Photoluminescence of Zero-Dimensional Perovskites and Perovskite

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Photoluminescence of Zero-Dimensional Perovskites and Perovskite-Related Materials Sudipta Seth, and Anunay Samanta J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02931 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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The Journal of Physical Chemistry Letters

Photoluminescence of Zero-Dimensional Perovskites and Perovskite-Related Materials Sudipta Seth and Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad 500046, India

Corresponding Author

*

Anunay Samanta. E-mail: [email protected]

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ABSTRACT. Zero-dimensional (0-D) perovskites and perovskite-related materials are an emerging class of optoelectronic materials exhibiting strong excitonic properties and quite often, high photoluminescence (PL) in the solid state. Here we highlight two different classes of 0-D perovskites with contrasting structural and optical properties focusing mainly on the less explored, but rapidly growing bulk quantum materials termed as 0-D perovskite related materials (0-D PRMs), whose PL properties are quite intriguing and a topic of recent debate. We attempt to present here a comprehensive picture to rationalize the contrasting properties of the 0-D PRMs and provide an understanding of the mechanism of exciton dynamics and PL of this class of materials. We hope that exciting PL and tunable composition of these systems will help design of new materials with versatile optical properties suited for practical applications.

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Quantum confinement, a frequently used term for describing the nature of a nanoscale semiconductor, which largely dictates the electronic and optical properties of the material,1 arises when the electron and hole wave functions in a semiconductor are constricted within a spatial limit of de Broglie wavelength (or exciton Bohr radius, typically a few nanometers). This confinement modifies the electronic band structure (in bulk) into discrete energy levels and enhances the PL behavior of the material.2 Depending on the extent of confinement in different spatial directions the semiconductors can be classified into different categories starting from three-dimensional (3-D, not confined in any dimension) up to 0-D (confined in all three dimensions). Last few decades have witnessed a resourceful use of the low dimensional metal chalcogenide nanostructures, specifically the 0-D quantum dots (QDs) in nanotechnology.3 Metal halide perovskites have become a burgeoning topic of research in the field of photovoltaic and optoelectronics since the first successful use of nanocrystalline CH3NH3PbX3 (X= Br, I) materials as light harvester in solar cells.4 Soon after metal halide perovskites of diverse compositions and physical forms have been utilized in the fabrication of highly efficient photovoltaic cells,5 light emitting diodes (LEDs),6-8 optically pumped lasers,9 photodetectors,10 radiation detectors,11 and memory devices.12 Such diverse applications of the material stem mainly from its broad absorption, long diffusion length, high charge carrier mobility and tunable narrow PL with high quantum yield (QY).13-16 Generally, the hybrid organic-inorganic lead halide perovskites are more useful as light harvester in high efficiency solar cells for their long diffusion length and charge carrier mobility,5,

13, 17

whereas, its all inorganic counterpart in

nanocrystalline form shows promise in photon emitting applications.7-9,

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In majority of the

cases, the perovskites used are 3-D in nature. As these perovskites have low exciton binding energies (∼20-50 meV),16, 17, 19 the excitons can dissociate easily at room temperature to form


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free charge carriers, which decreases the efficiency of the devices that are based on photon emission. In this context, low dimensional perovskites, especially the 0-D perovskites, whose exciton binding energy is much higher, are expected to be highly useful.20-23 Quite naturally, recent research focuses more on the low dimensional perovskite materials. However, very few comprehensive accountings have been made so far on low dimensional perovskites24-26 with a scarcely populated 0-D perovskite family27 as compared to numerous review articles on the 3-D perovskites describing structural, optical and electronic properties.28-31 In this perspective, we majorly focus on the intriguing crystal structure and optical properties of the 0-D perovskites and attempt to unveil the characteristics and potential of these materials. Interest on metal halide perovskites started with the 3-D perovskites having a general formulae of ABX3 (A is monovalent cation such as Cs, CH3NH3; B is bivalent metal cation like Pb, Sn, and X is halide, usually Cl, Br or I), where [BX6]4− octahedral units form a 3-D network by a corner-sharing connectivity and the A cations (with size within the limit of Goldsmith tolerance factor) reside in the octahedral voids.16 Depending on the size of the cations and resultant stress on the octahedral network, these materials exist in different crystal phases like cubic, orthorhombic, tetragonal, etc.32 Theoretical studies have shown that energies of the frontier electronic states of these perovskites are mainly controlled by the [BX6]4− octahedral units.16,

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The highest occupied valence electronic state is composed of the B-metal (s) and

halogen (p) valence orbitals, whereas the lowest unoccupied conduction band state is formed exclusively by the B-metal outer p-orbital; the A-site cation does not play any direct role in determining the frontier energy levels.16, 34 This is experimentally established by following the dependence of the band gap on the B-site metal cation and halide.16,

35, 36

Therefore, photo-

excitation of these substances generates the excitons in the individual octahedron and the overall


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band gap and optical properties of these materials are determined by the connectivity of these octahedra. A low dimensional semiconductor can be achieved by confining the excitons within a volume of radius comparable or less than the Bohr radius. This can be achieved by two means. Firstly, since the excitons created in the BX6 octahedra are connected to each other in ABX3 type of perovskites, a 0-D perovskite material is obtained when the particle size is below its exciton diameter, as illustrated in Figure 1a. Considering CsPbBr3 as a representative 3-D perovskite with exciton diameter of ∼7 nm, a 0-D perovskite will form when the size of the particle is comparable or less than 7 nm.16 Because of similarity with the metal chalcogenide QDs, such 0D perovskites are termed as perovskite QDs. Secondly, a 0-D perovskite can also be realized when the BX6 octahedral units are isolated by introduction of structural barrier inside a bulk crystal lattice. Such a 0-D structure is best demonstrated in rhombohedral Cs4PbBr6 crystal, where the PbBr6 octahedral units are spatially isolated by the intervening Cs atoms (Figure 1b).20, 33

This kind of 0-D materials can have a physical dimension of several millimeters or larger,

similar to a bulk material, but because of their intrinsic confined nature they act as a bulk quantum material. Such low dimensional bulk material is difficult to design, but can be easily obtained for perovskite material using an appropriate synthetic methodology. Unlike the 3-D perovskites, these are commonly termed as 0-D perovskite related materials (0-D PRMs) as these bulk quantum materials do not have corner sharing connectivity. The beauty of the 0-D PRMs is that these can have diverse morphologies and sizes, but properties similar to a single constituent octahedral unit.


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This kind of 0-D materials can have a physical dimension of several millimeters or larger, similar to a bulk material, but because of their intrinsic confined nature they act as a bulk quantum material. Such low dimensional bulk material is difficult to design, but can be easily obtained for perovskite material using an appropriate synthetic methodology.

Figure 1. Schematic representation of two types of 0-D perovskites. Perovskite QD with general formulae of ABX3 and size less than exciton Bohr diameter (represented by the diameter of the circle) and 0-D PRM with spatially isolated octahedra in a bulk crystal lattice.

Perovskite QDs. The hybrid and all-inorganic metal tri-halide perovskite QDs are generally obtained by hot injection or ligand assisted anti-solvent precipitation methods.7, 8, 16, 37 Apart from their halogen identity these QDs show size dependent band gap and optical properties with very high PL QY (∼90%). As exciton binding energies of these QDs are quite low (∼20-50 meV),16,

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an equilibrium exists between the excitons and free charge carriers at room

temperature, which can reduce the electron-hole recombination efficiency.38, 39 Even though the PL QY of these QDs are quite high in their colloidal form, it diminishes significantly in the solid state, e.g. thin film form necessary for device fabrication, due to opening up of new nonradiative


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decay channels.40, 41 Despite some of these deficiencies these QDs have already shown promising use in lighting and display technology.7, 8 0-D PRM and Controversy over its PL. Of late the 0-D PRMs, in particular Cs4PbBr6, have received considerable attention because of their intriguing structure associated optical properties. Generally a decrease in dimensionality of the material leads to an increase in the band gap. As the BX6 octahedra are electronically decoupled in 0-D PRMs one expects a larger band gap in these materials as compared to their higher dimensional counterparts. This is indeed demonstrated by Manna and coworkers in a recent report, where they have shown that Cs4PbBr6 nanocrystals (NCs) exhibit a sharp absorption peak at 314 nm corresponding to a band gap of 3.95 eV (Figure 2).33 According to this report, such a high band gap material cannot show PL in the visible region for band to band pure excitonic transitions. Similar non-luminescent Cs4PbBr6 NCs are also observed by Alivisatos and coworkers while studying ligand mediated post synthetic transformation of green emitting CsPbBr3 NCs.42 The NCs reported in these works vary in sizes, but all of them show the first absorption onset at ∼315 nm (Figure 2) indicating that the optical properties of these materials are determined by the individual PbBr6 octahedral unit. A few other studies dealing with transformation of crystal structure and composition between CsPbBr3 and Cs4PbBr6 also indicate that Cs4PbBr6 is a PL inactive material.43-46


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Figure 2. (a-d) TEM images, (scale bars: 100 nm), (e) HRTEM image, (scale bar: 2 nm), (f) XRD pattern and (g) absorption spectra of 0-D PRM Cs4PbBr6 of different sizes. (h) Band gap calculation using DFT and (i) demonstration of CsPbBr3 NCs as the probable impurity responsible for fluorescence in Cs4PbBr6 NCs. (Adapted from reference 33. Copyright 2017 American Chemical Society) On the contrary, several other reports indicate intense green emission of Cs4PbBr6 with particle size ranging from NC to single crystal.20-22, 47-51 PL of the powder form of Cs4PbBr6 with λmax of 520 nm and QY of ∼45% is also reported.20 Another report indicates high PL QY (∼65%) of Cs4PbBr6 NCs in colloidal form.21 An interesting point to note in this context is that unlike the other perovskite NCs, it is observed that Cs4PbBr6 thin films retain high PL QY (∼54%) when prepared from colloidal dispersions.21 Single particle PL imaging of Cs4PbBr6 microdisks47 and few other reports on Cs4PbBr6 single crystals confirm the PL of these 0-D PRMs (Figure 3).22, 48, 50

Irrespective of their shapes and sizes, these crystals exhibit PL in the green region (515-524

nm) with an average full width at half-maximum (FWHM) of ∼22 nm and QY of 38% or higher.


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Figure 3. Fluorescent 0-D PRMs. (A) Electron (FESEM, upper panel) and fluorescence microscope (FLIM, lower panel) images of Cs4PbBr6 microdisks of different shapes. Single hexagonal microdisk demonstrates exciton dynamics. (Adapted from ref. 47. Copyright 2017 American Chemical Society.) (B) Fluorescent (top) and optical microscope pictures of the single crystals (bottom). Red scale bar: 500 µm. (Adapted from ref. 22. Copyright 2017 American Chemical Society.) (C) Images of 1D C4N2H14SnBr6 and 0D (C4N2H14Br)4SnBr6 under ambient light and UV irradiation. Absorption spectra of 1D C4N2H14SnBr6 and 0D (C4N2H14Br)4SnBr6, as well as emission spectrum of 0D (C4N2H14Br)4SnBr6. (Reproduced with permission from ref. 52. Copyright 2017 Willey-VCH) Apart from Cs4PbBr6, a good number of reports is also available on 0-D PRMs with isolated octahedral units comprising other metal atoms (Sn, Bi, Sb) in place of lead.23, 52-58 However, all these 0-D structures cannot be generalized with a specific empirical formulae as the valency of the central metal ion determines the nature of isolated structural unit and overall composition of the 0-D PRM. In a majority of cases, these lead-free 0-D PRM structures are fluorescent with broad PL spectrum, low PL QYs and often, feature a long absorption tail (Figure 3C).52, 55, 56 Origin of PL. The discussion above shows that the PL of 0-D PRMs is quite intriguing and the literature presents two contrasting pictures, especially for Cs4PbBr6. As the composition and crystal structure are same for both luminescent and nonluminescent samples of Cs4PbBr6 the


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questions that naturally arise are the following: Are these materials expected to be nonluminescent? If so, then how do the photo-generated charge carriers relax? For luminescent samples, one can ask why is the PL band Stokes-shifted with respect to the anticipated band edge PL position and what could be the origin of PL in green emissive Cs4PbBr6 0-D PRM? The As the composition and crystal structure are same for both luminescent and nonluminescent samples of Cs4PbBr6 the questions that naturally arise are the following: Are these materials expected to be nonluminescent? If so, then how do the photo-generated charge carriers relax? For luminescent samples, one can ask why is the PL band Stokes-shifted with respect to the anticipated band edge PL position and what could be the origin of PL in green emissive Cs4PbBr6 0-D PRM? reports demonstrating nonluminescent Cs4PbBr6 do not provide information on the recombination dynamics of the photo-generated charge carriers in these materials. Absence of any PL band indicates that the excitons in these materials recombine nonradiatively. This brings additional questions such as the following: What are the factors that contribute to this efficient nonradiative recombination process? Are these NCs full of mid band gap trap states due to their high surface to volume ratio? The answers to these pertinent questions are not available so far through any experimental and/or theoretical works. Absence of any PL band indicates that the excitons in these materials recombine nonradiatively. This brings additional questions such as the following: What are the factors that contribute to this efficient nonradiative recombination process? Are these NCs full of mid band gap trap states due to their high surface to volume ratio?


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As far as the luminescent Cs4PbBr6 samples are concerned, one should note that the PL characteristics of these materials, in particular, the green emission around 515-520 nm with a small FWHM and high PL QY, matches closely with those of the corner-shared CsPbBr3 material and that the two structures are interconvertible.16, 20-22, 33, 42-47 Considering these points Manna and coworkers conjectured this PL to originate from contamination of trace amount of nanoscale CsPbBr3 impurity which could not be detected through powder X-ray diffraction (PXRD).33 However, the presence of trace quantity of PXRD-undetectable CsPbBr3 impurity giving rise to such a high PL QY (≥ 38%) in luminescent Cs4PbBr6 samples appears very unusual. As a matter of fact, the presence of less fluorescent (≤20 times) CsPbBr3 in Cs4PbBr6 is reported to lower the PL efficiency of the latter.20, 48, 50 As single crystals are generally free from morphological defects, the observation of PL from single crystals of Cs4PbBr6 does indicate that they are inherently fluorescent.22,

48, 50

A detailed structural characterization of the fluorescent

and non-fluorescent Cs4PbBr6 NCs by Bakr and coworkers indicated that these two types are structurally same and free from contamination.22 Our recent PL imaging study on individual Cs4PbBr6 microdisks (Figure 3A) confirmed PL and revealed charge carrier dynamics in the single particle level.47 PL intensity and lifetime imaging showed that fluorescence photons were not emitted from some localized domains containing trace amount of emissive impurity; rather, they came from the entire body. Moreover, the PXRD data confirmed that the PL was not due to any co-existing crystalline impurity. These studies unambiguously establish that Cs4PbBr6 particles are inherently green emissive. Large exciton binding energy and PL is rather common for high band gap 0-D PRMs.23, 52, 56, 57

Lead free 0-D PRMs exhibiting below band gap absorption and sharp/broad emission

bands with large Stokes shift values are already documented.23, 52-57 Generally, electron-phonon


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coupling plays an important role in determining the bandwidth and Stokes shift of the PL. This effect is mainly observed in large deformable crystals with low activation energy of defect formation. Strong electron-phonon coupling generates polarons in the crystals that bind the charge carriers causing self-trapping of excitons, which leads to broad emission band.57, 59 PL of lead free 0-D PRMs is attributed to the recombination of such self-trapped excitons. For the metal halide perovskites, it is shown that intrinsic point defects create mid band gap energy levels, which act as radiative recombination center.60-62 This is evident from multi-exponential PL decay and dispersive PL blinking kinetics of the single particles.62 Hence, the PL of 0-D PRMs originates either from self-trapped excitons or from emissive intrinsic trap states or could be a combined effect of both. We examine closely the difference in optical properties of structurally very similar nonemissive and emissive Cs4PbBr6. We note that luminescent Cs4PbBr6 exhibits (i) a long absorption tail extending up to 550 nm or beyond, and (ii) less intensity around 314 nm in the excitation spectrum where maximum absorption takes place (Figure 4). The long absorption tail is similar to an extended form of Urbach tail, which is common for disordered or poorly crystalline semiconductors. This arises from localized sub band gap electronic (trap) states mediated absorption of photons.23, 63 A large Stokes shift of the PL is in agreement with the mid band gap states mediated recombination. Since the perovskite crystals are prone to deformation, these mid band gap states generally arise from crystal defects.60-62 The observation of a blue shift, broadening of the PL band accompanied by a reduction in PL intensity with increase in temperature is a reflection of the electron-phonon coupling, which in other words, provides the signature of the trap states mediated recombination.22, 64, 65 The second (ii) point, high PL QY and sharp PL band suggest weak electron-phonon coupling and direct creation of exciton in the


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intrinsic trap states as the major factor for PL. Photo-generated carrier recombination processes are summarized in Figure 5. Band edge excitation (λexc ≤ 315 nm) shows less intense PL (as evident from excitation spectrum) indicating leakage of the charge carriers through nonradiative

Figure 4. Steady-state absorption, excitation, PL spectra, and band gap alignment of Cs4PbBr6 NCs colloidal; insets show a photograph of a thin-film sample and a colloidal sample under the illumination of 365 nm UV lamp. (Adapted from ref. 21. Copyright 2017 American Chemical Society) decay channels with a less prominent contribution from self-trapped exciton. Whereas, excitation at the sub band gap levels (500 nm ≥ λexc ≥ 350 nm) shows much stronger green emission indicating that major recombination in fluorescent Cs4PbBr6 occurs via direct absorption followed by emission from intrinsic permanent trap states. Hence, we think that mid band gap defect state mediated excitonic radiative recombination is responsible for the observed PL in emissive Cs4PbBr6 0-D PRM. The same has been speculated in some recent reports.22, 47 Hence, we think that mid band gap defect state mediated excitonic radiative recombination is responsible for the observed PL in emissive Cs4PbBr6 0-D PRM. The origin of these defects creating emissive mid band gap trap states are not clear at the moment. However, one can identify some of the factors, which can in principle contribute to


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these trap states. It is well known that halogen vacancy creates shallow electronic levels in the CsPbX3 material.60, 61 Density functional theory (DFT) calculations on tin-based perovskite have shown that iodine vacancy, which creates shallow trap states in CsSnI3, becomes deep on transformation of the structure to Cs2SnI6, which has a 0-D structure.66 A similar trend can be expected for Cs4PbBr6 as well. However, a detailed theoretical study on defect generation and corresponding energy level distribution is required to shed light on the actual origin of these emissive trap states. Further, to determine whether electron-phonon coupling or intrinsic emissive defect is the dominating factor it is necessary to study the Raman spectroscopy and/or excitation intensity dependent PL behavior of the 0-D PRMs.

Figure 5. Schematic illustration of photo-generated carrier recombination processes in Cs4PbBr6 0-D PRM. Applications and outlook. Fluorophores with high electron-hole recombination efficiency are essential in photon emitting applications. In most of these applications, the emitting materials are required in their solid form, where the PL efficiency is usually lower than in their stable colloidal form. The high exciton binding energy of Cs4PbBr6 favors the radiative


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electron-hole recombination resulting in high PL QY. More importantly, this bulk quantum material retains a high PLQY in the solid state with a much enhanced stability towards light or moisture than its 3-D analogous.20,

21, 58

Hence, 0-D PRM, Cs4PbBr6 can be considered as a

promising solid state luminophore in future applications. For the same reason, non-toxic leadfree 0-D PRMs can also be employed in the fabrication of environmentally friendly devices even though their PL QYs are considerably lower. Because of high environment stability and less structural mismatch these materials can be used as a protective layer for other perovskite materials and devices for enhancing their efficiency and long term stability.67 Crystal structures of the perovskite materials are versatile as the 0-D and 3-D forms are reversibly inter-convertible by simple means such as exposure to heat, photon or chemicals.42-46, 52

Transformation of nonfluorescent Cs4PbBr6 NCs to highly fluorescent CsPbBr3 NCs may be

useful in sensing applications. Recent report shows that a change of the monovalent cation in Bibased perovskites can influence the optical and electronic properties significantly.58 Thus by changing the monovalent cation in Pb or Sn-based 0-D PRMs one can obtain a library of compounds with versatile optical properties. As the octahedral units are electronically decoupled in 0-D PRMs one would expect these materials to be insulator towards carrier transport. However, efficient hole transport in Cs2SnI6 is observed in dye-sensitized solar cells demonstrating enough charge carrier mobility inside the crystals.68 Cs2SnI6, MA3Bi2I9 and MA3Sb2I9 are also successfully employed as absorber layer in solar cells thus extending the scope of fabrication of nontoxic and stable photovoltaic devices.56, 69, 70

In addition, because of high band gaps these materials could be useful in tandem solar cells.


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In summary, the 0-D PRMs provide an opportunity to study the unique optical and electronic properties of bulk quantum materials. Crystal defects in such materials play a crucial role in determining their applications. These defects which are generally considered as disadvantageous, turn an insulator band gap 0-D PRM to a highly luminescent material. Strong These defects which are generally considered as disadvantageous, turn an insulator band gap 0-D PRM to a highly luminescent material. PL and high exciton binding energy bring uniqueness to these materials. Applications of such material in optoelectronic and photovoltaic devices are still in their infancy. Understanding of the defect chemistry, exciton formation and dissociation mechanism can bring this exceptional material in the forefront of future energy related applications. The discussion presented in this perspective reconfirms the PL of these 0-D PRMs, provides an understanding of its origin and resolves the controversy on this topic. This may be helpful in extending the scope of utility of these bulk quantum materials. AUTHOR INFORMATION Notes The authors declare no competing financial interests. Biographies Sudipta Seth is a doctoral student of Chemistry at the University of Hyderabad, under the supervision of Prof. Anunay Samanta. He obtained his B.Sc. degree in 2012 from Vivekananda Mahavidyalaya (University of Burdwan) and M. Sc. degree in 2014 from the School of Chemistry, University of Hyderabad. His doctoral research work is focused on synthesis and


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photophysical investigation of metal halide perovskite and perovskite related materials employing single molecule and ultrafast techniques. Anunay Samanta is a Professor of Chemistry at the University of Hyderabad and J. C. Bose National Fellow of the Department of Science and Technology (DST), Government of India. He is elected Fellow of the National Science Academies of India. His current research interests focus on ultrafast dynamics of charge separation and recombination in photo-excited perovskite and related materials, solvation and rotational dynamics in deep eutectic solvents, ionic liquids and biological systems.

ACKNOWLEDGMENT We thank SERB Grant No EMR/2015/000582 and J.C. Bose Fellowship (to A.S.), DSTPURSE support to the School and UGC-UPE support to the University. S.S. thanks Council of Scientific and Industrial Research (CSIR) for Fellowship. REFERENCES (1) Brus, L. E. Electron–Electron and Electron-Hole Interactions in Small Semiconductor Crystallites: The Size Dependence of the Lowest Excited Electronic State. J. Chem. Phys. 1984, 80, 4403-4409. (2) Norris, D. J.; Bawendi, M. G. Measurement and Assignment of the Size-Dependent Optical Spectrum in CdSe Quantum Dots. Phys. Rev. B 1996, 53, 16338. (3) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; et al. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012-1057. (4) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (5) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. HighPerformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234-1237. (6) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; et al. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light Eemitting Diodes. Science 2015, 350, 1222-1225.


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Photoluminescence of Zero-Dimensional Perovskites and Perovskite

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