Handbook of Modern Coating Technologies

Organic photovoltaics

Currently a lot of research in photovoltaics has focused on organic—based photovoltaics (OPVs) due to their low-cost and low impact on environment when compared with conven­tional silicon based devices [64]. Power conversion efficiencies have reached more than 10% and the most efficient OPVs are based on thin («100—300 nm) light-absorbing layers con­sisting of a mixture of electron donating and accepting molecules. These so-called bulk het­erojunctions often consist of a mixture of a fullerene—based polymer serving as an electron acceptor and another polymer acting as a donator. Due to the large scattering contrast between carbon (the main component of the fullerene) and hydrogen (large fraction of the electron donating polymer) and its nanometer resolution in the direction perpendicular to the surface NR became increasingly important in the field of OPVs.

NR unraveled a depletion of the widely used electron acceptor phenyl-C₆₁-butyric acid methyl ester (PCBM) at the cathode, which diminished with thermal annealing explaining the higher efficiency of these devices after annealing [65]. This study triggered a series of NR—based investigations of the vertical phase separation in these types of OPVs on thermal annealing [66—68], light annealing [69—71], and followed by investigations of its sensitivity to water penetration [72].

Recent advances in neutron flux on state-of-the-art NR machines opened the possibility of following the stratification of these systems on in situ annealing in real time. We will review one recent study realized by time-resolved TOF-NR on bilayer films made of PCBM and polystyrene (PS) on annealing [73]. Optical microscopy on in situ heated samples revealed needle shaped PCBM crystals growing with time (on the time scale of hours) in the bilayer reaching several tens of micrometers in length for the thick PS films, eventually. Obviously such large crystal structures are to be avoided in OPVs as they block the hetero­junction. Optical microscopy showed a strong dependence of the crystal growth on the

FIGURE 4-16 (A) NR profiles (logarithmic scale) and fits (solid lines) from PCBM/PS (7 nm) samples annealed for 10 and 60 min at 170°C and measured ex situ. (B) Corresponding SLD profiles. (C) NR (logarithmic scale) and (D) SLD profiles from a PCBM/PS (35 nm) bilayer annealed at 170°C and measured in situ. Reflectivity curves are offset for clarity. NR, Neutron reflectometry; PCBM, phenyl-C₆₁-butyric acid methyl ester; PS, polystyrene; SLD, scattering length density. Adapted from D. Mon, A.M. Higgins, D. James, M. Hampton, J.E. Macdonald, M.B. Ward, et al., Bimodal crystallization at polymer-fullerene interfaces, Phys. Chem. Chem. Phys. 17 (2015) 2216-2227. doi:10.1039/ C4CP04253K. https://doi.org/10.1039/C4CP04253K.

thickness of the PS layers suggesting that the concentration of PCBM inside the PS layer may be responsible for this effect. NR on the in situ heated samples showed, however, that the PCBM concentration in the PS layer and the PS/PCBM interface roughness are independent of the film thickness and are stabilized in less than 5 min on annealing evidencing a liquid/ liquid equilibrium. Fig. 4-16 shows the time-resolved NR measurements and the corre­sponding SLD profiles for a selected sample. The reflectivities were recorded in 30 s slices.

• Surfactant monolayers at solid/liquid interfaces

Due to the possibility to access buried interfaces, the power of contrast enhancement/match- ing by solvent exchange and its nanometer resolution NR was extensively used to study the adsorption of various surfactants in aqueous solutions on different types of materials. This is important in Materials Science as those layers can act as protective or anticorrosive coatings, lubricants, detergents or wetting agents to name just a view.

A commonly studied surfactant is sodium dodecyl sulfate (SDS) which was studied on alumina [74,75] and silica [76]. Other NR examples include the adsorption of

• pentadecyl-pyridine on gold [77] or aerosol-OT adsorption on a charged silicon surface in the presence of divalent salts [78] or on calcium carbonate surfaces [79]. The adsorption of didodecyldimethylammonium bromide was studied by NR on Quartz [80,81] and on mica surfaces [82]. Sodium bis(2-ethylhexyl) phosphate adsorption on alumina was studied by NR as well [83].

Here we will review a recent study implying PNR on the adsorption of SDS and dodecyltri- methylammonium bromide (DTAB) on Ni as a function of pH [84]. This study is a particular informative example as it demonstrates two big advantages of neutron scattering, namely, the powerful use of contrast matching of the interface by mixing heavy and normal water in the bulk phase and the magnetic interaction of neutrons with the ferromagnetic Ni. The sample in this study was a 10-nm Ni film deposited on a silicon substrate in contact with water at different acidic conditions to probe the corrosion of the Ni film as a function of time. The two surfactants were added to the aqueous solution to probe their corrosion protection. Fig. 4—17 (left) shows representative SLD profiles of the system in heavy water and light water. It is evident that the SLD of H₂O is very close to the one of the surfactant, whereas the D₂O is almost contrast matched to the Ni layer if measured with a neutron spin antiparallel to the magnetization. The two spin-polarizations with the neutron spin parallel (up-spin) and antiparallel (down-spin) to the Ni magnetization give an additional contrast as seen in Fig. 4—17 (left). On the right-hand side of Fig. 4—17 the NR profiles of the H₂O contrast for the two spin states are shown for an almost neutral environment and for a strong acidic solution. Clearly visible are the Kiessig-oscillations stemming from the Ni layer, which is highlighted in this contrast for both surfactants in the mild solution. On addition of a strong acid the two surfactants show clear differences in their corrosion inhibition: the SDS data (Fig. 4—17C) shows almost no change of the reflectivity, whereas the DTAB reflectivity (Fig. 4—17D) looses the fringes completely evidencing the loss of the Ni layer. Also the difference between the two spin-state reflectiv­ities is lost which corroborates the loss of the magnetic layer.

• Cellulose degradation

Apart from the fundamental scientific interest of plant biology, enzymatic breakdown of bio­mass is one promising way of producing fuel, plastics, or other chemicals. However, natu­rally plants produce a complex composition and structure to protect their lingocellulose from degradation. The effectiveness of novel types of enzymes can be studied by NR by using multilayers of cellulose and xyloglucane [85]. The structure of cellulose nanocrystals has also been studied using NR [86].

• Thermosensitive coatings

The transition of a polymer with a lower critical solution temperature (LCST) from its swol­len to collapsed state can be used in thin films as nanosensor, artificial pump or muscle. Most of these polymers are water solvable in the right temperature range and thus NR is an

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FIGURE 4-17 Left: Sum of nuclear and magnetic SLD profiles for the two neutron spin states of the system under investigation: by using (A) D₂O or (B) H₂O as the bulk solvent, the surfactant layer or metal surface may be selectively emphasized as shown. Right: H₂O NR data (points) and fits (lines) on log scale at pH 6 (black) and pH 2 (gray) for (C) SDS data and (D) DTAB data. The reflectivities on the top correspond to up-spin and the lower ones to the down-spin state, the latter are offset for clarity. DTAB, Dodecyltrimethylammonium bromide; NR, neutron reflectometry; SDS, sodium dodecyl sulfate; SLD, scattering length density. Adapted from M.H. Wood, R.J. Welbourn, A. Zarbakhsh, P. Gutfreund, A.M. Clarke, Polarized neutron reflectometry of nickel corrosion inhibitors, Langmuir 31 (25) (2015) 7062-7072.

excellent tool to characterize the structure of the polymer while undergoing the swelling by the use of heavy water. It was even possible to follow the kinetics of the phase transitions on the second time scale with poly(N-isopropylacrylamide) and acrylate—based layers [87—92].