In contrast to silicon-based p-n junction photovoltaic solar cells (PVSCs), a silicon rich silicon carbide (SixC1-x)-based thin-film PVSC with enhanced absorption at the visible wavelength region. The silicon rich SixC1-x films are synthesized by using a low-substrate temperature and low-power plasma-enhanced chemical vapor (PECVD) system in a silane-rich environment. The molar ratio of the silicon atoms in SixC1-x grown at 500°C was tunable from 0.63 to 0.66 when reducing RF plasma power from 100 to 20 W. The low-plasma PECVD growth gave the SixC1-x an enhanced broadband absorption at 400–600 nm, where the highest optical absorption coefficient was 1.3 × 105 cm−1. The silicon rich composition also reduced the optical bandgap energy from 2.05 to 1.49 eV. This type of red-shifted cutoff wavelength promoted solar energy conversion at the near-infrared region. Consequently, an ITO/p-SixC1-x/n-SixC1-x/Al PVSC with a silicon molar ratio of 66% enhanced its conversion efficiency from 5 × 10−3 to 4.7% when the n-type SixC1-x thickness was reduced from 150 to 25 nm, which is attributed to the reduced series resistance to 0.6 Ω and the increased shunt resistance to 1500 Ω.
Finite Si atom diffusion induced size limitation of self-assembled Si quantum dots (Si-QDs) in silicon-rich silicon carbide (SiC) is demonstrated. After annealing, the Si-QDs with a size of 3 ± 1 nm are precipitated in the matrix of SiC 0.51 deposited by low-temperature plasma-enhanced chemical vapor deposition with Argon-diluted silane and methane mixture. The amorphous-Si dependent Raman scattering peak at ∼470 cm −1 is narrowing with increasing temperature, and the Si-CH 3 rocking-mode absorption line is shifted by dehydrogenation after high-temperature treatment. The self-assembled Si-QDs in SiC 0.51 with a volume density of 4.4 × 10 18 cm −3 transfer from amorphous to crystalline phase by increasing annealing temperature from 850 • C to 1050 • C. The calculated Si atom diffusion coefficient of 3-4 × 10 −4 nm 2 s −1 in Si-rich SiC 0.51 is 7 orders of magnitude larger than that in pure SiC, which coincides with the linear extrapolation from pure Si and SiC and reveals nonlinear proportionality with C/Si composition ratio and Si-QD size.
Without the need of single-layer graphene, the graphite nanosheet powder electrochemically exfoliated from graphite foil can also be employed as a stable saturable absorber and mode-locker for fiber lasers. High-quality graphite nano-sheets containing few graphene layers can be obtained by slow electrochemical exfoliation without the need of post annealing procedure. With reducing the electrochemical exfoliation bias of the graphite foil based anode from + 6 and + 3 volts, the electrochemically exfoliated graphite nano-sheets reveals a decreased D-band intensity in Raman scattering spectrum, and the 2D-band intensity is concurrently enlarged by two times to support the improved quality with suppressed oxidation during the exfoliation reaction. The X-ray photoelectron spectroscopy also confirms the suppression of the C-O bonds in the graphite nano-sheets obtained with decreasing the exfoliation bias. After centrifugation, the average diameter of the exfoliated graphite nano-sheets extracted from the acetone solution is shrunk from 7 μm to 100 nm as the anode bias decreases from 6 to 3 volts. Both the quality and size distribution of the graphite nano-sheets are improved with such slow but refined electrochemical exfoliation. In application, the graphite nano-sheets obtained at different exfoliation bias show relatively stable saturable absorption and passive mode-locking performance in Erbium doped fiber lasers (EDFLs). Benefiting from the advantages of high-gain and strong pulse compression in the EDFL, the graphite nano-sheets with different modulation depths only behave as a mode-locking starter and show trivial influence to the pulse shortening in the mode-locked EDFL, indicating that the strong soliton compression mechanism dominates the generation of 430-450 fs pulsewidth in the EDFL passively mode-locked by graphite nano-sheets.
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