Atomistic modeling of structure and dynamics of sheared atactic polystyrene films

Hydrogenated microcrystalline silicon (µc-Si:H) is a mixed-phase material consisting of crystalline silicon grains, hydrogenated amorphous silicon (a-Si:H) tissue, and voids. Microcrystalline silicon is extensively used as absorber layer in thin-film tandem solar cells, combining the advantages of a low (indirect) band gap (1.1 eV), which results in an enhanced absorption of red and (near) infrared light, with an improved stability under light exposure (reduced Staebler–Wronski effect). However, due to the indirect nature of the band gap, relatively thick (1–2 µm) µc-Si:H films are necessary to achieve an efficient absorption of red and (near) infrared light, even when light trapping concepts are applied. Therefore, from a cost-perspective point of view, high growth rates (>1 nm/s) are required, preferably in combination with large-area (roll-to-roll) processing. The most common, and so far most successful, deposition technique is the capacitively-coupled plasma (CCP) in parallel plate configuration using radio or very high excitation frequencies (RF or VHF, respectively) and highly hydrogen diluted hydrogen and silane gas mixtures. Approaches to increase the growth rate include an increase of the plasma power, moving from a low-pressure to a high-pressure depletion (LPD to HPD, respectively) regime, and/or by increasing the excitation frequency from 13.56 MHz to 27–300 MHz. The combination of HPD-VHF has resulted in high deposition rates (2-3 nm/s) while maintaining high solar cell efficiencies (7-8%). In this work, the use of an ultra-fast (2-20 nm/s) deposition technique, i.e. the expanding thermal plasma, has been explored for the deposition of µc-Si:H films. Characteristic for ETP-grown µc-Si:H films is the lack of a sufficient amount of a-Si:H tissue, which is necessary to passivate the grain boundaries and fill the intergranular space, resulting in a network of (inter-connected) cracks and voids. As a consequence, the µc-Si:H films are prone to post-deposition oxidation, resulting in low solar cells efficiencies (<2%). The post-deposition oxidation has been monitored by means of Fourier transform infrared (FTIR) spectroscopy over a period of 8 months. This study revealed a two-timescale oxidation: on short timescales (<3 months) the crystalline silicon grain boundaries oxidize, on longer timescales the oxidation involves also the a-Si:H tissue. This indicates that in order to prevent post-deposition oxidation, it is not sufficient to fill the intergranular space, but that the a-Si:H tissue needs to be of sufficient quality, i.e. dense and not susceptible for post-deposition oxidation. One process that could be responsible for the insufficient amount of a-Si:H tissue, is hydrogen-induced etching of a-Si:H tissue. Atomic hydrogen is, under µc-Si:H growth conditions, abundant in the plasma, and is known to preferentially etch a-Si:H over crystalline silicon (c-Si). In addition, the interaction of atomic hydrogen with the (growing) film can result in the formation of an hydrogen-rich sub-surface layer, caused by the insertion of atomic hydrogen into strained Si-Si bonds, which possibly explains the porous quality of the a-Si:H tissue. Monitoring the etch rate of a-Si:H films during Ar/H2 plasma exposure by real time spectroscopic ellipsometry showed that the hydrogen-induced etch rate was at least one order of magnitude lower than typical deposition rates. In addition, FTIR spectroscopy revealed that insertion of atomic H in the sub-surface layer (top ~30 nm) during Ar/H2 plasma exposure did not result in an increased porosity. These results suggest that the interaction of atomic hydrogen with the growing film is not responsible for the insufficient amount of (dense) a-Si:H tissue. The fact that the interaction of atomic hydrogen is not responsible for the poor material properties of ETP-grown µc-Si:H, the question "what mechanism is then responsible?" arises. To address this question the plasma chemistry and the resulting growth mechanism of ETP is compared to CCP, which so far is the only technique with which solar-grade µc-Si:H is obtained. One difference between the two techniques is the absence of an ion bombardment effect in ETP. In CCP the HPD and the use of VHF are employed to suppress a (potentially uncontrolled) ion bombardment effect, hypothesized to be responsible for an amorphization of the crystalline growth and defect incorporation. However, there is always some form of ion bombardment present. The extent to which ions contribute to the growth depends on the ion flux, the ion energy, and the chemical nature of the ion. Under HPD-VHF conditions SinHm+ is identified as the dominant ion in H2/SiH4 plasmas, but no direct ion energy and ion flux measurements under HPD conditions have been reported so far. Therefore, the ion energy and flux in a CCP reactor have been studied. For this purpose, a capacitively-coupled plasma reactor in parallel plate configuration has been designed and built, in close collaboration with the Institute of Photovoltaics at Forschungszentrum Jülich (Germany). This reactor has been especially designed for the implementation of plasma and (in situ) film diagnostics. Under solar-grade µc-Si:H deposition conditions the contribution of ions to the film growth has been studied by means of a capacitive probe. The ion to Si deposition flux ratio was found to be large, ~0.30. However, since the ion energy is rather low

Confinement and shear effects for atactic polystyrene film structure and mechanics

We have performed the molecular-dynamics simulations for the atactic polystyrene (aPS) films supported by one substrate (SF, supported film) and films capped by two substrates (CF, capped film). The simulations of supported films have been carried out with the purpose to study the influence of confinement on the glass-transition temperature (Tg). We define the Tg by measuring the film density and thickness. We show that the Tg value of aPS films weakly depends on the film thickness and remains almost constant for films down to 2 nm, which is in agreement with recent experimental study. The simulations of capped films have been performed to study the statistical and mechanical properties of polymer chains under shear. The capped film has been loaded with different normal pressures (25–170 MPa) and sheared with different shear velocities (5 × 10-4–1 × 10-1 nm ps-1). In the absence of shear the internal structure of the aPS SF and CF films is different. We found that the internal structure, density and order parameter of the aPS CF films do not change with the small shear deformations

Confinement and shear effects for atactic polystyrene film structure and mechanics

We have performed the molecular-dynamics simulations for the atactic polystyrene (aPS) films supported by one substrate (SF, supported film) and films capped by two substrates (CF, capped film). The simulations of supported films have been carried out with the purpose to study the influence of confinement on the glass-transition temperature (Tg). We define the Tg by measuring the film density and thickness. We show that the Tg value of aPS films weakly depends on the film thickness and remains almost constant for films down to 2 nm, which is in agreement with recent experimental study. The simulations of capped films have been performed to study the statistical and mechanical properties of polymer chains under shear. The capped film has been loaded with different normal pressures (25–170 MPa) and sheared with different shear velocities (5 × 10-4–1 × 10-1 nm ps-1). In the absence of shear the internal structure of the aPS SF and CF films is different. We found that the internal structure, density and order parameter of the aPS CF films do not change with the small shear deformations

Mechanical properties and local mobility of atactic-polystyrene films under constant-shear deformation

We have performed molecular-dynamics simulations of atactic polystyrene thin films to study the effect of shear rate, pressure, and temperature on the stress-strain behaviour, the relevant energetic contributions and non-affine displacements of polymer chains during constant-shear deformation. Under this deformation sliding motion is observed at high shear rates between the top substrate and top polymer layer, which disappears when the shear rate decreases. At low shear rates stick-slip motion of the whole film with respect to the bottom substrate takes place. We found that at low shear rates the yield stress logarithmically depends on the shear rate; this behaviour can be explained in terms of the Eyring model. It was also observed that an increase in the normal pressure leads to an increase in the yield stress in agreement with experiments. The contributions to the total shear stress and energy are mainly given by the excluded-volume interactions. It corresponds to a local translational dynamics under constant shear in which particles are forced to leave their original cages much earlier as compared to the case of the isotropic, non-sheared film. Moreover, it was observed that under constant-shear deformation the polymer glass is deformed non-affinely. As a result, the middle part of the film is much more deformed than the layers close to the supporting substrates, meaning that the well-known effect of shear-banding occurs

Competition of time and spatial scales in polymer glassy dynamics: Rejuvenation and confinements effects

We use molecular-dynamics (MD) simulations and an original lateral contact experiment to explore the influence of mechanical history on polymer mechanical behavior and segmental mobility. Two typical glassy polymers are considered: bulk acrylate (experiments) and atactic polystyrene (aPS) in a bulk and in thin films (simulations). Stress-strain behavior has been investigated both experimentally for sheared, 50 µm thick, acrylate films and by MD simulations of an aPS in a bulk for two different strain rates in a closed extension–recompression loops. Cyclic shear strains applied in the plastic regime were found experimentally to induce a progressive transition of the mechanical response of the polymer glass toward a steady state which is characterized by a strong reduction of the apparent – non linear – shear modulus. The dynamics of the polymer glass in this yielded state was subsequently analyzed from a measurement of the time dependent linear viscoelastic properties at various imposed frequencies. Immediately after the cyclic plastic deformation, mechanical "rejuvenation" of the polymer is evidenced by a drop in the storage modulus and an increase in the loss modulus, as compared to the initial values recorded before plastic deformation. A progressive recovery of the viscoelastic properties is also measured as a function of time as a result of the enhanced aging rate of the system. This experimentally observed mechanical rejuvenation of polymer has been for the first time connected to the drastic increase in the simulated segmental mobility. A simulated distribution of relaxation times shows a shift to shorter times of the a and ß relaxation processes which is consistent with the observed experimental changes in the viscoelastic modulus after rejuvenation. Finally, we present our first findings on the thickness- and substrate-dependence of the simulated glass transition temperature for thin aPS films. We observe the decrease of the glass transition temperature with film thickness, but for extremely thin (less than 2 nm) films

Rejuvenation, aging, and confinement effects in atactic-polystyrene films subjected to oscillatory shear

Molecular-dynamics simulations of 5 nm-thick atactic-polystyrene films have been used to study the influence of cyclic-shear deformation on the stress–strain behavior and local segmental mobility. Upon cyclic yield the stress–strain behavior of the films slowly evolves towards a steady state which is characterized by a decrease of the maximum stress and by an enhanced dissipative process. Immediately after plastic deformation the storage modulus is decreased and the loss modulus is increased as compared with their initial values. Such changes in the viscoelastic moduli reflect the mechanical rejuvenation of a polymer glass. This mechanical rejuvenation of polymers is connected to the increase in the simulated segmental mobility, which is calculated for the entire film as well as in different layers