Spectral Heterogeneity of Hybrid Lead Halide ... - ACS Publications

Subscriber access provided by UNIV OF WESTERN ONTARIO

Article

Spectral Heterogeneity of Hybrid Lead Halide Perovskites Demystified by Spatially-Resolved Emission Varun Mohan, and Prashant K. Jain J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08005 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Spectral Heterogeneity of Hybrid Lead Halide Perovskites Demystified by Spatially-Resolved Emission Varun Mohan1 and Prashant K. Jain2,3* 1

Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

2

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

3

Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Corresponding Author *Email: [email protected]

ABSTRACT: Solution-processed films of methylammonium lead bromide (MAPbBr3) perovskites have remarkable photoluminescence (PL), with utility in light emitting devices (LEDs) and photodiodes; however, the PL emission is often complex, heterogeneous, anomalous, or poorly understood. We provide a deeper understanding by studying PL spectra of single MAPbBr3 crystallites with intra-crystallite spatial resolution. We uncover an emission emanating from the crystallite boundaries that is spectrally distinct from the band-to-band-recombination based emission from the crystallite interiors. Both forms of emission contribute to spatially-averaged PL measured on heterogeneous samples. We also map the PL emission spectrum in a distant-dependent manner across a single crystallite. The systematic distance dependence observed reveals that a portion of the PL emission emanating from within a crystallite is waveguided and outcoupled from the boundaries of the crystallite in a form that is spectrally modulated by self-absorption. Spatial heterogeneities, self-absorption, and filtered-emission of PL are all processes that must be considered in the future design of perovskite-based LEDs.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 17

INTRODUCTION Hybrid organic-inorganic lead halide perovskites (with general formula ABX3; X = Cl, Br, or I; B = Pb; A = CH3NH3 or CH(NH2)2) have been extensively investigated in recent years, primarily as a new class of materials for ultra-efficient solar cells. 1–4 Increasingly, however, attention is expanding to the unique optical properties exhibited by these materials, driven in part by recent realizations of such perovskitebased optoelectronic devices as nanowire lasers, 5 resonant optical cavities, 6,7 light emitting diodes, 8 waveguides 7,9 and photodetectors. 10,11 These applications are motivated by the ease of solution processability and low cost of fabrication of these materials coupled with their remarkable photoluminescence (PL) quantum yields, charge carrier lifetimes, and carrier diffusion lengths. 12–15 In particular, the bromine-based perovskite: CH3NH3PbBr3 (MAPbBr3), studied here, has received attention due to its direct band gap of ~2.3 eV and bright green luminescence centered at ~540 nm, making MAPbBr3 a promising candidate for light emitting diodes (LEDs) in the true green region of the visible spectrum. Given the emphasis on optoelectronic applications of these perovskites, PL emission spectra have become principal probes of carrier transport properties, phase transitions, 16 band structure, 17 electronic defects, 18 microstructural segregation, 19 and device performance. 20 However, lead halide perovskite materials have exhibited anomalous or complicated features in their PL emission, which are not fully understood, but hold potential insights. For example, Fang et. al. 21 found that both individual crystallites and bulk thin films of MAPbBr3 perovskite exhibited a double-peaked PL emission. They assigned the higher energy peak (ca. 529 nm) to band-gap emission and the lower energy peak (ca. 549 nm) to emission from shallow-lying, optically active defects from excess PbBr2 precursor entrapped in the perovskite. Additionally, Priante et. al., 22 while studying amplified spontaneous emission in MAPbBr3 powders, encountered an asymmetric PL emission band (ca. 549 nm), which was suggested to consist of two peaks, each the result of recombination of a bound exciton from different surface states. There are other examples in the literature where anomalous or multi-peaked PL emission have been attributed to a range of originating phenomena: defects resulting from leftover precursors, 20 different bound excitonic states,21 radiative emission from surface or bulk trap states, 23 and even grain-size inhomogeneity related quantum confinement. 23 In this article, we elucidate the complex PL emission behavior of MAPbBr3 perovskite materials. From a systematic spatially-resolved PL study (Fig. 1) of common solution-phase-deposited MAPbBr3 morphologies, we conclude that anomalous, multi-peaked, and/or spatially heterogeneous behavior 21–23 results from outcoupled emission emanating from the boundaries of the crystallite. When compared to the primary PL emission from the crystal interior, this boundary emission has a red-shifted spectrum and/or an asymmetric shape due to the self-absorption the emission undergoes in its traversal from the interior to the edges of the crystallite. While self-absorption of PL emission in perovskite materials has been described in recent work 24,25, this phenomenon was never recognized as the cause of the oft-observed spatial heterogeneity 21,22 and complexity of the PL emission; rather other factors such as defects or structural inhomogeneities were implicated without evidence. When such heterogeneous or multi-peaked PL emission is encountered in future literature on polycrystalline thin films of perovskites or other emitters, the systematic findings of this work can provide the needed resolution. Our model system consisted of polycrystalline MAPbBr3 films that were drop-cast from solution onto glass substrates. Low-cost, facile solution-based processing is one of the advantages touted for device


2

Page 3 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


preparation from organic/inorganic lead halide perovskites. Simple methods such as spin coating, dip coating, and drop-casting have yielded highly emissive MAPbX3 (X = Cl, Br, I) films. 1,2,10 Our dropcasting-based sample preparation method is similar to such fabrication of perovskite thin-film emitters, making our findings relevant to a range of device applications. In addition, using our drop-casting-based methods, we were able to access different sample morphologies for study, differing in film microstructure and average crystallite size. The synthesis procedure for all samples is described in the Methods section. In one case, we obtained polycrystalline films with a dense coverage of μm-sized grains. These films served as ideal platforms for optical microscopy-based characterization of spatial heterogeneity. The other model sample consisted of well-defined, isolated crystallites, which are amenable to spatially-resolved measurements while being potential platforms for light emission applications like the single crystal nanowires of Zhu et al. 5 Using optical microscopy and spectroscopy (Fig. 1), we mapped with micronscale spatial resolution the PL emission of these drop-cast MAPbBr3 films. In particular, for individual

Figure 1. (A) Scheme of the experimental setup for spatially-resolved PL emission microscopy. A 457.9 nm laser beam, < 1 mW in power is focused using a 60x microscope objective lens to a ~2 micron-sized spot on a polycrystalline MAPbBr3 perovskite film. PL emission was collected from a wide-field region around the excitation spot. Owing to the sub-micron scale spatial resolution of the detection, the PL emission spectra could be mapped spatially both at the excitation spot and at locations distant from the excitation. Two different modes of PL emission collection were employed as illustrated in the color micrographs shown in (B) and (C). In (B; ‘Interior’ PL emission), the excitation location was set well within the boundaries of a crystallite identified by CCD images. PL emission was collected from the region of excitation and from either side of the excitation spot, along an axis parallel to the spectrometer slit, with a spatial resolution of 1 CCD pixel, equivalent to 0.33 µm at 60x magnification. In (C; ‘Boundary’ PL emission), the excitation location was set well within the boundaries of a crystallite. PL emission was collected from a specific boundary of the crystallite, as identified from an optical image. A series of such measurements were performed by varying the excitation location and thereby varying the distance between the excitation site and the boundary site where the emission was collected. The distance, in units of CCD pixels, was converted to units of µm by using the conversion factor of 0.33 μm/pixel at 60x magnification. Scale bars in (B) and (C) represent 50 µm. It must be noted that in our epi-illumination geometry, the excitation and collection are both conducted from below the glass-substrate mounted sample. Due to the limited working distance of the 60x objective and penetration depth of the laser excitation into the strongly absorbing MAPbBr3 sample, the majority of the collected emission originates from a thin region close to the bottom surface of the sample. Any propagation/waveguiding of the PL emission occurring along the axial (thickness) direction of the sample is less significant than that in the lateral direction, especially when the lateral distance between the excitation location and collection location is on the order of a few μm. (D) shows a scanning electron microscope (SEM) image of a representative crystallite, showing a prominent crystallographic etch pit. Scale bar represents 50 µm.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 17

crystallites identified from optical micrographs (Fig. 1 B, C), we measured the PL emission spectrum in a spatially dependent manner with intra-crystallite resolution. METHODS: Materials’ synthesis: CH3NH3Br (MABr) was synthesized by reacting 15.4 mL of 40% aqueous methylamine (178.6 mmol) and 20.3 mL of 48% aqueous HBr in a round-bottom flask at 0 °C for 2 hours. The excess solvent was subsequently removed by rotary evaporation at 60 °C. The powder obtained was washed several times in diethyl ether and stored in a vacuum desiccator overnight at 60 °C. PbBr2 was purchased from Alfa Aesar (Puratronic, 99+ %) and used as received. All precursors were stored in a vacuum desiccator. Samples for optical microscopy were prepared by drop-casting 100 μL of a solution of 1 M MABr and 1 M PbBr2 on a glass coverslip. The choice of solvent, which was either dimethylformamide (DMF) or γ-butyrolactone (GBL), dictated the morphology of the prepared sample. In the case of DMF, the equimolar solutions were mixed overnight; whereas in the case of GBL, the mixing time was 15 minutes. This simple sample preparation process was adopted to mimic conditions encountered in potential device fabrication applications. Upon heating at 60 °C, the solvent evaporated from the edges inwards, leaving behind a collection of well-formed MAPbBr3 crystallites in a coffee-ring pattern (Fig. S1). All samples were prepared on transparent glass coverslips (VWR, Inc., #1 Thickness) that were pre-cleaned with acetone and deionized water (Barnstead, 18.3 MΩ) and blow dried with pressurized air before use. A MAPbBr3 nano/microwire sample was fabricated using a procedure based on Zhu et al. 5 A 100 mg/mL aqueous solution of lead acetate trihydrate, Pb(CH3COO)2·3H2O (Alfa Aesar, Puratronic grade) was dropcast onto a glass coverslip and dried for 30 min at 70°C. The dried film was then dipped face up in a 2 mL solution of 5 mg/mL MABr in isopropyl alcohol for 16 hours, resulting in the growth of MAPbBr3 wires. Characterization: Electron micrographs of samples prepared by drop-casting precursor solution onto glass coverslips were obtained on a JEOL 6060 low-vacuum scanning electron microscope (SEM). Samples were coated with a thin layer of gold to improve surface conductivity and prevent charging during electron microscopy. Powder X-Ray diffraction (PXRD) was performed on a Rigaku Miniflex 600 system using Cu Kα radiation (wavelength of 1.54 Å), a step size of 0.02°, and a 1-s acquisition time per step. Crystallites deposited on a glass coverslip using DMF as the solvent were scraped off and transferred onto a glass substrate for PXRD measurements. The diffractogram showed relatively narrow, well defined peaks (Fig. S3), therefore no baseline or background subtraction was performed. Microscopy and Spectroscopy: Optical microscopy and spatially-resolved PL emission spectroscopy were performed on an Olympus IX-51 inverted microscope equipped with an AmScope 0.3 MP CMOS camera for acquisition of color images and a Princeton Instruments Acton SP 2300 spectrograph with a 300 g/mm, 500 nm blaze grating and a PyLoN 7570-0003 liquid nitrogen-cooled charge-coupled device (CCD) for imaging and spatially-resolved emission spectroscopy. The MAPbBr3 film deposited on a glass coverslip was excited by a 457.9 nm laser line (Spectra Physics 2017 Ar operating in TEM00 mode). The laser beam was attenuated with neutral density filters to a power < 1 mW to avoid damage to the sample under investigation.


4

Page 5 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For MAPbBr3 films deposited using GBL as the solvent, we performed PL microscopy and spectroscopy (Fig. 2) using an Olympus PLN Plan Achromat 10x 0.25 NA objective. For the individual crystallites prepared using DMF as the solvent, we used an Olympus UPlanApo 60x water immersion 1.2 NA objective and emission spectra (Figs. 3 and 4) were collected in two distinct measurement modes: (1) “Interior” PL emission: The excitation location was set well within the boundaries of a crystallite identified in a CCD image. PL emission was collected from the region of excitation and from either side of the excitation spot, along an axis parallel to the spectrometer slit, with a spatial resolution of 1 CCD pixel, equivalent to 0.33 µm at 60x magnification. (2) “Boundary” PL emission: The excitation location was set well within the boundaries of a crystallite. PL emission was collected from a specific boundary of the crystallite, as identified from an optical image. A series of such measurements were performed by varying the excitation location and thereby the distance between the excitation site and the boundary site where the emission was collected. The distance, in units of CCD pixels, was converted to units of µm by using the conversion factor of 0.33 μm/pixel at 60x magnification. Microscope-based absorbance measurements were carried out in transmission mode on MAPbBr3 films deposited on a glass coverslip using either DMF or GBL as the solvent. Broadband light from an Olympus U-LH100-3 100W halogen lamp was used to illuminate a selected crystallite (with intensity Io). The transmitted intensity (I) was collected using the Olympus UPlanApo 60x 1.2 NA water immersion objective. Bulk PL spectra were collected on a Cary Eclipse spectrophotometer with 1-nm wavelength resolution and fast scan speed. The excitation wavelength was set to 450 nm. Similar to the PL microscopy studies, bulk spectra were collected on a MAPbBr3 film deposited on a glass coverslip using GBL. The coverslip was placed 45° with respect to the incident excitation path and the collection path of the spectrograph. The excitation was focused down by the in-built optics to a rectangular spot of 8 mm x 2 mm on the sample. Data Analysis: Bulk PL measurements: The bulk PL spectrum obtained on the fluorimeter (Fig. 2C) was smoothed using 10-point Savitsky-Golay averaging prior to presentation. Microscope-based PL spectra: Each individual PL spectrum over a chosen location of the widefield region in Fig. 2A was obtained by binning the collected PL emission across 15 pixel rows about the excitation location. The spectrum generated from the binning procedure was normalized to its maximum intensity before plotting in Fig. 2B. The spectrum in Fig. 2C (black curve) was generated by averaging 100 local spectra (see Fig. S6). These local spectra were acquired by binning the collected PL emission across 11 or 22 pixel rows about the excitation location.. Hundred individual locations were picked for these measurements by manually displacing the sample stage between measurements. The average spectrum so obtained was normalized to its maximum intensity before it was plotted. PL emission spectra shown in Figs. 3, 4, S7, S8, and S9 are based on emission signal collected over a single, specified pixel row of the CCD. Peak wavelengths of PL spectra were obtained by fitting spectra to equal-width Gaussian functions using the MATLAB program peakfit.m. All fits had an error less than 5%; the error in peak position is expected to be significantly lower.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 17

Absorbance measurements: The absorbance spectrum A(λ) was determined using the LambertBeer formula: 𝐼(𝜆) ) 0 (𝜆)

𝐴(𝜆) = −𝑙𝑜𝑔10 (𝑇(𝜆)) = −𝑙𝑜𝑔10 (𝐼

(1)

where T is the transmittance. Since the glass coverslip contributes a background absorbance (Abkgd), it had to be subtracted out: 𝐴(𝜆) = 𝐴𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑 (𝜆) − 𝐴𝑏𝑘𝑔𝑑 (𝜆) (2) where: 𝐼𝑔𝑙𝑎𝑠𝑠 (𝜆)

𝐴𝑏𝑘𝑔𝑑 = −𝑙𝑜𝑔10 (

𝐼0 (𝜆)

) (3)

𝐼

𝐴𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑 = −𝑙𝑜𝑔10 ( 𝑠𝑎𝑚𝑝𝑙𝑒−𝑜𝑛−𝑔𝑙𝑎𝑠𝑠 𝐼0 (𝜆)

(𝜆)

) (4)

where Iglass is the intensity of light transmitted through a region of the glass coverslip with no sample. Isample-on-glass is the intensity of light transmitted through a crystallite supported on the coverslip. The intensity of light transmitted, without any intervening material, was considered to be the incident light intensity, Io. Simulation of spectra: The PL emission spectrum at a distance d from the excitation spot, Iem(λ;d) was modeled by taking into account the attenuation of the emitted light by the crystallite’s self-absorption: 𝐼𝑒𝑚 (𝑑; 𝜆) = 𝐼𝑒𝑚 (0; 𝜆). 𝑒 −𝛼(𝜆).𝑑 (5) where Iem(λ;0) represents the normalized PL emission spectrum measured at the excitation location and α(λ) is the absorption coefficient of MAPbBr3, which can be obtained from the measured absorbance spectrum of a representative crystallite, A(λ), and its thickness t as: 𝛼(𝜆) =

2.303 𝐴(𝜆) 𝑡

(6)

where factor 2.303 results from the conversion in the logarithmic base. Note, the objective of this model was to simulate the PL spectrum line-shape as a function of distance from the excitation spot. Therefore, we did not include factors such as radiative relaxation and carrier diffusion which decrease the overall intensity of the PL spectrum as one progresses away from the excitation location.

RESULTS AND DISCUSSION In our MAPbBr3 film syntheses, we found that the choice of the solvent, which was either dimethylformamide (DMF) or γ-butyrolactone (GBL), influenced the sample morphology. A film dropcast from DMF consisted of separate MAPbBr3 crystallites of 100-300 μm edge lengths. A photograph of the sample is shown in Fig. S1, a representative wide-area SEM image in Fig. S2A, and powder diffraction pattern in Fig. S3. Many of these crystallites show prominent pyramidal etch pits on the crystal faces (Fig. 1D), similar to what has been reported in other systems, and an outcome we attribute to growth kinetics in the early stages of crystal formation. 26,27 Each such crystallite appears to be surrounded by a field of debris consisting of smaller grains. However, the boundary between a crystallite and its debris field is clear-cut. With GBL as the solvent, the drop-cast sample was found to consist of a much denser film of smaller MAPbBr3 crystallites, < 15 μm in size (Fig. S2). This morphology/size difference is due to the lower solubility of the precursors in GBL, which leads to rapid precipitation, resulting in the nucleation of a larger number of crystallites that grow to a significantly smaller extent. The study of two


6

Page 7 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


widely disparate crystallite sizes is insightful, as described in the subsequent discussion, because the nature of the PL emission signal depends on the crystallite size of the sample relative to the spatial resolution of detection.

Figure 2. Spatial heterogeneity of PL emission of a MAPbBr3 film (A) Wide-field optical micrograph (10x objective; broad-band visible light illumination) of a MAPbBr3 film deposited using GBL as the solvent. The colored squares mark selected regions at which PL emission spectra were obtained. The sample comprises of a discontinuous carpet of deposited material (darker regions) with gaps in the deposit showing up as brighter areas. At 10x, individual crystallites are unresolvable. The actual sample morphology is best reflected in the representative SEM micrograph shown in Fig. S2B. (B) PL spectra from these individual locations show marked variation in the spectral shape and peak position. (C) An average (black curve) of local spectra collected from 100 individual locations from the film shown in (A). Also shown is a bulk PL spectrum (red curve) of the same sample, with a collection area of ca. 16 mm2. One example of a local PL spectrum (blue curve) from the film is shown for comparison; others are plotted in Fig. S6. All spectra are shown normalized to their maximum intensity.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 17

Spatial heterogeneity was observed to be a key feature of the MAPbBr3 film deposited from GBL (Fig. 2). Fig. 2B shows PL emission spectra from the film obtained with a 10x microscope objective. Representative local spectra collected from specific locations on the film (regions marked in Fig. 2A using the same color as the spectra shown in 2B, each sampling multiple crystallites), show considerable differences in spectral shape and peak position. The spectrum in blue shows a major peak at 540 nm, which can be assigned to the band-to-band transition, and a shoulder at longer wavelengths (centered about 552 nm). In contrast, the spectrum in red shows a primary peak at a longer wavelength (ca. 550 nm), with a shoulder corresponding to the band-to-band transition. The curve in green is an example of a local spectrum that is intermediate between the two previous cases. Several more local spectra, from 100 individual locations on the film, are shown in Fig. S6, clearly demonstrating the spatial heterogeneity in the PL spectrum. In every case, the spectrum is apparently double-peaked in line-shape; but the relative intensity of the two peaks and the precise position of the redder peak vary from one location to another. An average of these 100 local spectra (Fig. 2C, black curve) exhibits the effect of sample heterogeneity on the overall PL emission from the sample. The averaged spectrum is naturally broader, ostensibly comprised of two peaks that are not resolved as in the local spectra, and overall red-shifted relative to the band-to-band emission. An even more extreme manifestation of the effect of spatial heterogeneity is seen in a bulk PL spectrum of the film averaged over a much larger area: the bulk spectrum (Fig. 2C, red curve) is broad (FWHM ~30 nm) and even more red-shifted relative to the band-to-band transition. Thus, the greater the degree of spatial averaging over the heterogenous sample, the higher is the contribution of redder emission. It is instructive that bulk or averaged PL spectra are not necessarily pure representations of band-gap emission. In order to understand the origin of the spatial heterogeneity and the redder emission, we turned to micron-scale spatially resolved mapping of the PL emission. We used MAPbBr 3 samples deposited using DMF, because we could individually address a large (100-300 μm), well-separated and well-defined crystallite. The laser excitation was focused to a ca. 2.6 μm spot (Fig. S4) on a chosen crystallite. PL spectra were collected with a spatial precision of 0.33 μm (Fig. 1). This choice allowed insight into the spectrum of a particular individual crystallite, vis-à-vis spectra averaged over heterogeneous collections of many grains. We could also systematically investigate how the PL emission varied spatially across an individual crystallite. PL microscopy of an individual crystallite (see Fig. 1B, C for representative example) showed a strong PL emission from the excitation location, decaying with increasing distance away from the excitation location until the signal was nearly extinguished. However, significant PL emission re-emanated from the boundaries of the crystallite. This boundary emission is clearly distinguished in Fig. S9 from PL emission of small crystallites in the surrounding debris, which emit at a distinctly different wavelength. Using two detection schemes shown in Fig. 1, we measured the PL spectrum in a spatially-dependent manner from locations in the crystallite interior close to the excitation spot as well as from locations on a crystallite boundary. The results of this study are shown in Fig. 3. At the excitation location, the PL emission spectrum is narrow, symmetric and centered at ~540 nm, which corresponds to the band-to-band transition value. Moving further away from the excitation location, i.e., going from 0 to 2.33 μm, the ‘interior’ PL emission decreased in intensity (Fig. 3A) and the PL peak maximum red-shifted by a small amount (Fig. 3B, C). The peak PL wavelength and emission intensity of the ‘interior’ PL emission is shown as a function of distance from the excitation location in Fig. S5. The trend is mirror-image


8

Page 9 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Spatial mapping of PL emission from individual crystallites of a MAPbBr3 film prepared using DMF. (A) For a representative crystallite, the PL spectrum collected at the excitation location (0 μm, defined by the peak of the focused excitation spot) and those from ‘interior’ locations in the vicinity of the excitation (0-2.33 μm) are also shown. The spectrum at 0 μm is scaled by 1/8x for easy comparison. The inset provides a schematic of the method used to collect ‘interior’ PL emission. (B) For the same crystallite, the PL emission spectrum emanating from a boundary of the crystallite is shown as function of distance between the excitation location and the collection location. The mode of collection of ‘boundary’ PL emission is shown by an inset, with a clear distinction from the measurement method used for the spectra in (A). Similar data from several crystallites is shown in Fig. S7. (C) shows select spectra from (A) and (B), normalized to their maximum intensity, to emphasize the change in peak PL wavelength as a function of distance. (D) shows the change in the peak PL wavelength as a function of distance, combining both ‘interior’ PL emission (0-2.33 μm) and ‘boundary’ PL emission (5.33-15.66 μm). Data-points are averages of 8-10 measurements from individual crystallites. The standard error of mean is depicted as the error bar for each data-point. The curve was phenomenologically fit to an asymptotic function: y= 𝑎 − 𝑏𝑐 𝑥 , which yielded the asymptotic PL peak wavelength of 554 nm.

symmetric on either side of the excitation spot, further corroborating the robustness of the measured distance-dependence. The PL emission out-coupled from the crystallite boundaries allowed us to analyze the trend for the longer-distance-range (5.33-15.66 μm). These ‘boundary’ PL emission spectra are less intense and also considerably red-shifted compared to PL emission from the excitation spot (Fig. 3A, B). With increasing distance between the excitation location and the boundary location (Fig. 3B, C), the ‘boundary’ emission decreases in intensity and progressively red-shifts. The ‘interior’ and ‘boundary’ PL emission spectra jointly (Fig. 3D) exhibit a systematic trend: with increasing distance from the excitation site, the peak PL


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 17

wavelength shifts away from the band-to-band transition value of ~540 nm, reaching an asymptotic peak PL wavelength of ~554 nm. The bulk PL emission spectrum of the polycrystalline film in Fig. 2C can be better understood in the context of this distance-dependent spectral evolution measured on single crystallites (Fig. 3). The bulk PL spectrum is centered at ~549 nm, considerably red-shifted from the band-gap emission energy. In other words, the bulk PL spectrum, notwithstanding a small contribution from PL emission at the band-gap energy, appears to be dominated by emission contributions from locations distant from the respective carrier generation sites. Such a composition of the bulk PL spectrum of the polycrystalline film is also evident in Fig. S12, where the peak position and line-shape of the bulk spectrum are modeled by a weighted combination of the excitation-location PL spectrum and other distant PL spectra from the single crystallites.

Figure 4. Modeling of the distance-dependent evolution of PL spectrum line-shape: (A) Experimental and (B) Simulated PL emission spectra as a function of distance from the excitation spot for the representative crystallite from Fig. 3. Since the objective was to model the PL spectral lineshape as a function of distance, rather than to recover the trend in PL intensity, spectra are plotted normalized to their maximum intensity and stacked vertically for easy visualization. (C) Micro-absorbance spectrum (black curve) for the MAPbBr3 crystallite is shown alongside the PL emission spectrum (red curve) collected at the excitation location, which is centered at the band-to-band transition. We also show for comparison a micro-absorbance spectrum of a MAPbBr3 sample prepared using GBL. Note the oscillatory features in the high wavelength-region of this absorbance spectrum are thin-film interference fringes. The film prepared using GBL consists of smaller crystallites and is, therefore, thinner than the film prepared using DMF. The thicker sample made using DMF has a red-shifted absorbance onset, which is why the spectrum was corrected i.e., blue-shifted by 12 nm, prior to its use as the attenuation function in simulations shown in (B).

The preceding discussion also elucidates the origin of spatial heterogeneity of PL emission presented in Figs. 2, S6, and in the literature. 21–23 Each local PL spectrum from the polycrystalline MAPbBr3 film appears to consist of contributions from PL emission at the band-gap energy and ‘boundary’ PL emission. This is demonstrated by examples of modeled local spectra in Fig. S13. Simply, the relative contributions of the two types of emission change from one location to another. Depending on the local crystallite density and grain sizes, the relative population of boundary sites to interior sites varies from one location to another within the film. Consequently, local spectra in Fig. 2B (averaged over multiple grains) can vary in the relative contribution of band-gap emission to red-shifted emission. Next, we address the reason for the systematic distance-dependence of the PL spectrum. A crucial hint from Fig. 3A, B is the development of an asymmetry in the PL spectrum with increasing distance away from the excitation location: the spectra appear to be “clipped” on their low-wavelength or high-energy side. The development of this spectral asymmetry and associated red-shift suggests a mechanism whereby the high-energy component of the ‘boundary’ emission is attenuated relative to the lower-energy portion.


10

Page 11 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In other words, a portion of the PL emitted at the site of excitation travels through the body of the crystallite. In the course of this travel, the PL emission is attenuated by self-absorption within the crystal, but high-energy components of the PL undergo greater self-absorption compared to the lower-energy components. Such a self-absorption mechanism is further supported by considerable overlap (Fig. 3C) between the PL emission band, especially on the high-energy side, and the absorbance of the MAPbBr3 seen here and in recent literature. 28–30 Fang et al. 24 have likewise discussed the possibility of multiple reflections leading to wave-guiding of emitted light followed by out-coupling of attenuated, asymmetrically shaped PL emission through the edges of large MAPbBr 3 single crystals. Such processes are expected to be important, in general, for semiconductor systems with small Stokes shifts (considerable absorbance-emission overlap) and high refractive indices that support total internal reflection and wave guiding. 31 In a recent publication, Diab et al. 32 found a similar effect of absorption-based filtering on cathodoluminescence emission spectra from MAPbBr3 single crystals. Using the above-explained self-absorption mechanism we modeled the distance-dependent evolution of the PL emission spectrum. The PL emission emanating from the excitation location within the crystallite interior was subject to a Beer-Lambert-type attenuation as it traversed through the crystallite. The measured micro-absorbance spectrum of the crystallite was used as the attenuation function (see Methods) after some modification. Simulated spectra based on this model (Fig. 4B) reproduced the change in PL spectral shape as a function of distance (0 to 16 μm) observed in experiments on individual crystallites (Fig. 4A). With increasing distance, the simulated PL emission spectrum became asymmetric and also red-shifted. Essentially, the low-wavelength portion of the PL emission, which falls in the region of high MAPbBr3 absorbance (Fig. 4C), underwent attenuation to a greater degree than the higherwavelength components, leading to the asymmetry and red-shift. The magnitude of the overall red-shift (~ 12 nm) observed in the experiment is reproduced in the simulation (~ 10 nm). It is worth noting that this process of self-absorption-based spectral filtering of the emitted light is distinct from recently discovered photon recycling. 33 The latter phenomenon involves the re-absorption of the emitted light emission, resulting in the generation of excited charge carriers, which then recombine to result in PL re-emission. By successive cycles of re-absorption and re-emission, i.e., photon recycling, the radiation is proposed to propagate across the crystal with greater efficiency than it would otherwise. Fang et al. 24 have, however, found from a polarization dependent measurement that the efficiency of photon recycling is rather low (< 0.5%) and that recycled photons comprise a meager (< 0.5%) contribution to the PL emission measured from MAPbBr3 single crystals. On the other hand, self-absorption-based spectral filtering has a dominant effect on the measured PL emission. Consistent with these findings, we find that the measured distance-dependent trends in PL peak position and asymmetry are well simulated by a model with only self-absorption and no photon recycling. If the emitted PL emanating from crystallites were dominated by recycled photons, the PL spectra measured at edge locations would not have been as red-shifted and asymmetric, both effects of the self-absorption-based filtering. Our model did not also include another effect. Photogenerated carriers can diffuse away from the generation region and recombine at the measurement location, contributing to some of the PL emission measured. In thin films of MAPbBr3, without applied bias, under optical excitation, i.e., conditions matching our experiments, photogenerated carriers exhibit diffusion lengths of ca. 200 nm. 34 Thus, the distance over which carriers can travel before recombining is much smaller than the µm-scale distances (resolution limited to 0.33 µm) investigated in our work. At the µm length scales we investigate, optical phenomena including waveguiding and self-absorption-based spectral filtering dominate and any effect of


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 17

carrier diffusion is likely to be minimal. However, at the smallest distances considered in our study, carrier diffusion may be a contributing factor. The plot of the PL emission peak position as a function of distance (Fig. 3D) fits the asymptotic function well at the longer distances (5.33-15.66 µm); but there is a deviation at small distances (0-2.33 µm). This deviation may be caused by a fraction of carriers diffusing away from the excitation spot and recombining at the collection spot, thereby contributing to a fraction of the collected PL emission. This effect would lead to a reduced red-shift of the PL spectrum than that expected from a purely self-absorption-based modulation of the PL spectrum, thereby causing the observed deviation in Fig. 3D. We should note that the measured micro-absorbance spectrum used for the attenuation had to be blueshifted by 12 nm to effectively reproduce the experimental PL line-shape as a function of distance (see Fig. S8). Such a correction was justified on the basis of the reported thickness dependence of the absorbance spectrum by Tian and Scheblykin for the CH3NH3PbI3 perovskite, 35 and a common artifact in bulk measurements on polycrystalline films and single crystals. 36, 28 As per Tian and Scheblykin, 35 for thicker crystals, the measured absorbance spectrum is artifactually red-shifted relative to the inherent absorbance. Even in our measurements (Fig. 4C), the larger crystallites made using DMF exhibited a redder absorbance onset than the film with smaller crystallites made using GBL and also relative to the reported ~2.3 eV-band-gap of the material. 37,38 The offset we employed in the simulations served to correct for this thickness-dependent distortion of the absorbance spectrum measured for the thick crystallite. CONCLUSIONS In summary, spatial heterogeneity is an important feature of the PL emission of solution-phase deposited hybrid lead halide perovskites, which have potential uses in LEDs and other optoelectronic devices. We characterized this heterogeneity and elucidated its origin by studying the PL emission of single crystallites and spatial mapping the emission with intra-crystallite resolution. Maps revealed an emission emanating from the crystallite boundaries that is spectrally distinct from the band-to-band-recombination-based emission from the crystallite interiors. Study of the PL emission as a function of location on the crystallite and distance from the excitation spot provided a clear picture: a portion of the PL emission emanating at the excitation location is waveguided across a crystallite and outcoupled from the boundaries of the crystallite in a form that is spectrally modulated by self-absorption within the crystallite. Both interior emission and spectrally-modulated boundary emission contribute to bulk PL emission. This work provides lucidity about the line-shapes of heterogeneously-averaged PL emission spectra reported in the literature and allows proper attribution of anomalous features observed in complicated spectra. 21–23 These findings also have implications for optical microscopy-based characterization of polycrystalline, thin film samples of perovskites or other emissive materials. Future researchers will need to consider that measured PL emission characteristics can depend on the mode of microscopy. For instance, a confocal scanning mode of microscopy would yield local (location-dependent) PL emission from selected points of the sample, whereas a wide-field mode of measurement would provide aggregated PL characteristics containing the complicating contribution of filtered/attenuated PL emission components. Further, our findings indicate that PL self-absorption and out-coupling are processes that deserve consideration in design of thin film light emission devices.


12

Page 13 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. Representative sample photograph; SEM images for all the morphologies studied; PXRD for crystallites synthesized using DMF based crystals; calibration of excitation spot size at 60x, distance evolution of ‘interior’ PL emission spectra for 10 individual crystallites, all 100 local spectra used for generation of data in Fig. 2C; full set of PL spectra from edge locations for 10 individual crystallites; simulated spectra calculated on the basis of the as-measured absorbance spectrum; analysis of the PL emission from debris surrounding crystallites; distance-dependent PL spectra for a MAPbBr3 nanowire, FWHM of the PL emission spectrum as a function of distance from the excitation spot; simulation of the bulk spectrum and three local spectra of the polycrystalline MAPbBr3 film from Fig. 2. AUTHOR INFORMATION Website: http://www.nanogold.org/ E-mail: [email protected] Twitter: @plasmon Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS. V. M. performed experiments and data analysis and wrote manuscript. P.K.J. designed studies, performed data analysis, and modeling, and wrote manuscript. P. Tyagi is acknowledged for initial experiments and raw material synthesis. V. M. acknowledges A. J. Wilson for helpful discussions and J. Heo for assistance with PXRD. A part of this work was conducted at the Frederick Seitz Materials Research Laboratory at the University of Illinois. P. K. J. acknowledges financial support from the Arnold and Mabel O. Beckman Foundation.

REFERENCES (1)

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as VisibleLight Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 6050–6051.

(2)

Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316–320.

(3)

Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982-988.

(4)

Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542–547.

(5)

Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V; Trinh, M. T.; Jin, S.; Zhu, X.-Y. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636–642.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 17

(6)

Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q. Room-Temperature near-Infrared High-Q Perovskite Whispering-Gallery Planar Nanolasers. Nano Lett. 2014, 14, 5995–6001.

(7)

Li, Y. J.; Lv, Y.; Zou, C. L.; Zhang, W.; Yao, J.; Zhao, Y. S. Output Coupling of Perovskite Lasers from Embedded Nanoscale Plasmonic Waveguides. J. Am. Chem. Soc. 2016, 138, 2122– 2125.

(8)

Kim, Y. H.; Cho, H.; Heo, J. H.; Kim, T. S.; Myoung, N. S.; Lee, C. L.; Im, S. H.; Lee, T. W. Multicolored Organic/inorganic Hybrid Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 1248–1254.

(9)

Wang, Z.; Liu, J.; Xu, Z.; Xue, Y.; Jiang, L.; Song, J. C.; Huang, F.; Wang, Y.; Zhong, Y. L.; Zhang, Y.; et al. Wavelength-Tunable Waveguides Based on Polycrystalline Organic-Inorganic Perovskite Microwires. Nanoscale 2015, 8, 6258–6264.

(10)

Yakunin, S.; Sytnyk, M.; Kriegner, D.; Shrestha, S.; Richter, M.; Matt, G. J.; Azimi, H.; Brabec, C. J.; Stangl, J.; Kovalenko, M. V; et al. Detection of X-Ray Photons by Solution-Processed Lead Halide Perovskites. Nat. Photonics 2015, 9, 444-U44.

(11)

Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404.

(12)

Chen, Y.; Yi, H. T.; Wu, X.; Haroldson, R.; Gartstein, Y. N.; Rodionov, Y. I.; Tikhonov, K. S.; Zakhidov, A.; Zhu, X.-Y.; Podzorov, V. Extended Carrier Lifetimes and Diffusion in Hybrid Perovskites Revealed by Hall Effect and Photoconductivity Measurements. Nat. Commun. 2016, 7, 12253.

(13)

Yang, Y.; Yan, Y.; Yang, M.; Choi, S.; Zhu, K.; Luther, J. M.; Beard, M. C. Low Surface Recombination Velocity in Solution-Grown CH3NH3PbBr3 Perovskite Single Crystal. Nat Commun 2015, 6, 7961.

(14)

Zhang, W.; Pathak, S.; Sakai, N.; Stergiopoulos, T.; Nayak, P. K.; Noel, N. K.; Haghighirad, A. A.; Burlakov, V. M.; deQuilettes, D. W.; Sadhanala, A.; et al. Enhanced Optoelectronic Quality of Perovskite Thin Films with Hypophosphorous Acid for Planar Heterojunction Solar Cells. Nat Commun 2015, 6, 10030.

(15)

Guo, Z.; Manser, J. S.; Wan, Y.; Kamat, P. V.; Huang, L. Spatial and Temporal Imaging of LongRange Charge Transport in Perovskite Thin Films by Ultrafast Microscopy. Nat. Commun. 2015, 6 (May), 7471.

(16)

Dai, J.; Zheng, H.; Zhu, C.; Lu, J.; Xu, C. Comparative Investigation on The TemperatureDependent Photoluminescence of CH3NH3PbBr3 and CH(NH2)2PbBr3 Microstructures. J. Mater. Chem. C 2016, 4, 4408–4413.

(17)

Tauber, D.; Dobrovolsky, A.; Camacho, R.; Scheblykin, I. G. Exploring the Electronic Band Structure of Organometal Halide Perovskite via Photoluminescence Anisotropy of Individual Nanocrystals. Nano Lett. 2016, 16, 5087–5094.

(18)

Tian, Y.; Merdasa, A.; Unger, E.; Abdellah, M.; Zheng, K.; Mckibbin, S.; Mikkelsen, A.; Pullerits, T.; Yartsev, A.; Sundstrom, V.; et al. Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites. J. Phys. Chem. Lett. 2015, 6, 4171–4177.

(19)

Bischak, C. G.; Hetherington, C. L.; Wu, H.; Aloni, S.; Ogletree, D. F.; Limmer, D. T.; Ginsberg, N. S. Origin of Reversible Photo-Induced Phase Separation in Hybrid Perovskites. Nano Lett. 2017, 17, 1028–1033.

(20)

Eperon, G. E.; Moerman, D.; Ginger, D. S. Anticorrelation between Local Photoluminescence and


14

Page 15 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photocurrent Suggests Variability in Contact to Active Layer in Perovskite Solar Cells. ACS Nano 2016, 10, 10258–10266. (21)

Fang, X.; Zhang, K.; Li, Y.; Yao, L.; Zhang, Y.; Wang, Y.; Zhai, W.; Tao, L.; Du, H.; Ran, G. Effect of Excess PbBr2 on Photoluminescence Spectra of CH3NH3PbBr3 Perovskite Particles at Room Temperature. Appl. Phys. Lett. 2016, 108, 071109.

(22)

Priante, D.; Dursun, I.; Alias, M. S.; Shi, D.; Melnikov, V. A.; Ng, T. K.; Mohammed, O. F.; Bakr, O. M.; Ooi, B. S. The Recombination Mechanisms Leading to Amplified Spontaneous Emission at the True-Green Wavelength in CH3NH3PbBr3 Perovskites. Appl. Phys. Lett. 2015, 106, 081902.

(23)

Srimath Kandada, A. R.; Petrozza, A. Research Update: Luminescence in Lead Halide Perovskites. APL Mater. 2016, 4, 091506.

(24)

Fang, Y.; Wei, H.; Dong, Q.; Huang, J. Quantification of Re-Absorption and Re-Emission Processes to Determine Photon Recycling Efficiency in Perovskite Single Crystals. Nat. Commun. 2017, 8, 14417.

(25)

Zhang, S. Y.; Cheng, Y.; Bao, Q. Wavelength-Tunable Waveguides Based on Polycrystalline Organic–inorganic Perovskite Microwires. Nanoscale 2016, 8, 6258–6264.

(26)

Chen, Y.; Yang, S.; Chen, X.; Zheng, Y. C.; Hou, Y.; Li, Y. H.; Zeng, H. D.; Yang, H. G. Direct Insight into Crystallization and Stability of Hybrid Perovskite CH3NH3PbI3 via Solvothermal Synthesis. J. Mater. Chem. A 2015, 3, 15854–15857.

(27)

Hou, Y.; Kondoh, H.; Ohta, T. PbS Cubes with Pyramidal Pits: An Example of Etching Growth. Cryst. Growth Des. 2009, 9, 3119–3123.

(28)

Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519–522.

(29)

Walters, G.; Sutherland, B. R.; Hoogland, S.; Shi, D.; Comin, R.; Sellan, D. P.; Bakr, O. M.; Sargent, E. H. Two-Photon Absorption in Organometallic Bromide Perovskites. ACS Nano 2015, 9, 9340–9346.

(30)

Zhang, Z.-Y.; Wang, H.-Y.; Zhang, Y.-X.; Hao, Y.-W.; Sun, C.; Zhang, Y.; Gao, B.-R.; Chen, Q.D.; Sun, H.-B. The Role of Trap-Assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots. Sci. Rep. 2016, 6, 27286.

(31)

Liu, X.; Zhang, Q.; Xiong, Q.; Sum, T. C. Tailoring the Lasing Modes in Semiconductor Nanowire Cavities Using Intrinsic Self-Absorption. Nano Lett. 2013, 13, 1080–1085.

(32)

Diab, H.; Arnold, C.; Ledee, F.; Trippe-Allard, G.; Delport, G.; Vilar, C.; Bretenaker, F.; Barjon, J.; Lauret, J.-S.; Deleporte, E.; et al. Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals. J. Phys. Chem. Lett. 2017, 8, 2977–2983.

(33)

Pazos-Outon, L. M.; Szumilo, M.; Lamboll, R.; Richter, J. M.; Crespo-Quesada, M.; Abdi-Jalebi, M.; Beeson, H. J.; Vru ini, M.; Alsari, M.; Snaith, H. J.; et al. Photon Recycling in Lead Iodide Perovskite Solar Cells. Science 2016, 351, 1430–1433.

(34)

Adhyaksha, G. W. P.; Veldhuizen, L. W.; Kuang, Y.; Brittman, S.; Schropp, R. E. I.; Garnett, E. C. Carrier Diffusion Lengths in Hybrid: Processing, Composition, Aging, and Surface Passivation Effects. Chem. Mater. 2016, 28, 5259–5263.

(35)

Tian, Y.; Scheblykin, I. G. Artifacts in Absorption Measurements of Organometal Halide Perovskite Materials: What Are the Real Spectra? J. Phys. Chem. Lett. 2015, 6, 3466–3470.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 17

(36)

Saidaminov, M. I.; Abdelhady, A. L.; Murali, B.; Alarousu, E.; Burlakov, V. M.; Peng, W.; Dursun, I.; Wang, L.; He, Y.; Maculan, G.; et al. High-Quality Bulk Hybrid Perovskite Single Crystals within Minutes by Inverse Temperature Crystallization. Nat. Commun. 2015, 6, 7586.

(37)

Park, J.-S.; Choi, S.; Yan, Y.; Yang, Y.; Luther, J. M.; Wei, S.-H.; Parilla, P.; Zhu, K. Electronic Structure and Optical Properties of α-CH3NH3PbBr3 Perovskite Single Crystal. J. Phys. Chem. Lett. 2015, 6, 4304–4308.

(38)

Sharizal Alias, M.; Dursun, I.; Saidaminov, M. I.; Marwane Diallo, E.; Mishra, P.; Khee, T. N.; Bakr, O. M.; Ooi, B. S. Optical Constants of CH3NH3PbBr3 Perovskite Thin Films Measured by Spectroscopic Ellipsometry. Opt. Express 2016, 24, 201–208.


16

Page 17 of 17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

17

Spectral Heterogeneity of Hybrid Lead Halide ... - ACS Publications

Recommend Documents