Undoped a-SiC Research Articles

The Optical Spectra of a-Si:H and a-SiC:H Thin Films Measured by the Absolute Photothermal Deflection Spectroscopy (PDS)

The new absolute PDS setup allows to measure simultaneously the absolute values of the optical transmittance T, reflectance R and absorptance A spectra in the spectral range 280 2000 nm with the typical spectral resolution 10 nm in ultraviolet and visible spectral range and 20 nm in the near infrared region. The PDS setup provides the dynamic detection range in the optical absorptance up to 4 orders of magnitude using non-toxic liquid perfluorohexane Fluorinert FC72. Here we demonstrate the usability of this setup on a series of intrinsic as well as doped a-Si:H and a-SiC:H thin films deposited on glass substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (CVD) from hydrogen, silane and methane under various conditions. The increase of the Tauc gap with increasing carbon concentration in intrinsic a-SiC:H was observed. The defect-induced localized states in the energy gap were observed in doped a-Si:H as well as undoped a-SiC:H below the Urbach absorption edge.

Improvement of Single-Junction a-Si:H Thin-Film Solar Cells Toward 10% Efficiency

ABSTRACTThe hydrogenated amorphous silicon (a-Si:H) single-junction thin-film solar cells were fabricated on SnO2:F-coated glasses by plasma-enhanced chemical vapor deposition (PECVD) system. The boron-doped amorphous silicon carbide (a-SiC:H) was served as the window layer (p-layer) and the undoped a-SiC:H was used as a buffer layer (b-layer). The optimization of the p/b/i/n thin-films in a-Si:H solar cells have been carried out and discussed. Considering the effects of light absorption, electron-hole extraction and light-induced degradation, the thicknesses of p, b, n and i layers have been optimized. The optimal a-Si:H thin-film solar cell having an efficiency of 9.46% was achieved, with VOC=906 mV, JSC=14.42 mA/cm2 and FF=72.36%.

Effect of Buffer Structure on the Performance of a-Si:H/a-Si:H Tandem Solar Cells

ABSTRACTThe study focuses on the influence of the hydrogenated amorphous silicon carbide (a-SiC:H) buffer layer in hydrogenated amorphous silicon (a-Si:H) single-junction and tandem thin-film solar cells. By increasing the undoped a-SiC:H buffer layer thickness from 6nm to 12nm, the JSC in single-junction cell was significantly improved, and the efficiency was increased by 4.5%. The buffer layer also effectively improves the efficiency of the a-Si:H/a-Si:H tandem cells by 7% as a result of the increase in open-circuit voltage (VOC) and short-circuit current (JSC). Although the bottom cell absorbs less short-wavelength photons, the wider-bandgap doped and buffer layers were still necessary for improving the cell efficiency. Presumably, this is because these wider-bandgap layers allow more photons to reach the bottom cell. Also, they can reduce interface recombination.

Low dark current and blue enhanced a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode for imaging applications

This paper presents an a-Si:H∕a-SiC:H heterojunction n-i-δi-p photodiode with low dark current and enhanced short wavelength responsivity suitable for low-level light detection applications. Junction properties and carrier transport are investigated in terms of current-voltage characteristics, photocurrent transient measurements, and spectral photoresponse. It is demonstrated that introduction of a thin (∼40Å) undoped a-SiC:H buffer (δi) at the p-i interface significantly reduces the reverse dark current and recombination losses at this interface. A dark current density of ∼10pA∕cm2 at reverse bias of 1V is achieved for the n-i-δi-p structure, in which the p-type a-SiC:H window layer and the undoped δi buffer layer have a band gap of 2eV.

Reduction of Dark Current Under Reverse Bias in a-Si:H p-i-n Photodetectors

AbstractThis paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

Optical, electrical and structural properties of amorphous SiCN:H films prepared by rf glow-discharge decomposition

Amorphous quaternary alloy (a-SiCN:H) films composed of silicon, carbon, nitrogen and hydrogen have been prepared by the rf glow-discharge decomposition in a gas mixture of methane, silane, nitrogen and helium. Effects of nitrogen addition on the optical, electrical, structural and optoelectronic properties of the films have been investigated. Optical bandgap remains almost constant for a wide range of nitrogen addition. Dark and photoconductivity becomes greater by the nitrogen addition than those of undoped a-SiC:H films. Incorporation of a small amount of nitrogen in the film is likely to reduce the structural disorder and/or the density of defects, and some nitrogen atoms seem to work as a dopant. On the other hand, large incorporation deteriorates these properties. The nitrogen addition effect appears more remarkably in the films prepared under the low flow rate ratio of methane to silane.

Photocarrier collection in a-SiC:H/c-Si heterojunction solar cells

We measured the temperature dependent current–voltage ( J– V) characteristics of p a-SiC:H/n c-Si heterojunction solar cells with different doping levels in the p a-SiC:H layer. For heterojunction solar cells with an undoped a-SiC:H layer, S-shaped J– V characteristics are observed at room temperature leading to poor factors. Below room temperature, such S-shape also appears for the cells with a highly doped p a-SiC:H layer. Detailed simulation studies showed that the origin of this S-shape lies in a reduced electric field in the c-Si depletion region, which, in combination with a hole accumulation at the interface, causes an increase in recombination losses. As long as the p a-SiC:H layer in these heterojunction cells is highly doped (≥10 19 cm −3), these collection problems do not occur under standard operating conditions (i.e., room temperature and 100 mW/cm 2 illumination).

Amorphous Silicon-Carbon Alloys for Solar Cells

Recent efforts to optimize undoped, glow-discharge hydrogenated amorphous silicon-carbon alloys (a-SiC:H) with 1.9-2.0 eV bandgaps for solar cell applications are reviewed. Hydrogen dilution coupled with relatively low substrate temperatures (below 200 °C) have led to great improvements in the optical and phototransport properties of a-SiC:H films. The issue of alternative carbon feedstocks other than methane (CH4) will be explored. The improved a-SiC:H alloys have resulted in solar cells with high open circuit voltages (V∞ > 1.0 volt) and high fill factors (> 0.7). Further, the a-SiC:H solar cell instability upon prolonged light exposure has been much reduced. Correlation will be made between the properties of bulk undoped a-SiC:H films and the performance of p-i-nsingle junction solar cells using corresponding a-SiC:H thin i-layers.

Electrical properties of a-SiC/c-Si(p) heterojunctions

Electrical properties of heterojunctions consisting of amorphous silicon carbide (a-SiC) on p-type crystalline silicon (c-Si) have been investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) characteristics, as well as the temperature dependence of their current (I). The I-V characteristics of a-SiC/c-Si(p) heterojunctions exhibit a low turn-on voltage and a high reverse breakdown voltage (about 140 V). It was found that a model of an abrupt anisotype crystalline heterojunction is valid for the a-SiC/c-Si(p) structure. This model, for values of forward-bias voltage lower than a critical value VT, predicts that at low temperatures (T<250 K) recombination is the dominant transport mechanism, whereas at high temperatures (T>250 K) recombination-diffusion becomes the dominant transport mechanism. For higher voltages (V>VT), the tunnelling process dominates in the whole temperature range studied. From I-V measurements of a-SiC/c-Si(p) heterojunctions it was concluded that the conductivity of undoped a-SiC is n-type. Finally, from C-V measurements, the value of electron affinity of a-SiC was obtained, for the first time, and it was found to be chi 1=4.12+or-0.04 eV.

Electronic and optical effects of boron doping by plasma assisted diffusion in a-SiC : H

Hydrogenated amorphous SiC films have been p-type doped in an alternative process which is based upon plasma enhanced boron diffusion (PAD) from thin SiB layers on the substrate into the growing intrinsic film. In contrast to B 2H 6 doped films showing perceptible departures of optical properties in comparison to undoped a-SiC : H for PAD doping band gap, refractive index, Penn gap, and Urbach energy remain unchanged. The shift of Fermi level towards the valence band turns out to be comparable to gas phase doping.