Impact of Reabsorption on the Emission Spectra ... - ACS Publications

Subscriber access provided by HKU Libraries

Letter

The Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals Hiba Diab, Christophe Arnold, Ferdinand Lédée, Gaelle Trippe-Allard, Géraud Delport, Christèle Vilar, Fabien Bretenaker, Julien Barjon, Jean-Sébastien Lauret, Emmanuelle Deleporte, and Damien Garrot J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 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 Letters 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 22

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 Letters

The Impact of Reabsorption on the Emission Spectra and Recombination Dynamics of Hybrid Perovskite Single Crystals Hiba Diab,

Delport,

†

†

Christophe Arnold,

Christèle Vilar,

Lauret,

†

‡

‡

Ferdinand Lédée,

Fabien Bretenaker,

Emmanuelle Deleporte,

∗,†

†

†

Gaëlle Trippé-Allard,

Julien Barjon,

‡

†

Géraud

Jean-Sébastien

and Damien Garrot

∗,‡

†Laboratoire Aimé Cotton, CNRS, Univ. Paris-Sud, ENS Paris-Saclay, Université

Paris-Saclay, 91405 Orsay Cedex ‡Groupe d'Etude de la Matiére Condensée, CNRS, Université de Versailles Saint Quentin

En Yvelines, Université Paris-Saclay, 45 Avenue des Etats-Unis, 78035, Versailles, France E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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

Abstract Understanding the surface properties of organic-inorganic lead based perovskites is of high importance to improve the devices performance. Here, we have investigated the dierences between surface and bulk optical properties of CH3 NH3 PbBr3 single crystals. Depth-resolved cathodoluminescence was used to probe the near surface region on a depth of a few microns. In addition, we have studied the transmitted luminescence through thicknesses between 50 µm and 600 µm. In both experiments, the expected spectral shift due to reabsorption eect has been precisely calculated. We demonstrate that reabsorption explains the important variations reported for the emission energy of single crystals. Single crystals are partially transparent to their own luminescence and radiative transport is the dominant mechanism for the propagation of the excitation in thick crystals. The transmitted luminescence dynamics are characterized by a long rising time and a lengthening of their decay due to photon recycling and light-trapping.

Graphical TOC Entry

2


Page 2 of 22

Page 3 of 22

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 the last few years, hybrid organic-inorganic perovskites (HOPs) have emerged as a promising class of semiconductors due to their unique electronic and optical properties. Perovskite solar cells have achieved power conversion eciencies up to 22% 1 and HOPs present also a great potential for light-emitting devices 2,3 and photodetectors. 4,5 Recently, new methods have been developed to rapidly grow large single crystals of HOPs. 68 The latter are ideally suited to study the intrinsic properties of HOPs. 9 They are found to possess lower trap densities, longer diusion length and higher mobilities. 8,10,11 Additionally, HOPs have also a great potential as scintillators. Indeed, devices based on millimeter-scale single crystals have been developed since a signicant thickness is needed to absorb X-ray photons. 12,13 However, there are still important discrepancies in the literature concerning the basic optical properties of HOPs crystals such as the absorption and emission spectra. For example, the absorption edge of CH 3 NH3 PbBr3 have been reported with values ranging from 2.362 eV (525 nm) for thin lms to 2.175 eV (570 nm) for single crystals. 6,8,1416 We note that the higher energy values are measured in transmission and that due to high absorption coecient (α ≈ 104 − 105 cm−1 ), very few light could be transmitted through a thick crystal. It is likely that only the part of the spectrum corresponding to the absorption tail has been measured. In fact, ellipsometry and reectivity measurements show very similar absorption coecients for thin lms and single crystals. 1719 The variation of the emission spectra of HOPs is even more confusing. For CH 3 NH3 PbBr3 thin lm, photoluminescence (PL) maxima has been measured at approximately 2.317 eV (535 nm), while the emission of single crystals has been reported between 2.317 eV and 2.175 eV (570 nm). 8,15,16,20 Similar contradictions exist for CH 3 NH3 PbI3 , with emission maxima ranging from 1.59 eV (780 nm) to 1.512 eV (820 nm). 6,8,21,22 Additionally, a dual emission has been recently reported for single crystals of CH 3 NH3 PbX3 (X:I, Br) at room temperature. 5,15,23 As one-photon excitation PL is limited by a penetration depth of approximately 100 nm, dierent studies have used a combination of one and two-photon excitation to attempt to separate the optical properties of the surface and bulk. 15,2426 However, the con-

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

clusions of those studies are still conicting. The dierences between the surface and interior region emissions have been suggested to arise from reabsorption and photon recycling. 25 On the contrary, it has been suggested that a dierence of band gap exists between the bulk and surface of HOPs single crystals. 15,26 Regarding this question, a precise estimation of the impact of reabsorption on the optical properties of HOPs is necessary and could be helpful to further optimize the performance of devices. Herein, we propose a novel approach to study the surface and bulk properties of HOPs. Firstly, depth-resolved cathodoluminescence (CL) spectroscopy was employed to investigate locally the surface and bulk properties of single crystals on a depth of a few microns. Secondly, we study the transmitted photoluminescence through thicknesses ranging from 50 µm to several hundreds of micrometers. In both cases, we compare the experimental spectra with calculated spectra, corrected from the eect of reabsorption. We show that reabsorption eect is responsible for a large variation of the emission spectra energy and shape. We conclude that, if it exists, a dierence of optical band gap between surface and bulk should be inferior to . 10 meV. Unlike inorganic direct band gap semiconductor, we show that HOPs are partially transparent to their near-edge luminescence and that radiative transport (emission of a photon and its reabsorption at another location) is important in thick crystals. Additionally, photon recycling and light trapping cause a lengthening of transmitted PL decay and the presence of a long rising time. Figure 1a and 1b show SEM views of a millimeter-sized CH 3 NH3 PbBr3 single crystal grown by inverse crystallization temperature. 6 The surface of the single crystals presents terraces structures and pyramidal shapes due to spiral growth around screw dislocations (see Figure S1). We have performed depth-resolved CL spectroscopy to investigate locally the surface and bulk near-edge emission of CH 3 NH3 PbBr3 . The accelerating voltage determines the energy deposited by electrons as function of depth. CL intensity is proportional to the carrier density which can be approximated by the energy deposition prole. The latter can be modeled with the Monte Carlo simulation CASINO. 27 The depth prole range could be

4


Page 4 of 22

Page 5 of 22

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 1: (a) Scanning Electron Microscopy (SEM) image of a CH 3 NH3 PbBr3 single crystal (b) Higher-resolution SEM view of the surface of the crystal (c) Cathodoluminescence spectra at dierent accelerating voltages varied between a few tens of nanometers and a few microns (Figure S2). Figure 1c shows CL spectra measured at dierent accelerating voltages. At 2kV, the energy deposition prole is maximum at a depth of approximately 14 nm and we are then sensitive to the surface optical properties. The emission is then symmetric and centered at 2.317 eV (535 nm), close to the band edge. When the accelerating voltage is increased, CL is generated deeper inside the interior region of the crystal and we observe a progressive redshift of the spectra from 2.317 eV to 2.271 eV. Additionally, the spectra become asymmetric with a sharp edge on the high energy side and the full width at half maximum (fwhm) decreases from 100 meV to approximately 78 meV at 30 kV. With increasing voltage, the escape depth of CL photons is augmented and they are partially reabsorbed. In direct band gap semiconductors, the absorption edge is sharp and the high energy photons are strongly reabsorbed which induce a redshift of the emission for bulk samples. 28,29 In addition, a spectral shift may stem from variation of the physical properties throughout the material. A shift of the near-edge emission can originate from strain, defects or uctuation of the material composition. Recently, a dierence of optical band gap between the surface and the bulk has been suggested in dierent studies to explain the observations made at one and two-photon excitation. 15,26 Sarmah et al. have proposed, based on an experimental and

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

Figure 2: (a) Simulated CL spectrum at 30 kV (black squares) and experimental CL spectrum (red line) (b) Experimental (red dots) and simulated (black squares) CL maximum position energy as function of voltage theoretical approach, the existence of a dierence of approximately 60 meV between the optical band gap of the surface and bulk on CH 3 NH3 PbBr3 . 26 This dual band gap would be due to the appearance of a double charged surface layers, connected to ion migration. The predicted thickness of the surface layer is below 1 µm. In order to clarify this point, we have modeled the CL spectra, corrected of reabsorption eect, using Monte Carlo simulation for a homogeneous crystal. To this end, we have followed the procedure described by Gelhausen

et al.. 30 At 2 kV, the CL escape depth is very short and the spectrum is free of reabsorption. For higher accelerating voltage, CL proles were corrected for reabsorption using the BeerLambert law (see SI for details). Absorption coecients were derived from the study of Park

et al. 17 and from transmission measurements for the low energy Urbach tail (Figure S3). 6


Page 6 of 22

Page 7 of 22

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 2a shows the experimental and calculated spectra at 30 kV (Spectra at 5 kV and 10 kV are displayed in Figure S5). The shape of the measured peaks are correctly reproduced by the simulation. The maximum positions of the calculated and measured spectra as function of accelerating voltage are shown on Figure 2b. Simulated and experimental energy positions are in good agreement. The measured position at 10 kV is slightly redshifted of 10 meV (≈ 2.40 nm) from the calculated maximum. However, we note that the model neglects the eect of carriers diusion, which suppose that the carrier diusion length is small compared to the energy deposition range. With a diusion length of the order of 1 µm estimated for CH3 NH3 PbBr3 single crystals 20,25,31 the assumption is correct for the 30 kV measurement. The small shift between experimental and calculated values observed at lower voltage may be explained by the diusion of carriers deeper in the interior region of the crystal, which increases the redshift of the spectra. In any case, there is no evidence of an abrupt change of emission spectra or of a dual emission. 15,26,32 The spectral shift and shape evolution of the CL emission are mainly due to the eect of reabsorption. We note that, despite the surface irregularities we mentioned earlier, the crystal luminescence is homogeneous in terms of intensity (Figure S6) and energy position contrary to the emission of thin lms. 33 Hence, the spectral shift reported here is not due to variation of the surface properties. 34 However, we have observed locally the presence of specic defects, with a rod like shape, which present a strong emission, with maximum at an energy of 2.379 eV (521 nm), higher than the emission of CH 3 NH3 PbBr3 (Figure S7). Further studies are needed to identify the specic phase responsible of this luminescence. Additionally, under excitation by an electron beam, HOPs could be damaged. 35 In fact, we have observed that under irradiation, rstly the emission intensity decreases rapidly and secondly, new CL peaks appear at high energy (Figure S8). However, in agreement with the work of Xiao

et al. 35 on CH3 NH3 PbI3 , we found that the shape of the CL spectra is stable under short exposure time and weak currents ( I < 1nA) even at high voltage. The exposure time has been limited to 0.5 seconds with beam power inferior to 10 µW , to prevent the degradation

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

of material and each measurement has been made on a fresh, non-irradiated surface. In these conditions, no variation of the CL spectra with the current is observed (Figure S9). The eect of reabsorption could not be mistaken with an eventual material degradation as the latter causes a blueshift of the emission (Figure S8c).

Figure 3: (a) PL spectra measured at the surface (green) and in transmission through two thicknesses 60 µm (blue) and 600 µm (red) (b) Comparison of the calculated and experimental PL transmitted through 600 µm (c) Maximum energy position as function of thickness. In order to investigate further the bulk optical properties, we have collected the photoluminescence emerging from the excited surface of the single crystals, directly excited by the laser spot, referred as surface PL (S-PL), and the PL emerging from the opposite surface, referred as transmission PL (T-PL), for dierent distances between the excitation and collection points (Figure S10). The laser light penetration depth is approximately α−1 ≈ 81 nm. The initial carrier density follows the Beer-Lambert law and extends a few hundred nanometers from the surface. Hence, the T-PL, collected at distance comprised between 50 µm and 600 µm, results from indirect processes: carriers and photons diusion inside the crystals, with the possibility of photon recycling. 36 Here, we discern reabsorption from photon recycling dened as repeated reabsorption and re-emission processes. Figure 3a shows the surface PL spectra and the transmission PL spectra through a crystal thickness of 50 µm (blue) and 600 µm (red). The surface emission peak is somewhat redshifted with maximum at 2.287 eV (542 nm) relatively to the absorption edge (Figure S11) 8


Page 8 of 22

Page 9 of 22

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


and presents a slightly asymmetric shape with a tail on the low energy side. T-PL peaks are clearly asymmetric with a steep decay on the high energy side and show a redshift which is dependent of the thickness of the crystals. Neglecting carrier diusion, the transmitted spectra can be calculated from the BeerLambert law (equation (1)), where Is is the PL spectra at the surface, α is the absorption coecient.

IP L (hν) = Is (hν) ∗ e−α(hν)∗l

(1)

Figure 3b shows the experimental and calculated transmitted PL spectra through a thickness of 600 µm. The shape and position of the T-PL spectrum are correctly reproduced. We highlight that the spectrum is here directly calculated with no simulation or t parameters. Figure 3c presents the position of the emission maximum, measured and calculated, as function of the thickness. The calculated redshift due to reabsorption is strong on the rst tens of micrometers. Above 50 µm, the shift between the absorption band edge and the emission maximum energy is important and reabsorption occurs mainly due to the absorption tail of CH3 NH3 PbBr3 . The experimental redshift of the emission with increasing thickness is correctly reproduced by the Beer-Lambert law. The maximum deviation measured between experimental and calculated positions is of approximately 13 meV (3.3 nm). Reabsorption only, without the eect of carrier diusion and photon recycling, describes correctly the variation of PL spectra. This can be expected if the carrier diusion length is small compared to the thickness, which is coherent with an estimation of the diusion length of the order of 1 µm. 20,25,31,37 From the cathodoluminescence and transmitted PL experiments, we conclude that reabsorption mainly explains the variation reported in the literature regarding the emission energy of hybrid perovskite single crystals and between the luminescence measured at one and two-photon excitation. We cannot rule out completely the existence of a spectral shift between the near-edge emission of the surface and bulk, but it must be limited to . 10 meV. 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

We note that if, in a PL experiment, the collection volume is slightly shifted from the excitation volume, it results in the apparition of a characteristic dual emission with the presence of the direct and the indirect, reabsorbed, contribution (Figure S12). Finally, in a uorimeter, with a 90◦ excitation-collection conguration, the emission of a thick crystal will appear strongly redshifted. Interestingly, we highlight that HOPs single crystals, even with millimeter-size, are partially transparent to their own emission, whereas inorganic direct band gap semiconductors are generally opaque, unless they are heavily doped. 38 Once the emission is centered at 2.15 eV (≈ 560 nm), it is very weakly reabsorbed: the penetration depth of the light at 560 nm is α−1 ≈ 470µm. From the Beer-Lambert law and the measured surface PL spectra, we can estimate that approximately 8% of the PL intensity is transmitted through a 600 µm thickness. In comparison, for GaAs, we have evaluated that only 1% of the PL intensity is transmitted through a 150 µm thickness (Figure S13). This eect is due to the broad emission spectrum of CH 3 NH3 PbBr3 , with fwhm of 100 meV at room temperature (Figure S14a). Similarly to the observation of Wehrenfenning et al. on CH3 NH3 PbI3-x Clx , 39 we note that the luminescence of CH 3 NH3 PbBr3 is approximately twice as broad as the spectral width of the absorption onset (50.4 meV, Figure S14b) This important broadening of the emission of HOPs is likely caused by electron-phonon coupling. 9,39 Regarding the importance of the Stokes shift, a precise estimation will require a determination of the band gap energy from the absorption spectrum and there is still some uncertainty regarding the excitonic eect in CH3 NH3 PbBr3 . 19,4042 We measure a dierence of 47 meV (Figure S11) between the absorption maximum (2.361 eV, 525 nm) and the surface emission maximum (2.317 eV, 539 nm). We believe that this partial transparency should be taken into account when estimating the lifetime and the carrier diusion length in thick HOPs crystals. Additionally, this property is desirable for the realization of scintillators, as the photons generated in the bulk of the semiconductor need to reach the detector. Next, we study the kinetics of the surface and transmitted emission. In previous studies,

10


Page 10 of 22

Page 11 of 22

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 4: Time-resolved PL excited at 400 nm with a power density of 200 nJ/cm3 (a) Timeresolved S-PL at dierent wavelengths (b) PL dynamics of the S-PL and T-PL recorded at the emission maximum. Solid lines are the tted curves the one-photon excitation spectrum of crystals has been found to redshift with time, on a timescale of approximately 20 ns, from approximately 2.317 eV to 2.275 eV. 31 On the contrary, the two-photon excitation spectrum, with maximum redshifted at 2.18 eV, is found to be relatively stable. 20,25 These observations have been explained by the fast diusion of carriers created at the surface, inside the interior region. 31 Here, we monitor the surface PL decay at dierent energies (Figure 4a). A fast decay time appears for energies close to the absorption edge. The apparition of similar fast decay has been reported in GaAs. 43 This fast initial decay highlights the eect of reabsorption of the high energy photons and the diusion of excess carriers out of the initial excitation region. A longer decay is found at energy where photons are very weakly reabsorbed. This is consistent with the longer decay reported for two-photon excitation: the collected emission is then centered at an energy for 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

which reabsorption is very weak. Additionally, the fast component is absent from thin lms PL decay, which is consistent with lower reabsorption eect in thin lms compared to single crystals (Figure S15). Figure 4b compares the PL decays of CH 3 NH3 PbBr3 crystals for the transmission and surface congurations (measured at the emission maximum energy). The S-PL can be tted with a single exponential with characteristic time τs = 41.8 ± 0.1 ns . The T-PL kinetics ∗ = 1.8 ± 0.1 ns are characterized by a long rising time, tted here with two exponentials τt1

τt2∗ = 17.1 ± 0.4 ns and a decay time τt3 = 71 ± 3 ns. In the transmission conguration, we have maximized the eect of reabsorption and photon recycling on the recombination dynamics. These eects are known to lengthen the radiative lifetime 44,45 and we indeed observe a lengthening of the PL decay for the T-PL compared to the S-PL from approximately 50 ns to 70 ns. Additionally, the apparition of a long rising time on the T-PL is very similar to observations made on light emitting materials like uorophores in solutions or solid-state laser crystals. It has been found that photon recycling and light trapping lead to such rising time on the indirect emission, collected outside the directly excited region. 4648 In particular, this eect has been measured on lightly doped glasses, where the absorption length is large compared to the size of the sample, and where light undergoes many total internal reections (TIRs) and is evenly distributed inside the material. In our sample, the high energy photons are strongly absorbed and, near the surface, the carrier diusion length ( ≈ 1 µm) is superior to the emission average absorption length. However, after a few tens of microns, the red-shifted emission, with maximum centered at 2.214 eV (560 nm), is mainly composed of the low energy photons, weakly absorbed, and the absorption length ( α−1 ≈ 470µm at 2.214 eV) becomes largely superior to the carrier diusion length. Radiative transport, that is to say the emission of photons and their reabsorption at another location, is then the dominant mechanism for the excitation propagation in comparison with carrier diusion and the emission travels on distances relatively long compared to the crystal size. The refractive index at 2.214 eV is

12


Page 12 of 22

Page 13 of 22

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


approximately of 2.22 17,18 and the light with incident angle superior to 27◦ undergoes total internal reection (TIR). Hence, a fraction of the light is trapped inside the sample. The phenomenon is very easily observed during a PL experiment: the surface of the crystal emits a green light while the rest of the crystal emits a yellow glow (Figure S16). We observe that the indirect emission is evenly distributed inside thick crystals ( l ≥ 60µm): the T-PL rising time is relatively independent of the distance between the excitation spot and the collection. In these conditions, the measure of the intrinsic emission spectra and lifetime of HOPs requires to maximize the collection of the light from the directly excited region compared to the indirectly excited volume. In conclusion, we have used a combination of depth-resolved cathodoluminescence, steadystate and time-resolved PL to reveal the impact of reabsorption on the optical properties of CH3 NH3 PbBr3 . The discrepancies regarding the emission spectrum of HOPs single crystals at room temperature are mainly due to reabsorption eect and partial transparency of the HOP crystal to their near-edge luminescence. Reabsorption eects are important in CH3 NH3 PbX3 (X: I, Br, Cl) because of a sharp absorption edge and a broad emission spectrum. The intrinsic green emission of CH 3 NH3 PbBr3 could appear redshifted up to 570 nm in bulk material. The existence of a spectral shift between surface and bulk emission has been investigated and an upper limit for this shift has been estimated at ≈ 10 meV. Additionally, reabsorption leads to radiative transport being the dominant mechanism for the excitation transport inside large crystals. Transmitted PL is characterized by a long rising time and a decay time superior to the intrinsic lifetime due to photon recycling and light-trapping. In these conditions, special care must be taken to measure the emission spectrum and lifetime in large single crystals. The partial transparency together with the good mobility and long diusion length of HOPs single crystals reinforce the potential of single crystals for X-ray detection.

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

Experimental Section Precursor synthesis Methylammonium bromide CH 3 NH3 Br (called MABr hereafter) is synthesized by adding dropwise 4.6 mL (40 mmol) of hydrobromic acid HBr (47 % in water, Sigma Aldrich) to 10 mL (20 mmol) of methylamine CH 3 NH2 (2M solution in methanol, Sigma Aldrich) at

0◦ C. The reaction mixture is stirred at low temperature for 2 hours. The solvent is then evaporated at 60◦ C under vacuum using a rotary evaporator. The powder is subsequently washed several times with diethyl ether and dried overnight at 60◦ C. In order to increase purity, the white MABr microcrystals are nally recrystallized in a mix of ethanol and diethyl ether.

Thin lms 84 mg (0.75 mmol) of freshly made MABr and 275 mg (0.75 mmol) of lead bromide PbBr 2 are dissolved in 1 mL of N,N-dimethylformamide (DMF) (1:1 molar ratio). Quartz substrates (Neyco) were cleaned in an ultrasonic bath for 15 minutes successively with acetone, ethanol, then treated with a 10 %wt solution of KOH in ethanol. The slides were then rinsed with distilled water and dried with pressured air. CH 3 NH3 PbBr3 thin lms were obtained by spin coating the precursor solution at 2000 rpm for 15 seconds. The lms are then annealed at

90◦ C for 20 minutes in air.

Single crystals 448 mg (4 mmol) of MABr and 1.47 g (4 mmol) of lead bromide PbBr 2 are dissolved in 4 mL of DMF (1:1 molar ratio) in a small Teon capped vial. The vial is then placed on a hot plate at 90◦ C. Single crystals of CH 3 NH3 PbBr3 start to appear at the bottom of the vial after a couple of hours. There are recovered, dried and washed with dry diethyl ether several times. The single crystals have not been intentionally doped.

Scanning electron microscopy and cathodoluminescence The crystal was placed on conductive carbon tape. SEM views were acquired using a commercial JEOL JSM-7001F eld emission SEM. CL spectroscopy was conducted at room 14


Page 14 of 22

Page 15 of 22

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


temperature using an optical system (Horiba Jobin Yvon SA) installed on the SEM. The CL is collected by a parabolic mirror and focused with mirror optics on the entrance slit of TRIAX550 monochromator equipped with a silicon CCD camera. Energy deposition prole were simulated using CASINO 2.48 software.

Optical Spectroscopy Absorption was measured with a Perkin-Elmer spectrophotometer. The photoluminescence spectra were recorded using a Spectrapro 2500i spectrometer equipped with a Pixis: 100B CCD array detector (Ropers Scientic). Time-resolved photoluminescence was performed using the Time-correlated single Photon counting TimeHarp 260 system from PicoQuant. The excitation was the second harmonic of a pulse from a Ti:Sapphire laser (Mai Tai, SpectraPhysics). The emission was detected with a single photon avalanche diode (IDQuantique ID150).

Acknowledgement This work has received funding from the European Union's Horizon 2020 research and innovation programme under the grant agreement No 687008. The information and views set out in this paper are those of the author(s) and do not necessarily reect the ocial opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein. J.-S. Lauret is partially funded by Institut Universitaire de France.

Supporting information SEM views, CL energy deposition proles, absorption coecient, CL spectra simulation method, simulated spectra at 5 kV and 10 kV, CL maps, defect CL spectrum, dependence of the CL spectra with exposure time and current, schematic diagram of the S-PL and T15



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

PL measurement, direct and indirect emission measurements, fwhm of the CL emission and absorption onset, thin lms TRPL data and uorescence image of a single crystal

References (1) NREL Chart,

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg , Ac-

cessed 2017-05-29. (2) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476480. (3) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P. et al. Perovskite Energy Funnels for Ecient Light-Emitting Diodes. Nat. Nanotechnol. 2016, 11, 872877. (4) Lin, Q.; Armin, A.; Burn, P. L.; Meredith, P. Filterless Narrowband Visible Photodetectors. Nat. Photonics 2015, 9, 687694. (5) Fang, Y.; Dong, Q.; Shao, Y.; Yuan, Y.; Huang, J. Highly Narrowband Perovskite Single-Crystal Photodetectors Enabled by Surface-Charge Recombination. Nat. Pho-

tonics 2015, 9, 679686. (6) 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. (7) Saidaminov, M. I.; Abdelhady, A. L.; Maculan, G.; Bakr, O. M. Retrograde Solubility of Formamidinium and Methylammonium Lead Halide Perovskites Enabling Rapid Single Crystal Growth. Chem. Commun. 2015, 51, 1765861. 16


Page 16 of 22

Page 17 of 22

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


(8) Shi, D.; Adinol, 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 Diusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519 522. (9) Diab, H.; Trippé-Allard, G.; Lédée, F.; Jemli, K.; Vilar, C.; Bouchez, G.; Jacques, V. L.; Tejeda, A.; Even, J.; Lauret, J.-S. et al. Narrow Linewidth Excitonic Emission in Organic-Inorganic Lead Iodide Perovskite Single Crystals. J. Phys. Chem. Lett. 2016,

7, 50935100. (10) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole Diusion Lengths >175 µm in Solution Grown CH 3 NH3 PbI3 Single Crystals. Science

2015, 347, 967970. (11) Johnston, M. B.; Herz, L. M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diusion, and Radiative Eciencies. Acc. Chem. Res. 2015, 49, 146 154. (12) Wei, H.; Fang, Y.; Mulligan, P.; Chuirazzi, W.; Fang, H.-H.; Wang, C.; Ecker, B. R.; Gao, Y.; Loi, M. A.; Cao, L. et al. Sensitive X-ray Detectors Made of Methylammonium Lead Tribromide Perovskite Single Crystals. Nat. Photonics 2016, 10, 333339. (13) Birowosuto, M. D.; Cortecchia, D.; Drozdowski, W.; Brylew, K.; Lachmanski, W.; Bruno, A.; Soci, C. X-ray Scintillation in Lead Halide Perovskite Crystals. Sci. Rep.

2016, 6, 37254. (14) Green, M. A.; Jiang, Y.; Souani, A. M.; Ho-Baillie, A. Optical Properties of Photovoltaic Organic-Inorganic Lead Halide Perovskites. J. Phys. Chem. Lett. 2015, 6, 47744785. (15) Wu, B.; Nguyen, H. T.; Ku, Z.; Han, G.; Giovanni, D.; Mathews, N.; Fan, H. J.;

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

Sum, T. C. Discerning the Surface and Bulk Recombination Kinetics of OrganicInorganic Halide Perovskite Single Crystals. Adv. Energy Mater. 2016, 6, 1600551. (16) Zuo, Z.; Ding, J.; Zhao, Y.; Du, S.; Li, Y.; Zhan, X.; Cui, H. Enhanced Optoelectronic Performance on (110) Lattice Plane of MAPbBr 3 Single Crystal. J. Phys. Chem. Lett.

2017, 8, 684689. (17) 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-CH3 NH3 PbBr3 Perovskite Single Crystal. J. Phys. Chem. Lett. 2015, 6, 43044308. (18) Brittman, S.; Garnett, E. C. Measuring n and k at the Microscale in Single Crystals of CH3 NH3 PbBr3 Perovskite. J. Phys. Chem. C 2015, 120, 616620. (19) Kunugita, H.; Hashimoto, T.; Kiyota, Y.; Udagawa, Y.; Takeoka, Y.; Nakamura, Y.; Sano, J.; Matsushita, T.; Kondo, T.; Miyasaka, T. et al. Excitonic Feature in Hybrid Perovskite CH3 NH3 PbBr3 Single Crystals. Chem. Lett. 2015, 44, 852854. (20) Fang, H.-H.; Adjokatse, S.; Wei, H.; Yang, J.; Blake, G. R.; Huang, J.; Even, J.; Loi, M. A. Ultrahigh Sensitivity of Methylammonium Lead Tribromide Perovskite Single Crystals to Environmental Gases. Sci. Adv. 2016, 2, e1600534. (21) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. Photocarrier Recombination Dynamics in Perovskite CH 3 NH3 PbI3 for Solar Cell Applications. J.

Am. Chem. Soc. 2014, 136, 1161011613. (22) Liu, Y.; Sun, J.; Yang, Z.; Yang, D.; Ren, X.; Xu, H.; Yang, Z.; Frank Liu, S. 20mm-Large Single-Crystalline Formamidinium-Perovskite Wafer for Mass Production of Integrated Photodetectors. Adv. Opt. Mater. 2016, 4, 18291837. (23) Murali, B.; Dey, S.; Abdelhady, A. L.; Peng, W.; Alarousu, E.; Kirmani, A. R.; Cho, N.;

18


Page 18 of 22

Page 19 of 22

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


Sarmah, S. P.; Parida, M. R.; Saidaminov, M. I. et al. Surface Restructuring of Hybrid Perovskite Crystals. ACS Energy Letters 2016, 1, 11191126. (24) Yamada, Y.; Yamada, T.; Phuong, L. Q.; Maruyama, N.; Nishimura, H.; Wakamiya, A.; Murata, Y.; Kanemitsu, Y. Dynamic Optical Properties of CH 3 NH3 PbI3 Single Crystals as Revealed by One- and Two-photon Excited Photoluminescence Measurements. J.

Am. Chem. Soc. 2015, 137, 1045610459. (25) Yamada, T.; Yamada, Y.; Nakaike, Y.; Wakamiya, A.; Kanemitsu, Y. Photon Emission and Reabsorption Processes in CH 3 NH3 PbBr3 Single Crystals Revealed by TimeResolved Two-Photon-Excitation Photoluminescence Microscopy. Phys. Rev. Applied

2017, 7, 014001. (26) Sarmah, S. P.; Burlakov, V.; Yengel, E.; Murali, B.; Alarousu, E.; El-Zohry, A. M.; Yang, C.; Alias, M. S.; Zhumekenov, A. A.; Saidaminov, M. I. et al. Double Charged Surface Layers in Lead Halide Perovskite Crystals. Nano Lett. 2017, 17, 20212027. (27) Drouin, D.; Couture, A. R.; Joly, D.; Tastet, X.; Aimez, V.; Gauvin, R. CASINO V2.42-A Fast and Easy-to-use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users. Scanning 2007, 29, 92101. (28) Sermage, B.; Voos, M. Reabsorption of the Excitonic Luminescence in Direct Band Gap Semiconductors. Phys. Rev. B 1977, 15, 39353946. (29) Brillson, L. J. Applications of Depth-Resolved Cathodoluminescence Spectroscopy. J.

Phys. D: Appl. Phys. 2012, 45, 183001. (30) Gelhausen, O.; Phillips, M. R.; Toth, M. Depth-resolved Cathodoluminescence Microanalysis of Near-edge Emission in III-nitride Thin Films. J. Appl. Phys. 2001, 89, 35353537.

19



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

(31) Yamada, T.; Yamada, Y.; Nishimura, H.; Nakaike, Y.; Wakamiya, A.; Murata, Y.; Kanemitsu, Y. Fast Free-Carrier Diusion in CH 3 NH3 PbBr3 Single Crystals Revealed by Time-Resolved One- and Two-Photon Excitation Photoluminescence Spectroscopy.

Advanced Electronic Materials 2016, 2, 1500290. (32) Murali, B.; Dey, S.; Abdelhady, A. L.; Peng, W.; Alarousu, E.; Kirmani, A. R.; Cho, N.; Sarmah, S. P.; Parida, M. R.; Saidaminov, M. I. et al. Surface Restructuring of Hybrid Perovskite Crystals. ACS Energy Letters 2016, 1, 11191126. (33) Bischak, C. G.; Sanehira, E. M.; Precht, J. T.; Luther, J. M.; Ginsberg, N. S. Heterogeneous Charge Carrier Dynamics in Organic-Inorganic Hybrid Materials: Nanoscale Lateral and Depth-Dependent Variation of Recombination Rates in Methylammonium Lead Halide Perovskite Thin Films. Nano Lett. 2015, 15, 47994807. (34) Grancini, G.; D'Innocenzo, V.; Dohner, E. R.; Martino, N.; Srimath Kandada, A. R.; Mosconi, E.; De Angelis, F.; Karunadasa, H. I.; Hoke, E. T.; Petrozza, A. CH 3 NH3 PbI3 Perovskite Single Crystals: Surface Photophysics and its Interaction with the Environment. Chem. Sci. 2015, 6, 73057310. (35) Xiao, C.; Li, Z.; Guthrey, H.; Moseley, J.; Yang, Y.; Wozny, S.; Moutinho, H.; To, B.; Berry, J. J.; Gorman, B. P. et al. Mechanisms of Electron-Beam-Induced Damage in Perovskite Thin Films Revealed by Cathodoluminescence Spectroscopy. J. Phys. Chem.

C 2015, 119, 2690426911. (36) 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, 14301433. (37) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diusion Lengths

20


Page 20 of 22

Page 21 of 22

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


Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science

2013, 342, 341344. (38) Luryi, S.; Subashiev, A. V. Semiconductor Scintillator Based on Photon Recycling.

Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 652, 292294. (39) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH 3 NH3 PbI3−x Clx .

J. Phys. Chem. Lett. 2014, 5, 13001306. (40) Yang, Y.; Yang, M.; Li, Z.; Crisp, R.; Zhu, K.; Beard, M. C. Comparison of Recombination Dynamics in CH 3 NH3 PbBr3 and CH3 NH3 PbI3 Perovskite Films: Inuence of Exciton Binding Energy. J. Phys. Chem. Lett. 2015, 6, 46884692. (41) Galkowski, K.; Mitioglu, A.; atsuhiko Miyata,; paulina Plochocka,; oliver Portugall,; Eperon, G. E.; Wang, J. T.-W.; Stergiopoulos, T.; Stranks, S. D.; Snaith, H. et al. Determination of the Exciton Binding Energy and Eective Masses for Methylammonium and Formamidinium Lead Tri-halide Perovskite Semiconductors. Energy Environ. Sci.

2016, 9, 962970. (42) Tilchin, J.; Dirin, D. N.; Maikov, G. I.; Sashchiuk, A.; Kovalenko, M. V.; Lifshitz, E. Hydrogen-like Wannier-Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite. ACS Nano 2016, 6, 63636371. (43) Ahrenkiel, R. K.; Lundstrom, M. S. Minority Carriers in III-V Semiconductors:

Physics and Applications, Semiconductors and Semimetals, Volume 39 ; Academic Press, Boston, 1993. (44) Asbeck, P. Self-Absorption Eects on the Radiative Lifetime in GaAs-GaAlAs Double Heterostructures. J. Appl. Phys. 1977, 48, 820822.

21



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

(45) Staub, F.; Hempel, H.; Hebig, J.-C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Applied 2016, 6, 044017. (46) Pereira, E.; Berberan-Santos, M.; Martinho, J. Unidimensional Simulation of Radiative Transport of Electronic Energy. J. Lumin. 1995, 63, 259271. (47) Guy, S. Modelization of Lifetime Measurement in the Presence of Radiation Trapping in Solid-State Materials. Phys. Rev. B 2006, 73, 144101. (48) Stoita, A.; Vautey, T.; Jacquier, B.; Guy, S. Impact of the Radiation Trapping on Lifetime Measurement of Er 3+ Doped Glasses. J. Lumin. 2010, 130, 11191123.

22


Page 22 of 22

Impact of Reabsorption on the Emission Spectra ... - ACS Publications

Recommend Documents