Jean-Christophe Blancon

Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials

Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that, while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid density functional theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA = CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons indicates that in order to mitigate this problem, formamidinium (FA = HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.

Direct measurement of the absolute absorption spectrum of individual semiconducting single-wall carbon nanotubes.

The optical properties of single-wall carbon nanotubes are very promising for developing novel opto-electronic components and sensors with applications in many fields. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. Here we determine using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. These quantitative measurements permit determination of the oscillator strength of the different excitonic resonances and their dependencies on the excitonic transition and type of semiconducting nanotube. A non-resonant background is also identified and its cross-section comparable to the ideal graphene optical absorbance. Furthermore, investigation of the same single-wall nanotube either free standing or lying on a substrate shows large broadening of the excitonic resonances with increase of oscillator strength, as well as stark weakening of polarization-dependent antenna effects, due to nanotube-substrate interaction.

Ultrafast Optical Microscopy of Single Monolayer Molybdenum Disulfide Flakes

We use ultrafast optical microscopy to investigate carrier dynamics in single flakes of atomically thin molybdenum disulfide. By tuning the probe wavelength through the bandgap, we reveal the influence of layer thickness on carrier dynamics.

Design principles for electronic charge transport in solution-processed vertically stacked 2D perovskite quantum wells

State-of-the-art quantum-well-based devices such as photovoltaics, photodetectors, and light-emission devices are enabled by understanding the nature and the exact mechanism of electronic charge transport. Ruddlesden–Popper phase halide perovskites are two-dimensional solution-processed quantum wells and have recently emerged as highly efficient semiconductors for solar cell approaching 14% in power conversion efficiency. However, further improvements will require an understanding of the charge transport mechanisms, which are currently unknown and further complicated by the presence of strongly bound excitons. Here, we unambiguously determine that dominant photocurrent collection is through electric field-assisted electron–hole pair separation and transport across the potential barriers. This is revealed by in-depth device characterization, coupled with comprehensive device modeling, which can self-consistently reproduce our experimental findings. These findings establish the fundamental guidelines for the molecular and device design for layered 2D perovskite-based photovoltaics and optoelectronic devices, and are relevant for other similar quantum-confined systems.Solution-processed two-dimensional perovskite quantum-well-based optoelectronic devices have attracted great research interest but their electrical transport is poorly understood. Tsai et al. reveal that the potential barriers of the quantum wells dominate the transport properties in solar cell devices.

Making and breaking of the exciton in layered halide hybrid perovskites

Layered halide hybrid organic−inorganic perovskites [1] have been the subject of intense investigation before the rise of three-dimensional (3D) halide perovskites and their impressive performance in solar cells. Recently, layered perovskites have also been proposed as attractive alternatives for photostable solar cells [2] and revisited for light-emitting devices. Interestingly, these performances can be traced back to extremely efficient internal exciton dissociation through edge states identified on thin films and single crystals [3]. Layered perovskites present fascinating features with inherent quantum and dielectric confinements imposed by the organic layers sandwiching the inorganic core, and computational approaches have successfully help rationalized their properties (excitonic, Rashba effects, etc.) [4-6]. Here, we propose a joint spectroscopic and computational investigation to unravel the origin of the recently identified layer-edge states in layered Ruddlesden-Popper phases with inorganic layers containing n = 1 to 4 octahedra. We show that for n > 2, the system presents a localized surface state within the band gap. References [1] L. Pedesseau et al., ACS Nano (2016), 10, 9776. [2] H. Tsai et al., Nature (2016), 536, 312. [3] J.-C. Blancon et al., Science (2017), 355, 1288. [4] M. Kepenekian et al., ACS Nano (2015), 12, 11557. [5] D. Sapori, M. Kepenekian, L. Pedesseau, C. Katan, J. Even, Nanoscale (2016), 8, 6369. [6] M. D. Smith et al., Chem. Sci. (2017), 8, 1960.

High-performance, hysteresis free, ambipolar hybrid perovskite based field-effect transistors

Hybrid perovskites are solution processed crystalline materials with excellent electronic and optical properties, which enables high-efficiency optoelectronic devices. However, hybrid perovskites-based field effect transistor operation at room temperature has remained elusive. This is due to the non-reproducibility induced by polar nature of the structure coupled with ionic motions, which screens the capacitively coupled gate voltage. In this study, we report high-performance, hysteresis-free ambipolar FETs using highly crystalline hybrid perovskites thin films, operating at room temperature. We systematically improved the film quality, the effect of high-K dielectrics between the perovskites and gate. As a result, we obtained FETs with high trans-conductance with low subthreshold slopes leading to an on/off ration >104. Moreover, we achieve ambipolar transport at room temperature that strongly correlates to the choice of the gate-dielectric, that allow to tune the Fermi energy of perovskites for electrons and holes injections. We anticipate these results will open up the systematic investigation on the electronic properties in hybrid perovskites materials, for the opportunities to discover novel devices functionalities such as ultrasensitive photo-transistors and spin FETs.