Hirotada Inoue

Development and application of see-through a-Si solar cells

Abstract A new type of translucent amorphous silicon (a-Si) solar cell, called the see-through a-Si solar cell, is developed. It has multiple microscopic holes within its effective area to transmit light and it generates electric power. A series of technical data on the fabrication processing with various patterning and photovoltaic performance are presented. Some examples of application systems such as car sunroof and home interior are introduced and discussed on a wide variety of new areas of PV applications. The see-through a-Si solar cell was mounted on a car sunroof to drive the cars ventilating system or to charge its battery. The ventilating system reduced the interior temperature of the car from 61 to 47°C during daytime parking.

High-efficiency HIT solar cells with a very thin structure enabling a high Voc

To increase the competitiveness of HIT (Heterojunction with Intrinsic Thin-layer) solar cells, we have been working on the enhancing their conversion efficiency. This time, we improved the heterojunction of the HIT solar cell, which made it possible to enhance the cell conversion efficiency. In addition, we have developed module technologies such as a new tab design and anti-reflection coated glass. By combining these technologies, we have achieved 240-W model with module conversion efficiency of 19.0%. Those HIT solar cells have the worlds highest level of cell conversion efficiency 21.6 % at the mass-production stage. We have also been investigating the performance of thinner HIT solar cell using crystalline silicon (c-Si) wafers less than 100 μm in thickness. To minimize optical losses, such as the ultraviolet light absorption in the front transparent conductive oxide (TCO) layer and amorphous Si (a-Si) layers, and the near-infrared light absorption in the rear TCO layer, we have improved the deposition conditions of a-Si, and developed the TCO material respectively. To improve the surface passivation quality of the a-Si/c-Si heterointerface, we have examined our fabrication process from these three viewpoints: (1) the cleanliness of the c-Si surface, (2) the damage in the deposition process, and (3) the quality of the deposited a-Si layer. As a result, we have achieved an excellent Voc of 0.747 V with 58- and 75-μm-thick cells.

High-performance HIT solar cells for thinner silicon wafers

We have been researching and developing HIT (Heterojunction with Intrinsic Thin-layer) solar cells to obtain high conversion efficiency. Last year, we updated the worlds highest conversion efficiency, which was previously 22.3%, to 23.0% with a practical-sized HIT solar cell at the R&D stage. We have also been investigating the performance of thinner HIT solar cells using less than 100-µm-thick crystalline Si (c-Si) wafers in order to effectively reduce the production cost. By using improved technologies, we succeeded in gaining the high conversion efficiency of 22.8% in a HIT solar cell with a 98-µm-thick c-Si wafer and an excellent Voc of 743 mV at the R&D stage. The accomplishment of the 22.8% cell demonstrates that HIT solar cells are advantageous to the use of thinner Si wafers because of certain HIT solar cell features.

High-Rate Deposition of Amorphous Silicon Films by Microwave-Excited High-Density Plasma

In this study, the deposition of amorphous silicon (a-Si) thin films using a microwave-excited high-density plasma system is described. We investigate the effects of plasma excitation gas species (argon or helium), total gas pressure, silane (SiH4) flow rate, and substrate stage temperature, estimating the resultant films from cross-sectional morphology, photoconductivity, and dark conductivity measured without light-induced degradation. It is confirmed that high-quality a-Si films (photosensitivity= 1.29 ×106) can be formed in the plasma excitation gas helium at a pressure of 13.3 Pa by relatively high rate (1.1 nm/s) deposition. At the same time, we measure the plasma emission derived from various radicals such as Si and SiH radicals in order to discuss the mechanism of radical generation in the plasma. The result of the measurement implies that when argon is used as plasma excitation gas, metastable states of argon markedly dissociate silane, which produces low-quality a-Si films. On the other hand, it seems that electrons dissociate silane mainly, which produces high-quality a-Si films, in helium.

Development of an ultralight, flexible a-Si solar cell submodule

Abstract An ultralight, flexible a-Si solar cell fabricated on a polyimide film substrate has been developed. It was found that prebaking the polyimide film substrate improved the output performance of the cell by reducing the amount of H 2 0 and polymers released from the substrate. Also, using an i-layer fabricated at high temperature (250°C) and a p-layer fabricated at low temperature (80°C) improved the cells output performance to a level of over 10%. A maximum output of 782.4 mW and a power-to-weight ratio of 340 mW/g were obtained for an integrated-type flexible a-Si solar cell submodule with a size of 113 mm × 120 mm.

New-type of ultralight flexible a-Si solar cell and its application on an airplane

An ultralight flexible a-Si solar cell fabricated on a transparent film substrate has been developed. High photoconductivity ( sigma ph) of 2*10/sup -5/ Omega /sup -1/ cm/sup -1/ and high photosensitivity ( sigma ph/ sigma d) of 1*10/sup 6/ were obtained with a-Si films deposited by H/sub 2/-dilution of SiH/sub 4/ on a transparent film substrate at 180 degrees C. Prebaking the transparent film substrate effectively reduced the impurity content of the a-Si film. A maximum output power of 550 mW and a power-to-weight ratio of 275 mW/g were obtained for an integrated-type a-Si solar cell with a size of 110*115 mm. As the first application of this flexible a-Si solar cell, a solar-powered airplane has been developed, and it completed a transcontinental flight across the United States.<<ETX>>

New type structure of a-Si solar cell submodules fabricated by microscopic hole spacing technique

A laser patterning method is investigated as a fabrication technique for microscopic holes on a-Si solar cells. The optimum laser patterning conditions calculated from a three-dimensional thermal analysis were confirmed experimentally. It is shown that microscopic holes can be fabricated on an a-Si solar cell using this method without any significant damage to the photovoltaic performance around the holes. This technique was applied to two unique structures for a-Si solar cell submodules: a see-through integrated-type submodule, and a through-hole contact (THC) integrated-type submodule. The hole pattern of see-through submodules was optimized for the purpose of obtaining both high transmittance and light uniformity. The performance for THC submodules is estimated by numerical simulation based on a distributed equivalent circuit model. It is found that a 10% increase in the maximum output power can be expected using this structure.<<ETX>>