Toshihiro Kinoshita

Influence of Oxygen and Nitrogen in the Intrinsic Layer of a-Si:H Solar Cells

The effects of oxygen and nitrogen on hydrogenated amorphous silicon (a-Si:H) films and solar cells are systematically studied in the initial and light-soaked states. The oxygen and nitrogen concentrations were varied from 1018 to 1021 cm-3 and 1016 to 1019 cm-3, respectively. In the initial state, dark conductivity and photoconductivity increase, and the activation energy of dark conductivity decreases with increase in the oxygen and nitrogen concentrations, because of the creation of donorlike states. These film properties after light-soaking, however, are independent of the oxygen and nitrogen concentrations, probably because they are dominated by light-induced states. On the other hand, the conversion efficiency of solar cells in both the initial and light-soaked states drops as the oxygen and nitrogen concentrations increase. Collection efficiency measurements show that the electric field distribution in the i-layer is affected by the donorlike states even after light-soaking. The photovoltaic properties of solar cells more sensitively reflect the effect of oxygen and nitrogen incorporation than the film properties.

INFLUENCE OF OXYGEN IMPURITY IN THE INTRINSIC LAYER OF AMORPHOUS SILICON SOLAR CELLS

The influence of oxygen impurity in the i layer of hydrogenated amorphous silicon (a‐Si:H) solar cells is studied. At the initial state, the dark conductivity, and photoconductivity of a‐Si:H films increase and the conversion efficiency of the solar cells drops as the oxygen concentration increases. After the light soaking, these film properties become independent of the oxygen concentration, but the conversion efficiency of a‐Si:H solar cells is still influenced by the oxygen impurity. The dominant effect of the oxygen impurity in a‐Si:H solar cells is a modification of the electric field distribution in the i layer, due to the donorlike states created by oxygen.

Excellent power-generating properties by using the HIT structure

We are developing HIT solar cells with high conversion efficiency, which was achieved the worlds highest conversion efficiency of 22.3% in a practical size solar cell in July 2007. We have four main approaches to reducing power-generating cost: improve the conversion efficiency, apply the HIT structure to a thin wafer, improve the temperature coefficient, and apply HIT solar cells to a bifacial solar module. Using these approaches, we have achieved the remarkably high conversion efficiency of 21.4% due to a high Voc of 0.739 V with an 85-μm cell, which was measured at SANYO. A thinner Si wafer brings not only high Voc but also generating more output power at high temperature for a better temperature coefficient. We have confirmed that the HIT structure is suitable for use in thinner wafers, allowing us to reduce power-generating cost.

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.

Boron‐compensation effect on hydrogenated amorphous silicon with oxygen and nitrogen impurities

Oxygen and nitrogen impurities increase the dark conductivity of hydrogenated amorphous silicon (a‐Si:H), due to their donorlike behavior. An appropriate amount of boron doping recovers these values to the original ones unless the effect of band gap widening appears. The midgap absorption spectra of the compensated a‐Si:H are identical to those of intrinsic a‐Si:H both at initial and light‐soaked states. The major effect of the oxygen and nitrogen is the creation of donors, which show a similar behavior to those by phosphorus—only a small fraction of the oxygen and nitrogen produces donors, the rest is included in the a‐Si:H network without causing any other significant effect until alloying effects appear.

Optical confinement and optical loss in high-efficiency a-Si solar cells

The worlds highest stabilized conversion efficiency of 9.5% has been achieved for a 30/spl times/40 cm/sup 2/ a-Si/a-SiGe glass superstrate solar cell submodule. However, significant optical loss still exists even in these high-efficiency a-Si solar cells. FEM numerical simulation has shown that the primary origin of the optical loss in textured a-Si solar cells at about /spl ges/800 nm is absorption in SnO/sub 2/ which is enhanced by the optical confinement effect. Optical confinement also results in increased absorption in the metal electrode, which is another source of optical loss.

High-Efficiency HIT Solar Cells for Excellent Power Generating Properties

In order to achieve the widespread use of HIT (Hetero-junction with I etero-Intrinsic T ntrinsic Thin-layer) solar cells, it is important to reduce the power generating cost. There are three main approaches for reducing this cost: raising the conversion efficiency of the HIT cell, using a thinner wafer to reduce the wafer cost, and raising the open circuit voltage to obtain a better temperature coefficient. With the first approach, we have achieved the highest conversion efficiency values of 22.3%, confirmed by AIST, in a HIT solar cell. This cell has an open circuit voltage of 0.725 V, a short circuit current density of 38.9 mA/cm 2 and a fill factor of 0.791, with a cell size of 100.5 cm 2 . The second approach is to use thinner Si wafers. The shortage of Si feedstock and the strong requirement of a lower sales price make it necessary for solar cell manufacturers to reduce their production cost. The wafer cost is an especially dominant factor in the production cost. In order to provide low-priced, high-quality solar cells, we are trying to use thinner wafers. We obtained a conversion efficiency of 21.4% (measured by Sanyo) for a HIT solar cell with a thickness of 85μm. Even better, there was absolutely no sagging in our HIT solar cell because of its symmetrical structure. The third approach is to raise the open circuit voltage. We obtained a remarkably higher Voc of 0.739 V with the thinner cell mentioned above because of its low surface recombination velocity. The high Voc results in good temperature properties, which allow it to generate a large amount of electricity at high temperatures.

What causes the inverse Staebler-Wronski effect in p-type a-Si:H?

An inverse Staebler-Wronski effect in boron-doped a-Si:H is a bulk property and occurs due to the dopant activation of boron via light-soaking. It is suppressed by a considerably high hydrogen content. These phenomena can be explained by a hypothesis considering the equilibrium between three- and four-fold boron sites due to hydrogen passivation, and the trapping of the hydrogen related to the metastable boron sites (e.g. by clustered hydrogen complexes) at a very high hydrogen content.

Correlation between material properties and photovoltaic performance in amorphous silicon solar cells

We have focused on the i-layer material in our efforts to improve in the conversion efficiency of a-Si:H solar cells. Reductions in the defect density has been also investigated from the viewpoints of extrinsic (impurities) and intrinsic effects. The main incorporated impurity in a- Si:H is oxygen, which affects the conversion efficiency of a-Si:H solar cells by increasing the defect density and its donorlike behavior. A unified relationship can be observed among the properties of intrinsic (pure) a-Si:H. The film deposition rate plays an essential role in controlling the properties. A lower or higher deposition rate results in a narrower or wider bandgap, respectively. Therefore, the properties of a-Si:H can be controlled independent of the substrate temperature in a certain range by varying the film deposition rate. The controllability of the a-Si:H properties can be improved by applying vibrational / rotational energy to SiH4 molecules or related radicals by heating the source gas, and a-Si:H with the same properties as the best conventional one can be deposited at a lower substrate temperature and/or a higher film deposition rate. The highest conversion efficiency of 12% for an integrated a-Si solar cell submodule of 100 cm2 has been achieved by combining the high-quality i-layer and other technologies.