The Effect of Wannier and Frenkel Exciton Resonance on the

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The Effect of Wannier and Frenkel Exciton Resonance on the Luminescence Properties of Organic−Inorganic Layered PerovskiteType Compounds Naoki Kawano, Masanori Koshimizu,* and Keisuke Asai Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ABSTRACT: We investigated the optical properties of organic−inorganic layered perovskite-type compounds that contained naphthylmethyl moieties in the organic layer. The Wannier exciton energy in the inorganic layer can be modulated by varying the halogen content. The Wannier or Frenkel exciton emissions in the organic layer were observed in compounds where the exciton energy was lower or higher than that of the triplet energy of the naphthylmethyl moiety, respectively. For the compounds with near-resonant Wannier and Frenkel exciton energies, luminescence was only observed at 490 and 500 nm from the naphthylmethyl moiety while luminescence at 520−566 nm was not detected. This selective quenching of the luminescence can be attributed to an enhancement in oscillator strength of near-resonant transition energies between Wannier and Frenkel excitons.

I. INTRODUCTION

In these compounds, excitons in the inorganic layer possess large binding energies and oscillator strengths due to quantum confinement and the image charge effect.4,5 These stable excitons in the inorganic layer display unique optical properties such as excellent scintillation,6 distinguished optical nonlinearity,7 and efficient electroluminescence.8 In previous studies, it has been demonstrated by electroabsorption spectroscopy that the excitons in the inorganic layer are of the Wannier-type and exhibit pseudo-two-dimensional character.9,10 The optical properties of compounds containing alkyl ammonium molecules in the organic layer are characterized almost entirely by the electronic structure and excitonic behavior of the inorganic layer owing to the larger excitation energy of the organic molecule. On the other hand, it has been shown that hybrid compounds with π-conjugated moieties (e.g., pyrene11or azobenzen12) incorporated into the organic layer display distinct optical properties. The optical properties in such compounds depend on the relative positions of the Wannier exciton energy levels in the inorganic layer and the singlet and triplet states of the π-conjugated moieties.13 Among them, compounds containing naphthylmethyl molecules have been of great interest due to their efficient energy transfer from the inorganic layer to the organic layer (Figure 1).14 Timeresolved photoluminescence measurements revealed that the energy transfer in such systems is of the direct electron exchange (Dexter) type with almost unit efficiency.15 This result indicates that a strong interaction exists between the

It has been predicted that a novel excited state, referred to as a hybrid exciton, can be formed when Wannier and Frenkel excitons located in close proximity to each other are in resonance. Such hybrid excitons are characterized by their distinct optical properties that originated from the parent properties of both Frenkel (e.g., high oscillator strengths) and Wannier excitons (e.g., a large Bohr radius).1 In previous studies, it has been demonstrated that photon-mediated hybridization between Frenkel and Wannier excitons occurred in a hybrid organic−inorganic optical microcavity.2 This hybrid state consisted of 10% Frenkel and Wannier exciton and 80% cavity photon character at 100 K. In order to achieve stronger hybridization, resonance between Wannier and Frenkel excitons should be achieved not only by matching the absorption energies but also by ensuring the formation of a high quality interface between the organic and inorganic components of such hybrid materials. Lead-halide-based organic−inorganic layered perovskite compounds are good candidates for investigations into hybrid excitons. These compounds are known to form self-organized multiple quantum well structures with alternating organic− inorganic layers.3 The general formula of these compounds is (RNH3)2PbX4, where R and X represent the hydrocarbon group and halogen, respectively. The inorganic layers of the quantum well consist of corner-sharing PbX6 octahedra that are sandwiched between the organic barrier layers. The interface between the organic and inorganic layers is formed in a selforganized manner unrestricted by lattice mismatch. These compounds are therefore appropriate for research into hybrid excitons. © 2012 American Chemical Society

Received: May 9, 2012 Revised: August 12, 2012 Published: October 17, 2012 22992

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Figure 1. Schematic structure of metal halide-based layered perovskite with naphthylmethyl moieties.

Wannier excitons and the triplet state of the naphthylmethyl moiety. In this work, we investigated the optical properties of a hybrid system containing naphthylmethyl with a Wannier exciton level that is in resonance with the excited states of the organic layer. According to a previous study on the optical properties of (C6H5C2H4NH3)2PbBrxI4−x, the peak positions of Wannier exciton absorption and luminescence can be controlled by manipulating the halogen content of the inorganic layer.16 Thus, we prepared compounds with varying halogen content and monitored their resulting energy transfer properties. Furthermore, the luminescence was also investigated in the compounds with near-resonant Wannier and Frenkel exciton energies.

Figure 2. X-ray diffraction patterns of (C10H7CH2NH3)2PbBrxI4−x spin-coated films at room temperature.

Figure 3 shows the absorption spectra of (C10H7CH2NH3)2PbBrxI4−x at room temperature. A sharp

II. EXPERIMENTAL METHODS Stoichiometric quantities of C10H7CH2NH2 and HX {X = Br, I} were dissolved in N,N-dimethylformamide (DMF) and stirred for 0.5 h. After evaporation of the solvent, C10H7CH2NH3X powders were obtained and subsequently dissolved in DMF with PbX2 {X = Br, I} in a molar ratio of 1:2 and then stirred for 3 h under dry argon flow. Powders of (C10H7CH2NH3)2PbBrxI4−x were then obtained by evaporating the solvent, and thin film samples were prepared by spincoating the DMF solution onto glass or silicon substrates. The crystal structure and orientation of the films was investigated by X-ray diffraction (XRD) over a 2θ range of 3° to 40° at room temperature using Cu Kα radiation. The optical absorption spectra of the spin-coated films on glass substrates were measured using a spectrophotometer (Hitachi U-3500) at room temperature. The photoluminescence spectra of the spincoated films on Si substrates were measured at 10 K using a 325 nm He−Cd laser as the excitation source.

Figure 3. Optical absorption spectra of (C10H7CH2NH3)2PbBrxI4−x spin-coated films at room temperature.

peak at 382 nm was observed in the film with x = 4, which corresponds to the absorption of the Wannier excitons. The observed peak position is consistent with the finding of a previous study.14 Wannier exciton absorption peaks were also observed in the films with x = 0−3. The peak position of such excitons shifted toward lower energy in the films with higher iodine content in the inorganic layer. These results indicate that the variable range of the Wannier exciton energy level is approximately 2.5−3.3 eV. Figure 4 shows the photoluminescence spectra of (C10H7CH2NH3)2PbBrxI4−x at 10 K. Luminescence peaks at 520 and 508 nm were detected in the film with x = 0 and 1, respectively. These luminescence peaks can be attributed to the

III. RESULTS AND DISCUSSION Figure 2 includes the XRD patterns of (C10H7CH2NH3)2PbBrxI4−x films. Only the (0 0 l) diffraction peaks were observed in each of the samples. Also, the lack of separation of these diffraction peaks suggests that there is no phase separation in the samples. These results indicate that the layers within the sample films were single phase and oriented parallel to the surface of the substrate. 22993

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Figure 4. Luminescence spectra of (C10H7CH2NH3)2PbBrxI4−x spincoated films at 10 K.

Figure 5. Dependence of the optical properties on Wannier exciton energy level.

recombination of Wannier excitons, which occurs due to their small Stokes shift. On the other hand, several peaks at 487−560 nm were observed in the films with x = 2−4, and such were ascribed to the phosphorescence from the triplet excited state of the naphthylmethyl moiety. Wannier exciton emission was not observed from these films. Because only the inorganic layer was excited by the He−Cd laser at a wavelength of 325 nm, the observed phosphorescence is caused by energy transfer from the inorganic to the organic layer as has been reported in previous studies.14,15 The luminescence scheme of the samples is summarized in Figure 5. These results indicate that the triplet excited state of the naphthylmethyl moiety lies between 2.6 and 2.9 eV, which overlaps with the Wannier exciton energies in the film with x = 1 and 2, respectively. Based on the luminescence properties dependent on the composition in the inorganic layer, we fabricated the films with x = 1.75 and 1.87 in which it was supposed that the Wannier exciton energy level and the triplet excited level in the naphthylmethyl moiety exhibit near-resonance. Figure 6 shows the photoluminescence spectra of the films with x = 1.75 and 1.87 at 10 K. Luminescence peaks at 490 and 500 nm were observed in the films with x = 1.75−4. Such peaks correspond to the transition from a triplet excited state to each vibrational level in the ground state of the naphthlymethyl moiety. On the other hand, luminescence peaks at 520−566 nm were not observed in the films with x = 1.75 and 1.87. The luminescence emitted by the naphthylmethyl moiety in these compounds, particularly in the films with x = 3, was quite similar to that observed in one-dimensional organic−inorganic hybrid structures where the naphthylmethyl moiety is incorporated into the organic layer.17,18 In both cases, the phosphorescence occurs due to the efficient transfer of energy from the Wannier excitons in the inorganic layer to the triplet state in the naphthylmethyl moiety. The quenching in the luminescence peaks at 520−566 nm can be attributed to one of the following processes: (1) enhancement in the nonradiative decay rate of the correspond-

Figure 6. Luminescence spectra of (C10H7CH2NH3)2PbBrxI4−x spincoated films at 10 K.

ing transition, (2) symmetry-forbidden transition, or (3) enhancement in the radiative or nonradiative decay rate of other competing transition. In the case of (1), the excited state undergoes transition into the ground state via phonon emission. In this case, the intensity of the phosphorescence over the entire wavelength range would decrease, and thus, case (1) can be excluded as a possible explanation for the observed behavior. In the case of (2),19 quenching due to the transitions being asymmetry-forbidden is not probable. Thus, with regards to case (3), the transitions at 490 and 500 nm were promoted by enhancement in the radiative transition rate. This selective 22994

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(17) Papavassiliou, G. C.; Mousdis, G. A.; Raptopoulou, C. P.; Terzis, A. Z. Naturforsch. 1999, 54B, 1405−1409. (18) Goto, T.; Ohshima, N.; Mousdis, G. A.; Papavassiliou, G. C. Non-linear Optics 2002, 29, 379−384. (19) Ferguson, J.; Iredale, T.; Taylor, J. A. J. Chem. Soc. 1954, SEP, 3160−3165.

enhancement of the luminescence can be related to the enhancement in the oscillator strength of these transitions stemming from the resonance between the Wannier excitons and the triplet excited state. These results suggest that resonance was operative between Wannier and Frenkel excitons in the films with x = 1.81 and 1.87.

IV. CONCLUSION The optical properties of organic−inorganic layered perovskitetype compounds in which Wannier and Frenkel exciton energies exhibit near-resonance have been investigated. Selective quenching of the luminescence at 520−566 nm was observed in these compounds, which is attributed to the enhancement in the oscillator strength of the resonant transitions at 490 and 500 nm. These results suggest that resonance between Wannier and Frenkel excitons indeed existed. In order to elucidate the effect of resonance, further studies on the optical properties of such materials are required.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-22-795-7219. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by a Grant-in-Aid for Challenging Exploratory Research (Grant No. 23655168, 2011−2012) from the Ministry of Education, Culture, Sports, Science and Technology.

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