Asok K. Barua

Control of Crystallization at Low Thickness in µc-Si:H Films Using Layer-by-Layer Growth Scheme

Hydrogen plasma treatment of stacking layers in a layer-by-layer (LBL) growth scheme effectively modulates the network structure from the surface into the bulk through the growth zone by abstraction of hydrogen from the Si:H matrix. It is an efficient way of reducing the microcrystalline transition layer so that virtual saturation of the crystallization may be obtained at a significantly low thickness of the sample compared to that obtained by a continuous mode of deposition. The growth of a highly conducting undoped µc-Si:H film at a stacked layer thickness of ~650 A is described. The film has a dark conductivity, σD, of ~4×10-3 Scm-1 and exhibits a very high crystallinity, as determined by Raman scattering and transmission electron microscope studies.

Development of High Quality P-Type Hydrogenated Amorphous Silicon Oxide Film and Its Use in Improving the Performance of Single Junction Amorphous Silicon Solar Cells

By using radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) method (13.56 MHz) we have developed high quality wide band gap p-type hydrogenated amorphous silicon oxide (p-a-SiO:H) films having characteristics suitable for use as the window layer for single and multijunction amorphous silicon (a-Si) solar cells. The films have been characterized in detail. The p-a-SiO:H films having thickness ≥100 A has photoconductivity (σph) one order of magnitude higher than that for p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) films having similar band gap. However, at thickness ~100 A, required for window layer, the σph of p-a-SiO:H film is ~102 times higher than that of p-a-SiC:H film. This difference in thickness dependence of σph has been attributed to the structural difference of the two types of window layers. We have fabricated single junction p-i-n structure a-Si solar cells on transparent conducting oxide (TCO) coated glass substrates with both p-a-SiO:H and p-a-SiC:H film as the window layer. In the former case the Fill Factor of the solar cell is higher by ~10%. The improvement in Fill Factor is due to the higher σph of p-a-SiO:H resulting in lower series resistance.

Diamond-Like Carbon Films Prepared by Photochemical Vapour Deposition

Diamond-like carbon films have been successfully grown at comparatively low substrate temperature (250°C) on single-crystal silicon substrates by the photochemical vapour deposition technique. Raman spectroscopy and scanning electron microscopy studies have shown the formation of diamond particles embedded in the diamond-like carbon film. However, some amount of graphitic carbon is present in the material. The presence of sp3 as well as sp2 hybridised carbon atoms is also confirmed by infrared vibrational spectroscopy. Extremely high transparency (>90%) for 500 A film from the near-infrared to visible regions is an indication of the high optical gap of the film.

Reduction of Thickness of N-Type Microcrystalline Hydrogenated Silicon Oxide Film Using Different Types of Seed Layer

By using a seeding technique it has been possible to reduce the thickness of n-µc-SiO:H film for use at the tunnel junction of a double-junction a-Si solar cell from ∼ 300 A to ∼ 185 A with acceptable optoelectronic properties. We have used two types of seed layer, i.e., undoped µc-SiO:H and µc-Si:H. The layers were prepared by the radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) method (13.56 MHz) at low rf power density (14 mW/cm2) and low substrate temperature (200°C). The ultrathin seed layer (∼ 40 A) enhances the growth of microcrystallinity of the n-type µc-SiO:H film as confirmed by the results of transmission electron microscopy (TEM) analysis and Raman spectroscopy.

The Growth of Crystallinity in Undoped SiO:H Films at Low RF-Power Density and Substrate Temperature

Microcrystalline SiO:H films have been prepared by the usual RF plasma enhanced chemical vapor deposition (PECVD) method (13.56 MHz) at low rf-power density and substrate temperature which are essential for the fabrication of a-Si solar cell. The transition from amorphous to microcrystalline structure of SiO:H film has been shown to depend sensitively on hydrogen dilution and rf-power density. In the amorphous state, concentration of CO2 in the source gas mixture plays crucial role in determining optical gap whereas hydrogen dilution and rf-power density play important role in determining the characteristics of the microcrystalline SiO:H films.

Highly Conducting Undoped µc-SiO:H Films Prepared by RF Glow Discharge

Highly conducting undoped µc-SiO:H film of σD=3×10-2 Scm-1, ΔE=0.14 eV and σPh=8×10-3 Scm-1 was obtained from high H2-diluted plasma at a moderately high RF power and substrate temperature (Ts) by the plasma enhanced chemical vapour deposition (PECVD) process. At higher Ts and in improved µc-networks H-content reduced, however, O-incorporation increased. Crystallinity of the films was identified by Raman scattering and transmission electron microscope (TEM) studies. Sharp crystallographic rings in the electron diffraction pattern identified (111), (220), (311) planes of c-Si and the TEM micrograph exhibited a uniform and dense distribution of crystallites with a range of diameters from 50–200 A.

Growth and Characterization of Indium Tin Oxide Films Grown on Polymer Substrates by DC Magnetron Sputtering

Thin films of indium tin oxide (ITO) have been deposited on unheated polymer substrates by the DC magnetron sputtering technique. The sheet resistance of the films increased with time on exposure to atmosphere for stretched and cast acrylic substrates which was not seen in the case of polycarbonate substrate. The scanning electron microscopy studies carried out on these films corroborated the result. The transmission of the films was ~80% in the visible region for all the films. The X-ray diffraction pattern in all the three cases showed preferred orientation of In2O3 in the (440) planes.

Comparison of structural and optoelectronic properties of N-type microcrystalline silicon and silicon oxide films with lowering of thickness

We have compared the structural and optoelectronic properties of n-type microcrystalline hydrogenated silicon oxide (n-µc-SiO:H) and n-type microcrystalline hydrogenated silicon (n-µc-Si:H) films with lowering of thickness, prepared by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD, 13.56 MHz) method. At thickness ≤ 300 A, the n-µc-SiO:H film has higher optical gap (E05) and lower optical absorption while retaining the photoconductivity (σph) and activation energy (Ea) similar to those for n-µc-Si:H film. Due to these advantages of n-µc-SiO:H film over that of n-µc-Si:H at low thickness this material has potential for use in improving the performance of single and double junction amorphous silicon solar cells.

Lowering of Thickness of N-Type Microcrystalline Hydrogenated Silicon Film by Seeding Technique

By using a seeding technique it has been possible to lower significantly the thickness of n-type µc-Si:H film having acceptable dark conductivity for use at the tunnel junction/s of multijunction a-Si solar cells. The µc-Si:H films have been prepared by radio frequency plasma enhanced chemical vapor deposition (RFPECVD) method (13.56 MHz) at low power and substrate temperature suitable for the fabrication of a-Si solar cells. The seed layer used is an ultrathin layer of undoped µc-Si:H film which facilitates the growth of microcrystallinity in n-µc-Si:H film. The positive effect of seed layer in the growth of microcrystallinity of n-µ-Si:H film has been established by the study of dark conductivity and transmission electron microscope (TEM) of films prepared under different deposition conditions.

Band Offset Reduction at Defect-Rich p/i Interface Through a Wide Bandgap a-SiO:H Buffer Layer

In single junction p-i-n solar cells, the optical losses can be mitigated by inserting the wide band gap amorphous silicon oxide layer at the defect-rich p/i interface. In this paper, a simulation and experimental study on the performance of p-i-n solar cells by inserting the intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) buffer layer at the p/i interface is reported. The i-a-SiO:H film has been deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) (13.6 MHz) at a low substrate temperature of approximately 230°C. The p/i interface is crucial to solar cell performance because the first few nanometers of the intrinsic layer are a defect-rich layer, having the band gap discontinuity, resulting in band offset. Thus, the carrier recombination probability increases in the vicinity of p/i interface because of high defect density and short carrier lifetime. Aided by optically calibrated simulations and with the support of experimental results, this study shows that a wide band gap thin undoped a-SiO:H buffer layer with higher photoconductivity reduces the band-gap offset and minimizes the recombination of photogenerated charge carriers at the defect-rich p/i interface. It has also been found that the a-SiO:H buffer layer augmented the electric field inside the device. As a result, the overall performance of the a-Si:H-based single junction solar cell has significantly improved. By employing a ∼5 nm thick a-SiO:H buffer layer, the blue response of the cell has been improved, resulting in 7.34% and 18.62% enhancement in fill factor (FF) and power conversion efficiency (η), respectively, as compared to the buffer-less cell.