Hirofumi Hazama

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

In Cu2ZnSnS4 (CZTS) photovoltaic cells, a low open-circuit voltage (VOC) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the VOC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the VOC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%.

Bottom Coverage of Cu Deposit for 200-nm-Class Circular Vias with High Aspect Ratios Investigated by Magnetron Sputtering Activated Using Superconducting Bulk Magnet

We have investigated the bottom coverage effect of circular vias with different aspect ratios patterned on Si substrates by depositing Cu using a magnetron sputtering apparatus equipped with a superconducting bulk magnet. The superconducting SmBa2Cu3O7-δ bulk of 60 mm diameter was magnetized up to 4.5 T at 52 K and cooled to 40 K in a refrigerator. It served as a permanent magnet producing magnetic fields about 10 times as strong as that obtained from a conventional Nd–Fe–B permanent magnet, resulting in a marked enhancement in plasma density on the Cu target. Indeed, the present magnetron sputtering apparatus was capable of sputtering under Ar gas pressures of at least 3.33 ×10-3 Pa with a throw distance Dst of 300 mm when a Cu target of 150 mm diameter was employed. Practical sputtering was possible under 2.66 ×10-2 Pa even when Dst was extended to 500 mm. Under this condition, a uniform Cu film with a bottom coverage of 58% was successfully deposited at the bottom of a circular via of 250 nm diameter and 1.15 µm depth with an aspect ratio of 4.6 within a circle of 120 mm diameter on the substrate.

Solar Thermal Cogeneration System Using a Cylindrical Thermoelectric Module

We propose a solar thermal cogeneration system using a cylindrical thermoelectric module for efficient solar energy convergence. Numerical simulations are presented to evaluate the system efficiency compared with a conventional pillar-type thermoelectric cogeneration system. We consider the effects of thermal radiation, contact resistances, and heat flux in the connecting wire, which significantly affect the system efficiency. Compared with the pillar-type device, the cylindrical device can achieve a higher heat flux and lower thermal radiation loss from the sides. In particular, the thermal radiation loss from the sides becomes negligible in a scaled-up cylindrical device. When the areas of the light-absorbing layer are the same in both devices, the power efficiencies, which are defined as power extracted from the module over input heat to the module, are comparable, but the system efficiency, which is defined as extracted heat from the module over input heat to the module, of the cylindrical device is higher than that of the pillar-type device. In the case of the unileg cylindrical device, where the hot side is connected to the cold side by the Cu wire, the system efficiency increased but the power efficiency decreased owing to the heat flux through the Cu wire. On the other hand, the p-n couple cylindrical device can overcome the trade-off and achieve system efficiency as high as 91.6%, including 8.1% power efficiency.