Saleem H. Zaidi

Characterization of random reactive ion etched-textured silicon solar cells

Hemispherical reflectance and internal quantum efficiency (IQE) measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using reactive ion etching (RIE) random texturing techniques. Random RIE-textured surfaces typically exhibit broadband anti-reflection behavior with solar-weighted-reflectance (SWR) of /spl ap/3% over 300-1200-nm spectral range. RIE-texturing has been demonstrated over large areas (/spl sim/180 cm/sup 2/) of both single and multicrystalline Si substrates. Due to the surface contamination and plasma-induced damage, as formed RIE-textured solar cells do not provide enhanced short-circuit current. However, improved surface cleaning combined with controlled wet-chemical damage removal etches provide a significant improvement in the short-circuit current. For such textures, the internal quantum efficiencies are comparable to the random, wet-chemically-textured solar cells. In both the UV and near-IR wavelength regions, the RIE-textured subwavelength surfaces exhibit superior performance in comparison with the wet-chemically-textured surfaces. Due to their large area, low-reflection capability, random, RIE-texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells.

Amorphous Silicon Single-Junction Thin-Film Solar Cell Exceeding 10% Efficiency by Design Optimization

The conversion efficiency of a solar cell can substantially be increased by improved material properties and associated designs. At first, this study has adopted AMPS-1D (analysis of microelectronic and photonic structures) simulation technique to design and optimize the cell parameters prior to fabrication, where the optimum design parameters can be validated. Solar cells of single junction based on hydrogenated amorphous silicon (a-Si:H) have been analyzed by using AMPS-1D simulator. The investigation has been made based on important model parameters such as thickness, doping concentrations, bandgap, and operating temperature and so forth. The efficiency of single junction a-Si:H can be achieved as high as over 19% after parametric optimization in the simulation, which might seem unrealistic with presently available technologies. Therefore, the numerically designed and optimized a-SiC:H/a-SiC:H-buffer/a-Si:H/a-Si:H solar cells have been fabricated by using PECVD (plasma-enhanced chemical vapor deposition), where the best initial conversion efficiency of 10.02% has been achieved ( V,  mA/cm2 and ) for a small area cell (0.086 cm2). The quantum efficiency (QE) characteristic shows the cell’s better spectral response in the wavelength range of 400 nm–650 nm, which proves it to be a potential candidate as the middle cell in a-Si-based multijunction structures.

Diffraction grating structures in solar cells

Sub-wavelength periodic texturing (gratings) of crystalline-silicon (c-Si) surfaces for solar cell applications can be designed for maximizing optical absorption in thin c-Si films. The authors have investigated c-Si grating structures using rigorous modeling, hemispherical reflectance and internal quantum efficiency measurements. Model calculations predict almost /spl sim/100% energy coupling into obliquely propagating diffraction orders. By fabrication and optical characterization of a wide range of 1D and 2D c-Si grating structures, they have achieved broadband, low (/spl sim/5%) reflectance without an antireflection film. By integrating grating structures into conventional solar cell designs, they have demonstrated short-circuit current density enhancements of 3.4 and 4.1 mA/cm/sup 2/ for rectangular and triangular 1D grating structures compared to planar controls. The effective path length enhancements due to these gratings were 2.2 and 1.7, respectively. Optimized 2D gratings are expected to have even better performance.

Plasma-texturization for multicrystalline silicon solar cells

Multicrystalline Si (mc-Si) cells have not benefited from the cost-effective wet-chemical texturing processes that reduce front surface reflectance on single-crystal wafers. The authors developed a maskless plasma texturing technique for mc-Si cells using reactive ion etching (RIE) that results in much higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while reducing front reflectance to extremely low levels. Internal quantum efficiencies higher than those on planar and wet-textured cells have been obtained, boosting cell currents and efficiencies by up to 11% on monocrystalline Si and 2.5% on multicrystalline Si cells.

RIE-Texturing of Industrial Multicrystalline Silicon Solar Cells

We developed a maskless plasma texturing technique for multicrystalline Si (mc-Si) cells using Reactive Ion Etching (RIE) that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to low levels. Internal quantum efficiencies higher than those on planar and wet-textured cells have been obtained, boosting cell currents and efficiencies by up to 6% on tricrystalline Si cells.

The role of physical techniques on the preparation of photoanodes for dye sensitized solar cells

Dye sensitized solar cells (DSSCs) have attracted numerous research, especially in the context of enhancing their efficiency and durability, due to the low-cost and environmentally friendly nature of photovoltaic (PV) technology. The materials in DSSCs are vital towards the realization of these goals, since many of the important components are influenced by their respective preparation and deposition methods. This review aims to detail the research and development aspects of the different physical methods with the purpose of evaluating their prospects and corresponding limitations. The diversity of consideration and criteria includes thin film applications, material characteristics, and process technology that need to be taken into account when selecting a specific deposition method. Choosing a deposition method is not as simple as it seems and is rendered quite complicated due to various factors. Usually, a researcher will evaluate techniques based on factors such as the different preparations and deposition technology with materials’ and substrates’ type, specified applications, costs, and efficiencies.

Characterization of a Bifacial Photovoltaic Panel Integrated with External Diffuse and Semimirror Type Reflectors

Silicon wafer accounts for almost one-half the cost of a photovoltaic (PV) panel. A bifacial silicon solar cell is attractive due to its potential of enhancing power generation from the same silicon wafer in comparison with a conventional monofacial solar cell. The bifacial PV cell is able to capture solar radiation by back surface. This ability requires a suitable reflector appropriately oriented and separated from the cell’s rear surface. In order to optimize the bifacial solar cell performance with respect to an external back surface reflector, diffuse and semimirror reflectors were investigated at various angles and separations from the back surface. A simple bifacial solar panel, consisting of four monocrystalline Si solar cells, was designed and built. Reflection from the rear surface was provided by an extended semimirror and a white-painted diffuse reflector. Maximum power generation was observed at 30° with respect to ground for the semimirror reflector and 10° for diffuse reflector at an optimized reflector-panel separation of 115 mm. Output power enhancement of 20% and 15% from semimirror and diffuse reflectors, respectively, were observed. This loss from diffuse reflector is attributed to scattering of light beyond the rear surface capture cross-section of the bifacial solar panel.

Deeply etched grating structures for enhanced absorption in thin c-Si solar cells

Sub-wavelength periodic structures in crystalline-silicon (c-Si) for solar cell applications can be designed for maximizing optical absorption in thin films. We have investigated optical response of deeply etched c-Si grating structures using rigorous modeling, hemispherical reflectance, one-sun LIV, and internal quantum efficiency measurements. Model calculations predict that almost /spl sim/ 100 % optical absorption can be achieved in subwavelength 2D structures etched to a depth of /spl sim/ 15 /spl mu/m. Using advanced reactive ion etching techniques, subwavelength deeply etched grating structures have been fabricated and integrated into solar cells. Preliminary one-sun solar cell measurements from /spl sim/ 10-/spl mu/m 2D period structures have demonstrated short-circuit current enhancement of /spl sim/ 10 mA. The cell efficiencies were poor due to the lack of surface passivation and emitter optimization. Subwavelength grating solar cells failed to provide any performance boost probably due to the lack of surface passivation. Optimization of emitter formation on these types of deeply etched grating surfaces is expected to lead to high-efficiency, thin-film c-Si solar cells.

Enhanced near IR absorption in random, RIE-textured silicon solar cells: the role of surface profiles

We report on the role of surface profiles exhibiting comparable reflectance in random reactive ion etched textured Si solar cells. Internal quantum efficiency measurements demonstrate significant near IR absorption enhancement with peaks at /spl lambda/ /spl sim/ 1120 nm and 1050 nm. Using Fourier analysis of random surfaces, we find a majority of spatial structures in /spl sim/ 0.3-5-/spl mu/m range. This random distribution of subwavelength periodic structures leads to enhanced oblique coupling into Si through diffractive optics mechanisms. The random surfaces supporting finer features create diffraction orders propagating at larger angles enhancing near IR absorption through oblique light propagation. Random surfaces with larger features create almost vertically propagating diffraction orders resulting in little oblique coupling, and for some structures, almost no enhanced near IR absorption.

RIE-texturing of industrial multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline Si (mc-Si) cells using reactive ion etching (RIE) that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to low levels. Internal quantum efficiencies higher than those on planar and wet-textured cells have been obtained, boosting cell currents and efficiencies by up to 6% on tricrystalline Si cells.