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A “Cocktail” Approach to Effective Surface Passivation of Multiple Surface Defects of Metal Halide Perovskites Using a Combination of Ligands Jin Z. Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01166 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry Letters

A “Cocktail” Approach to Effective Surface Passivation of Multiple Surface Defects of Metal Halide Perovskites Using a Combination of Ligands Jin Zhong Zhang Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064 USA Abstract. Surface passivation of metal halide perovskites (MHPs) is essential for their stability and various properties as well as functionalities, including optical and electronic. Passivation is important for both stabilizing intrinsic defects and preventing extrinsic damaging species from reaching the perovskite (PVK), such as water and oxygen. Due to the ternary nature of their chemical composition, multiple surface defects exist for both bulk and nanostructured PVKs, with the latter particularly prominent due to their extremely large surface-to-volume (S/V) ratio. To effectively passivate the different surface defects, a multitude of different ligands are necessary since each type of defect likely requires a different ligand for optimal passivation, as has been successfully demonstrated in a number of systems in essentially a “cocktail” approach. Characters of the ligands that affect effectiveness of passivation include size, shape, charge and charge distribution, orientation, conductivity, and inter-ligand interaction. Examples of ligands for MHPs include both cationic and anionic or zwitterionic species with varied valences. The challenge is to identify the most effective ligand for each type of defect, and addressing this will require further experimental and theoretical study.

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Metal halide perovskites (MHPs) have attracted tremendous attention in recent years due to their highly interesting properties and promising applications in fields including solar cells, light emitting diodes (LEDs), detectors, and sensing.1-8 Their unique properties include long charge carrier diffusion length and lifetime, high photoluminescence quantum yield, flexibility in tuning electronic and optical properties by varying chemical composition and physical size (quantum confinement effect), and relative ease of generating different structures in zero dimension (0D), 1D, 2D or 3D for diverse applications.9-14 In particular, their ternary chemical structure, ABX3, affords tremendous opportunity and flexibility in varying A, B or C to attain various desired properties and functionalities. The same traits also present a challenge as more possible defects are present compared to binary or single component materials such as ZnS or Si, which will demand different passivation approaches. Their highly ionic character also imposes requirements different from that of covalent solids such as Si or Ge. Furthermore, the combination of organic and inorganic components, e.g., in MAPbX3, also requires different methods for passivation, possibly a blend of methods appropriate for both the organic and inorganic defects on the surface. Similar to other materials, passivation of surface defects in MHPs is critical to their stability and functionality.15-22 This is often achieved by using different materials including molecular or ionic ligands. Figure 1 illustrates the key idea behind surface passivation in terms of bandgap or trap states with respect to the valence band (VB) and conduction band (CB) edges. In this context, the main goal of passivation is to remove bandgap states, which often trap charge carriers and affect the electronic and optical properties, either by reducing the density of trap states or pushing them out of the bandgap, e.g., above the CB edge or below the VB edge, so they have no or little influence on the properties of the materials. 2


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For MHPs, the multiple and varied defects expected require a combination of different ligands for effective passivation. While it is relatively easy to identify a specific ligand for effectively passivating a particular type of defect, it is not an easy task to determine simultaneously multiple ligands for optimal passivation of multiple defects. It is like attempting to make a good “cocktail” with each ingredient in its optimal or most desired state. The challenge is partly due to the intricate interaction between ligands.

Structurally, passivation involves using another material, e.g., ions, molecules or solids, to stabilize or protect the surface of the perovskites (Figure 1 right). Figure 2 shows a structural model for surface passivation of perovskite quantum dots (PQDs) with a combination of ligands in a “cocktail” approach. The main idea generally applies to bulk perovskites (PVKs) as well with some subtle differences in certain situations, as will be discussed later. Passivation can improve both the properties of the materials, including charge carrier dynamics,23-24 and their functionality for applications.20-22,25 Ligands used for passivation also affect coupling or “communication” between PQDs, e.g., charge transfer and transport that are critical to device applications based on PQD solids, such as solar cells and LEDs,26 or phase transition at low temperature.27 As shown in Figure 2, a “cocktail” approach is needed to passivate the multiple types of defects that can arise from vacancy, interstitial, substitutional, or anti-site. Most methods and

Due to the high density of surface defects expected for PNCs or PQDs, efforts have been made recently to develop strategies using multiple types of ligands or single ligand molecule with multiple anchoring functional groups to target their multiple 3

types of surface defects.



ligands reported for passivating both bulk and nanostructured MHPs rely on a single ligand approach, where a specific ligand is used to target a particular type of defect. One may not expect this to be very effective generally given that MHPs are anticipated to have multiple types of defects and therefore likely requite different types of ligands for passivation at the same time. Of course, for bulk MHPs with a relatively low density of defects, a single component approach may be expected to work better than for nanostructures with a much higher density of defects. However, somewhat surprisingly, a single ligand approach actually seems to work reasonably well for both bulk and PQDs. The first reported PQDs used octadecylammonium (ODA) bromide, CH3(CH2)17NH3Br, as a passivation ligand or sometimes called passivator.28 A recent example of single ligand passivation is the use of protonated melamine to passivate MAPbBr3 perovskite nanocrystals (PNCs).29 These studies suggest that perhaps it is not critical to passivate all the defects for achieving a certain level of, albeit not optimal, stability and functionality. One approach to addressing multiple types of defects is to overload a particular constituent ion of the perovskite, e.g., X-, so its surface is halide rich and one can then focus on using appropriate ligands to target the surface-enriched halide defect sites (X- in this example), as illustrated in Figure 3. As a result, instead of having to use multiple types of ligands, one can essentially use one primary ligand to target the dominant surface defects. For better results, one could add other ligands to target the remaining defects related to other defects. For example, inorganic passivation using metal halides such as ZnX2 (X=Cl, Br, or I)30-31} or PbBr232 has been demonstrated for improving stability of PQDs post-synthesis involving organic amine and acid ligands and for changing the ionic characters of the PQD surface. To simultaneously target multiple types of defects using a “cocktail” approach as shown in Figure 2, one method is to use a combination of organic molecules containing carboxylic 4


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acids (Lewis acid), e.g., oleic acid (OA), and molecules containing amines (Lewis base), e.g., oleyamine (OLA or OAm).33-34 The acid and base molecules react via proton transfer from the acid to base, as shown in Eq. 1 below, resulting in -COO- and -NH3+ ions that can target cationic and anionic defects, such as MA+, Pb2+, and halide (X-), respectively. R-COOH + R’-NH2 = R-COO- + R’-NH3+

(1)

The first study using such binary ligands was reported by Protesescu et al. in early 2015 for passivating CsPbX3 (X=Cl, Br, I) PQDs using OLA and OA for LED applications.33 Figure 4 shows some of the electronic absorption, photoluminescence (PL), and time-resolved PL (TRPL) results. Around the same time, an independent study by Zhang et al. used a combination of noctylamine (OTA) and OA to passivate CH3NH3PbX3 PQDs.34 The same approach has since been used by others to stabilize PQDs using OA and OLA, including doped PQDs such as CsPbxMn1-xCl3.35-36 This relatively simple approach is quite effective and successful. Luo et al. extended this approach by using (3-aminopropyl)triethoxysilane (APTES) or PSS-[3-(2-aminoethyl)amino]propyl-heptaisobutyl substituted (POSS), instead of OLA or OTA, to generate MAPbBr3 PQDs with much better stability against protonic solvent molecules compared to PQDs synthesized using straight-chain amine ligands, octylammonium bromide (OABr) and octadecylammonium bromide (ODABr).37 The enhanced stability is attributed to the cone or umbrella shape of APTES and POSS that provides better protection of the surface. APTES turned out to be better than POSS due to its additional ability to hydrolyze in air and generate a thin layer of silica on the PQD surface, further protecting from species such as water, oxygen or other protonic solvent molecules. It should be made clear that, besides APTES and POSS, oleic acid is also important in turning the amine groups into ammonium ions (Eq. 1) for effective passivation of the ionic defects on the PQD surface, as discussed above. Interestingly, 5



round 140 K, PQDs passivated with OABr underwent a structural phase transition from tetragonal to orthorhombic, established by the emergence of a higher energy band in the photoluminescence (PL) spectrum, which was not observed in PQDs passivated with APTES, despite cooling down to 20 K.27 In a recent study, a combination of hydroxybenzene sulfonic acid or salt and SnCl2 complex layer has been used successfully to passivate in FASnI3 (FA for formamidinium) solar cells with significantly improved air or oxidation stability.38 A major factor is the interaction between the sulfonate group and the Sn2+ ion. Likewise, the binary approach has been applied to other pairs of ligands such as benzylamine (BZA) and benzoic acid (BA) for passivating MAPbBr3 PQDs.26 These aromatic ligands are designed and demonstrated to enhance charge delocalization and transfer/transport, desired for device applications, via π-π stacking interaction between the benzene rings of the ligands on different PQDs. A similar dual passivation approach has very recently been demonstrated using quaternary ammonium salts, namely benzyltrimethylammonium bromide (BTABr), phenyltrimethylammonium bromide (PTABr) and benzyltrimethylammonium chloride (BTACl), to passivate MAPbBr3 perovskite films for PeLED applications.39 Both the ammonium cation and halide anion from these salt additives contribute to passivating the anionic and cationic defect sites, respectively. Instead of using two different ligand molecules to target two types of defects, one can also use bidentate ligand molecules that can provide two different functional anchoring groups in one ligand molecule to target two different types of defects. One example is the use of peptidelike molecules that contain both amine and carboxylic acid functional groups and, upon proton transfer, turn into ammonium ion and ionized carboxyl group that are known to be effective for passivating PQDs or PNCs.40 Figure 5 shows the electronic absorption, PL, and photographs of 6


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PNCs passivated using peptide-like bidentate ligands containing ammonium and ionized carboxyl functional groups. The use of a single ligand with two or more functional groups simplifies the synthesis process and provides stronger binding to the PNC surface when considered on a per ligand basis. The distance between the two functional groups is an important factor, with the ideal distance matching the distance between two defect sites to be passivated. More than two ligands have also been demonstrated in PQD or PNC passivation. For example, oleyamine and oleic acid have been used in conjunction with paraffin to passivate CsPbBr3 PNCs with different shapes including 0D QDs, 1D nanowires, 2D nanoplatelets, and 3D nanocubes.41 Ethyl acetate (EA), when used to replace oleic acid, was found to be useful in tuning the nanostructures while paraffin helps to stabilize the PNCs. In some situations, reactions between ligands generate multiple species that can act to passivate more than two types of defects. These can be done using either a combination of multiple ligands or single multidentate ligands. For example, phosphonic acids (PAs), in conjunction with APTES, have been found to be highly effective in passivating PQDs, partly due to the multiple species generated from reaction between the PAs and APTES that can simultaneously passivate multiple types of defects on the PQD surface.42 Figure 6 shows a model for the surface passivation of MAPbBr3 PQDs using PAs in conjunction with APTES.42 Compared to -COO-, -PO32- has a tripod resonance structure that is likely more stable and can thereby better passivate the surface. PA is also more versatile as it can be in the -PO2OH- or PO32- anionic state that can passivate different defects. Instead of using multiple ligands, one can also use a single ligand with multiple functional anchoring groups to target multiple defects. For example, Wang et al. used L-cysteine that contains three functional groups (-SH, -COOH and -NH2) to passivate defects in MAPbBr3 7



PQDs.43 Interestingly, the multiple functional groups of a single ligand allow both passivation and linking between PQDs to assemble into super crystals that maintain largely the properties of isolated PQDs, attributed to relatively weak interaction between PQDs in the super crystals. The ability to form such unique super crystals is a direct result of the three functional groups in the Lcysteine ligand. Likewise, polymers with multiple anchoring groups (same or different types) for passivating PQDs. For instance, poly(pactic) acid with -C=O and -COOH functional groups has been used to passivate CsPbX3 PNCs.44 Similarly, polyethylenimine (PEI) and conductive polyaniline (C-PANI) with multiple N-H related functional groups have been successfully used to passivate MAPbI3 films.45-46 Figure 7 shows an illustration of interaction between C-PANI and the MAPbI3 film surface. With C-PANI, both the photovoltaic (PV) performance and stability of the perovskite solar cell (PSC) are improved, attributed to the favorable interfacial interaction between –NH2+– groups of C-PANI and I− inMAPbI3. The C-PANI-rich surfaces of the MAPbI3/PNAI perovskite film may also assist hole transport in the device. In another study, carbon quantum dots (CQDs) with -C=O and -OH groups were used to passivate uncoordinated Pb2+ at grain boundaries of MAPbI3 to reduce nonradiative recombination and enhance PL and charge carrier lifetime.47 However, usually the types of functional groups in a given ligand molecule such as polymer or CQD are somewhat limited. Therefore, the approach of using multiple ligands affords a higher degree of flexibility by providing a variety of functional groups for situations where there are many different types of defects. It is, of course, possible to use multiple ligands, with some of them containing multiple functional groups. An interesting example is a recent

Compared to PQDs, bulk MHPs have a much lower density of defects, especially surface defects. However, multiple defects are still expected for bulk MHPs. The 8

ACSusually Paragonbe Plus Environment approaches developed for PQDs can applied to their bulk counterparts.

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study that uses quintenary ammonium halides (QAHs), including choline chloride, choline iodide and L-α-phosphatidylcholine, to passivate MAPbI3 solar cells to achieve improved open circuit voltage, PCE, and stability.48 These ligands contain both cations and anions (zwitterionic) as well as functional groups such as -O-H and -C=O, but the zwitterionic character was determined to be mainly responsible for the successful passivation, as shown in Figure 8. Similarly, a more recent study using three squaraine molecules (SQ) with zwitterionic structures has shown that such ligands can passivate under-coordinated Pb2+ and Pb−I anti-site defects, substantially improving solar cell PCE and stability.49 One interesting exception found is that while APTES with OA is effective in passivating PQDs, the combination does not work well for bulk MAPbI3 films; however, OA alone works well for passivating the bulk film and solar cells.50 The large curvature of the PQDs can be well accommodated by the cone-shaped APTES that may be too crowded for the bulk surface, while OA alone does not passivate the PQD effectively due to spacing between OA molecules but can possibly form a self-assembled monolayer on the bulk surface, thereby providing effective passivation. Figure 9 shows a schematic illustration of this effect. This demonstrates that the detailed molecular structure of the ligands, especially steric hindrance, may play a critical role in the effectiveness of surface passivation. In addition to organic molecular ligands, combinations of inorganic ions have been used as passivators to passivate PVKs in a manner consistent with the “cocktail” approach. For example, ZnX2 and PbBr2 have been found effective in passivating CsPbX3 PQDs.30-32 The cations and anions from the passivators are expected to passivate the anion and cation defects on the surface of the PVKs, respectively. It is conceivable that a combination of organic and inorganic passivators, e.g., metal salt with organic anions, can be highly effective for passivation. 9



Appropriate solvent will need to be selected to solubilize both the organic and inorganic components. In some sense, when passivators such as ODABr are used, both the cationic and anionic components are likely contributing to passivation.37 It would be interesting to better understand the relative contributions from each of the two components. Theoretical and computation studies have also been carried out to understand surface passivation of MHPs. For example, first-principles calculations conducted on halogen bond passivated MAPbI3 surface, using iodopentafluorobenzene (IPFB) as a passivator show that IPFB can interact with the PVK surface via halogen bonds, which modifies the interfacial structures of the PVK surface.51 The potential energy surface of the PVK surface involving the adsorbed IPFB molecule changes along different axes of the unit cell, demonstrating the potential of using molecular engineering approaches to modify and improve the properties of MHPs. Surface passivation has important implications in applications of MHPs besides fundamental research. For example, thiourea has been successfully used to passivate MAPbI3 PVK and to improve stability towards oxygen, light, and heat.52 Solar cells based on the passivated films achieved a power conversion efficiency (PCE) of 19.57% and 17.67% for active areas of 0.1 and 1.0 cm2, respectively, and the unencapsulated devices retain 98% and 93% of original efficiencies after two months under air conditions for active areas of 0.1 and 1.0 cm2, respectively. Figure 10 shows some of the spectroscopic characterizations that reveal interaction between the thiourea ligand and the PVK. The passivation mechanism was mainly attributed to the formation of the Pb-S bond on the outermost layer of the PVK. If the thiourea is protonated in an acidic environment, -NH3+ functional groups can be generated and may passivate halide anions effectively and result in further improvement. 10


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Likewise, MAPbI3 PSC passivated with CQDs shows reduced nonradiative recombination by grain boundary passivation related to uncoordinated Pb2+, improved power conversion efficiency (PCE) (18.24%), and good device stability towards humidity in the dark at room temperature.47 Another very recent study embedded surfactant-like monoammonium zinc porphyrin (ZnP) compound into the MAPbI3 film for PSCs and showed a PCE of 18.3% for blade-coating large-area (1.96 cm2) and 20.5% for small-area (0.1 cm2 ) device.53 Detailed analyses reveal the functions of ZnP in crystallization control and defect passivation of the PVK surfaces and grain boundaries. As a consequence, the ZnP-encapsulated PSCs retained over 90% of its initial efficiency after 1000 h with a humidity of about 45% at 85 °C. This study demonstrates a simple molecular encapsulation strategy for controlling morphology and defect suppression, both of which are important in improving the PV performance and stability of PSCs. The ZnP salt used is effectively a multi-dentate ligand given the ammonium ion, Zn2+and I- counter ion in the salt used. Similarly, surface passivation has been used to modify and improve properties of PVKs for PeLEDs. In a recent study, trioctylphosphine oxide (TOPO) has been used to passivate quasi two-dimensional (2D) PEA2(FAPbBr3)n-1PbBr (PEA for phenylethylammonium or C6H5C2H4NH3+) and FA for formamidinium or HC(NH2)2+) PVK, and green LEDs based on these materials (n=3) reached a current efficiency of 62.4 cd A−1 and external quantum efficiency of 14.36%.25 Figure 11 shows some of the spectroscopic characterizations that demonstrate the dependence of optical, structural and dynamic properties on the TOPO passivation. For instance, an absorption peak located at 1150 cm−1 in the FTIR observed for TOPO, attributed to P=O bond stretching vibrations, shifts to approximately 1100 cm−1 in the film comprised of TOPO and PbBr2, which is taken as an indication of bonding between perovskite and TOPO. The 11



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passivation is attributed to bonding between the PVK and TOPO but the nature of bonding was not reported. Table 1 shows a summary of some examples of different ligands used to passivate both bulk and nanostructures of MHPs. It is clear that effective passivation requires multiple ligands or multidentate ligands. Important characters of the ligands to consider include their size, shape, charge and charge distribution, orientation, density, and inter-ligand interaction. To reach optimal passivation, we will need to have a deeper fundamental understanding of ligandperovskite interaction at the molecular or atomic level, which will require a combination of advanced experimental and theoretical techniques. The potential for emerging technological applications of MHPs is tremendous, and passivation for stability and enhanced functionality will undoubtedly play a key role in their applications. Table 1. A summary of some examples of surface passivation of perovskites Examples 1 2 3

Perovskite CsPbX3 PQDs MSPbX3 PNCs MAPbX3 PNCs

4

MAPbBr3 PQDs

5

MAPbI3 bulk film

6 7 8 9

MAPbI3 film MAPbBr3 PNCs MAPbI3 bulk film MAPbBr3 PDQs

10

MAPbI3 bulk film

11

MAPbBr3 bulk film

Ligands OLA and OA OTA and OA APTES or NH2POSS with oleic acid peptide-like bidentate ligand quintenary ammonium halides (QAHs) polyaniline L-cysteine oleic acid (OA) benzylamine (BZA) and benzoic acid (BA) CQDs with -C=O and -OH groups quaternary 12


References 33 34 37

40 48

45 43 50 26

47 39

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12 13

MAPbI3 bulk film FASnI3 bulk film

14

MAPbI3 film

15 16

CsPbX3 CsPbBr3

ammonium salts suaraine hydroxybenzene sulfonic acid or salt and SnCl2 monoammonium zinc porphyrin (ZnP) ZnX2 PbBr2

49 38

53

30 32

AUTHOR INFORMATION Corresponding Authors *To whom correspondence should be addressed. E-mail: [email protected] Biography Jin Zhong Zhang received his BS degree in Chemistry from Fudan University, Shanghai, China, in 1983 and his PhD in physical chemistry from University of Washington, Seattle, in 1989. He was a postdoctoral research fellow at University of California Berkeley from 1989 to 1992. In 1992 he joined the faculty at University of California Santa Cruz, where he is currently full professor of chemistry and biochemistry. Zhang’s recent research interests focus on design, synthesis, characterization, and applications of advanced materials including semiconductor, metal, and metal oxide nanomaterials, particularly in the areas of solar energy conversion, solidstate lighting, sensing, and biomedical detection/therapy. He has authored over 330 publications and three books. Zhang has been serving as a senior editor for JPC(L) published by ACS since 2004. He is a Fellow of AAAS, APS, and ACS. He is the recipient of the 2014 Richard A. Glenn Award of the ACS Energy and Fuel Division.

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Acknowledgment We are grateful to financial support from NASA through the MACES Center at University of California Merced (NNX15AQ01A) and helpful discussions with students, postdoctors, and collaborators, including Binbin Luo, Ying-Chih Pu, Evan Vickers, Sara Bonabi, Sarah Lindley, Ke Xu, Qi Pang, and Zhiguo Xia.

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Figure 1. Illustration of removal of bandgap trap states by surface passivation with ligands: before (left) and after (right).

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Figure 2. A structural model for surface passivation of PQDs with multiple defects using a combination of ligands in a “cocktail” approach.

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Figure 3. An illustration for using excess constituent ions to enrich the surface for specific targeting with appropriate ligands.

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Figure 4. Colloidal perovskite CsPbX3 NCs (X = Cl, Br, I) exhibit size- and composition-tunable bandgap energies covering the entire visible spectral region with narrow and bright emission: (a) colloidal solutions in toluene under UV lamp (λ = 365 nm); (b) representative PL spectra (λexc = 400 nm for all but 350 nm for CsPbCl3 samples); (c) typical optical absorption and PL spectra; (d) timeresolved PL decays for all samples shown in (c) except CsPbCl3.33

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Figure 5. (a) UV-Vis absorption, (b) PL spectra (λex = 365 nm) and (c) UV light (λex = 365 nm) photographs of PNCspeptide prepared with 0.025, 0.05, 0.1, 0.15 mmol/L 12-aminododecanoic acid (12-AA).40

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Figure 6. A model for binding between PAs and MAPbBr3 PQDs.42

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Figure 7. Schematic diagram of modification of MAPbI3 with PANI.45

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Figure 8. Schematic illustration of QAHs assembled on the defect sites of the MAPbI3 perovskite. The red and blue symbols represent the N atom and O atom of the choline chloride molecule, respectively.48

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Figure 9. Illustration for the need of different ligand shapes for PQD versus bulk surfaces. While (a) and (d) are effective, (b) and (c) are not.

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Figure 10. a) FTIR spectrum of thiourea, PbI2·thiourea, perovskite, and perovskite with thiourea. b) S=C stretching in different materials. c) XPS spectra of perovskite film with and without thiourea. d) 207Pb NMR spectra of PbI2 and perovskites with and without thiourea in dimethyl sulfoxide (DMSO).52

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Figure 11. Passivation effect of TOPO on the perovskite with n = 3 composition. a. Photoluminescence quantum yield (PLQY) of the PEA2(FAPbBr3)n-1PbBr4 (n = 3 composition) perovskite films with and without TOPO passivation. b. Time-resolved photoluminescence (TRPL) of the PEA2(FAPbBr3)n-1PbBr4 (n = 3 composition) films with and without TOPO passivation layer. c. Fourier transform infrared (FTIR) spectroscopy measurement for TOPO, PbBr2 and TOPO-PbBr2 films prepared on silicon wafers.25

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