Modulation of Broadband Emissions in Two- Dimensional ⟨100⟩-Oriented Ruddlesden− Popper Hybrid Perovskites

Two-dimensional (2D) Ruddlesden−Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in ⟨100⟩-oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike ⟨110⟩-oriented RP perovskites, the broadband emission of the 2D ⟨100⟩-oriented RP (PEA)2PbI4 (PEA = C6H5C2H4NH3 ) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both undercoordinated Pb ions on the surface and I vacancies in the bulk through I− ion migration. Two-dimensional (2D) Ruddlesden−Popper (RP) hybrid perovskites have attracted great attention in the field of optoelectronic devices, especially lightemitting diodes (LEDs), because of their tunable photoluminescence wavelength, high photoluminescence quantum yield (PLQY), and excellent photoand moisture stability. Conceptually, 2D RP perovskites can be derived by slicing three-dimensional (3D) perovskites along different orientations (i.e., ⟨110⟩, ⟨100⟩, and ⟨111⟩), in which the bulky organic spacers and inorganic layers are periodically arranged. The periodic arrangement of organic spacers and inorganic layers, having differing energetics and polarizability, leads to enlarged electron−hole Coulomb interactions and the exciton binding energies (relative to the parent 3D perovskite) due to the quantum and dielectric confinement effects. Moreover, the reduction of dimensionality from 3D to 2D also leads to a sharp excitonic optical absorption peak and an extremely narrowband emission from free excitons. The latter is a desired feature for many light-emitting applications. However, besides the narrowband luminescence, 2D RP hybrid perovskites also often show a very broad emission band and large Stokes shifted photoluminescence. For instance, Br-based ⟨110⟩-oriented perovskites show prominent broadband emission at room temperature. These corrugated 2D perovskites usually have highly distorted structures, in which two undercoordinated halide atoms between organic and inorganic layers allow large lattice rearrangements in the excited states. In ⟨110⟩-oriented 2D perovskites, the strong interaction between charge carriers and phonons leads to the formation of self-trapped electrons or self-trapped holes. These self-trapped holes [electrons] tend to capture the surrounding electrons [holes] via Coulombic attraction, generating self-trapped excitons (STEs) that can radiatively decay with broadband emission. On the other hand, the origin of the broadband emission in ⟨100⟩-oriented 2D RP perovskites is still hotly debated. Two mechanisms have recently been proposed to explain the origin of such emission. Similar to the ⟨110⟩-oriented cases, Smith et Received: May 14, 2020 Accepted: May 28, 2020 Published: May 28, 2020 Leter http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 2149 https://dx.doi.org/10.1021/acsenergylett.0c01047 ACS Energy Lett. 2020, 5, 2149−2155 This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. D ow nl oa de d vi a 67 .1 .2 05 .6 8 on J ul y 6, 2 02 0 at 2 0: 23 :3 9 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.

* sı Supporting Information ABSTRACT: Two-dimensional (2D) Ruddlesden−Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in ⟨100⟩-oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike ⟨110⟩-oriented RP perovskites, the broadband emission of the 2D ⟨100⟩-oriented RP (PEA) 2 PbI 4 (PEA = C 6 H 5 C 2 H 4 NH 3 + ) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both undercoordinated Pb 2+ ions on the surface and I vacancies in the bulk through I − ion migration.
T wo-dimensional (2D) Ruddlesden−Popper (RP) hybrid perovskites have attracted great attention in the field of optoelectronic devices, 1−6 especially lightemitting diodes (LEDs), 7−9 because of their tunable photoluminescence wavelength, high photoluminescence quantum yield (PLQY), and excellent photo-and moisture stability. 10,11 Conceptually, 2D RP perovskites can be derived by slicing three-dimensional (3D) perovskites along different orientations (i.e., ⟨110⟩, ⟨100⟩, and ⟨111⟩), in which the bulky organic spacers and inorganic layers are periodically arranged. The periodic arrangement of organic spacers and inorganic layers, having differing energetics and polarizability, leads to enlarged electron−hole Coulomb interactions and the exciton binding energies (relative to the parent 3D perovskite) due to the quantum and dielectric confinement effects. 12 Moreover, the reduction of dimensionality from 3D to 2D also leads to a sharp excitonic optical absorption peak and an extremely narrowband emission from free excitons. The latter is a desired feature for many light-emitting applications.
However, besides the narrowband luminescence, 2D RP hybrid perovskites also often show a very broad emission band and large Stokes shifted photoluminescence. 13 For instance, Br-based ⟨110⟩-oriented perovskites show prominent broad-band emission at room temperature. 14,15 These corrugated 2D perovskites usually have highly distorted structures, in which two undercoordinated halide atoms between organic and inorganic layers allow large lattice rearrangements in the excited states. In ⟨110⟩-oriented 2D perovskites, the strong interaction between charge carriers and phonons leads to the formation of self-trapped electrons or self-trapped holes. 16,17 These self-trapped holes [electrons] tend to capture the surrounding electrons [holes] via Coulombic attraction, generating self-trapped excitons (STEs) that can radiatively decay with broadband emission. 18,19 On the other hand, the origin of the broadband emission in ⟨100⟩-oriented 2D RP perovskites is still hotly debated. Two mechanisms have recently been proposed to explain the origin of such emission. Similar to the ⟨110⟩-oriented cases, Smith et al. proposed that the photoinduced local deformation of the crystal lattice leads to the formation of STEs in ⟨100⟩ lead bromide perovskites. 20 On the other hand, Zhang et al. showed that the intensity of the broadband emission in (PEA) 2 PbI 4 (PEA = C 6 H 5 C 2 H 4 NH 3 + ) can be tuned by adjusting the concentration of excess iodine during crystal growth. In this case, the authors attributed the broadband emission to iodine interstitials. 21 Park et al. indicated that the broadband emission in the (BA) 2 PbBr 4 (BA = C 4 H 9 NH 3 + ) crystal may originate from defect-assisted radiative recombination, i.e., organic cation vacancies formed by intercalated water molecules. 22 These contradicting explanations reported in the literature motivated us to decipher the source of the broadband emission. Given the interest in exploiting these materials in light-emitting applications, differentiating between self-trapped exciton emission and intrinsic defect emission will enable the development of synthesis strategies that fully control the material's luminescence and defect properties.
Here, we experimentally and theoretically explore the optical properties of the prototype ⟨100⟩-oriented 2D RP perovskite crystal (PEA) 2 PbI 4 to reveal the origin of its broadband emission. We demonstrate that the broadband emission in the (PEA) 2 PbI 4 crystals arise from defect-assisted radiative recombination, unlike ⟨110⟩-oriented 2D RP perovskites. By studying crystals grown using different precursors (e.g., PbI 2 versus PbO), particularly with excitation power-dependent PL measurements, we identify the defect nature of the broadband luminescence and rule out any substantive role for self-trapped excitons. To clarify the nature of the emissive defect centers, we conducted density functional theory (DFT) calculations on a variety of possible intrinsic defects and found that in-plane iodine vacancies in the bulk and on the surface are the most likely source of the broadband emission. Guided by this insight, we show a postsynthetic treatment strategy based on PEAI that substantially suppresses the broadband emission, as the excess I − ions can effectively passivate these vacancies. (a) Schematic illustration of (PEA) 2 PbI 4 single-crystal synthesis using PbO+PEAI or PbI 2 +PEAI precursors; (b) optical micrographs and corresponding fluorescence images of (PEA) 2 PbI 4 single crystals collected at 473 nm excitation; (c) theoretical and experimental X-ray powder diffraction (XRD) patterns of freshly cleaved (PEA) 2 PbI 4 single-crystal flakes without and with broadband emission (the inset gives the crystal structure of (PEA) 2 PbI 4 ); (d) normalized absorption and photoluminescence spectra of (PEA) 2 PbI 4 crystals without and with broadband emission.
As illustrated in Figure 1a, (PEA) 2 PbI 4 crystals were synthesized via similar approaches but with different precursors of (i) lead oxide (PbO) and ammonium salt (PEAI) and (ii) lead iodide (PbI 2 ) and PEAI. The precursors were dissolved in HI and heated to 100°C; then, the crystals were grown by gradually cooling the solution to room temperature (see details in Experimental Methods). Optical micrographs and corresponding fluorescence images of the as-synthesized crystals with green and red fluorescence are shown in Figure 1b. The X-ray diffraction (XRD) patterns of the as-grown (PEA) 2 PbI 4 crystals with different precursors show clear (00l) diffraction peaks, l = even number, confirming the layered perovskite structure is aligned along the z-axis (the calculated d spacing is ∼1.65 nm). For the 2D layered perovskite, the distortion of the [PbI 6 ] 4− octahedra was evaluated by the mean octahedral elongation (λ oct ) and the octahedral angle variance (σ 2 oct ). 23 The calculated λ oct and σ 2 oct of (PEA) 2 PbI 4 are 1.006 and 16, respectively, indicating less octahedral distortion compared to ⟨110⟩-oriented perovskites, such as (EDBA) 2 PbCl 4 (λ oct = 1.008 and σ 2 oct = 260). 23 Thus, based on the distortion of the octahedra, the ⟨100⟩-oriented (PEA) 2 PbI 4 crystals are categorized as narrowband emissive perovskites.
The absorption spectra of the two types of (PEA) 2 PbI 4 crystals show similar features, with an excitonic peak at approximately 533 nm and continuous absorption in the highenergy region (the additional absorption peak around 480 nm for crystals grown with PEAI and PbI 2 may originate from the few-layer PbI 2 with a calculated band gap of 2.55−2.64 eV 24 ). However, the (PEA) 2 PbI 4 crystals grown with PEAI and PbI 2 exhibit two distinct emission peaks: a narrowband emission centered at 527 nm and a broadband emission ranging from 550 to 800 nm (centered at approximately 690 nm). The narrowband emission is characteristic of exciton emission from the inorganic [PbI 4 ] 2− layers of the 2D perovskite crystal, 25 while the broadband emission is similar to the white-light emission in ⟨110⟩-oriented 2D perovskites but with a much smaller Stokes shift (0.48 eV). Interestingly, broadband emission is not observed in the control (PEA) 2 PbI 4 crystals grown with PbO and PbI 2 , indicating that the origin of the broadband emission in the former case is associated with crystal defect states rather than with photogenerated species, such as self-trapped excitons. Moreover, the broadband emission observed here is different from the red-to-nearinfrared emission of (PEA) 2 PbI 4 crystals that is triggered by Sn dopants (the large lattice deformation around the Sn impurities can accommodate the STEs). 19,26 The different optical features of the (PEA) 2 PbI 4 crystals prepared using different precursors suggest that the intrinsic defects of the (PEA) 2 PbI 4 crystals are controlled by the stoichiometric reaction between the precursors in the HI acid. For instance, when PbO is used, PbO reacts with HI and fully converts to pure PbI 2 , which further reacts with PEAI. In this case, defect-free (PEA) 2 PbI 4 crystals are formed. However, when PbI 2 is used, PbI 2 directly reacts with PEAI as well as with HI. This leads to the formation of polyiodide species, e.g., PbI 3 − and PbI 4 2− . 27,28 The overall reaction equilibrium in the final solution is then different because of the many possible reaction pathways. Hence, such changes in the nature and concentration of lead iodide species in the reaction mixture affect the final product and contribute to the generation of iodine vacancies in the (PEA) 2 PbI 4 crystals produced via the PbI 2 route.
To understand the origin of the broadband emission in (PEA) 2 PbI 4 crystals, we first performed excitation powerdependent PL measurements. Figure 2a shows the PL spectra of the (PEA) 2 PbI 4 crystals for different excitation powers ranging from 104 to 1319 μW. We can estimate the origin of both emissions from (PEA) 2 PbI 4 crystals using the power-law equation, 29 which is defined as I PL = nL k , where I PL is the integrated PL intensity; L represents the excitation power; and n is the emission efficiency. In semiconductor materials, the coefficient k in this equation represents the recombination mechanism: k = 2 corresponds to band-to-band recombination; k = 1−2 is related to free or bound exciton recombination, and k < 1 corresponds to impurity-related recombination. 29 Figure 2b shows the excitation power dependence of the PL intensity for the narrowband and broadband emissions of the (PEA) 2 PbI 4 crystals at room temperature. Similar to previous reports, 21,22 both emissions show a linear dependence on the excitation power density: the fitted k value for the narrowband emission is 1.28, which is consistent with a free exciton mechanism, while the fitted k value for the broadband emission is 0.96, which can be considered as an indicator of defect-related exciton emission. Note that such defect-induced broadband emission is not saturated in our case because further increasing the excitation power density may damage the crystal surface and create new

ACS Energy Letters
http://pubs.acs.org/journal/aelccp Letter surface defects that contribute to the broadband emission (this will be discussed in detail later).
To decipher the luminescent centers of the defect-induced broadband emission of the (PEA) 2 PbI 4 crystal, we performed density functional theory (DFT) calculations of the formation energy of native point defects and corresponding charge transition levels. As illustrated in Figure 3a, we considered two donor defects (out-of-plane and in-plane iodine vacancies, V I1 and V I2 ), two neutral defects (PEAI and PbI 2 vacancies, V PEAI and V PbI 2 ), and two acceptor defects (PEA vacancy and iodine antisite, V PEA and I I ), as they are expected to be the dominant defects in 2D layered perovskites. 30 The computational details can be found in Experimental Methods. As shown in Figure 3b, the overall defect formation energies of neutral defects (V PEAI and V PbI 2 ) are much lower than those of the other defects. However, such neutral defects would not introduce any deep trap state within the bandgap (see Figure S1 of the Supporting Information). Under I-rich conditions, the two kinds of V I defects have larger formation energies than those of the neutral and acceptor defects, especially out-of-plane V I1 . As the chemical potential point shifts from I-rich to I-poor conditions, the defect formation energies of I I and V PEA increase (projected density of states and electronic charge densities are shown in Figure S2), and those of V I1 and V I2 largely decrease. Thus, under I-poor conditions, the donor defects become the dominant defects, with formation energies of 1.08 eV for out-of-plane V I1 and 0.84 eV for in-plane V I2 .
The charge transition levels of the donor and acceptor defects are shown in Figure 3c. Both out-of-plane and in-plane V I in (PEA) 2 PbI 4 lead to (deep donor) states in the gap with transition levels 1.89 eV for V I1 and 1.73 eV for V I2 above the valence band maximum (VBM). In contrast, V PEA and I I exhibit shallow transition levels of 0.15 and 0.34 eV above the VBM. Panels d and e of Figure 3 show the projected density of states (PDOS) for (PEA) 2 PbI 4 after introducing donor defects. For perfect (PEA) 2 PbI 4 , the electronic charge densities at the VBM and CBM (mainly composed of I 5p, Pb 6s, and Pb 6p orbitals) of pristine (PEA) 2 PbI 4 are delocalized within the inorganic layers ( Figure S3). In the case of the (PEA) 2 PbI 4 supercell with V I1 , the bandgap of (PEA) 2 PbI 4 remains almost unchanged, and the charge densities at both the CBM and VBM retain the delocalized feature. This is different from the PDOS of the (PEA) 2 PbI 4 supercell containing V I2 , which shows a new intraband state (∼0.3 eV below the CBM), as well as a delocalized charge density surrounding the V I2 center. Overall, we can conclude that the in-plane I vacancy is a reasonable origin for the broadband emission observed in (PEA) 2 PbI 4 crystals because it has lower defect formation energy under I-poor conditions and its charge transition level (1.73 eV) is close to the broadband emission center (1.80 eV). To support that the broadband emission of ⟨100⟩-oriented 2D RP perovskites originates from the halide vacancies, we further carried out DFT calculations on ⟨100⟩-oriented (BA) 2 PbBr 4 , which has a similar PL feature with a narrowband emission at 3.06 eV and a broadband emission centered at 2.25 eV. 22 As shown in Figures S4−S6  In addition to the intrinsic point defects in the bulk of the 2D crystal, I vacancies also form on the surface during the growth of the (PEA) 2 PbI 4 crystal, especially under I-poor conditions. The resulting surface states capture photogenerated charge carriers and trigger radiative decay. We further evaluated the impact of the surface states on the electronic and optical properties of the (PEA) 2 PbI 4 crystal by constructing a slab model terminated with surface iodine vacancies (i.e., a PbI 2 -rich surface, Figure 4a). Similar to the bulk case with a V I2 defect, the PDOS of the (PEA) 2 PbI 4 slab shows a midgap state 0.34 eV below the CBM (Figure 4b).
The electronic charge density distributions at the CBM and VBM of the (PEA) 2 PbI 4 slab still present a delocalized feature

ACS Energy Letters
http://pubs.acs.org/journal/aelccp Letter as in the bulk, while the additional surface charge state shows a delocalized density on the top layers of the slab. In this case, the surface state might represent another radiative channel involved in the broadband emission, as charge carriers can decay via surface defect states located within the bandgap. A similar surface state is observed in the (BA) 2 PbBr 4 slab, with an intraband level 0.36 eV below the CBM ( Figure S7 of the Supporting Information).
To verify our theoretical results, we treated the (PEA) 2 PbI 4 crystals with excess PEAI (i.e., by placing them in PEAI solvent) and found that the broadband emission is largely suppressed (Figure 5a), with a strong decrease in the PL intensity ratio between the broadband and narrowband emissions, compared to the as-grown (PEA) 2 PbI 4 crystals. Figure 5b shows the time-resolved PL decays of the two types of (PEA) 2 PbI 4 crystal (the excitation fluence is 6.47 μJ/cm 2 ). All the fitting parameters are given in Table S1. The control (PEA) 2 PbI 4 crystal without broadband emission has an average PL lifetime of 0.39 ns [τ 1 = 0.32 ns (91.2%) and τ 2 = 1.1 ns (1.1%)], which agrees well with previous reports. 31 For the asgrown (PEA) 2 PbI 4 crystals with broadband emission, the average PL lifetimes of the narrowband and broadband emissions are ∼5.1 and ∼69 ns, respectively. This indicates that the appearance of defect states prolongs the exciton emission lifetime because of reverse electron transfer from defect states to the free excitonic state. After PEAI treatment, the average lifetime of the broadband emission decreases to ∼45 ns, but the lifetime of the narrowband emission slightly increases to ∼8.2 ns because of the suppression of nonradiative decay (induced by shallow trap states), providing new experimental evidence that PEAI treatment could reduce the defect states of (PEA) 2 PbI 4 crystals.
It is worth noting that the as-grown crystals were washed with the antisolvent diethyl ether (DEE) to remove any possible excess acid and dried at 60°C for both synthesis routes. However, the (PEA) 2 PbI 4 crystals prepared with PbO +PEAI (i.e., the control sample) did not show any broadband emission after washing, and the sample prepared with PbI 2 +PEAI still had broadband emission. Figure S8 shows PL and TRPL spectra for the control sample as a function of the DEE washing cycle. We find that the PL intensity sequentially decreases and the average TRPL lifetime increases from 0.51 to 0.83 ns with increasing number of cycles (Table  S2). The average TRPL lifetime of the control sample after four DEE washes is 1 order of magnitude lower than that of the broadband emitting (PEA) 2 PbI 4 crystals. Therefore, we can rule out the idea of extrinsic defect formation due to the antisolvent washing step.
Therefore, the experimental PEAI treatment plays a dual role in passivating the I vacancies of the (PEA) 2 PbI 4 crystal. On the one hand, the excess I − ions can remove the surface states by coordinating to Pb 2+ ions; on the other hand, I vacancies in the bulk can be filled through I − ion migration. To validate this last point, we calculated the migration activation energies for two iodine migration paths, one within the inorganic plane and one between inorganic layers, as illustrated in Figure 5c,d. We find that the lowest activation energy (E a ) for the I − ion is 0.49 eV within the inorganic plane, which is comparable to the calculated E a for the I − ion of MAPbI 3 (0.36 eV, Figure S9 of the Supporting Information), this suggests active vacancy diffusion of I − ions from the surface to the bulk within inorganic layers in the 2D (PEA) 2 PbI 4 crystal, which can significantly reduce the number of defect states. Note that the E a for I − ions between inorganic layers is much greater (1.76 eV, Figure 5d), pointing to very difficult I − ion migration across layers.
In conclusion, we have synthesized ⟨100⟩-oriented 2D perovskites without and with broadband emission using the same chemical approach but different reaction precursors. We have found that the broadband emission of the (PEA) 2 PbI 4 crystal originates from intrinsic defect states, as confirmed by power-dependent PL and lifetime measurements. It should be emphasized that this behavior is different from the self-trapped exciton-induced broadband emissions in ⟨110⟩-oriented 2D perovskites. The results of DFT calculations underline that the iodine vacancies both in the bulk and on the surface are responsible for introducing the sub-bandgap states leading to the broadband emission. We have also demonstrated that the broadband emission can be largely suppressed by treatment of excess PEAI, which effectively passivates the I vacancies. By unraveling the origin of the broadband emission in ⟨100⟩oriented 2D perovskites, we not only provide a detailed understanding of how to control the optical properties according to the crystal growth conditions but also point the way to the synthesis of highly efficient pristine narrowband emissive 2D perovskite crystals. Author Contributions § J.Y. and R.N. contributed equally to this work.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
This work was supported by the King Abdullah University of Science and Technology (KAUST) and the College of Science of the University of Arizona. We acknowledge the Supercomputing Laboratory at KAUST for their computational and storage resources, as well as their efficient technical assistance.