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Lannion, France

Fang H.-H.,Zernike Institute for Advanced Materials | Raissa R.,Zernike Institute for Advanced Materials | Abdu-Aguye M.,Zernike Institute for Advanced Materials | Adjokatse S.,Zernike Institute for Advanced Materials | And 3 more authors.
Advanced Functional Materials | Year: 2015

Hybrid organometal halide perovskites have been demonstrated to have outstanding performance as semiconductors for solar energy conversion. Further improvement of the efficiency and stability of these devices requires a deeper understanding of their intrinsic photophysical properties. Here, the structural and optical properties of high-quality single crystals of CH3NH3PbI3 from room temperature to 5 K are investigated. X-ray diffraction reveals an extremely sharp transition at 163 K from a twinned tetragonal I4/mcm phase to a low-temperature phase characterized by complex twinning and possible frozen disorder. Above the transition temperature, the photoluminescence is in agreement with a band-edge transition, explaining the outstanding performances of the solar cells. Whereas below the transition temperature, three different excitonic features arise, one of which is attributed to a free-exciton and the other two to bound excitons (BEs). The BEs are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon at high power excitation. The long lifetime and the saturation effect make us attribute these low temperature features to bound triplet excitons. This results in a description of the room temperature recombination as being due to spontaneous band-to-band radiative transitions, whereas a diffusion-limited behavior is expected for the low-temperature range. Low-temperature photophysical investigations of CH3NH3PbI3 single crystals indicate that the recombination in these perovskites is due to spontaneous band-to-band radiative transition at room temperature and to singlet-free-exciton and bound-triplet excitons below the phase transition temperature. The bound-triplet excitons are characterized by a decay dynamics of about 5 μs and by a saturation phenomenon due to many-body interactions. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ding Y.,Technical University of Denmark | Peucheret C.,CNRS Foton Laboratory | Ou H.,Technical University of Denmark | Yvind K.,Technical University of Denmark
Optics Letters | Year: 2014

We design and fabricate an ultrahigh coupling efficiency (CE) fully etched apodized grating coupler on the siliconon- insulator (SOI) platform using subwavelength photonic crystals and bonded aluminum mirror. Fabrication error sensitivity and coupling angle dependence are experimentally investigated. A record ultrahigh CE of -0.58 dB with a 3 dB bandwidth of 71 nm and low back reflection are demonstrated. © 2014 Optical Society of America.

Dumeige Y.,CNRS Foton Laboratory
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We theoretically analyze the second-harmonic generation process in a sequence of unidirectionnaly coupled doubly resonant whispering gallery mode semiconductor resonators. By using a convenient design, it is possible to coherently sum the second-harmonic fields generated inside each resonator. We show that resonator coupling allows the bandwidth of the phase-matching curve to be increased with respect to single-resonator configurations simultaneously taking advantage of the resonant feature of the resonators. This quasi-phase-matching technique could be applied to obtain small-footprint nonlinear devices with large bandwidth and limited nonlinear losses. The results are discussed in the framework of the slow-light-effect enhancement of second-order optical nonlinearities. © 2011 American Physical Society.

Even J.,CNRS Foton Laboratory | Pedesseau L.,CNRS Foton Laboratory | Katan C.,CNRS Chemistry Institute of Rennes
Physical Chemistry Chemical Physics | Year: 2014

Yun Wang et al. used density functional theory (DFT) to investigate the orthorhombic phase of CH3NH3PbI3, which has recently shown outstanding properties for photovoltaic applications. Whereas their analysis of ground state properties may represent a valuable contribution to understanding this class of materials, effects of spin-orbit coupling (SOC) cannot be overlooked as was shown in earlier studies. Moreover, their discussion on optical properties may be misleading for non-DFT-experts, and the nice agreement between experimental and calculated band gap is fortuitous, stemming from error cancellations between SOC and many-body effects. Lastly, Bader charges suggest potential problems during crystal structure optimization. © 2014 the Owner Societies.

Even J.,CNRS Foton Laboratory | Pedesseau L.,CNRS Foton Laboratory | Katan C.,CNRS Chemistry Institute of Rennes
Journal of Physical Chemistry C | Year: 2014

Solution-processable metal-halide perovskites recently opened a new route toward low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites, but despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials the appropriate absorption and transport properties are afforded by the multibandgap and multivalley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain. © 2014 American Chemical Society.

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