Temperature dependent polariton emission from strongly coupled organic semiconductor microcavities

We investigated the absorption and photoluminescence (PL) of J-aggregates of a cyanine dye both in a thin film format and when used as the active layer in a strongly-coupled microcavity. We show that as temperature is reduced, the absorption linewidth of the J-aggregates narrows and shifts to higher energy. When the J-aggregate is placed in a microcavity we find that the energy of the polariton modes also shifts to higher energies as temperature is reduced. We compare the intensity of PL emission from the upper and lower branches at resonance as a function of temperature, and find that it can be described by an activation energy of 25 meV. PL emission spectra at resonance also suggest that uncoupled excitons inside the microcavity populate the upper polariton branch states

Detecting 6 MV X-rays using an organic photovoltaic device

An organic photovoltaic (OPV) device has been used in conjunction with a flexible inorganic phosphor to produce a radiation tolerant, efficient and linear detector for 6 MV Xrays. The OPVs were based on a blend of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). We show that the devices have a sensitivity an order of magnitude higher than a commercial silicon detector used as a reference. Exposure to 360 Grays of radiation resulted in a small (2%) degradation in performance demonstrating that these detectors have the potential to be used as flexible, real-time, in vivo dosimeters for oncology treatments. (C) 2009 Elsevier B.V. All rights reserved

An optical nanocavity incorporating a fluorescent organic dye having a high quality factor

We have fabricated an L3 optical nanocavity operating at visible wavelengths that is coated with a thin-film of a fluorescent molecular-dye. The cavity was directly fabricated into a pre-etched, free-standing silicon-nitride (SIN) membrane and had a quality factor of Q = 2650. This relatively high Q-factor approaches the theoretical limit that can be expected from an L3 nanocavity using silicon nitride as a dielectric material and is achieved as a result of the solvent-free cavity-fabrication protocol that we have developed. We show that the fluorescence from a red-emitting fluorescent dye coated onto the cavity surface undergoes strong emission intensity enhancement at a series of discrete wavelengths corresponding to the cavity modes. Three dimensional finite difference time domain (FDTD) calculations are used to predict the mode structure of the cavities with excellent agreement demonstrated between theory and experiment

Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities

We have developed a model accounting for the photo-excitation dynamics and the photoluminescence of strongly coupled J-aggregate microcavities. Our model is based on a description of the J-aggregate film as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton-polaritons are formed, mixing superradiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a CW non resonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a paritticular set of discrete optical molecular vibrations active in relaxation processes are investigared.Comment: 8 pages, 6 figure

Bose condensation in a model microcavity

We study the equilibrium properties of a system of dipole-active excitons coupled to a single photon mode at fixed total excitation. Treating the presence or absence of a trapped exciton as a two-level system produces a model that is exactly soluble. It gives a simple description of the physics of polariton condensation in optical cavities beyond the low-density bosonic regime.Comment: 5 pages, 3 figures, uses RevTeX and psfig. Revised version: (1)Corrects an error in our treatment of the constraint, leading to a rescaled transition temperature, and (2)Extends our discussion of the relevance of the model to real system

The development of spray-coated perovskite solar cells

Over the past six years, researchers have investigated the use of spray coating to fabricate perovskite solar cells (PSCs), with the aim of demonstrating its viability as an industrial manufacturing process. This spotlight on applications outlines the key benefits of this coating technology and summarizes progress made to date, with attention focused on varied efforts to control the crystallization and uniformity of the perovskite layer. The emerging understanding of processes required to create smooth, dense spray-cast perovskite films has recently led to the demonstration of fully spray-cast PSCs with a power conversion efficiency of 19.4%

A review of non-fullerene polymer solar cells: from device physics to morphology control

The rise in power conversion efficiency of organic photovoltaic (OPV) devices over the last few years has been driven by the emergence of new organic semiconductors and the growing understanding of morphological control at both the molecular and aggregation scales. Non-fullerene OPVs adopting p-type conjugated polymers as the donor and n-type small molecules as the acceptor have exhibited steady progress, outperforming PCBM-based solar cells and reaching efficiencies of over 15% in 2019. This review starts with a refreshed discussion of charge separation, recombination, and V OC loss in non-fullerene OPVs, followed by a review of work undertaken to develop favorable molecular configurations required for high device performance. We summarize several key approaches that have been employed to tune the nanoscale morphology in non-fullerene photovoltaic blends, comparing them (where appropriate) to their PCBM-based counterparts. In particular, we discuss issues ranging from materials chemistry to solution processing and post-treatments, showing how this can lead to enhanced photovoltaic properties. Particular attention is given to the control of molecular configuration through solution processing, which can have a pronounced impact on the structure of the solid-state photoactive layer. Key challenges, including green solvent processing, stability and lifetime, burn-in, and thickness-dependence in non-fullerene OPVs are briefly discussed

The crossover between lasing and polariton condensation in optical microcavities

We study a model of a photon mode dipole-coupled to a medium of two-level oscillators in a microcavity in the presence of dephasing processes introduced by coupling to external baths. Decoherence processes can be classified as pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic impurities in superconductors. In the absence of dephasing, the ground state of the model is a polariton condensate with a gap in the excitation spectrum. Increase of the pair-breaking parameter $\gamma$ reduces the gap, which becomes zero at a critical value $\gamma_{C1}$; for large $\gamma$, the conventional laser regime is obtained in a way that demonstrates its close analogy to a gapless superconductor. In contrast, weak non-pair-breaking processes have no qualitative effect on the condensate or the existence of a gap, although they lead to inhomogeneous broadening of the excitations

Optical nanolithography using a scanning near-field probe with an integrated light source

An ultracompact near-field optical probe is described that is based on a single, integrated assembly consisting of a gallium nitride (GaN) light-emitting diode (LED), a microlens, and a cantilever assembly containing a hollow pyramidal probe with a subwavelength aperture at its apex. The LED emits ultraviolet light and may be used as a light source for near-field photolithographic exposure. Using this simple device compatible with many commercial atomic force microscope systems, it is possible to form nanostructures in photoresist with a resolution of 35 nm, corresponding to λ/10. © 2008 American Institute of Physics