Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.

We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir–Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the three-dimensional electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 μm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted by the simulations. The frequency positions of the absorption features indicate that only the NC surface optical phonons are activated in the SEIRA spectra.

The presence of strong spin–orbit coupling in the valence band and weak spin‐splitting in the conduction band result in the lowest energy exciton in WX2 (X = S, Se) being spin forbidden and optically dark. Because of their long lifetimes, dark excitons are highly attractive for quantum optics and optoelectronic applications. To date, studying dark excitonic emissions is limited to cryogenic temperatures or requires very complex experimental configurations to observe them at room temperature (RT). Here, the radiative decay of dark exciton related emission in WSe2 monolayers is studied using both conventional and tip‐enhanced photoluminescence (TEPL) at RT. Monolayer WSe2 flakes are sandwiched between noble metal substrates and polydimethylsiloxane nanopatches providing a strong local electrostatic out‐of‐plane dipole moment with respect to the 2D plane resulting in the observation of dark excitonic emission at RT. The spatial distribution of this dark exciton related emission is studied by TEPL with a spatial resolution of <10 nm confirming the confinement of these excitons within the polydimethylsiloxane nanopatches. The tip‐enhanced Raman scattering (TERS) investigation excludes any local strain induced effects and reveals a direct correlation between dark excitons and defects in WSe2. Finally, removal of the nanopatches led to the recovery of bright excitonic emission in WSe2.

The study of infrared absorption by linear gold nanoantennas fabricated on a Si surface with underlying SiO2 layers of various thicknesses allowed the penetration depth of localized surface plasmons into SiO2 to be determined. The value of the penetration depth derived experimentally (20 ± 10 nm) corresponds to that obtained from electromagnetic simulations (12.9–30.0 nm). Coupling between plasmonic excitations of gold nanoantennas and optical phonons in SiO2 leads to the appearance of new plasmon–phonon modes observed in the infrared transmission spectra the frequencies of which are well predicted by the simulations.

Twisted 2D bilayer materials are created by artificial stacking of two monolayer crystal networks of 2D materials with a desired twisting angle θ. The material forms a moiré superlattice due to the periodicity of both top and bottom layer crystal structure. The optical properties are modified by lattice reconstruction and phonon renormalization, which makes optical spectroscopy an ideal characterization tool to study novel physics phenomena. Here, we report a Raman investigation on a full period of the twisted bilayer (tB) WSe2 moiré superlattice (i.e. 0° ≤ θ ≤ 60°). We observe that the intensity ratio of two Raman peaks, B2g and E2g/A1g correlates with the evolution of moiré period. The Raman intensity ratio as a function of twisting angle follows an exponential profile matching the moiré period with two local maxima at 0° and 60° and a minimum at 30°. Using a series of temperature-dependent Raman and photoluminescence (PL) measurements as well as ab initio calculations, the intensity ratio IB2g /IE2g/A1g is explained as a signature of lattice dynamics in tB WSe2 moiré superlattices. By further exploring different material combinations of twisted hetero-bilayers, the results are extended for all kinds of Mo- and W-based TMDCs.