Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two-dimensional optoelectronic technologies. These ...processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K valleys in each layer. Because these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technique of femtosecond pump–probe spectroscopy. Upon photoexcitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and thus momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiative recombination with rates varying by up to three-orders of magnitude from sample to sample but with no correlation with interlayer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.
Singlet fission, the splitting of a singlet exciton into two triplet excitons in molecular materials, is interesting not only as a model many-electron problem, but also as a process with potential ...applications in solar energy conversion. Here we discuss limitations of the conventional four-electron and molecular dimer model in describing singlet fission in crystalline organic semiconductors, such as pentacene and tetracene. We emphasize the need to consider electronic delocalization, which is responsible for the decisive role played by the Mott-Wannier exciton, also called the charge transfer (CT) exciton, in mediating singlet fission. At the strong electronic coupling limit, the initial excitation creates a quantum superposition of singlet, CT, and triplet-pair states, and we present experimental evidence for this interpretation. We also discuss the most recent attempts at translating this mechanistic understanding into design principles for CT state-mediated intramolecular singlet fission in oligomers and polymers.
Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient ...scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH₃NH₃PbBr₃, CH(NH₂)₂PbBr₃, and CsPbBr₃. We observed hot fluorescence emission from energetic carriers with ~10²-picosecond lifetimes in CH₃NH₃PbBr₃ or CH(NH₂)₂PbBr₃, but not in CsPbBr₃. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.
Triplet Pair States in Singlet Fission Miyata, Kiyoshi; Conrad-Burton, Felisa S; Geyer, Florian L ...
Chemical reviews,
03/2019, Letnik:
119, Številka:
6
Journal Article
Recenzirano
This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S1 ) splitting into two triplet states (2 × T1 ) in molecular chromophores. ...Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, 1(TT) , from one which does not, 1(T···T) . Only the rate of formation of 1(T···T) is defined as that of singlet fission. We present distinct experimental evidence for 1(TT) , whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.
Two-dimensional materials from layered van der Waals (vdW) crystals hold great promise for electronic, optoelectronic, and quantum devices, but technological implementation will be hampered by the ...lack of high-throughput techniques for exfoliating single-crystal monolayers with sufficient size and high quality. Here, we report a facile method to disassemble vdW single crystals layer by layer into monolayers with near-unity yield and with dimensions limited only by bulk crystal sizes. The macroscopic monolayers are comparable in quality to microscopic monolayers from conventional Scotch tape exfoliation. The monolayers can be assembled into macroscopic artificial structures, including transition metal dichalcogenide multilayers with broken inversion symmetry and substantially enhanced nonlinear optical response. This approach takes us one step closer to mass production of macroscopic monolayers and bulk-like artificial materials with controllable properties.
Kiyoshi Miyata and X.-Y. Zhu analyse the ferroelectric-like dielectric response of lead halide perovskites in the terahertz region and discuss the potential role of polar nanodomains in accounting ...for the defect tolerance and low recombination rates of these materials.
Trap States in Lead Iodide Perovskites Wu, Xiaoxi; Trinh, M. Tuan; Niesner, Daniel ...
Journal of the American Chemical Society,
02/2015, Letnik:
137, Številka:
5
Journal Article
Recenzirano
Recent discoveries of highly efficient solar cells based on lead iodide perovskites have led to a surge in research activity on understanding photo carrier generation in these materials, but little ...is known about trap states that may be detrimental to solar cell performance. Here we provide direct evidence for hole traps on the surfaces of three-dimensional (3D) CH3NH3PbI3 perovskite thin films and excitonic traps below the optical gaps in these materials. The excitonic traps possess weak optical transition strengths, can be populated from the relaxation of above gap excitations, and become more significant as dimensionality decreases from 3D CH3NH3PbI3 to two-dimensional (2D) (C4H9NH3I)2(CH3NH3I) n−1(PbI2) n (n = 1, 2, 3) perovskites and, within the 2D family, as n decreases from 3 to 1. We also show that the density of excitonic traps in CH3NH3PbI3 perovskite thin films grown in the presence of chloride is at least one-order of magnitude lower than that grown in the absence of chloride, thus explaining a widely known mystery on the much better solar cell performance of the former. The trap states are likely caused by electron–phonon coupling and are enhanced at surfaces/interfaces where the perovskite crystal structure is most susceptible to deformation.
Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new ...strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general.
While the perovskite fever has focused on three-dimensional crystalline solids, this class of material can also self-assemble into two-dimensional (2D) layered structures that are natural quantum ...wells with tunable thickness and optoelectronic properties. Here we apply femtosecond transient absorption spectroscopy to study the many-body optical responses of 2D perovskites with the general formula of (C4H9NH3I)2(CH3NH3I) n−1(PbI2) n , where n = 1, 2, 3) is the number of lead iodide unit cells in the direction perpendicular to the 2D quantum well. In the thinnest quantum well (n = 1), above-gap optical excitation induces a blue shift but no population bleaching at the excitonic resonance; this is similar to the many-body optical response of conventional inorganic quantum wells. In contrast to inorganic quantum wells, we find the excitonic blue-shift in 2D perovskites to be independent of excitation power density. We take this as evidence for a Mott-Wannier exciton localizing into a “puddle”, which only exerts local influence on subsequent optical excitations. The excitonic puddles likely come from the disordered electronic energy landscape expected for the soft 2D hybrid organic–inorganic perovskite lattice. As the thickness of the quantum well increases to n = 3, free carrier characters start to show up for above band gap excitation; this is reflected in the broadening and bleaching of the excitonic resonance (in addition to blue-shift), attributed to carrier-exciton collision and screening of the Coulomb potential, respectively.
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