Tsuyoshi Uematsu

Design and characterization of flat-plate static-concentrator photovoltaic modules

Using a ray-tracing program to analyze the characteristics of monofacial-cell and bifacial-cell-types of flat-plate static-concentrator photovoltaic modules revealed that about 90% of the annual irradiation can be collected by a monofacial-cell module with a concentration ratio of 1.5 and by a bifacial-cell module with concentration ratio of 2.0. Preliminarily, small modules were fabricated and dependence of optical performance on light incident angles were measured. The optical collection efficiency of a monofacial-cell module with a concentration ratio of 1.5 was 87.6% and that of bifacial-cell module with a concentration ratio of 2.0 was 85.6%.

Static concentrator photovoltaic module with prism array

To obtain cost-effective photovoltaic modules, we have developed static prism-array concentrator modules consisting of prism concentrators about 4 mm thick assembled unidirectionally under a 3.2-mm-thick cover glass. Calculating the optical collection efficiency for the annual solar irradiation in Tokyo, we found that the theoretical efficiency of the modules is 94.4% when the geometrical concentration ratio is 1.88 and that it is 89.1% when that ratio is 2.66, respectively. Fabricating prism-array-concentrator modules with a geometrical concentration ratio of 2.66, we obtained a maximum optical collection efficiency of 82% with a flat reflector and 81.7% with a V-grooved reflector.

Optimization of thermal processing and device design for high-efficiency c-Si solar cells

Abstract To improve the cell performance of single-crystal silicon solar cells, the process conditions have been optimized by monitoring the bulk lifetime after each thermal step in the cell fabrication process. The emitter geometry, i.e., front and rear contact size and pitch were optimized, and the cells were fabricated through a set of environmentally considered processes, especially for surface treatment, oxidation, diffusion, and electrode fabrication. Conversion efficiency of 22.3% in a 4 cm 2 cell, and 22.6% in a 1 cm 2 cell, was attained, respectively, with structural features of SiO 2 single-AR, “inverted-pyramid” fron texture, point-contact with line-emitter for front electrodes, and locally diffused BSF for rear contacts.

Bifacial multicrystalline silicon solar cells

A bifacial solar cell structure is applied to multicrystalline substrates obtained from the OTC cast material. The double sided cathode configuration made it possible to collect photogenerated minority carriers from the nearest side of the cell, which allows the minority carrier not necessarily to travel across the substrate having short diffusion lengths. The rear cathode acts as a current booster for the front cathode for light entering from the front side, and vice versa. Because of this two fold current enhancement, cell outputs equivalent to nearly 20% efficiency are obtainable from a 15% cell when 30% albedo from the rear side is incorporated. A simplified screen printing process for cell fabrication provides a promising method of scaled production.<<ETX>>

Optimization of High-Efficiency n + - p - p + Back-Surface-Field Silicon Solar Cells

High-efficiency silicon solar cells are fabricated with back-surface-field (BSF) structures using medium-resistivity, float-zone substrates. The cell processes are optimized using a new BSF process in order to maintain a high minority-carrier lifetime. In addition, they are improved with a passivated, V-grooved surface to enhance collection efficiency. The resultant cells exhibit conversion efficiencies of 19.1% with a short-circuit current density of 38.4 mA/cm2. The minority-carrier lifetime and surface recombination velocity are estimated to be 341 µs and 3×105 cm/s using a computer program.

Applications and field tests of bifacial solar modules

In order to realize a photovoltaic (PV) system that is low cost and flexible in terms of installation, the authors have studied the vertical installation of bifacial solar modules. Simulations of output power and field testing were carried out to evaluate daily and yearly output power when bifacial modules are vertically installed at various azimuth angles. The yearly-generated power by a vertically installed bifacial module was found to be about 1.4 times larger than that of a vertically installed mono-facial module at the test location. Some applications of bifacial modules, such as fence-integrated PV systems and pole mounted PV systems, were successfully demonstrated on the basis of these results.

A new cell structure for very thin high-efficiency silicon solar cells

The cell has a corrugated structure, which is formed by aligned V grooves on both the front and back surfaces. The substrate thickness is reduced to 50 mu m while retaining high mechanical strength. This permits ease of handling during the fabrication process and subsequent procedures. This thin substrate promises a very high open-circuit voltage, and the structure is also beneficial to high optical performance. The surface reflectance is reduced in the same manner as that of V-grooved cells, but the optical path is lengthened by a minimum of four times the substrate thickness. Performance of experimental cells is also discussed. >

Very-low-temperature silicon epitaxy by plasma-CVD using SiH4-PH3-H2 reactants for bipolar devices

A novel plasma-CVD process has been developed for growing specular silicon epitaxial layers at very low temperatures of approximately 200°C. The epitaxial layers were deposited from SiH4-PH3 gases diluted with H2 on single-crystal substrates just after being etched in a HF dip. The highest electron mobility and lowest resistivity of the n+-layers were 30 cm2/V s and 4×10-4 Ωcm, respectively. This technology has been successfully applied to fabricate bipolar transistors with a high current gain of 370.

SiNx :H/SiO2 Double-Layer Passivation With Hydrogen-Radical Annealing For Solar Cells

A SiNx :H/SiO2 double-layer structure as the passivation layer and hydrogen-radical post annealing were introduced to decrease the surface recombination velocity on crystalline silicon. Effective lifetime was much increased by introducing a SiNx :H/SiO2 double-layer passivation structure instead of a SiNx :H or SiO2 single-layer, while the hydrogen radical post annealing improves the interface characteristics more effectively than N2 annealing. The results of C–V measurement suggest that many positive charges exist in the SiNx :H layer. The combination of field effect by these charges and the decrease of SiO2/c-Si interface defects by hydrogen-radical annealing effectively decrease the surface recombination velocity.

Effect of hydrogen radical annealing on SiN passivated solar cells

Abstract Remote plasma was used for PE-CVD of SiN films and it was found that hydrogen radical (H * ) annealing of c-Si cells with SiN films improved the efficiency of the cells. Cell efficiency of 21.8% was obtained by applying a SiN/SiO 2 double-layer structure on the emitter of a PERL-type solar cell. It was found that the H * annealing has two effects: it reduces surface recombination velocity (SRV); and it degrades bulk-lifetime of p-type c-Si. To apply SiN practically, it is effective to use a rear n-floating or a triode structure. Reducing the exposed area of the p-type substrate by using n-type diffused layer increases the efficiency of solar cells.