Arindam Sarker

Preparation and characterization of n-type microcrystalline hydrogenated silicon oxide films

We have developed n-type microcrystalline hydrogenated silicon oxide (n-µc-SiO : H) thin films by the radio frequency plasma enhanced chemical vapour deposition (RF-PECVD, 13.56 MHz) method having suitable characteristics for use in the fabrication of single or multijunction amorphous silicon (a-Si) solar cells. The films have been characterized in detail for the study of structural and optoelectronic properties. Transmission electron microscopy, Raman spectroscopy, x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy have been used for the structural studies. The dependence of the structure and optoelectronic properties of n-µc-SiO : H films on the various deposition parameters such as hydrogen dilution, chamber pressure, RF-power density etc have also been studied. Comparison of the properties between n-µc-SiO : H and n-µc-Si : H films have been studied, too, which shows that the former has higher optical gap (2.17 eV) and lower activation energy (0.015 eV) with similar electrical conductivity (12.08 S cm-1).

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.

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.

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.

Development of stabilized dual gap double junction a-Si solar cell using helium diluted a-Si : H intrinsic layer

Low gap (~1.6 eV) a-Si : H films have been prepared by using helium diluted silane as source gas mixture in the usual radiofrequency plasma enhanced chemical vapour deposition method (13.56 MHz). The films have characteristics suitable for use as active layer of the bottom cell of a dual gap double junction a-Si solar cell. The films have been prepared at different rf-power densities and chamber pressures and characterized in detail. The role played by helium in improving the structure of a-Si : H and lowering of the band gap has been analysed. The cells fabricated with the said material show higher short circuit current density (Jsc = 8.9 mA cm−2) and lower light induced degradation (9–10%) before stabilization.

Development of New Type of Polymer Based Low Cost Encapsulation Technique for Amorphous Silicon Solar Modules

We have developed a new material that can be used for encapsulation of a-Si modules fabricated on TCO coated glass substrates. This is a polymer based material which can be sprayed on the back side of the module. The encapsulation is done by curing it at 60deg-70degC for a period of two hours. The material is transparent over a wavelength region of 300 nm-200 nm (transmission >95%) and can withstand temperature range -30degC to 100degC without any change of property. The efficacy of this type of encapsulation has been tested on a number of single and double junction modules. It has been found that after encapsulation the initial wattage of the modules increase by 5-6%. This may be due to relaxation of the cell structure deposited on the glass substrate during the curing of the encapsulant. This type of encapsulation has also been tested by exposure to atmosphere. It has been seen that for light soaked modules there is virtually no environmental degradation of the encapsulated modules. Thus we have been able to develop a cheap and comparatively simple encapsulation method for a-Si modules deposited on glass substrates

Production of Amorphous Silicon Solar Modules in India Based on Indigenous Process and Plant

For increasing the market share of a-Si solar modules it is necessary to bring down its selling price significantly lower than that for c-Si modules. There is no problem in very large scale production of a-Si modules in terms of availability of materials and production technology. Steps have been taken to design a production line for large area single and double junction modules with capacity ges10 MW/Year. With indigenously built plant and machinery and comparatively low cost of operation and maintenance in India we expect to bring down the selling price of a-Si modules to les USD 1.5/pW which will be a significant breakthrough in solar electricity generated through the photovoltaic route