of the nonradiative recombination arises from the presence of surfactants on the MAPbBr3 surface. Local variations of chemical bonding and coordination were further analyzed by performing core-loss electron energy loss spectroscopy (EELS, Figure 3d,e). The carbon K-edge arising from transitions between the C 1s core electron to the unoccupied states just above the Fermi level was obtained along the line (yellow to black dots) for BS and OAm samples (Figure 3d). The overall EELS signature for the carbon K-edge exhibits a maximum peak for 1s → σ* transition at 292 eV and a prepeak for 1s → π* transition at 285 eV. C−C bonding of the organic ligands (i.e., OAm and OAmBr) contribute to a more significant σ* peak at 292 eV of the OAm sample, compared to the BS sample that contains only C−H and C−N bonding. It is interesting that the π* feature of the C K-edge of OAm shows a significant energy shift (∼0.9 eV) to lower binding energy (Figure 3e), which is related to the longer C−C bonding length than those of C−N and C−H bonding. The position of the C 1s π* and σ* peaks is varied by the local bond length and hybridization state. Density functional theory calculations show that longer C−C bonding length (about 15%) induces a shift of π* and σ* peaks to lower energies by 0.2 and 5 eV, respectively.43 The ratio changes of peaks at 287.7, 289.3, and 325 eV are probably due to changes in the local variations of chemical bonding associated with the deficient Br ions at the surface for BS sample. Because it is related to the valence electron transitions into the π* states, loosely bound electrons can be induced by metallic Pb clusters, nonstoichiometric MAPbBr3 surface defects, and strains in the crystal structure.44 These defects behave as the active sites for the nonradiative recombination. Thus, the enhanced PL 4225

DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228

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The Journal of Physical Chemistry Letters

Figure 4. Scheme of the basic surface reaction on the surface of perovskite NCs by (a) OAm and OAmBr, (b) oxygen molecules, and (c) oleic acid. Panels (a) and (b) exhibit the successful surface passivation via physisorption, while panel (c) represents the surface etching process of OLP NCs by oleic acid.

out from the BS, a formation of a Br-complete octahedral surface is expected. However, the fresh surface should also be involved in the dynamic equilibrium associated with the formation of VBr−. Therefore, when oleic acid is combined with OAm or OAmBr, it serves (i) to etch Pb2+ ions out of the defective sites, (ii) to react with an amine group and protonate it to an ammonium cation (R-NH3+), (iii) to shift the acid− base equilibrium of solution, resulting in the dissociation of OAmBr, and (iv) to be immobilized in the ligand shell as an ion-pair with n-octylamine, as we observed by NMR. In addition, we expect that the Pb2+ ions can coordinate with the adsorbed O2 molecules and prevent further reduction of Pb2+ to Pb0 (Figure 4, path b), based on the literature and our observation (Figure S10) of significant PL enhancement when BS is exposed to O2.40−42,46,52 In conclusion, defect formation on the surface of MAPbBr3 and their passivation by a surfactant molecule have been studied. It was found that MAPbBr3 NCs with the BS have a large amount of surface defects, exhibiting extremely poor optoelectronic properties. The amine groups are able to coordinate the MAPbBr3 surface, while carboxyl groups etch Pb2+ into solution when only acid is used. However, when the acid is combined with alkyl amine or alkyl ammonium bromide, it shows even stronger enhancement of PL output. The amine group immobilizes the Br− anion at the corner of the undercoordinated PbBr64− octahedron on the MAPbBr3 surface, resulting in the charge-balanced Pb−Br framework where the Br/Pb ratio is slightly higher than stoichiometry. High Br contents preserve metallic Pb0 formation from Pb2+ at the surface. An unprotected surface shows a significant interband transition due to the surface defect arising from the undercoordinated Pb ion and dangling bonds. These findings provide deeper insight into the underlying chemical processes of the surface passivation of MAPbBr3 NCs, which is of fundamental importance for the development of highly stable and defect-free MAPbBr3 NCs for optoelectronic devices.

intensity (Figure 1b−d) could be achieved by effective reduction of the above-mentioned defects by the organic ligands with amine groups. The limited solubility of MAPbBr3 in a poor solvent results in a dynamic equilibrium between MAPbBr3 precipitates and dissolved moieties such as PbBr2 and CH3NH3Br,26,45 which introduces Br− vacancies (VBr−) at the corner of PbBr6 octahedra, leaving undercoordinated Pb2+ ions at the MAPbBr3 surface.31,37,46 Here, the excitons can be easily dissociated into holes and free electrons due to low exciton binding energy (55 ± 20 meV),16 and the free electron converts the Pb2+ cation to metallic Pb0. In the presence of organic ligands, the amine end group (R-NH20) can be protonated to an ammonium (RNH3+) cation, which is able to coordinate with PbBr6 octahedra by electrostatic interactions including ionic bonding and hydrogen bonding (Figure 4, path a).47,48 As the noctylammonium cation is localized at the surface and forms a ligand shell, it also attracts and immobilizes Br− at the VBr− position of the octahedron, resulting in increased Br amounts (Figure 2a). Thus, Br− compensates the charge imbalance of the PbBr6 octahedron that contains vacancies and terminates Pb2+ → Pb0 conversion.49 This model successfully describes our observations where the increased amounts of Br enhance the PLQY similar to the prior reports.50,51 Furthermore, noctylammonium bromide consisting of ammonium cation and a Br− anion is expected to be more favorable for passivating the MAPbBr3 surface as it introduces more Br− ions. As shown in Figure 1, its PL intensity is much higher than the n-octylamine case. Therefore, we can expect that organic molecules with both a long alkyl chain and amine functional group potentially can passivate MAPbBr3 perovskites, reducing their surface defects. When oleic acid is used solely for the surfactant, the surface of MAPbBr3 crystals is not coordinated, and rather, it seems to be etched out to the solution (Figure 4, path c). Oleic acid reacts with Pb2+ and CH3NH3+ to form the corresponding oleate salts.21 This process is reversible and involved in their growth mechanism via Ostwald ripening. As the Pb2+ is etched 4226

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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.jpclett.9b01587.

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Experimental procedure, sample preparation, characterization techniques, SEM images, XPS and NMR spectra, time-resolved photoresponse to UV, and parameters used to fit the PL decay curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +82428216632. Fax: +82428215850. ORCID

Jihoon Choi: 0000-0003-2162-8895 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (No. NRF-2019R1I1A2A01060608, NRF-2016R1D1A1B03933212, NRF-2018R1A1A1A05078352). The New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP, Grant No. 20173010032080).

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DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228

Letter

The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.9b01587 J. Phys. Chem. Lett. 2019, 10, 4222−4228