Separation of Spin and Charge Transport in Pristine π-Conjugated Polymers.

We have experimentally tested whether spin-transport and charge-transport in pristine π-conjugated polymer films at room temperature occur via the same electronic processes. We have obtained the spin diffusion coefficient of several π-conjugated polymer films from the spin diffusion length measured by the technique of inverse spin Hall effect and the spin relaxation time measured by pulsed electrically detected magnetic resonance spectroscopy. The charge diffusion coefficient was obtained from the time-of-flight mobility measurements on the same films. We found that the spin diffusion coefficient is larger than the charge diffusion coefficient by about 1-2 orders of magnitude and conclude that spin and charge transports in disordered polymer films occur through different electronic processes.

Control of Whispering Gallery Modes and PT-Symmetry Breaking in Colloidal Quantum Dot Microdisk Lasers with Engineered Notches.

Whispering gallery mode resonators have been demonstrated to be a great way to achieve superior optical cavities with high quality factor and small mode volume. However, due to the high sensitivity of these modes to the properties of the resonator boundary, they are susceptible to parasitic splitting of clockwise and counterclockwise modes. In this work, we investigate the effect of implantation of an engineered notch into the boundary of a circular microdisk resonator fabricated from colloidal quantum dots, which are particularly sensitive to boundary defects. We observed a strong reduction of parasitic mode splitting with introduction of a large engineered notch, as well as enhanced directionality of laser emission. We further investigate the performance of these resonators in evanescently coupled pairs, where the modal interaction allows modulation of laser behavior through variation of the gain and loss induced by the optical pump. We show that two distinct cases of modal interaction can be achieved by adjusting the size of the engineered notch, providing a bridge between intra- and interdisk modal interactions for laser spectral control.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes.

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 µA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 µA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Hybrid-state emission in a polythienylenevinylene derivative with an electron deficient moiety.

The photoluminescence (PL) of a novel imide-substituted poly(3-thienylenevinylene) derivative (imidePTV) was studied in film and solution. PL quantum efficiency was measured to be more than two orders of magnitude larger than its nonluminescent counterpart, namely, alkyl-substituted PTV and was interpreted as evidence for a near degeneracy of optically allowed 1(1)Bu and optically forbidden 2(1)Ag excitonic states. As a result, coexistence of 2(1)Ag and 1(1)Bu emissions was observed, and the predominance was found to be sensitive to temperature and morphological environment. PL of solutions in solvents of higher polarity and polarizability and from low-temperature films was dominated by the transition from the dipole allowed 1(1)Bu state. On the other hand, the PL spectra of films at high temperature and solutions in solvents of low polarity and polarizability were primarily from the 2(1)Ag state that obtains a finite transmission moment from an asymmetric perturbation mixing with the 1(1)Bu.

Optical Studies of Doped Two-Dimensional Lead Halide Perovskites: Evidence for Rashba-Split Branches in the Conduction Band.

Two-dimensional (2D) hybrid organic/inorganic perovskites are an emerging materials class for optoelectronic and spintronic applications due to strong excitonic absorption and emission, large spin-orbit coupling, and Rashba spin-splitting effects. For many of the envisioned applications, tuning the majority charge carrier (electron or hole) concentration is desirable, but electronic doping of metal-halide perovskites has proven to be challenging. Here, we demonstrate electron injection into the lower-energy branch of the Rashba-split conduction band of 2D phenethylammonium lead iodide by means of n-type molecular doping at room temperature. The molecular dopant, benzyl viologen (BV), is shown to compensate adventitious p-type impurities and can lead to a tunable Fermi level above the conduction band minimum and increased conductivity in intrinsic samples. The doping-induced carrier concentration is monitored by the observation of free-carrier absorption and intraband optical transitions in the infrared spectral range. These optical measurements allow for an estimation of the Rashba splitting energy ER ≈38 ± 4 meV. Photoinduced quantum beating measurements demonstrate that the excess electron density reduces the electron spin g-factor by ca. 6%. This work demonstrates controllable carrier concentrations in hybrid organic/inorganic perovskites and yields potential for room temperature spin control through the Rashba effect.

Rashba splitting in organic-inorganic lead-halide perovskites revealed through two-photon absorption spectroscopy.

The Rashba splitting in hybrid organic-inorganic lead-halide perovskites (HOIP) is particularly promising and yet controversial, due to questions surrounding the presence or absence of inversion symmetry. Here we utilize two-photon absorption spectroscopy to study inversion symmetry breaking in different phases of these materials. This is an all-optical technique to observe and quantify the Rashba effect as it probes the bulk of the materials. In particular, we measure two-photon excitation spectra of the photoluminescence in 2D, 3D, and anionic mixed HOIP crystals, and show that an additional band above, but close to the optical gap is the signature of new two-photon transition channels that originate from the Rashba splitting. The inversion symmetry breaking is believed to arise from ionic impurities that induce local electric fields. The observation of the Rashba splitting in the bulk of HOIP has significant implications for the understanding of their spintronic and optoelectronic device properties.

Quantifying Exciton Heterogeneities in Mixed-Phase Organometal Halide Multiple Quantum Wells via Stark Spectroscopy Studies.

Solution-processable two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) quantum wells naturally self-assemble through weak van der Waals forces. In this study, we investigate the structural and optoelectronic properties of 2D-layered butylammonium (C4H9NH3+, BA+) methylammonium (CH3NH3+, MA) lead iodide, (BA)2(MA)n-1PbnI3n+1 quantum wells with varying n from 1 to 4. Through conventional structural characterization, (BA)2(MA)n-1PbnI3n+1 thin films showcase high-quality phase (n) purity. However, while investigating the optoelectronic properties, it is clear that these van der Waals heterostructures consist of multiple quantum well thicknesses coexisting within a single thin film. We utilized electroabsorption spectroscopy and Liptay theory to develop an analytical tool capable of deconvoluting the excitonic features that arise from different quantum well thicknesses (n) in (BA)2(MA)n-1PbnI3n+1 thin films. To obtain a quantitative assessment of exciton heterogeneities within a thin film comprising multiple quantum well structures, exciton resonances quantified by absorption spectroscopy were modeled as Gaussian features to yield various theory-generated electroabsorption spectra, which were then fit to our experimental electroabsorption features. In addition to identifying the quantum well heterostructures present within a thin film, this novel analytical tool provides powerful insights into the exact exciton composition and can be utilized to analyze the optoelectronic properties of many other mixed-phase quantum well heterostructures beyond those formed by OIHPs. Our findings may help in designing more efficient and reproducible light-emitting diodes based on 2D mixed-phase metal-organic multiple quantum wells.

Robust lasing modes in coupled colloidal quantum dot microdisk pairs using a non-Hermitian exceptional point.

Evanescently coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. It remains an open question how scalable and versatile such phenomena can be when carried over to other designs. Here we have studied the effect of gain/loss modulation in an evanescently coupled pair of microdisk optical resonators fabricated from solution-processed colloidal quantum dots. The emission spectra of these structures are sensitive to small imperfections, which cause frequency-splitting of the whispering gallery modes. Despite this inherent disorder, we found that when spatially modulating the optical pump to vary the gain differential between the coupled microdisks, the coupling drives the split parasitic intra-cavity modes into coalescence at an exceptional point of the resulting three-mode system. This unusual behavior is rationalized via a Hamiltonian that incorporates the intra-cavity coupling as well as the anisotropic inter-cavity coupling between modes in the microdisk pair.

Electroabsorption Spectroscopy Studies of (C4H9NH3)2PbI4 Organic-Inorganic Hybrid Perovskite Multiple Quantum Wells.

Two-dimensional (2D) organic-inorganic hybrid perovskite multiple quantum wells that consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies of order of 0.3 eV due to the dielectric confinement between the inorganic and organic layers. We have investigated the exciton characteristics of 2D butylammonium lead iodide, (C4H9NH3)2PbI4 using photoluminescence and UV-vis absorption in the temperature range of 10 K to 300 K, and electroabsorption spectroscopy. The evolution of an additional absorption/emission at low temperature indicates that this compound undergoes a phase transition at ≈250 K. We found that the electroabsorption spectrum of each structural phase contains contributions from both quantum confined exciton Stark effect and Franz-Keldysh oscillation of the continuum band, from which we could determine more accurately the 1s exciton, continuum band edge, and the exciton binding energy.

Role of Intrinsic Ion Accumulation in the Photocurrent and Photocapacitive Responses of MAPbBr3 Photodetectors.

We studied steady state and transient photocurrents in thin film and single-crystal devices of MAPbBr3, a prototype organic-inorganic hybrid perovskite. We found that the devices' capacitance is abnormally large, which originates from accumulation of large densities of Pb2+ and Br- in the active perovskite layer. Under applied bias, these ions are driven toward the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large capacitive response dominates the transient photocurrent and, therefore, obscures the weaker contribution from the photocarriers' drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photodetectors for visible light.

Generation and recombination kinetics of optical excitations in poly(3-dodecylthienylenevinylene) with controlled regioregularity.

Regioregularity in π-conjugated polymers is known to have desirable effects on chain organization in the solid state leading to improved characteristics for electronic device applications. In this report, we studied the photophysics of a new series of π-conjugated polymers, namely, poly(3-dodecylthienylenevinylene), with controlled regioregularity (PTV-CR), using the steady state photoinduced absorption (PIA), doping-induced absorption (DIA), thermally modulated absorption (TMA), and photoluminescence (PL) techniques in a broad spectral range, as well as Grazing incidence X-ray diffraction (GIXRD). PTV-CR has a low band gap of 1.65-1.70 eV from the most random PTV-5:5 to regular PTV-0:10. We have found that regioregularity affects optical properties of PTV-CR in different ways than in the case of well-studied poly(3-alkylthiophene). We discovered that even the most random PTV-5:5 shows submicrometer crystalline order, and there is no correlation between regioregularity and crystallinality. We found that PTV is intrinsically dark due to the reverse order of 1B(u) and 2A(g) excitons, with PL quantum efficiency ranging from 10(-5) to 2 × 10(-4). Spectroscopic data confirm the existence of long-lived neutral (triplets) and charged (polaron) excitations, and thus PTV is a unique nondegenerate ground state π-conjugated polymer which is intrinsically nonluminescent. The recombination mechanism and lifetime of such long-lived excitations are investigated through the pump intensity and pump frequency dependencies as well as temperature dependence of the sample. Increasing regioregularity is seen to reduce the thermal responsivity and intersystem crossing in these films. The novel, intrachain character of these effects and their implications for use of PTV derivatives for photovoltaic application are discussed.