Using nanosecond laser pulses to debond the glass-EVA layer from silicon photovoltaic modules.

The active silicon cell of a solar photovoltaic (PV) panel is covered by an ethylenevinylacetate (EVA) adhesive and a protective top glass layer. Separating this glass-EVA layer from the underlying silicon represents a bottleneck for recycling PV panels. Previous work has shown that the EVA-Si bond can be weakened by applying a continuous source of heat to melt the EVA. In this paper, a new method using nanosecond laser pulses is demonstrated to induce transient melting selectively at the EVA-Si interface. This impulsive heating method can cleanly separate the glass-EVA layer from the silicon in both model and commercial multicrystalline PV panels. The dependence of this debonding on parameters like laser pulse fluence (laser pulse energy per area), wavelength, applied pressure, and scan speed were characterized. For model PV panels, the single-pulse laser fluences required for spontaneous separation of the assembly under the force of gravity, were 0.23, 0.32 and 0.78 J/cm2 for 355 nm, 532 nm and 1064 nm, respectively. The use of shorter wavelengths reduces the laser fluence needed for debonding, while higher fluences can compensate for faster laser beam scanning rates. Optical and electron microscopy images of the Si surfaces before and after laser irradiation show that the textured antireflection layer is destroyed but the silver metal grid remains intact. Preliminary experiments using 532 nm pulses showed that the laser debonding method could remove the glass-EVA layer from sections of decommissioned commercial PV panels, even when the top glass layer was densely cracked.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene.

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 µm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Effects of solvent and micellar encapsulation on the photostability of avobenzone.

The photodegradation of avobenzone (AV), the only ultraviolet filter molecule approved by the Food and Drug Administration to absorb UVA radiation, is an important problem in sunscreen formulations. In this paper, the photophysics and photostability of AV in various solvent systems and in aqueous micelles are studied. AV in its keto-enol tautomer functions as an effective UVA protection agent. AV is highly susceptible to photoinduced diketonization in both nonpolar solvents and in aqueous aggregates but is considerably more stable in polar, protic solvents like methanol. By studying its stability in different surfactant solutions, we show that incorporation of AV into sodium dodecylsulfate (SDS) micelles can achieve stability levels comparable to neat methanol. Steady-state spectral shifts, fluorescence anisotropy, and time-resolved fluorescence decay measurements are all consistent with AV experiencing a polar environment after micellar encapsulation. It is proposed that AV is encapsulated in the palisade layer of the SDS micelles, which allows access to water molecules that facilitate the re-formation of the enol form after photon absorption and relaxation. Although the detailed mechanism of AV tautomerization remains unclear, this work suggests that tuning the chemical microenvironment of AV may be a useful strategy for improving sunscreen efficacy.

Correction: Photochemical degradation of the UV filter octyl methoxycinnamate in solution and in aggregates.

Correction for 'Photochemical degradation of the UV filter octyl methoxycinnamate in solution and in aggregates' by Kerry M. Hanson et al., Photochem. Photobiol. Sci., 2015, 14, 1607-1616.

Hybrid Molecule-Nanocrystal Photon Upconversion Across the Visible and Near-Infrared.

The ability to upconvert two low energy photons into one high energy photon has potential applications in solar energy, biological imaging, and data storage. In this Letter, CdSe and PbSe semiconductor nanocrystals are combined with molecular emitters (diphenylanthracene and rubrene) to upconvert photons in both the visible and the near-infrared spectral regions. Absorption of low energy photons by the nanocrystals is followed by energy transfer to the molecular triplet states, which then undergo triplet-triplet annihilation to create high energy singlet states that emit upconverted light. By using conjugated organic ligands on the CdSe nanocrystals to form an energy cascade, the upconversion process could be enhanced by up to 3 orders of magnitude. The use of different combinations of nanocrystals and emitters shows that this platform has great flexibility in the choice of both excitation and emission wavelengths.

Photochemical degradation of the UV filter octyl methoxycinnamate in solution and in aggregates.

The photodegradation of the ultraviolet (UV) filter octyl methoxycinnamate (OMC) is investigated in both dilute solution and in aggregated form. In dilute solution, the ratio of trans and cis isomers achieved at the photostationary state is solvent-dependent because of variations in the isomerization quantum yield. The two isomeric forms at the photostationary state are highly resistant to further photodegradation and no other UVA-absorbing species are formed. Aggregation of OMC, either in a neat film or in aqueous colloidal suspensions, leads to irreversible photodegradation of the molecule and the formation of multiple photoproducts. In addition to previously identified photoproducts like the UVB-absorbing cis and trans isomers and photodimers, we find photoproduct species whose absorption extends into the UVA. Characterization of the photophysical properties of these species indicates that they have long-lived excited-states (τf > 1 ns, 400 nm), unlike the isomeric forms of OMC (τf < 30 ps, 266 nm), and that excitation at 405 nm can sensitize the formation of singlet oxygen. These results show that the environment of OMC affects the photochemistry of the molecule and that the environmental conditions must be taken into account when considering the molecule's stability. In particular, aggregation of OMC molecules results in complex photochemistry that can produce species whose absorption extends into UVA and are capable of generating reactive oxygen species.

Synthesis and photophysical properties of a "face-to-face" stacked tetracene dimer.

A covalently linked tetracene dimer has been prepared and its molecular structure is characterized by (1)H NMR and MALDI-TOF mass spectroscopy, and elemental analysis. The minimized molecular structure reveals that the tetracene subunits in a dimer adopt a "face-to-face" stacked configuration. Its absorption spectrum differs significantly from that of the monomeric counterpart in solution, suggesting the presence of strong interactions between the two tetracene subunits. In solution, the fluorescence spectrum is dominated by a band at around 535 nm, due to an oxidative impurity. In the longer wavelength range, a short-lived lower energy emission can be identified as the intrinsic emission of the dimer. In a polystyrene matrix or at low temperatures, the lifetime of the lower energy emission lengthens and it becomes more prominent. We suggest that the interactions between the two tetracene subunits produce a short-lived, lower energy "excimer-like" state. The fluorescence decays show no observable dependence on an applied magnetic field, and no obvious evidence of significant singlet fission is found in this dimer. This research suggests that even though there are strong electronic interactions between the tetracene subunits in the dimer, singlet fission cannot be achieved efficiently, probably because the formation of "excimer-like" states competes effectively with singlet fission.

Application of nonlinear optical microscopy for imaging skin.

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti-Stokes Raman and two-photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one-photon excitation: intrinsic 3D imaging with <1 microm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1,000 microm with the use of near-infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time-consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation-induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.

Two-photon standing-wave fluorescence correlation spectroscopy.

A fluorescence correlation spectroscopy experiment that combines two-photon excitation and a standing-wave interference pattern is presented. The experimental correlation function can be analyzed using a simple expression involving (1) an exponential decay with time constant tau(f), which reflects diffusion across the interference fringes, and (2) a longer-lived decay with time constant tau(omega), which reflects diffusion in and out of the focal spot. The diffusion of Rhodamine 110 in water and ethylene glycol is measured using this method. The ability to simultaneously measure diffusion on two different time and lengthscales makes this experiment especially useful in environments where anomalous diffusion is suspected.

Sunscreen enhancement of UV-induced reactive oxygen species in the skin.

The number of UV-induced (20 mJ cm(-2)) reactive oxygen species (ROS) generated in nucleated epidermis is dependent upon the length of time the UV filter octocrylene, octylmethoxycinnamate, or benzophenone-3 remains on the skin surface. Two-photon fluorescence images acquired immediately after application of each formulation (2 mg cm(-2)) to the skin surface show that the number of ROS produced is dramatically reduced relative to the skin-UV filter control. After each UV filter remains on the skin surface for t=20 min, the number of ROS generated increases, although it remains below the number generated in the control. By t=60 min, the filters generate ROS above the control. The data show that when all three of the UV filters penetrate into the nucleated layers, the level of ROS increases above that produced naturally by epidermal chromophores under UV illumination.