Light Emission Enhancement by Tuning the Structural Phase of APbBr3

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Spectroscopy and Photochemistry; General Theory

Light Emission Enhancement by Tuning the Structural Phase of APbBr (A = CHNH, Cs) Perovskites 3

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Yung Ji Choi, DEBBICHI LAMJED, Do-Kyoung Lee, Nam-Gyu Park, Hyungjun Kim, and Dongho Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00829 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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The Journal of Physical Chemistry Letters

Light emission enhancement by tuning the structural phase of APbBr3 (A = CH3NH3, Cs) perovskites Yung Ji Choi,† Lamjed Debbichi, ‡ Do–Kyoung Lee, § Nam-Gyu Park,*, § Hyungjun Kim,*, ‡ Dongho Kim*,† †

Department of Chemistry, Yonsei University, Seoul 03722, Korea

‡

Graduate School of EEWS and Department of Chemistry, Korea Advanced Institute of Science

and Technology (KAIST), Daejeon 34141, Korea §

School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Korea

AUTHOR INFORMATION Corresponding Author *N.-G.P. Tel: +82 31 290 7241, E-mail: [email protected] *H.K. Tel: +82 42 350 1725, E-mail: [email protected] *D.K. Tel: +82 2 2123 2652, E-mail: [email protected]

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ABSTRACT: Lead halide perovskite (APbX3) has recently emerged as a promising active layer in light-emitting diodes (LEDs) as well as an absorber for photovoltaic devices. For better LED properties, it is important to understand the fundamental mechanism of the optoelectronic behaviors, e.g., how the nanostructure of APbX3 thin film correlates with its emitting properties. We investigated the effect of APbBr3 (A=CH3NH3,Cs) crystallite size on the photophysical properties regarding its crystallographic changes and spin-orbit coupling. Photoluminescence lifetime measurements, X-ray and electron diffraction analyses, and density functional theory calculations were performed. We demonstrate that the emitting properties of mesoscale APbBr3 crystallites are improved due to the formation of pure cubic phase that leads to the spin- and momentum-allowed carrier recombination. Our findings provide fundamental insights into the emitting behavior of APbBr3, which suggests a control of its optoelectronic properties by means of modulating the crystal morphology and resultant electronic band structures.

TOC Graphic.

KEYWORDS Lead halide perovskite, spin-orbit coupling, Rashba spitting, crystal structure, density functional theory, spectroscopy


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Lead halide perovskites (APbX3; A = CH3NH3 (MA), HC(NH2)2, or Cs, and X = Cl, Br, or I) have recently attracted significant research attention in fundamental, technological, and industrial fields due to their unique optoelectronic properties1–3 as well as a rapid increase in their photovoltaic power conversion efficiencies.4,5 In recent few years, APbX3 has also become a promising active layer of light-emitting diodes (LEDs) in virtue of its color purity, easily tunable band gap, and low fabrication cost.6–13 Especially, MAPbBr3-based green LEDs have largely reduced the technical gap between perovskite LEDs (PeLEDs) and organic or quantum dot LEDs.8– 10

On top of that, PeLEDs with the use of all-inorganic CsPbBr3 as an active layer showed a

breakthrough in terms of the stability and brightness of PeLEDs, reaching the current efficiency (CE) of 25.6 cd/A and external quantum efficiency (EQE) of 5.7%.11–13 Massive studies have been performed to enable a better understanding and further improve the luminescence of APbX3, which demonstrates that reducing the size of APbX3 crystallite below a hundred nanometer is a prerequisite for superior photo- and electroluminescence (PL and EL) properties, and efficient LEDs.8–13 These researches anticipated the spatially limited charge carriers and the promoted formation of exciton in the perovskite nanocrystallites. However, those with a size range that significantly exceeds a Bohr radius (r0) albeit below 100 nm exhibit ambiguous excitonic features. In order to explain this, several possibilities including quantum confinement effect,9–13 the effect of band gap,14 surface effect,15 and alignment of permanent dipole moment of A cation16 have been proposed; nevertheless, there is an absence of consensus on the fundamental mechanistic origin. To find the origin of the morphological effect on light-emitting properties, in the present study, the crystal structures and the electronic band structures were investigated. Especially, we focused on the change in inversion symmetry that is intimately related to the existence of Rashba splitting.


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Rashba splitting, which appears due to the strong spin-orbit coupling (SOC) effect, is known to provide a spin-forbidden recombination path by invoking a spin or momentum mismatch.17,18 This topic, the presence of Rashba couplings in APbX3 perovskites, has been recently issued as a field of broad interest.19 However, it is still controversial and of ongoing debate: Some researchers have studied on DFT calculations, and ARPES and magneto-optical measurements to prove the existence,17,20-22 but others suspect these arguments or limit its scope to only at surfaces.23,24 In this regard, it is important to understand the conditions where the Rashba effect occurs or not.19 Our crystallographic measurements and density functional theory (DFT) calculations indicate that the pure cubic structure (centrosymmetry) is effectively formed by changing the crystallite size into the mesoscale regime (> 2r0 and < 100 nm), which can significantly suppresses the Rashba splitting.18 This enables a spin- and momentum-allowed transition, resulting in improved lightemitting properties with superior luminescence quantum yield and faster radiative recombination, all of which are advantageous from a practical point of view as optoelectronic device applications. Morphology of stoichiometric and nonstoichiometric APbBr3 thin films. APbBr3 perovskite thin films (A = MA or Cs) with different sizes of crystallites were prepared (Figure 1) by adduct synthetic approaches as we previously reported (see Experimental Procedures, and Figure S1 for details of sample preparation).4,10 As shown in Figure S2, scanning electron microscopy (SEM) images represent the morphology of stoichiometric and nonstoichiometric MAPbBr3 (or CsPbBr3) thin films. While the stoichiometric MAPbBr3 (or CsPbBr3) sample shows a bulk polycrystallinity with average sizes of 250 (or 170) nm, the nonstoichiometric MAPbBr3 (or CsPbBr3) sample does not exhibit distinct grain boundary, hampering the measurement of the exact crystallite size from SEM images. The stoichiometric and nonstoichiometric MAPbBr3 (or CsPbBr3) thin films possess thicknesses of 204 and 212 nm (or 62 and 74 nm for CsPbBr3), respectively.


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The nanostructure and selective area electron diffraction (SAED) patterns of MAPbBr3 and CsPbBr3 thin films were investigated by spherical aberration corrected scanning transmission electron microscope (STEM) after samples were vertically etched by using focused ion beam (FIB) equipment (Figure S3). As shown in the high-resolution TEM images (Figure 1b-e), the nonstoichiometric MAPbBr3 (or CsPbBr3) sample shows a nanocrystallinity with an average size of 13 nm, while the stoichiometric sample does not exhibit any distinct features at this length scale (Figures S3 and S4). We thus denote stoichiometric and nonstoichiometric samples as bulk and mesoscale samples, respectively, to reflect the characteristics of their crystallite sizes (Figure S5). From the lattice fringes shown in the fast Fourier transformed (FFT) images (inset of Figure 1d,e), we find that the nanocrystalline domain of the mesoscale MAPbBr3 (or CsPbBr3) samples possesses inter-planar spacings of 0.34 and 0.31 nm (or 0.41 and 0.29 nm for CsPbBr3) that are indexed as (111) and (200) planes of MAPbBr3 (or (110) and (200) planes of CsPbBr3) crystals, respectively. These typical inter-planar spacings agree with the values of cubic APbBr3 perovskite structures obtained from our DFT calculations (Figure S6) and also with our X-ray diffraction (XRD) measurements that will be discussed later. All the structural and photophysical analyses in this work were conducted at room temperature (298K). Photophysical properties of bulk and mesoscale APbBr3 perovskites. Steady-state absorption spectra of bulk APbBr3 (A= MA, Cs) samples were measured (Figure 1f,g), where the onset values were 2.27 and 2.30 eV for MAPbBr3 and CsPbBr3, respectively, which agrees with the band gaps of MAPbBr3 and CsPbBr3 (2.2~2.3 eV);25-27 however, with respect to the mesoscale samples, the onset shows a slight blue-shift tendency. Especially, an excitonic component of transition, a sharp peak near the band edge, was significantly bleached in mesoscale sample regardless of the cation choice between MA and Cs (Figure 1f,g).16 The excitonic feature is distinguished from the


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absorption of continuum or free carriers by means of absorption spectra fitting using Elliot’s model (see Figure S7 and Notes in Supplemental Information).28 Recently, Hoffman et al.29 reported the similar absorption behavior – excitonic absorption grows sharply as CsPbBr3 nanocrystals are merged into a larger crystal during sintering process, which cannot be explained by quantum confinement effect.15 Independently of the A cation species, the mesoscale samples exhibited bright green PL emission (Figure 2a) under UV light exposure at 363 nm, while bulk samples exhibited weak PL emission. The steady-state PL spectrum and quantum yield of both bulk and mesoscale APbBr3 films were measured with photoexcitation at 470 nm (Figures 2b and 2c). Mesoscale APbBr3 films exhibited a drastic increase in PL intensity (Figure 2b) and quantum yield (132 and 42 times for A= MA and Cs, respectively) compared to bulk samples (Figure 2c). The PL maxima were shown at 545 and 538 nm (or 528 and 524 nm) for bulk and mesoscale MAPbBr3 (or bulk and mesoscale CsPbBr3). As the PL spectral shifts are not so significant (30 and 18 meV for MA and Cs, respectively), we can conclude that the strong quantum confinement effect is not dominant in the size range of our mesoscale crystalline samples (> 2r0). As shown in Figure 2d, the PL decay profiles of APbBr3 films were measured in Ar-purged atmosphere at the PL emission maximum of each sample by a time-correlated single photon counting (TCSPC) method. The decay profile of the bulk sample exhibits biexponential decay with a fast initial decay (τ1 ~ 5.5 and 6.1 ns for MAPbBr3 and CsPbBr3) followed by a much slower decay arising from well-known free carrier recombination in bulk perovskites (τ > 10 ns);10 the detailed fitting parameters are listed in Table S1. However, the fast decay components (τ ~ 5 ns) were manifested in mesoscale samples with a portion over 90%, irrespective of A cation species, which is indicative of the distinct charge recombination pathway of the mesoscale samples.


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Especially, as mentioned above, the mesoscale samples represent superior PL emission properties compared with the bulk samples, and this infers the accelerated radiative recombination pathway in the mesoscale samples.14 Furthermore, the excitation density dependent PL lifetime measurements have been conducted to elucidate the underlying mechanisms of PL increase and to provide a more comprehensive kinetic model. Figure S8 shows the PL lifetimes of our samples by modulating the excitation density. The relationship between the average PL lifetimes and the excitation density (n) is given as,30 𝜏"# =

% &

=

% '()*+, -

(1)

By fitting this equation to our experimental data (dotted lines in Figure S8), we evaluated both the monomolecular trapping rate (A) and the intrinsic bimolecular radiative recombination coefficient (Brad). We have found that the mesoscale crystallites, irrespective of A cation species, exhibit a much higher Brad of 5.28 x 10-9 s−1cm3 for MAPbBr3 (or 5.43 x 10-9 s−1cm3 for CsPbBr3) with respect to Brad of 3.26 x 10-10 s−1cm3 for bulk MAPbBr3 sample (or 4.41×10−10 s−1cm3 for bulk CsPbBr3). As Brad is known as intrinsic property related to the electronic structures of materials,31,32 we concluded that an inherent change of electronic structures, leading to allowed (radiative) transition, occurs. This indeed results in the shorter PL lifetime and a larger quantum yield. It should be noted that the monomolecular trapping rate (A) is even increased from 3.84×107 s−1 to 1.16 × 108 s−1 as the MAPbBr3 crystallite dimension decreases, which indicates that the improved quantum yield of mesoscale MAPbBr3 sample is essentially attributed to the increased Brad. In mesoscale CsPbBr3 sample, however, the lowered A, from 5.94× 107 s−1 to 4.26 × 107 s−1, can be one of the reasons for improving its emission property at low excitation carrier density (n

Light Emission Enhancement by Tuning the Structural Phase of APbBr3

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