Role of Polymer in Hybrid Polymer/PbS Quantum Dot Solar Cells

Hybrid nanocomposites (HCs) obtained by blend solutions of conjugated polymers and colloidal semiconductor nanocrystals are among the most promising materials to be exploited in solution-processed photovoltaic applications. The comprehension of the operating principles of solar cells based on HCs thus represents a crucial step toward the rational engineering of high performing photovoltaic devices. Here we investigate the effect of conjugated polymers on hybrid solar cell performances by taking advantage from an optimized morphology of the HCs comprising lead sulfide quantum dots (PbS QDs). Uncommonly, we find that larger photocurrent densities are achieved by HCs incorporating wide-bandgap polymers. A combination of spectroscopic and electro-optical measurements suggests that wide-bandgap polymers promote efficient charge/exciton transfer processes and hinder the population of midgap states on PbS QDs. Our findings underline the key role of the polymer in HC-based solar cells in the activation/deactivation of charge transfer/loss pathways

Ultrastrong Plasmon–Exciton Coupling by Dynamic Molecular Aggregation

Plasmon–exciton polaritons arise from the coherent coupling of the localized plasmon of metal nanoparticles and the exciton of nearby resonant nanoemitters. The behavior of such systems is strictly defined by the initial choice of the metallic and excitonic materials, with only weak control possibilities, essentially limited to polarization-related effects or photoswitchable molecules. Here we propose a new strategy to control the plasmon–exciton splitting, based on the number of excitonic dipoles involved in the interaction. By integrating plasmonic arrays in a microfluidic device and injecting a dilute near-infrared cyanine dye solution, we are able to probe in real time the emergence and evolution of the strong plasmon–exciton coupling regime. When dye molecules selectively aggregate on silver as a result of chemical affinity, we observe a continuous increase of the Rabi splitting up to an exciton energy fraction as high as 35%, compatible with an ultrastrong coupling regime

Exploring Light–Matter Interaction Phenomena under Ultrastrong Coupling Regime

Exciton-polaritons are bosonic quasiparticles that arise from the normal mode splitting of photons in a microcavity and excitons in a semiconductor material. One of the most intriguing extensions of such a light–matter interaction is the so-called ultrastrong coupling regime. It is achieved when the Rabi frequency (Ω_R, the energy exchange rate between the emitter and the resonant photonic mode) reaches a considerable fraction of the emitter transition frequency, ω₀. Here, we report a Rabi energy splitting (2ℏΩ_R) of 1.12 eV and record values of the coupling ratio (2Ω_R/ω₀) up to 0.6-fold the material band gap in organic semiconductor microcavities and up to 0.5-fold in monolithic heterostructure organic light-emitting diodes working at room temperature. Furthermore, we show that with such a large coupling strength it is possible to undress the exciton homogeneous linewidth from its inhomogeneous broadening, which allows for an unprecedented narrow emission line (below the cavity finesse) for such organic LEDs. The latter can be exploited for the realization of novel monochromatic sources and near-IR organic emitting devices

Ultrastrong Plasmon–Exciton Coupling by Dynamic Molecular Aggregation

Plasmon–exciton polaritons arise from the coherent coupling of the localized plasmon of metal nanoparticles and the exciton of nearby resonant nanoemitters. The behavior of such systems is strictly defined by the initial choice of the metallic and excitonic materials, with only weak control possibilities, essentially limited to polarization-related effects or photoswitchable molecules. Here we propose a new strategy to control the plasmon–exciton splitting, based on the number of excitonic dipoles involved in the interaction. By integrating plasmonic arrays in a microfluidic device and injecting a dilute near-infrared cyanine dye solution, we are able to probe in real time the emergence and evolution of the strong plasmon–exciton coupling regime. When dye molecules selectively aggregate on silver as a result of chemical affinity, we observe a continuous increase of the Rabi splitting up to an exciton energy fraction as high as 35%, compatible with an ultrastrong coupling regime

Exciton–Plasmon Coupling Enhancement <i>via</i> Metal Oxidation

In this paper, we report on the effect of metal oxidation on strong coupling interactions between silver nanostructures and a J-aggregated cyanine dye. We show that metal oxidation can sensibly affect the plexcitonic system, inducing a change in the coupling strength. In particular, we demonstrate that the presence of oxide prevents the appearance of Rabi splitting in the extinction spectra for thick spacers. In contrast, below a threshold percentage, the oxide layer results in an higher coupling strength between the plasmon and the Frenkel exciton. Contrary to common belief, a thin oxide layer seems thus to act, under certain conditions, as a coupling mediator between an emitter and a localized surface plasmon excited in a metallic nanostructure. This suggests that metal oxidation can be exploited as a means to enhance light–matter interactions in strong coupling applications

Toward Cavity Quantum Electrodynamics with Hybrid Photon Gap-Plasmon States

Combining localized surface plasmons (LSPs) and diffractive surface waves (DSWs) in metallic nanoparticle gratings leads to the emergence of collective hybrid plasmonic–photonic modes known as surface lattice resonances (SLRs). These show reduced losses and therefore a higher <i>Q</i> factor with respect to pure LSPs, at the price of larger volumes. Thus, they can constitute a flexible and efficient platform for light–matter interaction. However, it remains an open question if there is, in terms of the <i>Q</i>/<i>V</i> ratio, a sizable gain with respect to the uncoupled LSPs or DSWs. This is a fundamental point to shed light upon if such modes want to be exploited, for instance, for cavity quantum electrodynamic effects. Here, using aluminum nanoparticle square gratings with unit cells consisting of narrow-gap disk dimersa geometry featuring a very small modal volumewe demonstrate that an enhancement of the <i>Q</i>/<i>V</i> ratio with respect to the pure LSP and DSW is obtained for SLRs with a well-defined degree of plasmon hybridization. Simultaneously, we report a 5× increase of the <i>Q</i>/<i>V</i> ratio for the gap-coupled LSP with respect to that of the single nanoparticle. These outcomes are experimentally probed against the Rabi splitting, resulting from the coupling between the SLR and a J-aggregated molecular dye, showing an increase of 80% with respect to the DSW-like SLR sustained by the disk LSP of the dimer. The results of this work open the way toward more efficient applications for the exploitation of excitonic nonlinearities in hybrid plasmonic platforms