Biexciton Resonances Reveal Exciton Localization in Stacked Perovskite Quantum Wells.

The deployment of perovskite solar cells will rely on further progress in the operating and ambient stability of active layers and interfaces within these materials. Low-dimensional perovskites, also known as perovskite quantum wells (PQWs), utilize organic ligands to protect the perovskite lattice from degradation and offer to improve device stability; combining 2D and 3D perovskites in heterostructures has been shown to take advantage of the high efficiency of the majority 3D active layers and combine it with the improved stability of a thin 2D top layer. Prior PQWs have relied on relatively weak interwell van der Waals bonding between hydrophobic organic moieties of the ligands. Here we instead use the ligand 4-vinylbenzylammonium to form well-ordered PQWs atop a 3D perovskite layer. The ligand's vinyl group is activatedusing UV light which photochemically forms new covalent bonds among PQWs. UV-cross-linked 2D/3D devices show improved operational stability as well as improved long-term dark stability in air: they retain 90% of their initial efficiency after 2300 h of dark aging compared to a retention of 20% of performance in the case of 3D films. The UV-cross-linked PQWs and 2D/3D interfaces reduce device hysteresis and improve the open-circuit voltages to values up to 1.20 V, resulting in more efficient devices (PCE of up to 20.4%). This work highlights the exploitation of the chemical reactivity of PQW ligands to tailor the molecular properties of PQW interfaces for improved stability and performance in 2D/3D perovskite photovoltaics.

Electric field effect on hybrid exciton states in organic-inorganic quantum wells

Hybrid excitons in organic–inorganic semiconducting quantum wells in a microcavity

Solution-processed perovskite quantum wells have been used to fabricate increasingly efficient and stable optoelectronic devices. Little is known about the dynamics of photogenerated excitons in perovskite quantum wells within the first few hundred femtoseconds-a crucial time scale on which energy and charge transfer processes may compete. Here we use ultrafast transient absorption and two-dimensional electronic spectroscopy to clarify the movement of excitons and charges in reduced-dimensional perovskite solids. We report excitonic funneling from strongly to weakly confined perovskite quantum wells within 150 fs, facilitated by strong spectral overlap and orientational alignment among neighboring wells. This energy transfer happens on time scales orders of magnitude faster than charge transfer, which we find to occur instead over 10s to 100s of picoseconds. Simulations of both Förster-type interwell exciton transfer and free carrier charge transfer are in agreement with these experimental findings, with theoretical exciton transfer calculated to occur in 100s of femtoseconds.

In the past few years, with developing the technology of electromagnetically induced transparency (EIT) and improving the semiconductor technology, it has become possible to realize the application of optical soliton to communication device. Studies show the reduction of group velocity of the optical soliton in EIT medium under weak driving condition, which possibly realizes the storing of optical pulses in information storage. More importantly, semiconductor quantum wells have the inherent advantages such as large electric dipole moments of the transitions, high nonlinear optical coefficients, small size, easily operating and integrating. So it is considered to be the most potential EIT medium to realize the application of quantum devices. The optical soliton behavior in the semiconductor quantum well is studied, which can provide a certain reference value for the practical application of information transmission and processing together quantum devices. Although there has been a series of researches on both linear and nonlinear optical properties in semiconductor quantum wells structures, few publications report the effects of the cross-coupling longitude-optical phonon (CCLOP) relaxation on its linear and nonlinear optical properties. However, to our knowledge, the electron-longitude-optical phonon scattering rate can be realized experimentally by varying the sub-picosecond range to the order of a picosecond. According to this, we in the paper study the effects of the CCLOP relaxation on its linear and nonlinear optical properties in a cascade-type three-level EIT semiconductor quantum well. According to the current experimental conditions, we first propose a cascade-type three-level EIT semiconductor quantum well model. And in this model we consider the longitudinal optical phonons coupling between the bond state and anti-bond state. Subsequently, by using the multiple-scale method, we analytically study the dynamical properties of solitons in the cascade-type three-level EIT semiconductor quantum well with the CCRLOP. It is shown that when the CCRLOP strength is smaller, there exhibits the dark soliton in the EIT semiconductor quantum well. Only if the strength of the CCRLOP is larger, will in the system there exists bright soliton. That is to say, with increasing the strength of the CCRLOP, the soliton type of the system is converted from dark to bright soliton little by little. So, the temporal soliton type can be effectively controlled by the strength of the CCRLOP. In addition, we also find that the group velocity of the soliton can also be controlled by the strength of CCRLOP and the control light. These results may provide a theoretical basis for manipulating experimentally the dynamics of soliton in semiconductor quantum wells.

Semiconductor quantum wells underpin a great number of modern optoelectronics technologies. Here, we demonstrate directional photoluminescence arising from: 1) magnetic dipole transitions in layered two-dimensional hybrid organic-inorganic perovskite quantum wells and 2) phased array GaN quantum well metasurfaces.

We present a theoretical review of the properties of electronic excitations in nanostructures based on combinations of organic materials with inorganic semiconductors, having respectively Frenkel excitons and Wannier-Mott excitons with nearly equal energies. We show that in this case the resonant coupling between organic and inorganic quantum wells (or wires or dots) may lead to several interesting effects, such as splitting of the excitonic spectrum and enhancement of the resonant optical nonlinearities. First, we discuss the properties of hybrid Frenkel-Wannier-Mott excitons, which appear when the energy splitting of the excitonic spectrum is large compared to the width of the exciton resonances (the case of strong resonant coupling). Such peculiar excitations share at the same time both the properties of the Wannier excitons (e.g., the large radius) and those of the Frenkel excitons (e.g., the large oscillator strength). We discuss mainly two-dimensional configurations (interfaces or coupled quantum wells) which are the most extensively studied. In particular, we show that hybrid excitons are expected to have resonant optical nonlinearities significantly enhanced with respect to those of traditional inorganic or organic systems. We also consider analogous phenomena in microcavities where the exciton resonances are close to the cavity photon mode resonance. Next, we consider the case of weak resonant coupling and show the relevance of the Förster mechanism of energy transfer from an inorganic quantum well to an organic overlayer. Such an effect may be especially interesting for applications: the electrical pumping of excitons in the semiconductor quantum well can be used to efficiently turn on the organic material luminescence.

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 ...

Hybrid organic–inorganic perovskites have emerged as promising gain media for tunable, solution-processed semiconductor lasers. However, continuous-wave operation has not been achieved so far 1–3 . Here, we demonstrate that optically pumped continuous-wave lasing can be sustained above threshold excitation intensities of ~17 kW cm–2 for over an hour in methylammonium lead iodide (MAPbI3) distributed feedback lasers that are maintained below the MAPbI3 tetragonal-to-orthorhombic phase transition temperature of T ≈ 160 K. In contrast with the lasing death phenomenon that occurs for pure tetragonal-phase MAPbI3 at T > 160 K (ref. 4 ), we find that continuous-wave gain becomes possible at T ≈ 100 K from tetragonal-phase inclusions that are photogenerated by the pump within the normally existing, larger-bandgap orthorhombic host matrix. In this mixed-phase system, the tetragonal inclusions function as carrier recombination sinks that reduce the transparency threshold, in loose analogy to inorganic semiconductor quantum wells, and may serve as a model for engineering improved perovskite gain media. Optically pumped continuous-wave lasing is achieved in methylammonium lead iodide (MAPbI3) distributed feedback lasers that are maintained below the MAPbI3 tetragonal-to-orthorhombic phase transition temperature of 160 K.

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value $\ensuremath{\pi}\ensuremath{\alpha}$ where $\ensuremath{\alpha}$ is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of $\ensuremath{\pi}\ensuremath{\alpha}$, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that $\ensuremath{\pi}\ensuremath{\alpha}$ is also a metric of their absorption when normalized to a single period. We conclude that the $\ensuremath{\pi}\ensuremath{\alpha}$ quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by $\ensuremath{\pi}\ensuremath{\alpha}$ independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector $\mathbf{k}\phantom{\rule{4pt}{0ex}}(\ensuremath{\sim}\mathbf{V}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{k})$, which couples the conduction band to the valence bands. However, the prefactor in front of $\ensuremath{\pi}\ensuremath{\alpha}$ is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of $\ensuremath{\pi}\ensuremath{\alpha}$. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a ``massful gap'' is formed around the Dirac points.

Anti-crossing of organic exciton and photon modes and a room temperature Rabi splitting that is an order of magnitude larger than the highest values reported for inorganic semiconductor quantum wells are observed in organic microcavity embedded quantum wells. Also, this larger splitting due to the large oscillator strength of organic excitons is easily achieved with many organic systems and gives reasonable hope for reaching the strong coupling regime. However, dynamic condensation clearly involving stimulated emission of the k ≈ 0 lower polaritons is expected at relatively low exciton density, due to the very small polariton mass. We report on the theoretical study of non-linear emission of organic microcavity polaritons. In the ‘Boser’ model at low density, we include the acoustic phonon-assisted relaxation and polariton–polariton scattering stimulated by the quasi-bosonic polariton final state population.

The optical properties of nanostructures using composite organic–inorganic semiconductors, are dominated by a new type of excitonic states. These new hybrid excitations can be described as Frenkel-Wannier-Mott excitons. Frenkel excitons have very strong oscillator strength while Wannier-Mott excitons are very sensitive to external perturbations: static electric and magnetic fields. Our interest is centred on mixed exciton formation under magnetic field effects; calculations were performed for a system composed of a monolayer of organic semiconductor (anthracene) weakly adsorbed at a single parabolic quantum well of inorganic semiconductor (ZnSe/ZnCdSe). The application of a magnetic field leads to an additional confinement. With the transverse magnetic field, a changeover of the characteristic length resulting from inorganic well width and the cyclotron length is obtained from the application of the magnetic field. The lower states of the dispersion law for hybrid Frenkel-Wannier exciton possess a minimum near the center of the Brillouin zone. It is deepest with increase in the applied magnetic field.

We study the hybrid exciton-polaritons in a bad microcavity containing the organic and inorganic quantum wells. The corresponding polariton states are given. The analytical solution and numerical result of the stationary spectrum for the cavity field are finished.

Organic and inorganic quantum wells in a microcavity: Frenkel-Wannier-Mott excitons hybridization and energy transformation

Hybrid interface excitons in organic-inorganic quantum wells