Room Temperature Broadband Light Emission From Hybrid Lead

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Room Temperature Broadband Light Emission From Hybrid Lead Iodide Perovskite-Like Quantum Wells: THz Spectroscopic Investigation of Meta-Stable Defects Adedayo M. Sanni, Sydney N. Lavan, Aleksandr Avramenko, Federico A. Rabuffetti, Leopoldo Suescun, and Aaron S. Rury J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00743 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry Letters

Room Temperature Broadband Light Emission From Hybrid Lead Iodide Perovskite-Like Quantum Wells: THz Spectroscopic Investigation of Meta-Stable Defects Adedayo M. Sanni,† Sydney N. Lavan,† Aleksandr Avramenko,† Federico A. Rabuffetti,† Leopoldo Suescun,‡ and Aaron S. Rury∗,† †Department of Chemistry, Wayne State University, Detroit, MI, USA 48202 ‡Cryssmat-Lab/DETEMA, Facultad de Qu´ımica, Universidad de la Rep´ ublica, Montevideo 11800, Uruguay E-mail: [email protected]

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Abstract The properties of mid-bandgap electronic states are central to the potential application of self-assembled, hybrid organic-inorganic perovskite-like quantum wells in opto-electronic technologies. In this study we investigate broadband light emission from mid-bandgap states in fast-forming hybrid organic lead iodide quantum wells at room temperature. By comparing temperature and intensity dependent photoluminescence (PL) spectra emitted from butyl ammonium spaced inorganic layers, we propose structural defects in a meta-stable material phase trap excitons and cause broadband light emission spanning wavelengths between 600 nm and 1000 nm. We use temperature dependent THz time-domain spectroscopy to correlate changes in the sub-gap PL emission with changes in the chemical bonding of the inorganic octahedral layer. Our results provide new fundamental physical insights into the array of mechanisms capable of inducing broadband light emission from low dimensional perovskite-like materials central to their application in future opto-electronic technologies and novel spectroscopic tools to characterize these states.

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Hybrid organic-inorganic perovskite (HOIP) semiconductors have emerged as potentially transformative, solution processed opto-electronic materials. 1,2 These materials possess intriguing performance metrics when formed into devices for photo-voltaic, 3–6 light emitting diode, 7–9 and laser applications. 7,10,11 By modulating the size of the organic cation species, one can drive the spontaneous formation of non-interacting, 2-dimensional sheets of perovskite-like metal halide octahedra spaced by lower dielectric molecular cations. 12,13 The spatially separated sheets then behave similarly to extensively studied quantum wells of IIIV semiconductors. A large literature of studies on self-assembled perovskite quantum wells indicate they possess fascinating fundamental properties 14–22 in addition to being suitable for effective opto-electronic devices. 10,23–32 Despite their structural similarity to III-V semiconductor quantum wells, HOIP quantum wells possess different fundamental physical properties. For instance, several studies report the presence of sub-bandgap photoluminescence (PL) emission whose properties can be controlled via external forces like temperature or atomic substitution. 23,24,29,33–37 Even though this sub-gap emission was reported with early synthetic studies of HOIP quantum wells over 25 years ago, 12 significant interest has been stimulated in its properties more recently. The broad bandwidth and large emission Stokes shift indicate the sub-gap PL emission from selfassembled, HOIP quantum wells may be useful in white light emitting technologies 23,24,35–37 or as gain media in lasers. 10 Typically, one considers two physical extremes to explain broadband PL emission from quantum confined semiconductors. In one extreme, permanent defects can trap excitons, electrons, or holes, stabilize their energy through coupling to the material’s lattice vibrations, and cause emission of strongly red-shifted light upon charge carrier recombination. In the other, coupling between lattice phonons and electronic excitations is strong enough to form a trap through an elastic distortion the material’s excited state structure. This self-trapping process localizes the electronic excitation, reduces its energy, and causes broadband light emission possessing a significant Stokes shift.

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The fundamental explanation of the broadband, sub-gap PL emission from perovskite-like quantum wells varies on the particular material under consideration. 30,33–36,38,39 Zhu and coworkers use steady-state, ultrafast, and photoemission spectroscopies to propose molecular fluctuations due to relatively weak hydrogen-bonding interactions between inorganic and organic layers of butyl ammonium lead iodide (BA2 PbI4 ) quantum wells cause self-trapping at low temperatures. 33,34 These authors propose such traps cause broadband sub-bandgap light emission. Kang and Wang use Monte Carlo simulations to support the conclusion butyl ammonium (BA) cations can localize charge carriers in BA2 PbBr4 quantum well structures. 40 These results indicate a possible role for molecular fluctuations and disorder in exciton and carrier trapping processes of HOIP quantum wells that could be connected to subgap emission. Yangui et al. also propose self-trapping causes broadband, sub-gap light emission from benzyl ammonium-spaced PbBr4 octahedral layers. 39 In contrast, other studies propose sub-gap PL emission results from permanent defects in the inorganic lattice. 30,35 In particular, Cortecchia et al. propose structural distortions of inorganic octahedra of diammonium-spaced HOIP quantum wells stabilize self-trapped excitons or charge carriers at Pb vacancies whose excited state population relaxes via broadband emission at room temperature. 30 The deterministic design of sub-gap PL emission from perovskite-like quantum wells remains elusive due to an incomplete understanding of its fundamental, material specific driving forces. In particular, there remains no direct experimental tests capable of deciphering the role of permanent defects and structural reorganization in the trap state formation central to sub-gap light emission. Additionally, few studies have reported broadband subgap emission in the near-IR, a requisite for several important opto-electronic applications. Assigning the trapping mechanism is critical for developing design principles of broadband emission spectra central to the applications ranging from interior lighting to optical metrology and quantum information science. In this study we report spectroscopic measurements correlating the appearance and prop-

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erties of room temperature, broadband, sub-gap photoluminescence (PL) with changes to the presence of meta-stable structural defects in butyl ammonium lead iodide (BA2 PbI4 ) multiple quantum wells. Excitation intensity dependent PL measurements show the light emission saturates at high incident laser fluences and suggests permanent defects play a central role in emissive trap formation. By examining temperature dependent PL and vibrational spectra of BA2 PbI4 , we find the presence of broadband, sub-gap light emission does not correlate with vibrational density of states localized in the material’s inorganic octahedral layers. While, we find no changes in the low frequency Raman scattering spectra, temperature-dependent THz time-domain spectra differ significantly before and after extinguishing the sub-gap PL by thermal annealing. We develop a theoretical picture based on changes to the character of chemical bonding in the inorganic octahedral layer to explain these changes in the THz spectra. Our results provide new fundamental physical insights into the formation, properties, and characterization of mid-gap emissive states of hybrid organic-inorganic materials that could play an important role in future opto-electronic technologies. To begin, we synthesize BA2 PbI4 single crystals by combining the solution-phase approaches of Mitzi 41 and Stoumpos et al., 14 as detailed in the Methods section below. After boiling a solution containing our starting materials, large, bright orange crystals precipitate out of solution near room temperature following cooling over about 45 minutes. Rietveld analysis of X-ray patterns diffracted from powders ground from these crystals demonstrates the phase purity of our samples, as detailed in the Supporting Information (SI). We also collected room temperature single crystal X-ray diffraction patterns. We find the patterns can be sufficiently fit by a structure with Pbca space group symmetry, as detailed in the Methods section. This primitive structure is similar to that reported by Billing and Lemmerer 42 for the case of slowly cooled crystals. Analysis of powder X-ray diffraction patterns shows our fast-formed inorganic layers possess disorder relative to the structure of slowly cooled BA2 PbI4 previously reported. 42 Specifically, the apical I atoms are disordered as visualized in Figure 1 likely stemming from the short time it takes to

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crystalize our samples. All parameters of the crystal structure can be found in the associated crystallographic information file found in the SI.

Figure 1: The structure of quickly cooled BA2 PbI4 crystals reported here. Left panel: view of the crystal structure down the b-axis. Right panel: view of the crystal structure down the c-axis. Shading of the apical iodine atoms indicates the structural disorder we used to model the crystalline structure of the sample. The steady state absorption spectrum of an ensemble of sub-mm sized crystals in the left panel of Figure 2 indicates the presence of a broad range of states just below the exciton band-edge, likely due to the disorder present in sample caused by fast crystal formation. By subtracting the rapidly rising background we find a peak near 540 nm, which we assign as the 1S exciton and show in the right panel of Figure 2. 0.55

20 Measured Spectrum Baseline

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Figure 2: Left panel: comparison of the measured absorption spectrum of an ensemble of fast forming of butyl ammonium lead iodide (BA2 PbI4 ) micro-crystals and the polynomial baseline used for background subtraction. The baseline likely indicates the presence of significant disorder in the crystal, as established by single crystal X-ray diffraction. Right panel: excitonic absorption spectrum of the same crystal found by subtracting the baseline from the measured spectrum. The vertical scale has been changed to milli-O.D. (mO.D.)

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To further analyze these samples, we measured their temperature dependent PL spectra. Figure 3 compares the temperature dependent PL emission from a BA2 PbI4 single crystal between 625 nm and 1025 nm following 532 nm laser excitation. Details of the experiment are found in the Methods section. Given the absorbance spectra of Figure 2, excitation at this energy should drive population into the 1S exciton state. Figure S2 of the Supporting Information shows the full room temperature PL spectrum from 535 nm to 1025 nm, demonstrating the appearance of the strong room temperature emission peaking at 540 nm consistent with electron-hole recombination. 14000

Intensity [counts]

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298 K start 308 K 318 K 328 K 333 K 298 K end

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Emission Wavelength [nm] Figure 3: Comparison of sub-bandgap photoluminescence spectra of butyl ammonium lead iodide multiple quantum wells before, during, and after a thermal annealing cycle.

Figure 3 shows the evolution of the PL spectra when thermally cycling the sample from 298 K to 333 K and then back to 298 K at 5 K increments. Upon inspection one observes a peak near 750 nm with full width half maxima (FWHM) of ∼140 nm in the spectra initially measured at 298 K. Heating the sample causes this peak to gradually lose intensity until it is completely extinguished at 333 K. When we cool the sample back to 298 K from 333 K in steps of 5 K we find the sub-gap PL emission reduces to a much smaller peak on top of the tail typically observed in many classes of dynamically disordered semiconductors. Surprisingly, 7



we find no change in the structure extracted from single crystal or powder XRD before and after thermally cycling our samples. We propose the peak in sub-gap PL spectra of Figure 3 arises from exciton trapping rather than charge trapping following exciton dissociation. This proposal is based on the direct exciton excitation by our 532 nm laser and the lack of any discernible time-resolved THz signal when pumping the sample on the exciton resonance, in agreement with Lindenberg at co-workers. 38 In addition to the thermal properties of the PL peak, its spectral properties differ significantly from spectra reported in previous studies on HOIP quantum wells. In particular, Zhu and co-workers 33,34 and Booker et al. 35 report sub-gap emission from PbI4 quantum wells spaced by alkyl ammonium cations. This emission was centered near 600 nm, spanned over 200 nm, and was observed only at temperatures below 100 K. In contrast, we find emission peaked at wavelengths over 100 nm longer whose bandwidth spans over 400 nm. Furthermore, we observe this emission at room temperature. To understand the fundamental physical mechanism causing sub-gap PL in our samples, we undertook an analysis of intensity dependent sub-gap PL spectra excited at 532 nm. As shown in the SI, PL spectra are fit to a linear combination of wavelength-dependent Gaussian and exponentially decreasing contributions corresponding to mid-bandgap state light emission and dynamically disordered exciton emission, respectively. We then analyze the Gaussian contribution as a function of excitation intensity. The panels of Figure 4 show the results of this analysis. While we find a linear intensity trend up to 200 W/cm2 , the PL intensity flattens out at higher incident laser powers, as shown in the top panel of Figure 4. In addition, the peak of the Gaussian contribution to the sub-gap PL spectrum shifts to longer wavelength (lower energy) at the same intensity for which the PL saturates, as shown in the bottom of Figure 4. Above 325 W/cm2 the sample shows discernible signs of photo-degradation, which impede our ability to study the PL spectrum at higher intensities. To further assess the origin of the sub-gap PL emission we undertook two types of vibrational spectroscopic measurements before and after we thermally cycled the sample. First, we

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Iincident [W/cm2] Figure 4: Comparison of the integrated photoluminescence intensity (top panel) and peak position (bottom panel) as a function of incident intensity of 532 nm laser light extracted from nonlinear least squaring fitting, as detailed in Supporting Information. measured THz transmission spectra in the presence and absence of the sub-gap PL emission. Spectroscopy of infrared (IR) active vibrations has long been used to assess the character of chemical bonds through the connection between sum rules on the dielectric function and the modulation of the unit cell dipole moment, as explained below. 43 If particular emissive defects affect the character chemical bonds in the inorganic layer of our samples, then we

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should observe changes in measured far-IR vibrational spectra when the sub-gap PL is reduced. Second, we measured the non-resonant Raman spectrum. These measurements serve as a check on the density of vibrational states in the material as the sub-gap PL changes. If the density of Raman-active vibrations remains relatively unchanged as we reduce the PL intensity, then we have higher confidence any changes in the THz spectra result from changes in chemical bonding and not structural artifacts. We support our interpretations of these measurements with calculations of the vibrational spectra using density functional theory, as detailed in the Methods section. Permanent defects have recently been proposed as a critical part of sub-gap emission from alkyl ammonium spaced lead iodide perovskite quantum wells. 35 While optical methods such as time resolved photoluminescence spectroscopy have been used to determine the lifetime of sub-gap emissive states, they cannot clearly delineate the type of defect causing that light emission. In contrast, vibrational spectroscopy carried out in tandem with studies of sub-gap PL could in principle provide insights into the the presence and role of structural defects in HOIP quantum wells. An analysis of the connection between the macroscopic observables of a vibrational spectroscopic measurement and the microscopic physics of the material shows how changes to the amount of light absorbed in the region of the lattice vibrations can provide direct access to changes in the character of chemical bond in the lattice induced by changes in the density of permanent defects. The imaginary part of the dielectric function, = 1 + ı2 , contains information related to the absorption of light due to different degrees of freedom of a material. Using the Lorentz model, the microscopic origin of 2 can be written, 44

2 =

|µlm |2 h ¯ ωlm Γω 8πN X , 2 2 2 h ¯ l,m ωlm − ω 2 + Γ2 ω 2

(1)

where N is the density of modes per unit volume, µlm and h ¯ ωlm are the dipole moment and energy separation between states |mi and |li, Γ is a phenomenological damping rate of the

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mode, and ω is the angular frequency of the incident light. To decipher the contribution to 2 we anticipate from a material’s vibrations, we must expand µ in terms of the vibrational normal coordinates, Qi . To first order, the dipole moment becomes,

µ(Qi ) ≈ µ(Q0 ) +

X ∂µ Qi , ∂Q Q0 i i

(2)

where Q0 corresponds to the equilibrium structure of the material and the sub-script i denotes the different vibrational coordinates of the material dictated by the number of atoms in the unit cell and its symmetry. The purely vibrational contribution to 2 then becomes,

vib 2

2 ∂µ hm|Q |ni ¯ ωi Γi ω h i 8πN X X ∂Qi Q0 = 2 2 h ¯ ωi2 − ω 2 + Γ2i ω 2 i l,m

(3)

where ωi is the angular frequency of the ith vibrational mode and we have explicitly. One can define the oscillator strength of the ith mode between the states |li and |mi as flm = 2 2 2Mi h ¯ ωi /¯ h hm|Qi |li where Mi is the mode reduced mass. Based on the commutations relations between the vibrational normal coordinates and their canonical momenta the osP cillator strength must obey Thomas-Reiche-Kuhn sum rule, lm flm = n, where n is the number of unit cells in the materials illuminated by the light field. This sum rule leads to an imaginary part of the dielectric function due to vibrational coordinates written as,

vib 2

X 4πn ∂µ 2 Γi ω = 2 Mi ∂Qi Q0 ωi − ω 2 2 + Γ2i ω 2 i

(4)

As seen in the SI, the amount of energy absorbed by a sample between the angular frequencies ω0 and ωc can be related to an effective charge induced by changes to the unit cell dipole moment by atomic motions along the vibrational normal coordinates through the

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equation, Z

ωc

ω2 (ω)dω = ω0

X 1 ∂µ 2 nef f (ωi , ω0 , ωc ), M ∂Q Q0 i i i

(5)

where nef f (ωi , ω0 , ωc ) is a number on the order of 1 whose exact value depends on the width of the frequency band between ω0 and ωc . Despite this small ambiguity, Eq. (5) shows one can establish how changes to the amount of energy absorbed in the region of the lattice vibrations can be related to changes in the ability of vibrations to modulate the dipole moment of unit cell. Since this modulation sensitively depends on the character of the bonding in the lattice, we can now use changes to experimentally measured 2 to understand changes to the microscopic chemical bonding environment of our samples as we thermally anneal away the sub-gap photoluminescence.

Figure 5: Comparison of the atomic motions of B2u vibrations at 70 cm−1 (left panel), 77 cm−1 (middle panel), and 80 cm−1 polarized in the plane of BA2 PbI4 derived from density functional theory calculations of the material’s vibrational spectrum. As the material chemical bonds change, one should anticipate changes in the amount of charge induced by moving the constituent atoms along vibrational coordinates participating in those bonds. For example, if material bonds become more ionic, ∂µ/∂Qi becomes larger. As shown in Eq. (5), we expect increasing ∂µ/∂Qi would then result in an associated increase in the amount of light absorbed by the sample. If charge is localized near the constituents of the inorganic octahedral layer in our initially formed samples, then the electron density involved in the bonding between Pb and I will become distorted. These distortions would 12


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then change the ability of the Pb-I stretching vibrations to modulate the dipole moment of the unit cell and change ∂µ/∂Qi . By measuring the total energy absorbed in the region of the Pb-I vibrations coincidently with the presence or absence of the sub-gap PL, we can assess how permanent defects play a role in the trapping and energy stabilization of excitons in our samples. In particular, if integral in Eq. (5) extracted from experimental measurements increases as the sub-gap PL becomes extinguished, then this fact would suggest ∂µ/∂Qi in the band of probed frequencies also increases upon reducing the sub-gap PL. An increasing ∂µ/∂Qi could suggest a coincidental increase in the ionicity of the Pb-I bonds of the octahedral layer due to the migration of a charged defect away from that lattice site. DFT calculations suggest vibrations sensitive to presence of charged defects would be found below 150 cm−1 in BA2 PbI4 . In particular, our calculations suggest there are 3 intense IR-active, B2u vibrations centered at 70 cm−1 , 77 cm−1 , and 80 cm−1 largely centered on the Pb and I atoms of the octahedral layer. The atomic motions comprising these vibrations are shown in Figure 5. This figure shows the motion of the Pb atom stays within the octahedral plane of the material, making observation of these vibrations in a normal incidence transmission measurement possible. The bonding of these atoms would be significantly affected by charge within the lattice and affect the ability of the vibrations shown in Figure 5 to modulate the dipole moment of the unit cell. In such a case, any changes to the density of charge within the lattice should manifest itself in changes to the absorb of light by these vibrations, in accordance with Eq. (5). The frequencies of the lattice vibrations of BA2 PbI4 necessitate our use of THz time domain spectroscopy (THz-TDS). The specifics of our instrument are explained in the Methods section. Figure 6 shows the results of our THz-TDS measurements. In its top panel, Figure 6 shows the changes induced in the time-domain waveform of the THz pulse by thermally annealing the sample to reduce the sub-gap PL. Through a Fourier transformation and subsequent data analysis the bottom panel of Figure 6 shows 2 of the sample increases noticeably after the thermal annealing process, especially in the region between 20 cm−1

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/2 c [cm-1] Figure 6: Top panel: comparison between the THz time domain waveforms transmitted through a crystalline sample of butyl ammonium lead iodide (BA2 PbI4 ) before (blue) and after (red) thermally annealing the sample to extinguish sub-gap photoluminescence. Bottom panel: comparison between the imaginary part of the dielectric function of fast formed crystalline BA2 PbI4 derived from THz time domain waveforms in the top panel before (blue) and after (red) thermally annealing the sample to extinguish sub-gap photoluminescence. and 140 cm−1 . Based on the surrounding intensity, we assign the peaks centered at 54 cm−1 as the Pb-I stretching and wagging vibrations shown in Figure 5. When we focus on the spectral region between 34 cm−1 and 84 cm−1 , centered around the peak at 54 cm−1 , we find that integral in Eq. (5) increases by a factor of 1.13. We reiterate this increase in 2 occurs while there is no change in the most reasonable structure extracted from single crystal x-ray diffraction patterns collected before and after thermally annealing the samples.

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Raman Shift [cm-1] Figure 7: Comparison between the low frequency Raman scattering spectra of a 298 K butyl ammonium lead iodide (BA2 PbI4 ) quantum well single crystal excited at 785 nm before (blue) and after (red) annealing the sample at 333 K. This comparison shows a minimal change in the vibrational density of states that would likely participate in exciton self-trapping. To assess if other structural changes could explain the THz-TDS results in Figure 5 we excited our samples at 785 nm, significantly red-shifted from the exciton absorption edge, to measure the polarized reduced Raman spectra, written as, 45

Ired (¯ ν) =

I(¯ ν )¯ ν , e¯hν¯c/kB T + 1

(6)

where I(¯ ν ) is the raw Raman spectrum, T is the sample temperature, c is the speed of light, and ν¯ is the Raman shift frequency in cm−1 . Spectra measured for z(xx)¯z polarization before and after thermally annealing the sample are shown in Figure 7. Based on the presumption that exciton trapping should occur in the inorganic octahedral layers, we only analyze vibrations containing motion of the Pb and I atoms. By mapping the atomic motions of each vibration in the spectral window of Figure 7 we find two modes largely defined by the motion of inorganic atoms: an Ag mode at 100.63 cm−1 and a B1g mode at 71.06 cm−1 whose atomic motions are shown in Figure S5. These two calculated vibrations lie very close in energy to peaks observed in the z(xx)¯z-polarized spectrum at 97.35 cm−1 and 69.83 15



cm−1 , respectively. The large peak located near 135 cm−1 likely arises from the disordered structure consistent with single crystal x-ray diffraction measurements shown in Figure 1. 45 Close inspection of the spectra in Figure 7 shows there is significantly less change in the Raman intensity as a function thermally annealing our samples than observed in the THz spectra of Figure 6. In addition, whatever small changes we actually observe correspond to a reduction of the spectral intensity as we thermally anneal the material, contrasting with the increase in 2 upon thermal annealing shown in Figure 6. From Eq. (5) this increase in 2 could stem from either an increase in the magnitude of ∂µ/∂Qi along Pb-I vibrational coordinates like those shown in Figure 5 or from a reduction in the reduced mass of the vibration, Mi . In either case, thermally annealing the material must cause a change in the bonding of the material. Changes to the reduced mass of the vibration would necessitate changes in the identities of particular atoms bound together as the material temperature changes ∼40 K. Such changes seem unlikely given this small increase in the sample temperature. In contrast, it seems more likely that annealing could cause the movement of unbound, intercalated charge left in between lattice sites due to the fast cooling of the sample. Our crystals are formed in the presence of excess I− anions. These anions could get dispersed through the crystals in relatively low density. Booker et al. have shown interstitial I− anions can cause sub-gap light emission in films of alkyl ammonium spaced lead iodide perovskite quantum wells. 35 However, those authors reported light energies over 100 meV higher than the peaks in Figure 3 above and could only resolve the peaks at temperatures below 100 K. The assignment of sub-gap PL stemming from mobile, interstitial charges contrasts with recent reports that suggest octahedral distortions are necessary to stabilize emissive mid-bandgap electronic states in similar hybrid perovskite-like quantum wells. Specifically, Cortecchia et al. characterized the structure of slowly cooled (BA)2 PbI4 crystals reported by Billing and Lemmerer 42 and strongly distorted 2,2-(ethylenedioxyl)bis(ethylammonium)PbI4 with the octahedral elongation and octahedral angle variance parameters developed by

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Robinson at al . 46 These authors found that weakly distorted octahedra for (BA)2 PbI4 char2 = acterized by elongation and octahedral angle variance parameters λoct = 1.0016 and σoct

5.6, respectively, showed no sub-gap light emission. 30 In contrast, the octahedra of sub-gap PL emitting sample of (ethylenedioxyl)bis(ethylammonium)PbI4 are characterized by λoct = 2 1.0058 and σoct = 14.2. Based on these gross differences and those found in other materi-

als, these authors concluded distortions were a necessary ingredient to observe sub-gap PL. The lack of octahedral distortions in our samples implies a different mechanism explains the sub-gap PL emission shown above. Based on the lack of distortions to the PbI4 octahedra of our samples and our vibrational spectroscopic results, we propose intercalated charge drives the emission of a broadband spectrum of light below the bandgap of BAPbI4 quantum wells, even at room temperature. However, at least two attributes of this mechanism still remain unclear. First, our results do not provide direct insights into how these charges stabilize traps. These traps could lie below the conduction band or above the valence band. Second, we cannot identify if specific vibrational modes may participate in the exciton trapping process. These modes must play a critical role in stabilizing the trap states that emit the measured sub-gap PL. Further studies with more advanced spectroscopic techniques such as vibrational coherence spectroscopy can directly access experimental information central to completing the physical picture explaining the sub-gap PL emission in Figures 3 and 4. 47–50 Rury and co-workers have shown this technique can sensitively detect and characterize mid-gap states in organic semiconductors, 50 but have yet to apply it to HOIP materials. In combination with high level ab initio electronic structure calculations capable of identifying mid-gap trap states, vibrational coherence spectroscopy could unravel a novel physical mechanism that enables broadband HOIP emitters. The observation of broadband near-IR emission at room temperature indicates there may be a utility for disordered HOIP quantum wells in imaging and optical metrology applications. Low light illumination with near-IR light emitting diodes has grown as a desirable

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method for imaging in wide array of security systems ranging from building surveillance to baby monitors. Solution-based methods to fabricate active components in these devices could enable new capabilities. In addition, atomic transitions used in high precision time keeping techniques lie in the near-IR and are already predominantly interrogated with semiconductor diode lasers. Developing solution-based growth of laser gain media could help enable field configurable opto-electronic devices for high precision spatial mapping central to applications ranging from car navigation to locating missing military personnel. In conclusion, we have correlated steady-state light emission spectroscopies to uncover the connection between room temperature broadband visible/IR PL emission and permanent defects in perovskite quantum wells self-assembled from butyl ammonium lead iodide precursors. X-ray structure analysis suggests previously proposed distortion-driven broadband emission cannot account for our results. By examining time-domain THz absorption and polarized Raman spectra, we find broadband visible/IR emission coincides with reduced modulation of the unit cell dipole moment by vibrations in the inorganic layers of the material. We propose this coincidence suggests mobile charges intercalated into the initially formed material following rapid cooling cause substantial exciton trapping. Further theoretical studies using advanced electronic structure calculations will shed more light into the strength of these proposals. Our results highlight the general importance of permanent defects to the presence and properties of broadband, sub-bandgap photoluminescence emitted by hybrid perovskite quantum wells whose fundamental physical understanding is central to their suitability in an array of future opto-electronic technologies.

Methods Synthesis of (C4 H9 NH3 )2 PbI4 All chemicals were purchased from Sigma-Aldrich and used as received without further purification. Butylammonium lead iodide was prepared using a modified method earlier reported 18


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in Ref. [41], as described briefly. 1.16 mmol lead iodide were dissolved in 2 mL HI and heated to 130 ◦C in an oil bath. In a separate beaker, 2.32 mmol butylamine solution were neutralized in 3 mL HI. The butylamine solution was added to the lead iodide solution and stirred for about 20 minutes under dry nitrogen atmosphere to produce a yellow solution. The stirring was stopped, the solution allowed to cool to room temperature, and orange crystals formed. The solution was placed in an ice-bath for about 2 hours to complete the crystallization. The crystals were filtered, washed with cold diethyl ether, and dried in a vacuum oven at 60 ◦C for 24 hours.

Characterization X-ray Diffraction Analysis A suitable single crystal was selected and diffraction data were collected on a Bruker X8 APEX diffractometer using Mo Kα radiation. The crystal was kept at 298.1 K during data collection. Unit cell determination, data integration, scaling and processing was performed with Bruker APEX3 suite. Absorption correction and scaling were performed with SADABS 51 software. An additional additional spherical µr = 2.5 correction was used to account for the high absorption of the sample. The structure was solved with the SHELXT 52 structure solution program using intrinsic phasing and refined with the SHELXL 53 refinement package running under SHELXLE 54 using least squares minimization. Crystal data were deposited in the Cambridge Crystallographic Data Centre with number 1902790. Structures were visualized and figures were generated using the VESTA software. 55 Powder XRD patterns were collected using a Bruker D8 powder diffractometer operated at 40 kV and 40 mA. Cu Kα radiation (λ = 1.5418 ˚ A) was employed. Diffractograms for Rietveld analysis were collected in the 5-65◦ 2θ range using a step size of 0.02◦ and a step time of 1.0 s. Rietveld analysis 56,57 of powder XRD patterns was conducted using the General Structure Analysis System 2 (GSAS-II). 58 The following parameters were refined: (1) scale factor; (2) background, which was modeled using a shifted Chebyschev polynomial function; (3) peak 19



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shape, which was modeled using a modified Thompson-Cox-Hasting pseudo-Voight function; 59 (4) lattice constants; (5) atomic coordinates of X; and (6) an isotropic displacement parameter (Uiso ) for each atom. Difference curves and Rwp and residuals were employed to assess the quality of the refined structural models.

Raman Scattering Measurements Raman scattering spectra were collected using a Horiba XploRA Plus Raman micro-spectrometer excited at 785 nm .and dispersed in a 0.3 m imaging monochromator using a 1800 gr/mm diffraction grating to produce high resolution spectra. Rayleigh scattering was rejected using a interferometric stopgap filter centered at the laser wavelength and extending ∼35 cm−1 into each of its wings. Temperature-dependent Raman spectra were collected by a 10x microscope in combination with a Linkam THMS600 liquid nitrogen temperature probe. Temperatures were maintained within ±0.5 ◦C of each set point and the sample was allowed to settle at each temperature for 5 minutes before spectra were acquired.

Photoluminescence Spectra Measurements. Temperature-dependent photoluminescence measurements were carried out using a Horiba XploRA Plus Raman micro-spectrometer by exciting individual micro-crystals with a CW 532 nm laser focused with a 10x objectve. Broadband spectra were formed after dispersing the collected emission using a 600 gr/mm diffraction grating. Temperatures were controlled with a Linkam THMS600 liquid nitrogen temperature stage using the same approach as that of the Raman spectroscopy measurements described above. Power dependent measurements were carried out with a 5x objective, which allowed the use of external filters to more precise control the incident laser power.

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Time-domain THz Absorption Measurements THz radiation was driven by focusing 0.85 mJ, 35 fs pulses of 800 nm light combined with its second harmonic in dry N2 gas. The pulse derives from a regeneratively amplified Tidoped Sapphire oscillator (Spectra Physics Solstice Ace). After passing through the reference/sample, the THz waveform was detected with electro-optic sampling using a (110)-cut GaP wafer (MTI Corporation). The THz signal was filtered and amplified with a Stanford Research System SR530 lock-in amplifier graciously loaned to the authors by the Technical Applications Center of the Newport Corporation. To calculate the real and imaginary parts of the sample’s refractive index, we developed a numerical routine that reduced the difference between the measured complex transfer function of the THz radiation through the sample and a theoretical model of the transfer function, reported previously. 60

Calculation of the Vibrational Spectra of BA2 PbI4 Density functional theory calculations of the electronic structure of BA2 PbI4 were carried out using CRYSTAL14. 61 We used the Perdew-Burke-Ernzerhof generalized gradient functional for both exchange and correlation 62 and polarizable electronic basis sets for lead, 63 iodine, 64 carbon, 65 nitrogen, 65 and hydrogen. 65 The positions of the atoms of BA2 PbI4 were optimized from those found in completely ordered structure reported by Billing. 42 The irreducible Brillouin zone of BA2 PbI4 was set on a mesh according to Pack-Monkhorst sampling using a shrinking factor of 3 for the inverse of all three crystallographic directions. The Coupled Perturbed/Kohm-Sham algorithm was used to find the frequencies of both the IR and Raman-active vibrations of BA2 PbI4 . 66,67

Acknowledgement The authors gratefully acknowledge support from the Wayne State University Research Fund.

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The authors declare no competing financial interests.

Supporting Information Available Supplemental Information including wider band photoluminescence (PL) spectra, details of the PL fitting algorithm, comparison of experimental data and fitting functions, comparison of polarized and depolarized non-resonant Raman spectra, visualization of Raman-active vibrations, and the crystallographic information file from our X-ray diffraction analysis can be found on-line at:

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Room Temperature Broadband Light Emission From Hybrid Lead

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