We investigated the effect of a 3-nm-thick amorphous Si (a-Si) capping layer on the hole transport properties of BaSi2 films. The contact resistance decreased with decreasing resistivity of p-BaSi2 and reached a minimum of 0.35 Ω·cm2. The effect of the a-Si layer was confirmed by higher photoresponsivities for n-BaSi2 films capped with the a-Si layer than for those without the a-Si layer, showing that the minority carriers (holes) were extracted efficiently across the a-Si/n-BaSi2 interface. Under AM1.5 illumination, the conversion efficiency reached 9.9% in a-Si(3 nm)/p-BaSi2(20 nm)/n-Si solar cells, the highest value ever reported for semiconducting silicides.

We have successfully determined the bulk minority-carrier lifetime in BaSi2 epitaxial films by utilizing a drastic enhancement of lifetime by post-growth annealing at 800 °C, which is attributed to strain relaxation. From the film-thickness dependence of lifetime, we reveal that the bulk lifetime is 14 µs, which is long enough for thin-film solar cell applications. In addition, the sum of surface and interface recombination velocities is found to be as low as 8.3 cm/s presumably due to the ionic nature of BaSi2. This confirms that BaSi2 is promising as an absorption-layer material for earth-abundant thin-film solar cells.

Undoped 0.5-lm-thick BaSi 2 epitaxial films were grown on Si(111) substrates with various ratios of the Ba deposition rate to the Si deposition rate (R Ba /R Si) ranging from 1.0 to 5.1, and their electrical and optical properties were characterized. The photoresponse spectra drastically changed as a function of R Ba /R Si , and the quantum efficiency reached a maximum at R Ba /R Si ¼ 2.2. Hall measurements and capacitance versus voltage measurements revealed that the electron concentration drastically decreased as R Ba /R Si approached 2.2, and the BaSi 2 films with R Ba /R Si ¼ 2.0, 2.2, and 2.6 exhibited p-type conductivity. The lowest hole concentration of approximately 1 Â 10 15 cm À3 was obtained for the BaSi 2 grown with R Ba /R Si ¼ 2.2, which is the lowest value ever reported. First-principles calculations suggest that Si vacancies give rise to localized states within the bandgap of BaSi 2 and therefore degrade the minority-carrier properties.

We have fabricated approximately 0.5-lm-thick undoped n-BaSi 2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 lm 2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi 2 films using the decay time of the second decay mode, s SRH , caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi 2 films. The measured s SRH was grouped into two, independently of the average grain area of BaSi 2. BaSi 2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large s SRH (ca. 8 ls), whereas those with mirror surfaces much smaller s SRH (ca. 0.4 ls). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi 2 films.

p-BaSi 2 /n-Si heterojunction solar cells consisting of a 20 nm thick B-doped p-BaSi 2 epitaxial layer (p ¼ 2.2 Â 10 18 cm À3) on n-Si(111) (q ¼ 1-4 X 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 g 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/cm 2 , and a fill factor of 0.60 were obtained. These results demonstrate the high potential of BaSi 2 for solar cell applications. V