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 (