There is growing interest in using strongly coupled organic microcavities to tune molecular dynamics, including the electronic and vibrational properties of molecules. However, very little attention has been paid to the utility of cavity polaritons as sensors for out-of-equilibrium phenomena, including thermal excitations. Here, we demonstrate that non-resonant infrared excitation of an organic microcavity system induces a transient response in the visible spectral range near the cavity polariton resonances. We show how these optical response can be understood in terms of ultrafast heating of electrons in the metal cavity mirror, which modifies the effective refractive index and subsequently the strong coupling conditions. The temporal dynamics of the microcavity are strictly determined by carriers in the metal, including the cooling of electrons via electron-phonon coupling and excitation of propagating coherent acoustic modes in the lattice. We rule out multiphoton excitation processes and verify that no real polariton population exists despite their strong transient features. These results suggest the promise of cavity polaritons as sensitive probes of non-equilibrium phenomena.
We compute the short-circuit diffusion current of excitons in an organic solar cell, with special emphasis on fluorescence losses. The exciton diffusion length is not uniform but varies with its position within the device, even with moderate fluorescence quantum efficiency. With large quantum efficiencies, the rate of fluorescence can be strongly reduced with proper choices of the geometrical and dielectric parameters. In this way, the diffusion length can be increased and the device performance significantly improved.
We explore the generation of anti-Stokes fluorescence from strongly coupled organic dye microcavities following resonant ground-state excitation. We observe polariton emission along the lower polariton branch, with our results indicating that this process involves a return to the exciton reservoir and the absorption of thermal energy from molecules in a vibrationally excited ground-state. We speculate that the generation of a population of "hot" polaritons is enhanced by the fact that the cavity supresses the emission of Stokes-shifted fluorescence, as it is located energetically below the cut…
We report on the quantum confinement of zero-dimensional polaritons in perovskite-based microcavity at room temperature. Photoluminescence of discrete polaritonic states are observed for polariton localized in symmetric sphere-like defects which are spontaneously nucleated on the top dielectric Bragg mirror. The linewidth of these confined states are found much sharper (almost one order of magnitude) than that of photonic modes in the perovskite planar microcavity. Our results show the possibility to study organic-inorganic cavity polariton in confined microstructure and suggest a fabrication …
In organic photovoltaics, the mechanism by which free electrons and holes are generated overcoming the Coulomb attraction is a currently much debated topic. To elucidate this mechanism at a molecular level, we carried out a combined electronic structure and quantum dynamical analysis that captures the elementary events from the exciton dissociation to the free carrier generation at polymer/fullerene donor-acceptor heterojunctions. Our calculations show that experimentally observed efficient charge separations can be explained by a combination of two effects: First, the delocalization of charge…
The mechanism of electron-hole separation in organic solar cells is currently hotly debated. Recent experimental work suggests that these charges can separate on extremely short timescales (<100 fs). This can be understood in terms of delocalised transport within fullerene aggregates, which is thought to emerge on short timescales before vibronic relaxation induces polaron formation. However, in the optimal heterojunction morphology, electrons and holes will often re-encounter each other before reaching the electrodes. If such charges trap and cannot separate, then device efficiency will suffe…
Epsilon-near-zero (ENZ) materials have shown significant potential for nonlinear optical applications due to their ultrafast hot carriers and consequent optical nonlinearity enhancement. Modified poly(3,4-ethylenedioxythiophene) (PEDOT) films show metallic characteristics and a resultant ENZ wavelength near 1550nm through polar solvent treatment and annealing. The metallic PEDOT film exhibits an intrinsic optical nonlinear response that is comparable to gold and 100-fold higher than typical inorganic semiconductor ENZ materials due to π-conjugated delocalized electrons. Hot carriers generate a…
Methylammonium lead iodide (MAPI) is a benchmark hybrid organic perovskite material, which is used for the low-cost, printed solar cells with over 20 percent power conversion efficiency. Yet, the nature of light-matter interaction in MAPI as well as the exact physical mechanism behind device operation is currently debated. Here we report room temperature, ultrafast photocurrent and freespace terahertz (THz) emission generation from unbiased MAPI induced by 150 fs light pulses. Polarization dependence of the observed photoresponse is consistent with the Bulk Photovoltaic Effect (BPVE) caused by…
The effectiveness of molecular-based light harvesting relies on transport of optical excitations, excitons, to charg-transfer sites. Measuring exciton migration has, however, been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. In organic semiconductors, common bulk methods employ a series of films terminated at quenching substrates, altering the spatioenergetic landscape for migration. Here we instead define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native n…
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron accepto…
Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation rates. However, the membranes' use has been limited mostly to aqueous solutions because GO membranes appear to be impermeable to organic solvents, a phenomenon not fully understood yet. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from flakes with large sizes of ~ 10-20 micron. Without sacrificing their sieving characteristics, such membranes can be mad…
Real-time time-dependent density functional theory, in conjunction with the Ehrenfest molecular dynamics scheme, is becoming a popular methodology to investigate ultrafast phenomena on the nanoscale. Thanks to recent developments, it is also possible to explicitly include in the simulations a time-dependent laser pulse, thereby accessing the transient excitation regime. However, the complexity entailed in these calculations calls for in-depth analysis of the accessible and yet approximate (either "dressed" or "bare") quantities, in order to evaluate their ability to provide us with a realistic…
Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time ev…
Organic solar cells must separate strongly bound electron-hole pairs into free charges. This is achieved at interfaces between electron donor and acceptor organic semiconductors. The most popular electron acceptor is the fullerene derivative PCBM. Electron-hole separation has been observed on femtosecond timescales, which is incompatible with conventional Marcus theories of organic transport. In this work we show that ultrafast charge transport in PCBM arises from its broad range of electronic eigenstates, provided by the presence of three closely spaced delocalised bands near the LUMO level. …
We present an exciton/lattice model of the electronic dynamics of primary photoexcitations in a polymeric semiconductor heterojunction which includes both polymer pi-stacking, energetic disorder, and phonon relaxation. Results from our model are consistent with a wide range of recent experimental evidence that excitons decay directly to well-defined polarons on a sub-100 fs timescale, which is substantially faster than exciton relaxation processes. Averaging over multiple samples, we find that as the interfacial offset is increased, a substantial fraction of the density of electronic states in…
Low power photon upconversion (UC) based on sensitized triplet-triplet annihilation (sTTA) is considered as the most promising upwards wavelength-shifting technique to enhance the light harvesting capability of solar devices by recovering the low-energy tail of the solar spectrum. Semiconductor nanocrystals (NCs) with conjugated organic ligands have been proposed as broadband sensitizers for extending the light-harvesting capability of sTTA-UC molecular absorbers. Key to their functioning is efficient energy transfer (ET) from the NC to the triplet state of the ligands that sensitizes the trip…
Metal-corroles are macrocycle organic molecules with numerous practical applications. In particular, copper corroles exhibit an interesting saddled geometry, which has attracted significant attention from theoreticians and experimentalists over the years. The present work is dedicated to understand the effect of structural saddling in a copper corrole on potential probe signals via imaging ultrafast coherent electron dynamics. A linearly polarized pulse is used to trigger the electron dynamics and time-resolved x-ray diffraction is employed to image the triggered dynamics. It is found that the…
With hybrid organic-inorganic perovskites increasing its technological reach, from photovoltaics solar cells to light-emitting devices, to nanoscale transistors, it is critical to establish the role of microstructures in dictating how carrier dynamics dictate device efficiency. Here we report on the ultrafast dynamics of charge carriers in hybrid perovskites at various stages of nucleation and growth kinetics. A solution-processed fabrication technique, with spin-coating conditions optimized to control the nucleation density of an intermediate phase, converts to hybrid perovskites upon a tempe…
Resolving nonlinear recombination dynamics in semiconductors via ultrafast excitation correlation spectroscopy: Photoluminescence versus photocurrent detection
Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our…
The ultrafast dynamics of photon-to-charge conversion in an organic light harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C$_{60}$. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time …
We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.
In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photo- generation dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200 fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrati…
Hybrid organic-inorganic halide perovskites have shown remarkable optoelectronic properties (1-3), believed to originate from correlated motion of charge carriers and the polar lattice forming large polarons (4-7). Few experimental techniques are capable of probing these correlations directly, requiring simultaneous sub-meV energy and femtosecond temporal resolution after absorption of a photon (8). Here we use transient multi-THz spectroscopy, sensitive to the internal motions of charges within the polaron, to temporally and energetically resolve the coherent coupling of charges to longitudin…
Direct visualization of ultrafast coupling between charge carriers and lattice degrees of freedom in photo-excited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behavior of materials under extreme non-equilibrium conditions. Here, by monitoring the evolution of the wave-vector resolved ultrafast electron diffraction intensity following above-bandgap photo-excitation, we obtain a direct visual of the structural dynamics in monocrystalline 2D perovskites. Analysis reveals a surprising, light-induced ultrafast lattice ordering r…
A one-dimensional many-body model is established to mimic the charge distribution and dynamics in nonfullerene organic solar cells. Two essential issues are taken into account in the model: The alternating donor and acceptor structure and the local imbalance of the intrinsic electrons and holes. The alternating structure is beneficial for the direct generation of charge transfer state which enhances the local imbalance of intrinsic charges. The most remarkable outcome of the model is that, due to the strong Coulomb attractive potential energy, the intrinsic charges in the cells are self-accumu…
All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-…
Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly conce…
Driven by the rapid development of non-fullerene electron acceptors (NFAs), the power conversion efficiencies of organic solar cells (OSCs) have reached levels suitable for commercial applications. However, the poor operational stability of high-performance NFA OSCs is a remaining fundamental challenge that must be addressed. Whilst previous studies have primarily focused on the NFA component, we consider here the degradation pathways of both the donor and acceptor materials in the benchmark PM6:Y6 blend. Here, we show that light soaking greatly increases the energetic disorder and trap state …
The soft character of organic materials leads to strong coupling between molecular nuclear and electronic dynamics. This coupling opens the way to control charge transport in organic electronic devices by inducing molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such control has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be controlled by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superp…
