Recent Progress Toward Realization of High-Efficiency BaSi2 Solar Cells: Thin-Film Deposition Techniques and Passivation of Defects

Safe, stable, and earth-abundant materials for solar cell applications are of particular importance to realize a decarbonized society. Semiconducting barium disilicide (BaSi2), which is composed of nontoxic and earth-abundant elements, is an emerging material to meet this requirement. BaSi2 has a bandgap of 1.3 eV that is suitable for single-junction solar cells, a large absorption coefficient exceeding that of chalcopyrite, and inactive grain boundaries. This review is started by describing the recent progress of BaSi2 thin-film deposition techniques using radio-frequency sputtering and discuss the high photoresponsivity of BaSi2 thin films. Special attention is paid to passivation of the defects in BaSi2 films by hydrogen or carbon doping. Ab initio studies based on density-functional theory are then used to calculate the positions of the localized defective states and the Fermi level to discuss the experimentally obtained passivation effects. Finally, the issues that need to be resolved toward realization of high-efficiency BaSi2 solar cells are addressed

Effects of hydrogen on trap neutralization in BaSi2 with interstitial silicon atoms

Semiconducting barium disilicide (BaSi2) is one of emerging materials for solar cell applications. Therefore, defect neutralization is very important for improving its solar cell performance. Herein, the effect of atomic hydrogen (H) on the photoresponsivity of 500-nm-thick undoped n-BaSi2 films grown under Si-rich conditions by molecular beam epitaxy was examined, wherein interstitial Si atoms were likely to exist instead of common Si vacancies. The photoresponsivity reached a maximum of ∼1.3 A W−1 at room temperature (about twice as large as that for as-grown films) in BaSi2 films exposed to an atomic H for 10 s. This H treatment time is much smaller than that for BaSi2 films grown under stoichiometric conditions, indicating that interstitial Si atoms provide a smaller trap concentration with respect to the case of Si vacancy domination and incorporation of H atom can neutralize traps. The inverse logarithmic slope (E0) of the Urbach tail was investigated to discuss the dependence of the photoresponsivity of BaSi2 films on H treatment time. There was a clear negative correlation between E0 and photocurrent density obtained from the photoresponsivity. The detailed picture on how interstitial Si atoms affect the electronic structure of BaSi2 by defect state formation and how H atom incorporation modifies the structure is revealed by ab initio calculations that allowed interpreting and understanding of all experimentally observed trends

Theoretical analysis of electrochromism of Ni-deficient nickel oxide – from bulk to surfaces

The development of new electrochromic materials and devices, like smart windows, has an enormous impact on the energy efficiency of modern society. One of the crucial materials in this technology is nickel oxide. Ni-deficient NiO shows anodic electrochromism, whose mechanism is still under debate. We use DFT+U calculations to show that Ni vacancy generation results in the formation of hole polarons localized at the two oxygens next to the vacancy. In the case of NiO bulk, upon Li insertion or injection of an extra electron into Ni-deficient NiO, one hole gets filled, and the hole bipolaron is converted into a hole polaron well-localized at one O atom, resulting from the transition between oxidized (colored) to reduced (bleached) state. In the case of the Ni-deficient NiO(001) surface, the qualitatively same picture is obtained upon embedding Li, Na, and K into the Ni surface vacancy, reinforcing the conclusion that the electron injection, resulting in the filling of the hole states, is responsible for the modulation of the optical properties of NiO. Hence, our results suggest a new mechanism of Ni-deficient NiO electrochromism not related to the change of the Ni oxidation states, i.e., the Ni2+/Ni3+ transition, but based on the formation and annihilation of hole polarons in oxygen p-states

Orientation and size effects on phonon thermal conductivity in silicon/germanium multilayer structures

We study the effect of morphology on the in- and cross-plane phonon thermal conductivity of the (001), (110), and (111) oriented Si/Ge multilayer films by means of non-equilibrium molecular dynamics at 300 K. The extended comparison of the estimated values for the multilayer films to one for the appropriate homogeneous Si and Ge films has been performed. The results revealed a significant advantage in reducing the thermal conductivity of the Si/Ge multilayer films compared to the referenced homogeneous Ge and Si films for the cross-plane transport regardless of the film orientation, and for the in-plane transport only for (001)/[¯ 110 ,] (110)/[001] directions with an increase in the number of periods, which indicated the prospects of such layered structures

Mg(2)Si(x)Sn(1-x)heterostructures on Si(111) substrate for optoelectronics and thermoelectronics

Thin (50-90 m) non-doped and doped (by Al atoms) Mg2Sn0.6Si0.4 and Mg(2)Sn(0.4)Si(0.6)films with roughness of 1.9-3.7 nm have been grown by multiple deposition and single annealing at 150 degrees C of multilayers formed by repetition deposition of three-layers (Si-Sn-Mg) on Si(111) p-type wafers with 45 cm resistivity. Transmission electron microscopy has shown that the first forming layer is an epitaxial layer of hex-Mg2Sn(300) on Si(111) substrate with thickness not more than 5-7 nm. Epitaxial relationships: hex-Mg2Sn(300)parallel to Si(111), hex-Mg2Sn[001]parallel to Si[-112] and hex-Mg2Sn[030]parallel to Si[110] have been found for the epitaxial layer. But inclusions of cub-Mg2Si were also observed inside hex-Mg2Sn layer. It was found that the remaining part of the film thickness is in amorphous state and has a layered distribution of major elements: Mg, Sn and Mg without exact chemical composition. It was established by optical spectroscopy data that both type films are semiconductor with undispersed region lower 0.18 eV with n(o) = 3.59 +/- 0.01, but only two direct interband transitions with energies 0.75-0.76 eV and 1.2 eV have been determined. The last interband transition has been confirmed by photoreflectance data at room temperature. Fourier transmittance spectroscopy and Raman spectroscopy data have established the formation of stannide, silicide and ternary compositions

Effects of bipolarons on oxidation states, and the electronic and optical properties of W18O49

W18O49 has been studied by means of ab initio techniques in the framework of the density functional theory using the onsite Hubbard-U correction applied to the W-d states as well as using the hybrid potential. The existence of bipolarons is found to be an intrinsic feature of this oxide resulting in the presence of different oxidation states of W atoms (W6+ and W5+) and in the co-existence of localized and delocalized electrons. We also discuss possible switching from the W6+ to W5+ and from the W5+ to W4+ oxidation states in the presence of an O vacancy. It appears that O vacancy formation does not cause any additional charge localization at W sites but solely contributes to delocalized electrons. The calculated absorption and reflection coefficients manifest a transparency window in the visible region. At the same time, sizable absorption, occurring due to the presence of free carriers, is detected in the far and mid infrared regions. Additionally, in the near infrared region we confirm and explain an experimentally observed shielding effect originating from transitions involving the localized bipolaronic states

Effect of morphology on the phonon thermal conductivity in Si/Ge superlattice nanowires

We used nonequilibrium molecular dynamics to investigate the role of morphology in the phonon thermal conductivity of 〈100〉, 〈110〉, 〈111〉 and 〈112〉-oriented Si/Ge superlattice nanowires at 300 K. Such nanowires with 〈112〉 growth direction were found to possess the lowest values of the thermal conductivity [1.6 W/(m·K) for a Si and Ge segment thickness of ∼3 nm] due to the lowest average group velocity and highly effective {113} facets and Si/Ge(112) interface for phonon-surface and phonon-interface scattering, respectively. Comparison with homogeneous and core/shell Si and Ge nanowires showed that the superlattice morphology is the most efficient to suppress the thermal conductivity

Effects of lattice parameter manipulations on electronic and optical properties of BaSi2

We present comprehensive experimental and theoretical investigation on a band-gap engineering and modification in optical properties of BaSi2 – a new and very promising material for solar cell fabrication. BaSi2 thin films have been synthesized by molecular beam epitaxy with various deposition rates of Si and Ba. Changes in their band gaps and shifts in absorption edges with respect to alteration in the a lattice parameter have been investigated by optical measurements. It is possible to shrink a by about 0.003 nm (or 0.3%), while the other lattice parameters are locked by the epitaxial relationship with a Si(111) substrate, that leads to the gap reduction from 1.28 eV to 1.20 eV. By means of ab initio calculations we explore a possibility to manipulate bandgap values in BaSi2 along with the corresponding shift in the absorption edge by changing its a, b and c lattice parameters. It is revealed that an increase in any of the lattice parameters provides band-gap enlargement while the opposite trend is observed when decreasing the lattice parameters. Numerically uniaxial lattice strain of 3% can provide variations in the band gap up to 0.1 eV. We also discuss possible reasons for a variation and applicability of the band-gap engineering in BaSi2 by strain

Towards B-doped p-BaSi2 films on Si substrates by co-sputtering of BaSi2, Ba, and B-doped Si targets

BaSi2 is one of the emerging materials for thin-film solar cell applications; hence the conductivity control by impurity doping is of great importance. The formation of B-doped p-BaSi2 films has been achieved by molecular beam epitaxy and vacuum evaporation. We fabricated B-doped BaSi2 films on Si substrates at 600 °C by co-sputtering BaSi2, Ba, and B-doped Si targets, followed by post-annealing at 900 °C or 1000 °C for 5 min in an Ar atmosphere. Contrary to expectations, as-grown sample and the sample annealed at 900 °C showed n-type conductivity, while the sample annealed at 1000 °C showed p-type conductivity. The reason for the n-type conductivity was discussed based on first-principles calculationconsidering the presence of oxygen atoms in the order of 1021 cm−3. The n-type conductivity for B-doped BaSi2 is possible only when both the B and O atoms being a substitution impurity are in the same Si4 tetrahedron