We synthetize some new perovskite thin layers: p-fluorophenethylamine tetraiodoplumbate pFC6H4C2H4NH3)2PbI4 perovskite molecules, included in a PMMA matrix. We report on the optical properties of the perovskite doped PMMA thin layers and we show that these layers are much more stable under laser illumination and present a smaller roughness than the spin-coated (C6H5C2H4NH3)2PbI4 layers. These new layers are used as the active material in vertical microcavities and the strong-coupling regime is evidenced by a clear anti-crossing appearing in the angular-resolved reflectivity experiments at room temperature.
© 2012 Optical Society of America
Light-matter interaction in vertical microcavities, and more particularly the strong-coupling regime, is intensively studied since two decades [1–3], due to the coherent and stimulated optical effects which are expected in such systems and which can lead to realization of new optoelectronic devices such as low threshold polariton laser [4, 5]. In molecular materials, the strong coupling has been demonstrated recently at room temperature with zinc porphyrin , J-aggregates , sigma-conjugated polysilane , anthracene  and perovskite molecules [10–12]. Generally, the photobleaching of the molecules is an important drawback to overcome to observe stimulated optical effects. Another drawback to overcome concerns technological difficulties appearing in the realization of high quality factor cavities containing molecules, due to the fact that molecules are sensitive to the deposition conditions of the cavity mirrors. More particularly, in vertical microcavities containing two-dimensional layered perovskite-type semi-conductor such as (C6H5C2H4NH3)2PbI4 or (C6H5C2H4NH3)2PbCl4 [11, 12], we are seeking to overcome two difficulties. Firstly, due to the too large roughness of the spin-coated perovskite layers, the improvement of the quality factor by monolithic deposition of a dielectric mirror directly on the perovskite layer, is impossible. Secondly, the perovskite emission decreases with time under laser illumination. In this work, we synthetize some new perovskite thin layers: p-fluorophenethylamine tetraiodoplumbate, whose chemical formula is (pFC6H4C2H4NH3)2PbI4 (abbreviated as pFPEPI hereafter) perovskite molecules, included in a polymer matrix: PMMA (PolyMethylMetAcrylate). We report on the optical properties of the perovskite doped PMMA thin layers and we show that these layers are much more stable under laser illumination and present a smaller roughness than the spin-coated (C6H5C2H4NH3)2PbI4 (abbreviated as PEPI hereafter) layers previously used in our microcavities  These new layers are used as the active material in vertical microcavities and the strong-coupling regime is evidenced by a clear anti-crossing appearing in the angular-resolved reflectivity experiments at room temperature.
2. Synthesis and characterization of the fluorophenethylamine based perovskite compounds
Two-dimensional organic-inorganic layered perovskite compounds such as (R-NH3)2PbX4 where R is an organic group, and X an halogen ion (Cl−, Br−, I−). [14, 15], have been shown to form a molecular crystal, having a self-organized multiple quantum well structure when deposited by spin-coating on a substrate. The inorganic wells of thickness around 0.5 nm alternate with organic barriers of thickness around 1 nm. Because of the high contrast between the dielectric constants of the organic layers and the inorganic PbI64− octahedrons layers, which strengthens the Coulombic interaction between the electrons and the holes, large exciton binding energies as large as 300 meV are found, leading to a strong photoluminescence (PL) at room temperature. A commonly used perovskite, absorbing and emitting in the visible range, is (C6H5C2H4NH3)2PbI4, called PEPI [14, 16]. PEPI is used as the active material in different kinds of microcavities [10, 11, 17]. In references [11, 17], we have used PEPI as the active material in a vertical λ/2 microcavity closed by a bottom dielectric mirror and a top metallic mirror. Figures 1(a), 1(d), 1(e) and Figs. 2(a) show the optical and topological properties of a 50 nm thin PEPI layer prepared in the same conditions as in reference : a 10wt% solution of C6H5C2H4NH2HI and PbI2 dissolved in stoechiometric amounts in DMF is deposited by spin-coating on a quartz substrate at a speed of 2000 rpm for 30 s, and is annealed at 95 ºC during 1 minute. As seen in Fig. 1(a), this 50 nm PEPI thin layer absorbs at 2,39 eV and emits at 2,368 eV. Figure 1(d) shows the behavior as a function of time of the integrated PL intensity of PEPI, under a He-Cd laser illumination of 7.4 mW at 325 nm: it can be seen that 80 % of the PL is lost after 30 minutes. This can be a critical drawback for experiments involving some non-linear effects and so requiring strong incident power. Figure 2(a) shows an AFM (Atomic Force Microscopy) image of the PEPI 50 nm thin layer. The surface roughness is defined such as:
In order to improve the long-time stability under illumination and the roughness of the perovskite active layer embedded in the vertical microcavities, we have synthetized new per-ovskites included in a polymer matrix. Inspired by the work of [18, 19], we synthetize the para-fluorophenethylamine based perovskite, called hereafter pFPEPI. In this molecule, the hydrogen atom of the PEPI phenyl group in the para position is substituted by a fluorine atom. The presence of fluorine atom in the organic cation will polarize the electronic state of the organic ammonium and is expected to decrease redox-type reactions inside the molecule, which can be responsible for the degradation of the molecule under laser illumination. To prepare the ammonium salt, we use the commercially available para-fluorophenethylamine, purchased from Aldrich: gaseous HI, dried by dropping HI solution carefully on P2O5 powder, reacts with para-fluorophenethylamine in diethyl oxide under agitation at room temperature. The obtained ammonium salt is mixed with PbI2 in a stoechiometric ratio of 2:1, and then dissolved in dimethylformamide (DMF) at a 1:10 mass ratio.
The PL signal of a 50 nm pFPEPI spin-coated layer (deposited with the same spin-coating parameters as for the 50 nm PEPI thin layer) lies at the same energy as the PEPI spin-coated layer (see Fig. 1(b)) and Fig. 1(d) shows the behavior as a function of time of the integrated PL intensity of the pFPEPI spin-coated layer. It can be seen that the long-time stability under illumination is really improved since 60 % of the PL remains after 30 minutes. As the polymer matrix, we chose the PMMA because we control very well its thickness and roughness when deposited by spin-coating . Some amount of PMMA is then directly added to the pFPEPI molecules diluted in DMF solution: the weight ratio of pFPEPI and PMMA is 1:5 and the concentration of PMMA in DMF is 20% in weight. This mixture is then deposited by spin-coating on a substrate and the obtained thin layer is annealed in order to optimize its photoluminescence: the 1 μm thin layer presented in Figs. 1 and 2 is annealed at 150 ºC during 90 minutes and is deposited by spin-coating at a speed of 2000 rpm for 40 s. From the work of , it is thought that some nanosized inclusions of perovskite are formed inside the polymer. The optical and topographical properties of this new perovskite thin layers are compared to the 50 nm PEPI spin-coated thin layer. Figure 1(c) exhibits the absorbance and photoluminescence spectra of the 1 μm pFPEPI doped PMMA layer: the maxima of the absorption and photoluminescence peaks of pFPEPI doped PMMA lie at 2.386 eV and 2.357 eV respectively, that is to say practically at the same energies as PEPI and pFPEPI spin-coated layers. The Full Width at Half Maximum of the PEPI and pFPEPI doped PMMA peaks are quite similar. Figures 1(d) and 1(e) show that the stability of the pFPEPI doped PMMA layer under strong laser illumination is further improved compared to PEPI spin-coated layers: after 100 minutes, 60% of the PL signal remains for the pFPEPI doped PMMA layer whereas only 1% of the PL signal remains for the PEPI spin-coated layer. Additionnally, it can be seen that the long-time stability of the pFPEPI doped PMMA layer is better than the one of the pFPEPI spin-coated layer, probably because the PMMA protects the nanosized perovskite inclusions from the oxidation by ambiant air. Figure 2b allows the comparison of the topography of the two samples: Δ is found to be 0.8658 nm for the pFPEPI doped PMMA layer while it is 10.5516 nm for the PEPI layer. As a consequence, we can conclude that this method of deposition allows to improve the roughness by a factor of ten.
3. Microcavity containing fluorophenethylamine based perovskite compounds
A vertical microcavity is realized for which the active layer is the new perovskite layer presented above. The microcavity consists of a 350 nm pFPEPI doped PMMA layer embedded between two mirrors, the thin layer has been annealed at 150 ºC during 30 minutes and is deposited by spin coating with at a speed of 2000 rpm for 30 s. The bottom mirror is a commercial dielectric mirror, centered at 2.41 eV at normal incidence, the reflectivity at 2.41 eV is 98%, the stop-band extends from 1.98 eV to 2.95 eV. The top mirror of the cavity is produced by thermal evaporation of silver, its thickness is around 30–40 nm. The refractive index n of the pFPEPI doped PMMA layer has been measured by spectroscopic ellipsometry measurement (Sopra Model GESP5): n = 1.654, we have thus realized a λ-cavity. Angle-resolved reflectivity measurements of the microcavity are performed using a xenon lamp as the excitation source, between the angles 0 º and 65 º, at room temperature. Varying the angle of incidence θ relative to the surface normal allows to tune the relative energy separation between the active layer exciton energy (considered as dispersionless and so angle-independant) and the cavity photon mode energy, which is related to θ byFigure 3(a) shows a series of reflectivity spectra at room temperature. Two dips, whose energy position, intensity and linewidth are angle dependent, are observed. A clear anticrossing between the two transitions can be seen, which is the signature of the strong coupling between the active layer exciton and the cavity mode. The energy of the two minima observed in Fig. 3(a) are reported in Fig. 3(c) (as a function of k//, k// = (E/hc)sinθ. The experimental results are fitted to the dispersion using a standard two-level model (solid lines in Fig. 3(c)):
This relation is the same as the relation obtained for two coupled oscillators with a coupling energy V. V is a fitting parameter, assumed to be constant at all angles. A good agreement between the experimental results and the two level model is obtained for E0=2.18, neff = 1.624 and V = 44.8 meV. The value of neff obtained from the fit is quite coherent with the value of the refractive index of pFPEPI dozped PMMA obtained from spectroscopic ellipsometry measurements. The Rabi splitting is of the order of magnitude of 90 meV. This Rabi splitting is slightly smaller that the Rabi splitting obtained with a spin-coated PEPI layer , which is coherent with the fact that the spin-coated layer lies exactly under the maximum of the electric field, while in this new cavity, the perovskite nanosized inclusions are dispersed along the whole cavity.
Photoluminescence spectra have been performed for various detection angles, ranging from 0 º to 60 º, they are reported in Fig. 3(b). The energy position of the photoluminescence peaks have been reported as stars in Fig. 3(c). For angles up to 35 º, two peaks are present in the spectra. The position of the high energy peak is independant of the detection angle, whereas the position of the lower energy peak varies as the detection angle is tuned. The dispersionless data correspond to the noncoupled part of the perovskite exciton, since the energy position of this peak corresponds to the energy position of the photoluminescence of the active layer in figure 1(c). The variation of the low energy PL peak as a function of k// coincides with the disperson relation of the low energy polaritonic branch.
In summary, we have synthetized a new kind of perovskite material: pFPEPI doped PMMA layers, presenting a high stabilty under laser illumination and a very small roughness. We have realized a vertical microcavity containing this new material, demonstrated that the strong coupling can be observed at room temperature, and observed the emission of the low energy polaritonic branch. This opens the way to the realization of high quality factor cavities containing perovskites by closing the cavity with a dielectric mirror and then to the study of non-linear effects from experiments using strong laser illumination.
This work is supported by Agence Nationale pour la Recherche (grant PEROCAI) and by Triangle de la Physique (grant CAVPER). Laboratoire de Photonique Quantique et Moléculaire and Laboratoire de Photophysique et Photochimie Supramoléculaires et Macromoléculaires are unités mixtes de recherche associées au CNRS (UMR8537 and UMR8531 respectively).
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