Yoshihiko Nakagawa

Realization of single-phase BaSi2 films by vacuum evaporation with suitable optical properties and carrier lifetime for solar cell applications

We have realized BaSi2 films by a simple vacuum evaporation technique for solar cell applications. X-ray diffraction analysis shows that single-phase BaSi2 films are formed on alkali-free glass substrates at 500 and 600 °C while impurity phases coexist on quartz or soda-lime glass substrates or at a substrate temperature of 400 °C. The mechanism of film growth is discussed by analyzing the residue on the evaporation boat. An issue on the fabricated films is cracking due to thermal mismatch, as observed by secondary electron microscopy. Optical characterizations by transmittance and reflectance spectroscopy show that the evaporated films have high absorption coefficients, reaching 2 × 104 cm−1 for a photon energy of 1.5 eV, and have indirect absorption edges of 1.14–1.21 eV, which are suitable for solar cells. The microwave-detected photoconductivity decay measurement reveals that the carrier lifetime is approximately 0.027 µs, corresponding to the diffusion length of 0.84 µm, which suggests the potential effective usage of photoexcited carriers.

Fabrication of single-phase polycrystalline BaSi2 thin films on silicon substrates by vacuum evaporation for solar cell applications

We report on the successful fabrication of single-phase polycrystalline BaSi2 films on Si(111) substrates by a simple vacuum evaporation method using BaSi2 granules. Substrate heating was found to be the key, and high substrate temperatures are required to realize single-phase BaSi2. The underlying mechanism is discussed considering the composition of vapor flux, and we confirmed that the vapor flux is Ba-rich at the initial stage of evaporation. The reevaporation of excess Ba atoms or their reaction with Si substrates would be responsible for the realization of stoichiometric single-phase BaSi2, which explains why high substrate temperatures are necessary.

Photoresponse properties of BaSi2 film grown on Si (100) by vacuum evaporation

We have succeeded in the observation of high photoresponsivity of orthorhombic BaSi2 film grown on crystalline Si by a vacuum evaporation method, raising the prospect of its promising application in high-efficiency thin-film solar cells. Photocurrent was observed at photon energies larger than 1.28 eV, which corresponds to the band gap of evaporated BaSi2 film, indicating that the photoresponsivity originates from the BaSi2 film. The effect of the substrate temperature on the films properties was also investigated. The films grown at a substrate temperature larger than 500 °C are single-phase polycrystalline BaSi2 films, while those grown at a substrate temperature of 400 °C is a mixture of phases. We confirmed that undoped evaporated BaSi2 films are an n-type material with high carrier concentration. High carrier lifetime of 4.8 and 2.7 μs can be found for the films grown at 500 °C and 400 °C, respectively. BaSi2 film grown at a substrate temperature of 500 °C, which is crack-free and single-phase, shows the best photoresponsivity. The maximum value of photocurrent was obtained at photon energy of 1.9 eV, corresponding to an external quantum efficiency of 22% under reverse applied voltage of 2 V.

Ba Isotope Effect in a Ba2YCu3O7-δ Superconductor

The Ba isotope effect in Ba2YCu3O7-δ ceramic superconductors was studied by the Meissner effect. 134Ba-rich Ba (the weight of 134Ba being about 2.8 atomic units lighter than that of naturally obtained Ba) was introduced to obtain an isotope shift in Tc. No clear shifts in Tc are observed over the experimental resolution limit of temperature measurement.

Investigation of p-type emitter layer materials for heterojunction barium disilicide thin film solar cells

To achieve heterojunction thin film solar cells with high conversion efficiency using BaSi2 as a photoabsorption layer, we investigated the effects of electron affinity (χ) and the band gap (E g) of a p-type semiconductor on the performance of BaSi2 thin film solar cells with a one-dimensional device simulator, Afors-HET ver. 2.5. By simulation, we found that χ should be less than 3.5 eV and E g should be in the range of 1.5–2.5 eV to achieve a conversion efficiency of more than 20% in the case of p-type semiconductor/n-type BaSi2 active layer/n++-type BaSi2/electrode. Ultimately, we selected Zn3P2 (E g = 1.5 eV, χ = 3.2 eV) and SnS (E g = 1.3 eV, χ = 3.6 eV) as options for p-type materials. In particular, Zn3P2 maintains high efficiency even if a very thin oxide layer exists at the pn heterointerface. The highest conversion efficiency of p-type Zn3P2/n-type BaSi2 thin film solar cells was 23.17% without any light-trapping structure.