Daichi Tsukahara

p-BaSi2/n-Si heterojunction solar cells with conversion efficiency reaching 9.0%

p-BaSi2/n-Si heterojunction solar cells consisting of a 20 nm thick B-doped p-BaSi2 epitaxial layer (p = 2.2 × 1018 cm−3) on n-Si(111) (ρ = 1–4 Ω cm) were formed by molecular beam epitaxy. The separation of photogenerated minority carriers is promoted at the heterointerface in this structure. Under AM1.5 illumination, the conversion efficiency η reached 9.0%, which is the highest ever reported for solar cells with semiconducting silicides. An open-circuit voltage of 0.46 V, a short-circuit current density of 31.9 mA/cm2, and a fill factor of 0.60 were obtained. These results demonstrate the high potential of BaSi2 for solar cell applications.

Potential variations around grain boundaries in impurity-doped BaSi2 epitaxial films evaluated by Kelvin probe force microscopy

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi2 epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi2 films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 1017 cm−3. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 1018 cm−3. In the B-doped p-BaSi2 films, the upward band bending was observed for a hole concentration p ≈ 1018 cm−3. The average barrier height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 1018 to p = 4.0 × 1018 cm−3. These results are explained under the assumption that the position of the Fermi level Ef at GBs depends on the degree of occupancy of defect states at the GBs, while Ef approached the bottom of the conduction band or the top of the valence band in the BaSi2 grain ...

Influence of air exposure duration and a-Si capping layer thickness on the performance of p-BaSi2/n-Si heterojunction solar cells

Fabrication of p-BaSi2(20nm)/n-Si heterojunction solar cells was performed with different a-Si capping layer thicknesses (da-Si) and varying air exposure durations (tair) prior to the formation of a 70-nm-thick indium-tin-oxide electrode. The conversion efficiencies (η) reached approximately 4.7% regardless of tair (varying from 12–150 h) for solar cells with da-Si = 5 nm. In contrast, η increased from 5.3 to 6.6% with increasing tair for those with da-Si = 2 nm, in contrast to our prediction. For this sample, the reverse saturation current density (J0) and diode ideality factor decreased with tair, resulting in the enhancement of η. The effects of the variation of da-Si (0.7, 2, 3, and 5 nm) upon the solar cell performance were examined while keeping tair = 150 h. The η reached a maximum of 9.0% when da-Si was 3 nm, wherein the open-circuit voltage and fill factor also reached a maximum. The series resistance, shunt resistance, and J0 exhibited a tendency to decrease as da-Si increased. These results dem...

Potential variation around grain boundaries in BaSi2 films grown on multicrystalline silicon evaluated using Kelvin probe force microscopy

Potential variations across the grain boundaries (GBs) in a 100 nm thick undoped n-BaSi2 film on a cast-grown multicrystalline Si (mc-Si) substrate are evaluated using Kelvin probe force microscopy (KFM). The θ-2θ X-ray diffraction pattern reveals diffraction peaks, such as (201), (301), (410), and (411) of BaSi2. Local-area electron backscatter diffraction reveals that the a-axis of BaSi2 is tilted slightly from the surface normal, depending on the local crystal plane of the mc-Si. KFM measurements show that the potentials are not significantly disordered in the grown BaSi2, even around the GBs of mc-Si. The potentials are higher at GBs of BaSi2 around Si GBs that are formed by grains with a Si(111) face and those with faces that deviate slightly from Si(111). Thus, downward band bending occurs at these BaSi2 GBs. Minority carriers (holes) undergo a repelling force near the GBs, which may suppress recombination as in the case of undoped n-BaSi2 epitaxial films on a single crystal Si(111) substrate. The b...

Cross-sectional potential profile across a BaSi2 pn junction by Kelvin probe force microscopy

We grow a boron (B)-doped BaSi2 (0.7 µm)/undoped n-BaSi2 (1.7 µm) layered structure on a p-Si(111) substrate by molecular beam epitaxy, and observe the cross-sectional potential profile across the junction by Kelvin probe force microscopy (KFM). The potential increases when the KFM tip is moved from the B-doped BaSi2 to the n-BaSi2, and decreases in the p-Si. Inflection points are clearly observed in the potential profile at the B-doped BaSi2/n-BaSi2 and n-BaSi2/p-Si interfaces. Secondary ion mass spectrometry reveals that B atoms scarcely diffuse to the n-BaSi2 layer. These results show the formation of a pn junction at the B-doped BaSi2/n-BaSi2.

p-BaSi 2 /n-Si heterojunction solar cells with 9.0% efficiency

Summary form only given. We fabricated p-BaSi₂/n-Si solar cells by forming a 20-nm-thick B-doped p-BaSi₂ epitaxial layer (p = 2.2 × 10¹⁸ cm^-3) on an n-Si(111) substrate (ρ = 1-4 Ω·cm) by molecular beam epitaxy (MBE). 3-nm-thick amorphous-Si was deposited on the p-BaSi₂ surface to prevent the surface oxidation. According to the band alignment of p-BaSi₂/n-Si heterojunction, the separation of photogenerated minority carriers is promoted at the heterointerface. We recorded conversion efficiency of 9.0 % under AM1.5 illumination at room temperature. Short-circuit current density of 31.9 mA/cm², open-circuit voltage of 0.46 V, and fill factor of 0.60 were obtained. These results suggest the potential of BaSi₂ for solar cell application.

Investigation of surface potential distributions of phosphorus-doped n-BaSi thin-films by kelvin probe force microscopy

We investigated the surface potential distributions around grain boundaries (GBs) in phosphorus (P)-doped n-BaSi2 thin-films by Kelvin probe force microscopy (KFM) and the crystal planes constituting GBs by transmission electron microscopy (TEM). By KFM measurements, it was found that the GBs in P-doped n-BaSi2 are different from those in undoped BaSi2; undoped n-BaSi2 has a downward band bending around the GBs with barrier heights of approximately 30 meV. In contrast, P-doped n-BaSi2 has an upward band bending with barrier heights of approximately 15 meV. TEM observation revealed that most of the GBs in P-doped BaSi2 are composed of BaSi2 (011)/(0-11) planes. This result is the same as that in undoped BaSi2.