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We report on optical spectroscopy (photoluminescence and photoluminescence excitation) on two-dimensional self-organized layers of (C6H5C2H4-NH3)2-PbI4 perovskite. Temperature and excitation power dependance of the optical spectra gives a new insight into the excitonic and the phononic properties of this hybrid organic/inorganic semiconductor. In particular, exciton-phonon interaction is found to be more than one order of magnitude higher than in GaAs QWs. As a result, photoluminescence emission lines have to be interpreted in the framework of a polaron model.
©2010 Optical Society of America
1. Introduction
Optical properties of soft materials have attracted much attention for years thanks to their potential applications in optoelectronics devices. In particular, these last years an increasing number of studies are dedicated on hybrid organic-inorganic materials [1], due to the possibility of combining the properties both of inorganic materials (high mobility, electrical pumping, band engineering) and of organic materials (low cost technology, high luminescence quantum yield at room temperature). In this context, organic-inorganic perovskites, having a chemical formula (R-NH3)2MX4 where R is an organic chain, M is a metal and X an halogen, represent a natural hybrid system. Such perovskites present a great flexibility in their optical properties: the spectral position of the excitonic transitions can be tailored by subsituting different halides X [2,3], the photoluminescence efficiency can be tailored by changing the organic part R [4]. This kind of perovskites has been studied both for fundamental studies [2,3,5–27] and for applications in optoelectronics, for instance as the active material in a distributed feedback laser [28].
Recently the strong coupling regime between the perovskite exciton and the optical mode of a Perot-Fabry microcavity has been demonstrated at room temperature in the UV range [5] and in the visible range [6,7]. The physical properties of these new polaritons have now to be investigated. In particular, the demonstration of polariton-polariton interactions which lead to polariton scattering would be a breakthrough for the physics of these new devices in the context of the low threshold polariton lasers [29–31]. To evaluate these possibilities, a good knowledge of the perovskite material electronic properties is needed. As an example, the phonon energy and the strength of the electron-phonon coupling will indicate whether an efficient relaxation of perovskite polaritons is conceivable. Additionally, the origin of the different perosvkite luminescence lines has to be clarified to improve the knowledge about the excitons which couples to the cavity mode.
In this paper, we report on the optical properties of a particular perovskite molecule, namely [bi-(phenethylammonium) tetraiodoplumbate]:(C6H5C2H4-NH3)2-PbI4 (named PEPI), absorbing and emitting in the green part of the visible range (see Fig. 1 ). Photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) are performed at various temperatures ranging from 10 K to 300 K. The origin of the different PL lines is discussed and an extensive study of the electron-phonon coupling is performed, for the first time to our knowledge in this kind of perovskite molecular crystal. A Longitudinal Optical (LO) phonon energy of 14 meV and an electron-phonon coupling more than one order of magnitude higher than in GaAs quantum wells, have been estimated. These experiments suggest that the PL signal at low temperature has to be interpreted in the context of a polaron model.
Fig. 1 a) and b) Pictures of the green photoluminescence of a PEPI thin layer excited by a UV laser.
2. Sample
When a solution of (C6H5C2H4- NH3)2-PbI4 and PbI2 dissolved in steochiometric amounts in DMF (N,N-DiMethylFormamide) is deposited by spin-coating on a quartz substrate, a self-organization occurs, leading to the formation of a molecular crystal, consisting in an alternance of organic and inorganic layers (see Fig. 2(a) ). The self organization occurs for thicknesses from 3 nm to 100 nm.
Fig. 2 a) Sketch of the perovskite structure. The blue points at the center of the octahedrons represent the Pb atoms while I atoms are displayed in green. The red symbols represent the organic chains. b) XRD rocking curve of a 50 nm (C6H5C2H4-NH3)2PbI4 layer.
The thickness of the perovskite layers can be controlled by changing the concentration of the solution, the speed and the acceleration of the spin-coater. AFM (Atomic Force Microscopy) measurements show that a high quality polycristalline film is obtained, with a surface rugosity as small as 2 nm. X-ray diffraction has been used to characterize the ordering within the self-organized perovskite layers under study. Figure 2b shows an example of X ray rocking curve measured on a 50 nm PEPI thin layer (obtained by spin-coating a solution of 10 wt% C6H5C2H4-NH3I and PbI2 dissolved in steochiometric amounts in DMF). The observation of numerous diffraction orders proves the high crystallinity of the thin layer and the very good periodicity of the stacking. Each satellite peak being regarded as a diffraction plan, it is seen that we can observe diffraction planes from (002) to (0022) beyond 2*θ = 60°. As a consequence, a period of 16.4 Å can be accurately estimated. Knowing the lattice parameter of 6.4 Å of inorganic layer PbI62- octahedrons, we deduce that the organic part has then an extension of 10 Å along the growth direction. Similar values have been obtained from X ray diffraction spectra on samples of different thicknesses.
Because of the very large energy difference between the band gap of the inorganic layer and the HOMO-LUMO of the organic layer, a multi quantum-well (QW) electronic structure is formed in these self-organized perovskite structures with a strong 2D quantum confinement: the QWs are formed by the PbI62- octahedron monolayers and the barriers by the organic alkylammonium layers. Moreover, the barrier and the well of these natural QWs have very different dielectric constants (εbarrier ~2.1; εwell ~6.1) [17]. Thanks to this high contrast in dielectric constants, the Coulomb interaction in the QW is hardly screened by the presence of the barrier: it is the well known dielectric confinement effect [32,33]. Therefore, the electron-hole interaction within the exciton is strengthened, resulting in huge oscillator strengths and very large exciton binding energies (of a few hundred of meV).
In this paper, we will focus our attention on a PEPI layer of thickness ~5 nm, containing 3 QWs: this thin layer has been obtained by spin-coating a solution of 1 wt% C6H5C2H4-NH3I and PbI2 dissolved in steochiometric amounts in DMF.
3. Optical experiments
We report on PL and PLE experiments performed on this 3 QWs sample. The excitation source is a quartz-iode lamp spectrally filtered with a monochromator. The image of the exit slit of the monochromator is formed on the sample leading to a spot size of about 3 mm* 350 μm. PL experiments as a function of the excitation power are performed using a laser diode at 405 nm. The luminescence is then collected and dispersed in a 1 m double-spectrometer and detected with a photomultiplier. The sample can be placed either on the cold finger of a helium closed cycle cryostat (sample temperature between 10 K and 300 K) or in a bath of superfluid helium (2 K).
4. Results and discussion
Figure 3(a) exhibits the PL spectrum of the sample at 10 K: two lines can be observed at 2.355 eV (S1) and 2.337 eV (S2). Similar optical spectra were already reported in the literature and interpreted in several ways [10,15,17]. Fujisawa et al attribute the PL high energy line to free excitons and the lower energy lines to free bi-excitons [10]. Hong et al suggest the presence of free excitons, bound excitons and of phonon replica [15] and Ishihara et al raise the possibility of the existence of three exciton lines named A, B and C as observed in GaN [17].
Fig. 3 a) PL (black curve) excited at 2.8 eV and PLE (grey curve) detected at the energy of S2 (2.337 eV) at 10 K; b) Variation of the PL intensity of S1 and S2 measured as a function of the pump power at 2 K; c) Photoluminescence spectra excited at 2.5 eV for temperatures ranging from 10 K to 300 K.
In order to get more information about the two PL lines, we have performed PL spectra as a function of the excitation power at 2 K (see Fig. 3(b)). Excitation powers ranging between 3 μW and 4 mW have been explored, but results are presented only for excitation powers below 500 μW, because the sample presents some photobleaching at higher powers. As evidenced on Fig. 3b, the two PL lines have the same behavior in this range of power. We can thus deduce that the satellite line S2 has an intrinsic origin and should not be attributed to excitons bound to defects (which would correspond to a sublinear dependance on the excitation power) nor to bi-excitons (surlinear behavior).
Figure 3(a) shows also the PLE spectrum with a detection energy tuned in resonance with the low energy PL emission line S2. The PLE spectrum recorded with a detection energy tuned in resonance with S1 is exactly similar: this again rules out an interpretation of S2 in terms of bi-exciton. Moreover the independence of the PLE spectra from the detection energy demonstrates that the two emission lines are connected to the same excited states. The energy difference in the PLE spectra between the lowest energy line and the 2D absorption step corresponds to an exciton binding energy of 200 meV, in agreement with previous estimate deduced from optical absorption spectroscopy [15]. At 10K, the Stoke's shift between the PLE lower energy line and the PL line S1 is 10 meV. The origin of the Stokes shift as well as that of the different PLE lines will be discussed later in the paper.
We now investigate the optical properties of this perovskite multi-layer sample for various temperatures. Figure 3(c) shows PL spectra measured for temperatures ranging from 10 K to 300 K: the energy of the high energy PL line increases with temperature. This is in contrast with other perovskites, for instance (CnH2n + 1 -NH3)2PbI4 for which the emission energy has been reported to decrease with temperature [17]. Our result shows the importance of the barrier nature for the understanding of the perovskite electronic structure. Indeed, depending on the exact composition of the barrier, the dielectric constant difference εQW - εB between the barrier and the QW may increase or decrease with temperature. Moreover as detailed in ref [15], the exciton energy in perovskite QWs is mainly determined by εQW - εB through the dielectric confinement. Additionally, the strain in the QW might be different depending on the nature of the barrier [4] and might also influence the exciton energy dependance on temperature. As a result, changing the barrier composition may lead to opposite temperature behavior of the exciton transition energy. A detailed optical study of the temperature behaviour for several perovksites is under progress and should further clarify the exact role ofthe barrier composition.
Let us now discuss the temperature dependence of the S1 PL line, namely the evolution of its energy position, intensity, and Full Width at Half Maximum (FWHM). The S1 and S2 lines have been extracted from the total PL spectrum through a deconvolution with two Lorentzians (see Fig. 4(b) ). Figure 4(a) summarizes the evolution with temperature of the S1 PL integrated intensity. It remains stable between 10 K and 100 K and decreases by more than one order of magnitude between 100 K and 300 K, as exp(Ea/kT) with Ea ~ 60 meV. The activation energy we measure here is far from the exciton binding energy, so the decrease of the integrated PL intensity cannot be attributed in our sample to the partial ionization of the exciton, as it was the case in the samples studied in ref [15]. Performing similar measurements on several samples of same thickness but prepared at different days, Ea is found to significantly vary from sample to sample between 30 meV and 80 meV. The strong dispersion in the measured value of Ea demonstrates that the decrease of the integrated PL intensity should be related to activation of another carrier recombination channel. This could be a non radiative mechanism like carrier recombination on impurities inside the barrier, or at the interface between two areas of the inhomogeneous film. As shown on Fig. 4c, the measured full width at half maximum of the PL line S1 continuously increases when increasing the temperature between 10 K and 300 K. We attribute this broadening to exciton interaction with phonons and describe it within a phenomenological model, often used for inorganic QWs [34]. The dashed line represents the fit of the experimental data with the function:
In this Eq. (1), Γ0 is the linewidth at 0 K, a.T represents the broadening induced by acoustic phonons. The third term corresponds to the interaction of excitons with optical phonons. ΓLO is the exciton-phonon coupling and ħωLO the optical phonon energy. The last term is due to the exciton scattering on impurities with an activation energy Ea = 60 meV deduced from temperature dependant measurements. Some assumptions used in this model can be discussed in the case of organic-inorganic perovskite QWs. First, the model is valid within the framework of the envelope function [34], which requires that the variation of the excitonic wave function is smooth on several unit cells. In the perovskite molecular crystal, the exciton Bohr radius has been estimated to be 1.7 nm in the layers plane [15], that is to say 3 or 4 unit cells. Therefore, the limit of the envelope function approximation is reached in perovskite layers. Nevertheless, notice that perovskite layers present several features of 2D QW which are well described using the enveloppe function. Secondly, the thermal variation of the optical phonon energy with temperature is neglected in our simple approach. However in soft material, this assumption could be too strong. In fact, the Van der Walls bounds which create the self-assembled QWs are weak as compared to covalent bounds in inorganic QW systems. As a consequence, a weak perturbation could strongly deform the structure and affect the phonon energies. Anyway, using this simple approach we are able to give the first estimate of the phonon energy and of exciton-phonon interaction in this new system.Fig. 4 a) S1 Integrated PL intensity as a function of 1000/T; b) Example of deconvolution of the S1 and S2 PL lines at 80 K in the PL spectra. The scattered line is the experimental curve, the red solid line is the result of the deconvolution with two lorentzians shown with green solid lines; c) S1 FWHM as a function of temperature; the dashed line is the calculated FWHM following the model explained in the text.
In order to extract physical values from the model, we have chosen to fix the optical phonon energy ħωLO to 13.7 meV (the value of optical phonons in PbI2 [35]). A very good agreement between the experimental data and the theoretical model is then obtained providing some numerical values for the parameters Γ0, ΓLO and a. A value of Γ0 = 17 ± 1 meV is deduced: it varies from a sample to another between 12 meV and 17 meV, as expected if Γ0 is mainly dominated by inhomogeneous broadening. The parameter a is equal to 0.03 ± 0.01 meV.K−1. The uncertainty on this value is large because of the large inhomogeneous linewidth at 0 K. Nevertheless, the exciton interaction with acoustic phonons is found more than one order of magnitude higher than in GaAs QWs [34]. The coupling ΓLO with the optical phonons is estimated to be of the order of 70 meV which is also more than ten times higher than in inorganic quantum wells [34]. This value doesn't depend on the sample, showing the intrinsic nature of these measurements.
We can now go further in the interpretation of the PL lines S1 and S2 and of the lines observed in the PLE spectrum. Figure 5 shows that the Stoke's shift between S1 and the lower energy PLE line does not depend on temperature. In inorganic quantum wells, the Stokes shift is usually due to exciton localization in potential minima induced by disorder. The exciton trapping becomes less efficient as temperature is increased. As a result, the Stokes shift decreases with temperature and eventually vanishes when the thermal energy kT overcomes the trapping potential. The fact that in our perovskite sample the 10 meV Stokes shift is still observed when kT is larger than 10 meV (above 115 K) strongly suggests that the energy difference between emission and absorption is not due to exciton localization in disorder potential. It has been reported that lead halides such as PbI2, PbCl2 or PbBr2 exhibit a very strong exciton-phonon coupling leading to the existence of self-trapped excitons [36], and the Stoke's shift in PbI2 amounts to 7.4 meV [37]. Moreover, note that the independence of the Stokes shift with temperature can also be observed in InGaN layers [38] and has been interpreted as an evidence of a polaron origin of the emission line. Since we have shown previously that the exciton-phonon coupling is very high in perovskite layers, the independence of the Stokes shift with the temperature strongly suggests that the mean emission line S1 has to be described in a polaron model. Since the energy separation between S1 and S2 is 14 meV, which corresponds to the LO phonon energy we have determined in PEPI, S2 is probably a phonon replica of the main line S1. Considering this context of high exciton-phonon coupling, some phonon replicas are expected in the PLE spectra.The first phonon replica should be observed 14 meV above the lower energy line of the PLE spectrum. We can see that the lower energy PLE line is two times broader than the PL line S1: we attribute this broadening to the presence of the phonon replica, which can't be resolved.
Fig. 5 Energy of the S1 PL line (squares), and of the lower energy line in the PLE spectra (triangles) as a function of temperature.
The other lines seen in the PLE spectra at higher energy are surely related to the electronic structure of the conduction band. In PbI2, the conduction band is three times degenerated. The crystal field, the spin-orbit coupling or the exciton exchange interaction may lift this degeneracy [39] [40]. Polarization resolved experiments are under progress and calculations of the electronic structure are performed in order to get better understanding of the optical spectra.
4. Conclusion
To conclude, we have performed optical spectroscopy (photoluminescence and excitation of the photoluminescence) of (C6H5C2H4- NH3)2PbI4 perovskite self organized layers. An estimate of the optical phonon energy is obtained as well as of the exciton-phonon interaction. These results could be further confirmed using Raman spectroscopy and theoretical calculations of the Perovskite phonon structure. The exciton-phonon interaction is found to be more than one order of magnitude higher than in GaAs QWs, both for acoustic and optical phonons. This leads to an interpretation of the PL emission lines within the framework of a polaron model. The very strong exciton-phonon interaction in Perovskite QWs is very promising for the use of this material as an active medium in polaritonic systems such as microcavities operating in the strong coupling regime. It should allow efficient polariton relaxation, a key requirement to obtain polariton lasing.
Acknowledgments
Authors are grateful to M. Schott for helpful discussions. This work has been supported by the ANR grant 'MICHRY', the C'Nano “Region Ile de France” grant 'MICRORG' and the RTRA “Triangle de la Physique”grant 'MOSKITO'. The Laboratoire de Photonique Quantique et Moléculaire de l'Ecole Normale Superieure de Cachan is a Unité mixte de recherche associée au CNRS (UMR8537).
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