Excitonic properties and ultrafast carrier dynamics in hybrid organic-perovskites

International audienceDue to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites (HOPs) has emerged since 2012. HOPs can be arranged in such a way that carriers are free to move at 3D or at 2D. At 3D, the exciton binding energy has been shown to be of the order of tenth of meV. When they are arranged in two dimensions, these materials can be considered as hybrid multi-quantum wells. Both quantum and dielectric confinement lead to a huge exciton binding energy of several hundreds of meV.In this talk, we will first discuss about the excitonic properties of CH3NH3PbI3 single crystals at low temperature, and compare them to those of thin polycrystalline films. The main feature is the appearance of a sharp emission line (FWHM ~ 5 meV) at high energy that is tentatively attributed to the free exciton signature. Experiments as a function of temperature confirm the existence of a strong electron-phonon coupling in HOPs.Secondly, we will report on ultrafast pump-probe experiments performed on (C6H5C2H4NH3)2PbI4 thin layers at room temperature. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. The presence of a long tail in the pump/probe signal is attributed to trapped excitons on dark states, while an ultrafast intraband relaxation (τintra ≤ 150 fs) is reported. The analysis of the transient broadening and loss of oscillator strength demonstrates that 2D-HOPs share common behaviours with standard semiconductors quantum wells despite their huge exciton binding energy that is closer to the one reported for organic semiconductors. Finally, preliminary results on the intraband relaxation in CH3NH3PbI3 will be presented

Photoluminescence Tuning Through Irradiation Defects in CH3_3NH3_3PbI3_3 Perovskites

International audienceDefect engineering is applied to hybrid (CH3NH3)PbI3 organic–inorganic perovskites. These materials have become one of the most promising low‐cost alternatives to traditional semiconductors in the field of photovoltaics and light emitting devices. Here Helium ion irradiation at low energy has been used as a tool for the controlled introduction of point defects in both single crystals and polycrystalline thin films. The irradiation defects modify the opto‐electronic properties as probed using photoluminescence (PL) spectroscopy from 10 K to room‐temperature. Contrary to usual semiconductors, a very good resilience of the PL properties with irradiation is observed, even associated to an enhancement of the optical emission at low temperature. These results are discussed in relation with the tetragonal to orthorhombic low‐temperature phase transition below T = 160 K. A comparison between spectra from single crystals and polycrystalline films, both with and without irradiation defects, allows a better understanding of the light emission mechanisms in both kinds of samples. The authors thereby evidence radiation hardness of these materials and the specificity of defects and their impact on light emission properties

Photophysics of self-assembled luminophore-perovskite systems

International audienceDuring the past few years hybrid organic-inorganic perovskites (HOIPs) have attracted much interest as solution-processed semiconductors with high potentialities in optoelectronics. On the one hand “3D” HOIPs such as CH3NH3PbI3 have shown their outstanding performances when incorporated in solar cells [1]. On the other hand their “2D layered” counterparts (R-NH3)2PbI4, where R is an organic group, are promising materials for light emitting devices such as LEDs [2] and Lasers [3]. Indeed their self-assembled, multilayered structure allows strong excitonic emission at room temperature (fig. 1). They offer moreover much flexibility with respect to the organic group R. While most of the reported 2D layered perovskites have optically inactive organic moieties, a huge improvement of the performances could be done by introducing functional organic groups. This is why we propose in the present work two innovative luminophore-perovskite systems. First by introducing 2,3-naphthalimide-ethylammonium (NAAB) fluorescent molecules in the perovskite structure we efficiently improved its excitonic emission [4]. Secondly thanks to the perovskite self-assembling we succesfully templated tetrazine molecules into the perovskite structure and changed their optical properties (fig. 2)

Excitonic properties of hybrid organic perovskite crystals for photovoltaics and lasers.

International audienceSince 2012, hybrid organic perovskites (HOPs) represent a “material breakthrough” for photovoltaïcs, HOP-solar cells reaching an efficiency record of 21%, competing with the efficiencies of the silicon-based cells. The explanation of such a success is related to a combination of good transport properties for both electrons and holes and appropriate excitonic properties. More broadly, HOPs have also high potentialities for light-emitting devices such as electroluminescent diodes and lasers. Thus, it seems that HOPs combine strengths of the inorganic semiconductors and organic semiconductors, likely to solve the contradiction that high charge carrier mobility and large stimulated emissions are required for lasing devices but can’t be found, in general, in the same material.A decisive advantage of the HOPs crystals, allowing the access to this wide range of applications, come from the ability to tune the excitonic effects. In fact HOPs can be arranged in such a way that carriers are able to move at 3, 2, 1 or 0 dimensions. In particular, the exciton binding energy in 3D HOPs has been shown to be of the order of tenth of meV, well suited for an electron-hole separation at room temperature. When HOPs are arranged in two dimensions, they can be considered as hybrid multi-quantum wells in which both quantum and dielectric confinements lead to a huge exciton binding energy of several hundreds of meV stable at room temperature, presenting oscillator strengths one order of magnitude larger than in GaAs/(Ga,Al)As heterostructures.We will discuss here about the optical properties of HOPs single crystals and compare them to those of thin polycrystalline films. In particular, the main feature in CH3NH3PbI3 crystals is the appearance, in the low temperature luminescence spectra, of a sharp emission line (FWHM ~ 5 meV) at high energy which is attributed to the free exciton signature. Experiments performed as a function of temperature confirm the existence of a strong electron-phonon coupling both in 3D and 2D HOPs.We will report also on ultrafast pump-probe experiments performed on (C6H5C2H4NH3)2PbI4 thin layers at room temperature. The exciton dynamics is fitted with a bi-exponential decay with a free exciton lifetime of ∼100 ps. The presence of a long tail in the pump/probe signal is attributed to trapped excitons on dark states, while an ultrafast intraband relaxation (τintra ≤ 150 fs) is reported. The analysis of the transient broadening and loss of oscillator strength demonstrates that 2D HOPs share common behaviours with standard semiconductors quantum wells despite their huge exciton binding energy that is closer to the one reported for organic semiconductors

Photophysics of self-assembled luminophore-perovskite systems

International audienceDuring the past few years hybrid organic-inorganic perovskites (HOIPs) have attracted much interest as solution-processed semiconductors with high potentialities in optoelectronics. On the one hand “3D” HOIPs such as CH3NH3PbI3 have shown their outstanding performances when incorporated in solar cells [1]. On the other hand their “2D layered” counterparts (R-NH3)2PbI4, where R is an organic group, are promising materials for light emitting devices such as LEDs [2] and Lasers [3]. Indeed their self-assembled, multilayered structure allows strong excitonic emission at room temperature (fig. 1). They offer moreover much flexibility with respect to the organic group R. While most of the reported 2D layered perovskites have optically inactive organic moieties, a huge improvement of the performances could be done by introducing functional organic groups. This is why we propose in the present work two innovative luminophore-perovskite systems. First by introducing 2,3-naphthalimide-ethylammonium (NAAB) fluorescent molecules in the perovskite structure we efficiently improved its excitonic emission [4]. Secondly thanks to the perovskite self-assembling we succesfully templated tetrazine molecules into the perovskite structure and changed their optical properties (fig. 2)

Fast growth of monocrystalline thin films of 2D layered hybrid perovskite

International audienceDuring the past few years hybrid organic-inorganic perovskites (HOIPs) have attracted much interest as solution-processed semiconductors with high potentialities in optoelectronics and photovoltaics. On the one hand 3D HOIPs such as CH3NH3PbI3 have shown their outstanding performances when incorporated in solar cells [1]. On the other hand their 2D layered counterparts such as (C6H5C2H4NH3)2PbI4 (PEPI) are promising materials for light emitting devices because of their strong emission at room temperature [2]. However, the optoelectronic properties of hybrid perovskite polycrystalline films suffer from a microscale grain structure. In order to take advantage of the great potential of these materials for both photovoltaics and emitting devices, the synthesis of large monocrystalline films is a key issue. Here we propose a fast crystallization method for the 2D layered hybrid perovskite PEPI. A vapor-assisted process coupled with a capping of the precursor solution allows to grow 2-dimensionnal thin films with millimetric monocrystalline grains, a high aspect ratio and a good surface quality (Fig.1). Moreover, this growth is several orders of magnitudes faster than the other reported techniques. In addition, we highlight the benefits of using γ-butyrolactone (GBL) for the growth of layered perovskites monocrystalline grains

Using a confocal PL microscope to correlate the PL and structural properties of 2D-layered perovskites crystals and thin films

International audienceOver the past few years, hybrid organic perovskites (HOP) were found to be remarkable semiconductors with outstanding optoelectronics properties. These materials are very versatile, as their bandgap and the carrier confinement can be easily tuned chemically. Many studies have focused on 3D perovskites, that tend to be good absorbing materials for thin films photovoltaic devices. Alternatively, another class of perovskites, called 2D perovskites, is getting more attention. These 2D HOP have a natural quantum well structure and a high dielectric excitonic confinement. Nevertheless, many of the fundamental photophysics properties of these 2d HOP remain to be understood. In this presentation, we will study the photoluminescence properties of 2D perovskites based on phenylethylammonium (C6H5C2H4NH3)2PbI4 and their 2d/3d derivatives (including a proportion of methylammonium). A new method will be presented to grow large grains films as well as single crystals with large aspect ratios and low roughness [1]. These single crystals have a low proportion of defects and will be then used to unveil some of the intrinsic properties of these perovskites. In particular a cryogenic confocal study will be presented, to finely probe the optoelectronic properties (excitons/phonons, …) and the presence of defects in the produced samples

Excitonic properties and ultrafast carrier dynamics in hybrid organic-perovskites

International audienceDue to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites (HOPs) has emerged since 2012. HOPs can be arranged in such a way that carriers are free to move at 3D or at 2D. At 3D, the exciton binding energy has been shown to be of the order of tenth of meV. When they are arranged in two dimensions, these materials can be considered as hybrid multi-quantum wells. Both quantum and dielectric confinement lead to a huge exciton binding energy of several hundreds of meV.In this talk, we will first discuss about the excitonic properties of CH3NH3PbI3 single crystals at low temperature, and compare them to those of thin polycrystalline films. The main feature is the appearance of a sharp emission line (FWHM ~ 5 meV) at high energy that is tentatively attributed to the free exciton signature. Experiments as a function of temperature confirm the existence of a strong electron-phonon coupling in HOPs.Secondly, we will report on ultrafast pump-probe experiments performed on (C6H5C2H4NH3)2PbI4 thin layers at room temperature. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. The presence of a long tail in the pump/probe signal is attributed to trapped excitons on dark states, while an ultrafast intraband relaxation (τintra ≤ 150 fs) is reported. The analysis of the transient broadening and loss of oscillator strength demonstrates that 2D-HOPs share common behaviours with standard semiconductors quantum wells despite their huge exciton binding energy that is closer to the one reported for organic semiconductors. Finally, preliminary results on the intraband relaxation in CH3NH3PbI3 will be presented

Fast growth of monocrystalline thin films of 2D layered hybrid perovskite

International audienceHybrid organic-inorganic perovskites are on the way to deeply transform the photovoltaic domain. Likewise, it recently appears that this class of materials have a lot of assets for other optoelectronic applications. One key aspect is to be able to synthesize large areas of monocrystalline thin films. Here, we report on the development of a new synthesis method of 2D hybrid organic-inorganic perovskite called "Anti-solvent Vapor-assisted Capping Crystalliza-tion". This method allows to grow monocrystalline thin films with high aspect ratio in less than 30 minutes