Yasufumi Tsunomura

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-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.