- Email: [email protected]
ZnS quantum dots on temperature-dependent luminescence properties in mixed halide perovskites
Journal Pre-proof Effect of CdSe/ZnS quantum dots on temperature-dependent luminescence properties in mixed halide perovskites Il-Wook Cho, Mee-Yi Ryu PII:
S0022-2313(19)31781-8
DOI:
https://doi.org/10.1016/j.jlumin.2019.116940
Reference:
LUMIN 116940
To appear in:
Journal of Luminescence
Received Date: 10 September 2019 Revised Date:
27 November 2019
Accepted Date: 2 December 2019
Please cite this article as: I.-W. Cho, M.-Y. Ryu, Effect of CdSe/ZnS quantum dots on temperaturedependent luminescence properties in mixed halide perovskites, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116940. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of CdSe/ZnS quantum dots on temperature-dependent luminescence properties in mixed halide perovskites Il-Wook Cho and Mee-Yi Ryu*
Department of Physics, Kangwon National University, Kangwon-Do 24341, Korea *
Corresponding author: [email protected]
Abstract The temperature-dependent dynamics of organic cations are known to influence the optical properties of lead halide perovskites (CH3NH3PbX3) significantly. The Rashba splitting effect and internal electric field induced due to the strong spin-orbit coupling and absence of inversion symmetry in the lead halide perovskites (PS) lead to a decrease in the radiative recombination rates. This study has investigated the luminescence properties of the mixed halide PS (CH3NH3PbI2Br) covered with CdSe/ZnS quantum dots (QDs) (QD/PS hybrid structure) using temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. A shorter radiative lifetime obtained in the QD/PS hybrid structures as compared to the bare PS film was attributed to the screening effects of the Rashba splitting and internal electric field due to the charge carrier transfer from QDs to PS. The PL intensity and the PL decay times of the QD/PS hybrid structures increased when compared to those of the bare PS film because of the surfacepassivation and charge transfer (CT) effects. The CT from the QDs to PS leads to an enhancement in the radiative recombination due to the enhanced oscillator strength by the screened internal electric field and Rashba splitting, which is induced by an increase in the charge carriers. 1
Introduction The dynamics of organic molecules play a central role in determining the optoelectronic properties of organic-inorganic lead halide perovskites (PS). Extensive theoretical studies on the motion of organic cations have been conducted over the last few years [1–6]. In particular, the temperature-dependent rotational dynamics in lead halide PS are discussed by Mattoni et al. [4]. As the temperature increases, the distortion of the lead (Pb)-iodine (I) octahedra in CH3NH3PbI3 induces a higher degree of symmetry, leading to the phase transition at a finite temperature at which the rotation of organic molecules is significant [4]. The dynamics of organic cations induce longer carrier lifetimes due to the carrier localization effect [7], ferroelectric effect [8,9], and Rashba effect [2,5]. A random orientation of the organic molecules in PS causes an electrostatic potential fluctuation and ferroelectric polarization, leading to the spatial separation of electrons and holes [7]. In addition, the rotational motion of the CH3NH3+ cation creates a strong internal electric field due to its dipole moment, which causes a reduced band gap and charge separation in the PS. The Rashba band splitting effect has also been used to explain the long carrier lifetimes. Lead halide PS has a strong spin-orbit coupling (SOC) due to heavy elements such as Pb, I, and bromine (Br). The strong SOC effect and the breaking of inversion symmetry give rise to splitting of the degenerate valence band (VB) and conduction band (CB) near the bandgap into opposite directions for each band. This splitting in the degenerate bands for these halide PS creates an indirect band gap, resulting in a slow recombination process due to the different momentum states of the photo-generated
2
electrons and holes. These processes often deteriorate the performance of optoelectronic devices. Here, we investigated the effect of CdSe/ZnS core-shell quantum dots on the dynamics of organic cation in the lead halide PS (CH3NH3PbI2Br). Previously, we reported the synthesis of a quantum dots-perovskite heterostructure (QD/PS hybrid structure) [10]. In this hybrid structure, the photo-excited electrons and holes were transferred from QDs to the PS [10], and the internal electric field induced by the motion of the organic cation can be reduced by the screening effect of the charge carriers migrated from QDs. In addition, the surface passivation effects, including protection of surface and decrease in the defect states, were demonstrated [10]. Thus, the hybrid structures using CdSe/ZnS QDs presented a promising approach to improve the optical properties of the PS films. In this study, we investigated the temperature-dependent luminescence properties of QD/PS hybrid structure by using photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. To analyze the effects of CdSe/ZnS core-shell QDs, the PL and TRPL results of QD/PS hybrid structure were compared with those of the pristine PS film (bare PS). The integrated PL intensities were much larger in the QD/PS hybrid structures than in the bare PS at all temperatures, and the PL decay times in the QD/PS hybrid structures were also longer than those in the bare PS, at temperatures higher than 200 K. In addition, the optical properties of the PS were altered by controlling the energy level alignment between QDs and PS, which is achieved by varying the diameter of the CdSe core structure.
3
Experimental Details Indium tin oxide (ITO)-treated glass substrates were washed by an ultrasonic cleaner with acetone and subsequently with isopropyl alcohol. The substrates were then dried using an air blower. The cleaned ITO substrates were treated with ultraviolet-ozone at 100 °C for 15 min. The methylammonium bromide (120 mg, Great Cell Solar Ltd.) and lead iodide (480 mg, 99%, Sigma-Aldrich) powders were dissolved in a mixture of dimethyl sulfoxide (1 ml, ≥99.5%, Sigma-Aldrich) and N,N-dimethylformamide (2 ml, ≥99.9%, Sigma-Aldrich), and the PS precursor solution was prepared by stirring this mixture at 60 °C for 1 h. The PS precursor solution was coated by a two-step spin-coating process onto the ITO substrates; the first and the second steps were performed at 1000 rpm for 10 s and 5000 rpm for 20 s, respectively. During the second step, toluene (Sigma-Aldrich) was dropped onto the surface. Finally, the PS film (CH3NH3PbI2Br) was annealed at 100 °C for 2 min. A CdSe/ZnS core–shell QD powder (5 mg, PlasmaChem GmbH) was dissolved in toluene (1 ml) and stirred at 60 °C for 1 h. The CdSe/ZnS QD solution was then spincoated onto the PS films at 3000 rpm for 30 s, and the QD/PS hybrid structure was finally annealed at 60 °C for 2 min. The QD/PS hybrid structures were synthesized with different CdSe core diameters (dQD = 2.7 and 4.2 nm) in the CdSe/ZnS core-shell QDs . X-ray diffraction (XRD) analysis and field-emission scanning electron microscopy (FE-SEM) were performed by using an X’pert Pro X-ray diffractometer (Malvern Panalytical) and a Supra55VP (Zeiss) scanning electron microscope to examine the crystallinity and surface morphology of the PS, respectively. The PL spectra of the samples were acquired using the 325-nm continuous-wave laser, as an excitation source, 4
equipped with an Andor DV420A-BU2 CCD detector. The TRPL spectra of the samples were collected by a microchannel plate photomultiplier tube (FLS 920, Edinburgh Instrument), where a 375-nm pulsed-diode laser (pulse width = 90 ps) was used as an excitation source.
Results and discussion Figure 1(a) shows the XRD pattern of the bare PS film. The characteristic PS diffraction peaks are clearly visible 2θ values of 14.3° and 28.8°, which were assigned to the (100) and (200) planes of the cubic phase of PS, respectively [11]. The surface morphology of the bare PS film was studied using SEM images, as shown in the inset of Fig. 1(a). The temperature-dependent PL spectra of the bare PS film and the QD/PS hybrid structure with dQD of 2.7 nm are shown in the Figs. 1(b) and (c), respectively. The temperaturedependent PL of the QD/PS hybrid structure with dQD of 4.2 nm (not shown here) shows a similar behavior as the hybrid structure with dQD of 2.7 nm. At 300 K, the PL spectra of the bare PS (QD/PS hybrid structure) exhibited two PL peaks at approximately 1.677 eV and 1.804 eV (1.691 eV and 1.805 eV), which were assigned to the emission from the cubic (PC) and the tetragonal (PT) phases of PS, respectively. The formation of PT in the bare PS and the hybrid structure is attributed to the photo-induced phase segregation, which is the light-soaking effect [13–15]. The increase in the PL peak energy of the PC in the hybrid structure relative to the bare PS film can be attributed to the decrease in the defect states due to the surface passivation by QDs [12] and to the reduced internal 5
electric field screened by charge carriers transferred from the QDs to the PS layer. In the mixed halide PS (CH3NH3PbI2Br) materials, the CB and the VB split into two spinpolarized bands in k-space due to the Rashba effect caused by a strong SOC from the heavy elements (Pb, I, and Br) and the internal electric field induced by the absence of inversion symmetry [5, 16]. In addition, a dynamical Rashba effect is observed in CH3NH3PbI2Br due to the distortion of lead-halide bonds and thermal vibrations of the organic (CH3NH3+) ions [17]. The PL intensity of the PC in the QD/PS hybrid structure was observed to be more dominant than that of the PC in the bare PS, which is attributed to the combined effect of the surface passivation and charge carrier transfer from QDs. Therefore, the following discussion will focus on the PC peak because the main contribution of the luminescence mechanism is arising from the cubic phase of PS, as seen in the XRD pattern and the PL spectra shown in Fig. 1. In addition, the phase transformation from PC to PT occurred at the critical temperatures of 180 K and 160 K in the bare PS and the QD/PS hybrid structures, respectively. The temperature-dependent integrated PL intensities of the PC in the bare PS and the QD/PS hybrid structures are shown in Fig. 2(a). At 300 K, the integrated PL intensity of the QD/PS hybrid structure was two orders of magnitude higher than that of the bare PS due to the charge transfer (CT) from QDs to the PS layer. As the temperature increased from 160 to 300 K, the integrated PL intensities of the bare PS and the QD/PS hybrid structures decreased gradually. Such a behavior can be attributed to the increase in the non-radiative recombination process. It is noteworthy that the integrated PL intensities of the QD/PS hybrid structures decreased at a much slower rate with an increase in the 6
temperature, compared to that of the bare PS films, which can be explained by the CT from QDs to PS [10]. Figure 2(b) shows the PL peak energies of the bare PS and the QD/PS hybrid structures. As the temperature increased from 180 to 240 K, the PL peak energy of the bare PS was redshifted to approximately 14 meV. This redshift is ascribed to the reduction of the band gap in PS due to the internal electric field induced by the rotational dynamics of the CH3NH3+ cation. The PL peak energy of the QD/PS hybrid structure with dQD of 4.2 nm was redshifted to approximately 10 meV with an increase in the temperature; this shift was smaller than that of the bare PS. In addition, no redshift in the PL peak energy of the QD/PS hybrid structure with dQD of 2.7 nm was observed with an increase in the temperature from 160 to 240 K, which can be explained by the completely screened internal field due to the higher CT rate from QDs to PS compared with that of the hybrid structure with dQD of 4.2 nm. The CT efficiencies of 63% and 44% from QDs to the PS layer were determined in the QD/PS hybrid structures with dQD of 2.7 and 4.2 nm, respectively, at 300 K using the PL decay times of the QD peak in the bare QDs and the QD/PS hybrid structure [18]. The blueshift of the PL peak for all the samples at higher temperatures (> 240 K) is attributed to the thermal expansion of the lattice [19]. The temperature-dependent integrated PL intensities of the PT in the bare PS and the QD/PS hybrid structures are also shown in Fig. S1(a). (See Supplementary Information (SI)). The integrated PL intensities of the PT in the QD/PS hybrid structures decreased at a much slower rate compared to that in the bare PS films, which can be explained by reduced light-soaking effect caused by the surface passivation effect. As 7
shown in Figs. S1(b) and (c), the PT peak in the bare PS was relatively dominant than the PC while the PC in the QD/PS hybrid structure was more dominant than the PT. In addition, the PT peak in the bare PS and the QD/PS hybrid structure was blueshifted due to thermal expansion of the lattice with increasing temperature from 160 to 300 K as shown in Fig. S1(d) in SI. Figures 3(a) and (b) show the temperature-dependent PL decay curves measured for the PC in the bare PS and the QD/PS hybrid structures, respectively. The PL decay in the bare PS hastens with an increase in the temperature from 180 to 300 K; whereas, the PL decay in the QD/PS hybrid structures slows gradually, as the temperature increases from 160 to 300 K. The QD/PS hybrid structure with dQD of 4.2 nm exhibited a similar temperature-dependent PL decay behavior as that of the hybrid structure with dQD of 2.7 nm (not shown here). The average decay times were estimated by τave=∑Aiτi2/∑Aiτi using the fast and slow decay times (τ1 and τ2) and the corresponding pre-exponential constants (A1 and A2), which were determined by using I(t) = A1 exp(-t/τ1) + A2 exp(-t/τ2). The average decay times obtained at the PL decay peak of the PC in the bare PS and the QD/PS hybrid structures as functions of temperature are shown in Fig. 4(a). While the PL decay time for the bare PS film gradually decreased from 4.68 to 1.44 ns with increasing temperatures (180–300 K), the QD/PS hybrid structures with dQD of 2.7 and 4.2 nm displayed an increase in the PL decay times from 4.28 to 6.04 ns and 4.07 to 4.53 ns, respectively, with an increase in the temperature from 160 to 300 K. Figures 4(b) and (c) show the temperature-dependent radiative (τR) and non-radiative lifetimes (τNR), respectively, which are estimated using the relationships: η(T) = τPL(T)/τR ≈ I(T)/I0, and 8
1/τPL(T) = 1/τR(T) + 1/τNR(T), where η is the internal quantum efficiency; I is the integrated PL intensity; and τPL is the PL decay time. The experimental values of the integrated PL intensity and the PL decay time shown in Figs. 2(a) and 4(a) were used for the calculation of I(T) and τPL(T), respectively. Here, we assumed η(Tmax) = 1 and I(Tmax) = I0, where Tmax is the temperature with the maximum integrated PL intensity. At 300 K,τR for the bare PS was much longer than for the QD/PS hybrid structures, which is discussed a little later. We observed an increase in the τR and decrease in the τNR values with an increase in the temperature for all samples, which is a typical temperaturedependent behavior of semiconductors. Therefore, the PL decay times of the QD/PS hybrid structures were mainly dominated by τR at all temperatures, whereas the PL decay time of the bare PS was dominated by τNR. The increase in the PL decay time of the QD/PS hybrid structures with increasing temperatures can be explained by the CT from QDs to PS [10]. In contrast, the decrease in the PL decay time of the base PS with increasing temperature was attributed to an increase in the non-radiative recombination processes caused by the phonon scattering, thermally escaped carriers, and/or the trapped carriers in the defect states [20]. It is important to note that the radiative lifetimes of the bare PS were longer than those of the QD/PS hybrid structures in all temperature regions as seen in Fig. 4(b). The slow recombination rate (longer lifetime) in halide PS materials was ascribed to the Rashba effect and/or charge separation due to potential fluctuations caused by the random orientation of the dipole moments of the organic molecules (CH3NH3+) [7]. Therefore, a longer radiative lifetime τR observed in the bare PS relative to that of the hybrid structures 9
can be attributed to the stronger Rashba effect and larger potential fluctuations, which were screened in the QD/PS hybrid structures by the charge carriers transferred from QDs to the PS layer. In addition, an increase in the radiative lifetime with increasing temperatures can be explained by the larger potential fluctuations at higher temperatures, which hinder the delocalization of photo-generated carriers and decrease the electron-hole recombination [21]. The higher integrated PL intensities and the faster radiative recombination rates in the QD/PS hybrid structures compared with those in the bare PS are ascribed to an increase in the capture cross-section (increase of delocalized carriers) due to the CT from QDs to the PS layer.
Conclusions The temperature-dependent photoluminescence (PL) and time-resolved PL studies were carried out in the mixed halide perovskite (CH3NH3PbI2Br) films with CdSe/ZnS core-shell QDs in order to investigate the effects of QDs on the optical properties of PS films. The integrated PL intensity of the QD/PS hybrid structures decreased at a much slower rate with increasing temperatures from 180 to 300 K than that of the bare PS, which is attributed to the weakened Rashba effect and the reduced charge separation due to the charge transfer from QDs to the PS layer. The QD/PS hybrid structures exhibited the enhanced PL intensities and decay times (fast recombination rate) compared with those of the bare PS. This observation suggests an improvement in the charge recombination of the QD/PS hybrid structures by the enhanced oscillator strength, caused by the decreased internal electric field and the reduced Rashba effect 10
observed due to the carrier migration from QDs to the PS, and a reduction in the defect states by the surface passivation effect. Our results help in identifying the role of CdSe/ZnS QDs in the luminescence mechanisms of PS films, and the QDs can be used as an effective material for the surface passivation and the enhancement of optical properties. Therefore, the QD/PS hybrid structures are powerful prospective candidates for PS-based optoelectronic applications.
Figures and Captions
Figure 1. (a) XRD patterns of the bare PS film. Inset shows the SEM image of the bare PS. Temperature-dependent PL spectra of (b) the bare PS and (c) the QD/PS hybrid structure with dQD of 2.7 nm. The PL peaks of the PC (red lines) and the PT (blue lines) were extracted using the Gaussian fitting.
Figure 2. Temperature-dependent (a) integrated PL intensities and (b) PL peak energies for the PC of the bare PS and the QD/PS hybrid structures.
Figure 3. PL decay curves acquired for the PC in (a) the bare PS and (b) the QD/PS hybrid structures with dQD of 2.7 nm as functions of temperature.
Figure 4. Temperature-dependent (a) PL decay times, (b) radiative lifetimes, and (c) nonradiative lifetimes of the bare PS and the QD/PS hybrid structures. Each PL decay time was obtained at the PC peak energy of the respective temperature.
Corresponding Author *M.-Y. R. E-mail: [email protected]
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Notes The authors declare no competing financial interests. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2019R1A2C1086813). Field-emission scanning electron microscopy analyses were performed at the Korea Basic Science Institute. Time-resolved photoluminescence measurements were performed at the Central Lab of Kangwon National University.
References [1] C. Motta, F. El-Mellouhi, S. Kais, N. Tabet, F. Alharbi, S. Sanvito, Nat. Commun. 6 (2015) 7026. [2] F. Zheng, L.Z. Tan, S. Liu, A.M. Rappe, Nano Lett. 15 (2015) 7794-7800. [3] T. Chen, B.J. Foley, B. Ipek, M. Tyagi, J.R.D. Copley, C.M. Brown, J.J. Choi, S.-H. Lee, Phys. Chem. Chem. Phys. 17 (2015) 31278-31286. [4] A. Mattoni, A. Filippetti, M.I. Saba, P. Delugas, J. Phys. Chem. C, 119 (2015) 17421-17428. [5] T. Etienne, E. Mosconi, F. De Angelis, J. Phys. Chem. Lett., 7 (2016) 1638-1645. [6] Z. Xiao, Y. Yan, Adv. Energy Mater. 7 (2017) 1701136. [7] J. Ma, L.-W. Wang, Nano Lett. 15 (2015) 248-253. [8] Z. Fan, J. Xiao, K. Sun, L. Chen, Y. Hu, J. Ouyang, K.P. Ong, K. Zeng, J. Wang, J. Phys. Chem. Lett., 6 (2015) 1155-1161. [9] S.M. Vorpahl, R. Giridharagopal, G.E. Eperon, I.M. Hermes, S.A.L. Weber, D.S. Ginger, ACS Appl. Energy Mater. 1 (2018) 1534-1539. [10] I.-W. Cho, M.-Y. Ryu, APL Mater. 7 (2019) 051112. 12
[11] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Nano Lett., 13 (2013) 1764-1769. [12] M. Kim, S.G. Motti, R. Sorrentino, A. Petrozza, Energy Environ. Sci. 11 (2018) 2609-2619. [13] S.J. Yoon, S. Draguta, J.S. Manser, O. Sharia, W.F. Schneider, M. Kuno, P.V. Kamat, ACS Energy Lett. 1 (2016) 290-296. [14] D.J. Slotcavage, H.I. Karunadasa, M.D. McGehee, ACS Energy Lett. 1 (2016) 1199-1205. [15] M.C. Brennan, S. Draguta, P.V. Kamat, M. Kuno, ACS Energy Lett. 3 (2018) 204-213. [16] Y. Zhai, S. Baniya, C. Zhang, J. Li, P. Haney, C.-X. Sheng, E. Ehrenfreund, Z.V. Vardeny, Sci. Adv. 3 (2017) e1700704. [17] G. Rajendra Kumar, A. Dennyson Savariraj, S.N. Karthick, S. Selvam, B. Balamuralitharan, H.-J. Kim, K.K. Viswanathan, M. Vijaykumar, K. Prabakar, Phys. Chem. Chem. Phys. 18 (2016) 7284-7292. [18] I.-W. Cho and M.-Y. Ryu, submitted to journal. [19] M.I. Dar, G. Jacopin, S. Meloni, A. Mattoni, N. Arora, A. Boziki, S.M. Zakeeruddin, U. Rothlisberger, M. Grätzel, sci. adv. 2 (2016) e1601156. [20] D. Valerini, A. Cretí, M. Lomascolo, L. Manna, R. Cingolani, M. Anni, Phys. Rev. B 71 (2005) 235409. [21] K.T. Munson, G.S. Doucette, E.R. Kennehan, J.R. Swartzfager, J.B. Asbury, J. Phys. Chem. C, 123 (2019) 7061-7073.
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Highlights -
Temperature-dependent luminescence properties of perovskite (PS) were investigated.
-
Improved optical properties of surface-passivated PS films by quantum dots (QDs).
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Enhanced optical properties in the QD/PS due to the charge transfer from QDs to PS.
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Reduced Rashba effect and internal electric field in the QD/PS structures.
Author Statement M.-Y. R. conceived the idea presented in the text. I.-W. C. carried out all the experiments. Both authors contribute equally and approve the submission of the manuscript.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2313(19)31781-8
DOI:
https://doi.org/10.1016/j.jlumin.2019.116940
Reference:
LUMIN 116940
To appear in:
Journal of Luminescence
Received Date: 10 September 2019 Revised Date:
27 November 2019
Accepted Date: 2 December 2019
Please cite this article as: I.-W. Cho, M.-Y. Ryu, Effect of CdSe/ZnS quantum dots on temperaturedependent luminescence properties in mixed halide perovskites, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116940. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of CdSe/ZnS quantum dots on temperature-dependent luminescence properties in mixed halide perovskites Il-Wook Cho and Mee-Yi Ryu*
Department of Physics, Kangwon National University, Kangwon-Do 24341, Korea *
Corresponding author: [email protected]
Abstract The temperature-dependent dynamics of organic cations are known to influence the optical properties of lead halide perovskites (CH3NH3PbX3) significantly. The Rashba splitting effect and internal electric field induced due to the strong spin-orbit coupling and absence of inversion symmetry in the lead halide perovskites (PS) lead to a decrease in the radiative recombination rates. This study has investigated the luminescence properties of the mixed halide PS (CH3NH3PbI2Br) covered with CdSe/ZnS quantum dots (QDs) (QD/PS hybrid structure) using temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. A shorter radiative lifetime obtained in the QD/PS hybrid structures as compared to the bare PS film was attributed to the screening effects of the Rashba splitting and internal electric field due to the charge carrier transfer from QDs to PS. The PL intensity and the PL decay times of the QD/PS hybrid structures increased when compared to those of the bare PS film because of the surfacepassivation and charge transfer (CT) effects. The CT from the QDs to PS leads to an enhancement in the radiative recombination due to the enhanced oscillator strength by the screened internal electric field and Rashba splitting, which is induced by an increase in the charge carriers. 1
Introduction The dynamics of organic molecules play a central role in determining the optoelectronic properties of organic-inorganic lead halide perovskites (PS). Extensive theoretical studies on the motion of organic cations have been conducted over the last few years [1–6]. In particular, the temperature-dependent rotational dynamics in lead halide PS are discussed by Mattoni et al. [4]. As the temperature increases, the distortion of the lead (Pb)-iodine (I) octahedra in CH3NH3PbI3 induces a higher degree of symmetry, leading to the phase transition at a finite temperature at which the rotation of organic molecules is significant [4]. The dynamics of organic cations induce longer carrier lifetimes due to the carrier localization effect [7], ferroelectric effect [8,9], and Rashba effect [2,5]. A random orientation of the organic molecules in PS causes an electrostatic potential fluctuation and ferroelectric polarization, leading to the spatial separation of electrons and holes [7]. In addition, the rotational motion of the CH3NH3+ cation creates a strong internal electric field due to its dipole moment, which causes a reduced band gap and charge separation in the PS. The Rashba band splitting effect has also been used to explain the long carrier lifetimes. Lead halide PS has a strong spin-orbit coupling (SOC) due to heavy elements such as Pb, I, and bromine (Br). The strong SOC effect and the breaking of inversion symmetry give rise to splitting of the degenerate valence band (VB) and conduction band (CB) near the bandgap into opposite directions for each band. This splitting in the degenerate bands for these halide PS creates an indirect band gap, resulting in a slow recombination process due to the different momentum states of the photo-generated
2
electrons and holes. These processes often deteriorate the performance of optoelectronic devices. Here, we investigated the effect of CdSe/ZnS core-shell quantum dots on the dynamics of organic cation in the lead halide PS (CH3NH3PbI2Br). Previously, we reported the synthesis of a quantum dots-perovskite heterostructure (QD/PS hybrid structure) [10]. In this hybrid structure, the photo-excited electrons and holes were transferred from QDs to the PS [10], and the internal electric field induced by the motion of the organic cation can be reduced by the screening effect of the charge carriers migrated from QDs. In addition, the surface passivation effects, including protection of surface and decrease in the defect states, were demonstrated [10]. Thus, the hybrid structures using CdSe/ZnS QDs presented a promising approach to improve the optical properties of the PS films. In this study, we investigated the temperature-dependent luminescence properties of QD/PS hybrid structure by using photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. To analyze the effects of CdSe/ZnS core-shell QDs, the PL and TRPL results of QD/PS hybrid structure were compared with those of the pristine PS film (bare PS). The integrated PL intensities were much larger in the QD/PS hybrid structures than in the bare PS at all temperatures, and the PL decay times in the QD/PS hybrid structures were also longer than those in the bare PS, at temperatures higher than 200 K. In addition, the optical properties of the PS were altered by controlling the energy level alignment between QDs and PS, which is achieved by varying the diameter of the CdSe core structure.
3
Experimental Details Indium tin oxide (ITO)-treated glass substrates were washed by an ultrasonic cleaner with acetone and subsequently with isopropyl alcohol. The substrates were then dried using an air blower. The cleaned ITO substrates were treated with ultraviolet-ozone at 100 °C for 15 min. The methylammonium bromide (120 mg, Great Cell Solar Ltd.) and lead iodide (480 mg, 99%, Sigma-Aldrich) powders were dissolved in a mixture of dimethyl sulfoxide (1 ml, ≥99.5%, Sigma-Aldrich) and N,N-dimethylformamide (2 ml, ≥99.9%, Sigma-Aldrich), and the PS precursor solution was prepared by stirring this mixture at 60 °C for 1 h. The PS precursor solution was coated by a two-step spin-coating process onto the ITO substrates; the first and the second steps were performed at 1000 rpm for 10 s and 5000 rpm for 20 s, respectively. During the second step, toluene (Sigma-Aldrich) was dropped onto the surface. Finally, the PS film (CH3NH3PbI2Br) was annealed at 100 °C for 2 min. A CdSe/ZnS core–shell QD powder (5 mg, PlasmaChem GmbH) was dissolved in toluene (1 ml) and stirred at 60 °C for 1 h. The CdSe/ZnS QD solution was then spincoated onto the PS films at 3000 rpm for 30 s, and the QD/PS hybrid structure was finally annealed at 60 °C for 2 min. The QD/PS hybrid structures were synthesized with different CdSe core diameters (dQD = 2.7 and 4.2 nm) in the CdSe/ZnS core-shell QDs . X-ray diffraction (XRD) analysis and field-emission scanning electron microscopy (FE-SEM) were performed by using an X’pert Pro X-ray diffractometer (Malvern Panalytical) and a Supra55VP (Zeiss) scanning electron microscope to examine the crystallinity and surface morphology of the PS, respectively. The PL spectra of the samples were acquired using the 325-nm continuous-wave laser, as an excitation source, 4
equipped with an Andor DV420A-BU2 CCD detector. The TRPL spectra of the samples were collected by a microchannel plate photomultiplier tube (FLS 920, Edinburgh Instrument), where a 375-nm pulsed-diode laser (pulse width = 90 ps) was used as an excitation source.
Results and discussion Figure 1(a) shows the XRD pattern of the bare PS film. The characteristic PS diffraction peaks are clearly visible 2θ values of 14.3° and 28.8°, which were assigned to the (100) and (200) planes of the cubic phase of PS, respectively [11]. The surface morphology of the bare PS film was studied using SEM images, as shown in the inset of Fig. 1(a). The temperature-dependent PL spectra of the bare PS film and the QD/PS hybrid structure with dQD of 2.7 nm are shown in the Figs. 1(b) and (c), respectively. The temperaturedependent PL of the QD/PS hybrid structure with dQD of 4.2 nm (not shown here) shows a similar behavior as the hybrid structure with dQD of 2.7 nm. At 300 K, the PL spectra of the bare PS (QD/PS hybrid structure) exhibited two PL peaks at approximately 1.677 eV and 1.804 eV (1.691 eV and 1.805 eV), which were assigned to the emission from the cubic (PC) and the tetragonal (PT) phases of PS, respectively. The formation of PT in the bare PS and the hybrid structure is attributed to the photo-induced phase segregation, which is the light-soaking effect [13–15]. The increase in the PL peak energy of the PC in the hybrid structure relative to the bare PS film can be attributed to the decrease in the defect states due to the surface passivation by QDs [12] and to the reduced internal 5
electric field screened by charge carriers transferred from the QDs to the PS layer. In the mixed halide PS (CH3NH3PbI2Br) materials, the CB and the VB split into two spinpolarized bands in k-space due to the Rashba effect caused by a strong SOC from the heavy elements (Pb, I, and Br) and the internal electric field induced by the absence of inversion symmetry [5, 16]. In addition, a dynamical Rashba effect is observed in CH3NH3PbI2Br due to the distortion of lead-halide bonds and thermal vibrations of the organic (CH3NH3+) ions [17]. The PL intensity of the PC in the QD/PS hybrid structure was observed to be more dominant than that of the PC in the bare PS, which is attributed to the combined effect of the surface passivation and charge carrier transfer from QDs. Therefore, the following discussion will focus on the PC peak because the main contribution of the luminescence mechanism is arising from the cubic phase of PS, as seen in the XRD pattern and the PL spectra shown in Fig. 1. In addition, the phase transformation from PC to PT occurred at the critical temperatures of 180 K and 160 K in the bare PS and the QD/PS hybrid structures, respectively. The temperature-dependent integrated PL intensities of the PC in the bare PS and the QD/PS hybrid structures are shown in Fig. 2(a). At 300 K, the integrated PL intensity of the QD/PS hybrid structure was two orders of magnitude higher than that of the bare PS due to the charge transfer (CT) from QDs to the PS layer. As the temperature increased from 160 to 300 K, the integrated PL intensities of the bare PS and the QD/PS hybrid structures decreased gradually. Such a behavior can be attributed to the increase in the non-radiative recombination process. It is noteworthy that the integrated PL intensities of the QD/PS hybrid structures decreased at a much slower rate with an increase in the 6
temperature, compared to that of the bare PS films, which can be explained by the CT from QDs to PS [10]. Figure 2(b) shows the PL peak energies of the bare PS and the QD/PS hybrid structures. As the temperature increased from 180 to 240 K, the PL peak energy of the bare PS was redshifted to approximately 14 meV. This redshift is ascribed to the reduction of the band gap in PS due to the internal electric field induced by the rotational dynamics of the CH3NH3+ cation. The PL peak energy of the QD/PS hybrid structure with dQD of 4.2 nm was redshifted to approximately 10 meV with an increase in the temperature; this shift was smaller than that of the bare PS. In addition, no redshift in the PL peak energy of the QD/PS hybrid structure with dQD of 2.7 nm was observed with an increase in the temperature from 160 to 240 K, which can be explained by the completely screened internal field due to the higher CT rate from QDs to PS compared with that of the hybrid structure with dQD of 4.2 nm. The CT efficiencies of 63% and 44% from QDs to the PS layer were determined in the QD/PS hybrid structures with dQD of 2.7 and 4.2 nm, respectively, at 300 K using the PL decay times of the QD peak in the bare QDs and the QD/PS hybrid structure [18]. The blueshift of the PL peak for all the samples at higher temperatures (> 240 K) is attributed to the thermal expansion of the lattice [19]. The temperature-dependent integrated PL intensities of the PT in the bare PS and the QD/PS hybrid structures are also shown in Fig. S1(a). (See Supplementary Information (SI)). The integrated PL intensities of the PT in the QD/PS hybrid structures decreased at a much slower rate compared to that in the bare PS films, which can be explained by reduced light-soaking effect caused by the surface passivation effect. As 7
shown in Figs. S1(b) and (c), the PT peak in the bare PS was relatively dominant than the PC while the PC in the QD/PS hybrid structure was more dominant than the PT. In addition, the PT peak in the bare PS and the QD/PS hybrid structure was blueshifted due to thermal expansion of the lattice with increasing temperature from 160 to 300 K as shown in Fig. S1(d) in SI. Figures 3(a) and (b) show the temperature-dependent PL decay curves measured for the PC in the bare PS and the QD/PS hybrid structures, respectively. The PL decay in the bare PS hastens with an increase in the temperature from 180 to 300 K; whereas, the PL decay in the QD/PS hybrid structures slows gradually, as the temperature increases from 160 to 300 K. The QD/PS hybrid structure with dQD of 4.2 nm exhibited a similar temperature-dependent PL decay behavior as that of the hybrid structure with dQD of 2.7 nm (not shown here). The average decay times were estimated by τave=∑Aiτi2/∑Aiτi using the fast and slow decay times (τ1 and τ2) and the corresponding pre-exponential constants (A1 and A2), which were determined by using I(t) = A1 exp(-t/τ1) + A2 exp(-t/τ2). The average decay times obtained at the PL decay peak of the PC in the bare PS and the QD/PS hybrid structures as functions of temperature are shown in Fig. 4(a). While the PL decay time for the bare PS film gradually decreased from 4.68 to 1.44 ns with increasing temperatures (180–300 K), the QD/PS hybrid structures with dQD of 2.7 and 4.2 nm displayed an increase in the PL decay times from 4.28 to 6.04 ns and 4.07 to 4.53 ns, respectively, with an increase in the temperature from 160 to 300 K. Figures 4(b) and (c) show the temperature-dependent radiative (τR) and non-radiative lifetimes (τNR), respectively, which are estimated using the relationships: η(T) = τPL(T)/τR ≈ I(T)/I0, and 8
1/τPL(T) = 1/τR(T) + 1/τNR(T), where η is the internal quantum efficiency; I is the integrated PL intensity; and τPL is the PL decay time. The experimental values of the integrated PL intensity and the PL decay time shown in Figs. 2(a) and 4(a) were used for the calculation of I(T) and τPL(T), respectively. Here, we assumed η(Tmax) = 1 and I(Tmax) = I0, where Tmax is the temperature with the maximum integrated PL intensity. At 300 K,τR for the bare PS was much longer than for the QD/PS hybrid structures, which is discussed a little later. We observed an increase in the τR and decrease in the τNR values with an increase in the temperature for all samples, which is a typical temperaturedependent behavior of semiconductors. Therefore, the PL decay times of the QD/PS hybrid structures were mainly dominated by τR at all temperatures, whereas the PL decay time of the bare PS was dominated by τNR. The increase in the PL decay time of the QD/PS hybrid structures with increasing temperatures can be explained by the CT from QDs to PS [10]. In contrast, the decrease in the PL decay time of the base PS with increasing temperature was attributed to an increase in the non-radiative recombination processes caused by the phonon scattering, thermally escaped carriers, and/or the trapped carriers in the defect states [20]. It is important to note that the radiative lifetimes of the bare PS were longer than those of the QD/PS hybrid structures in all temperature regions as seen in Fig. 4(b). The slow recombination rate (longer lifetime) in halide PS materials was ascribed to the Rashba effect and/or charge separation due to potential fluctuations caused by the random orientation of the dipole moments of the organic molecules (CH3NH3+) [7]. Therefore, a longer radiative lifetime τR observed in the bare PS relative to that of the hybrid structures 9
can be attributed to the stronger Rashba effect and larger potential fluctuations, which were screened in the QD/PS hybrid structures by the charge carriers transferred from QDs to the PS layer. In addition, an increase in the radiative lifetime with increasing temperatures can be explained by the larger potential fluctuations at higher temperatures, which hinder the delocalization of photo-generated carriers and decrease the electron-hole recombination [21]. The higher integrated PL intensities and the faster radiative recombination rates in the QD/PS hybrid structures compared with those in the bare PS are ascribed to an increase in the capture cross-section (increase of delocalized carriers) due to the CT from QDs to the PS layer.
Conclusions The temperature-dependent photoluminescence (PL) and time-resolved PL studies were carried out in the mixed halide perovskite (CH3NH3PbI2Br) films with CdSe/ZnS core-shell QDs in order to investigate the effects of QDs on the optical properties of PS films. The integrated PL intensity of the QD/PS hybrid structures decreased at a much slower rate with increasing temperatures from 180 to 300 K than that of the bare PS, which is attributed to the weakened Rashba effect and the reduced charge separation due to the charge transfer from QDs to the PS layer. The QD/PS hybrid structures exhibited the enhanced PL intensities and decay times (fast recombination rate) compared with those of the bare PS. This observation suggests an improvement in the charge recombination of the QD/PS hybrid structures by the enhanced oscillator strength, caused by the decreased internal electric field and the reduced Rashba effect 10
observed due to the carrier migration from QDs to the PS, and a reduction in the defect states by the surface passivation effect. Our results help in identifying the role of CdSe/ZnS QDs in the luminescence mechanisms of PS films, and the QDs can be used as an effective material for the surface passivation and the enhancement of optical properties. Therefore, the QD/PS hybrid structures are powerful prospective candidates for PS-based optoelectronic applications.
Figures and Captions
Figure 1. (a) XRD patterns of the bare PS film. Inset shows the SEM image of the bare PS. Temperature-dependent PL spectra of (b) the bare PS and (c) the QD/PS hybrid structure with dQD of 2.7 nm. The PL peaks of the PC (red lines) and the PT (blue lines) were extracted using the Gaussian fitting.
Figure 2. Temperature-dependent (a) integrated PL intensities and (b) PL peak energies for the PC of the bare PS and the QD/PS hybrid structures.
Figure 3. PL decay curves acquired for the PC in (a) the bare PS and (b) the QD/PS hybrid structures with dQD of 2.7 nm as functions of temperature.
Figure 4. Temperature-dependent (a) PL decay times, (b) radiative lifetimes, and (c) nonradiative lifetimes of the bare PS and the QD/PS hybrid structures. Each PL decay time was obtained at the PC peak energy of the respective temperature.
Corresponding Author *M.-Y. R. E-mail: [email protected]
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Notes The authors declare no competing financial interests. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2019R1A2C1086813). Field-emission scanning electron microscopy analyses were performed at the Korea Basic Science Institute. Time-resolved photoluminescence measurements were performed at the Central Lab of Kangwon National University.
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Highlights -
Temperature-dependent luminescence properties of perovskite (PS) were investigated.
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Improved optical properties of surface-passivated PS films by quantum dots (QDs).
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Enhanced optical properties in the QD/PS due to the charge transfer from QDs to PS.
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Reduced Rashba effect and internal electric field in the QD/PS structures.
Author Statement M.-Y. R. conceived the idea presented in the text. I.-W. C. carried out all the experiments. Both authors contribute equally and approve the submission of the manuscript.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: