Significant improvement of photoresponsivity of BaSi2 passivated by atomic H and exploring the H states via muon spin rotation

Solar cells have been developed over half of a century and applied in daily life.A high conversion efficiency (η) is always expected to achieve via new materials or technologies.Nowadays, thin-film solar cells composed of materials such as CdTe, CIGS, and organic-inorganic perovskite have attracted increasing attention owing to their low cost and high η.However, these materials contain toxic or scarce elements.Considering these circumstances, we have focused on semiconducting material barium disilicide (BaSi2).It has attractive features for solar cell applications such as a suitable band gap of 1.3 eV for a single-junction solar cell, high absorption coefficients exceeding those of CIGS, a long minority-carrier lifetime above 10 μs, and a large minority-carrier diffusion length of 10 μm due to inactive grain boundaries.Furthermore, epitaxial BaSi2 thin films can be grown on Si(111) and Si(100) substrates by molecular beam epitaxy.The lattice mismatch between the Si substrate and BaSi2 thin film is small.Because of these advantages, BaSi2 has a great potential to be solar cells or a light absorber layer in thin-film solar cell applications.Recently, we achieved an η approaching 10% in p-BaSi2/n-Si.However, in this solar cell, the built-in potential is as small as 0.2 V because of the low electron affinity of BaSi2 (3.2 eV).The experimentally obtained open-circuit voltage is thus smaller than 0.5 V, limiting the η.Hence, we next moved on to BaSi2 homojunction solar cells and first demonstrated that the operation of BaSi2 homojunction solar cells on the p + -Si substrate.To achieve high η in homojunction solar cells, the formation of high-quality BaSi2 light absorber layer is important.According to a first-principle calculation, from the viewpoint of formation energy, Si vacancies are more likely to locate at Si(3) sites in the Si tetrahedron and act as point defects in BaSi 2 regardless of whether the growth conditions are rich or poor in Si.The calculation also revealed that Si vacancies generate dangling bonds and thereby localized states within the band gap, leading to the degradation of the minority-carrier properties of BaSi2.The defects in BaSi2 films and bulk polycrystalline BaSi2 has also been measured by deeplevel transient spectroscopy, Raman spectroscopy, electron paramagnetic resonance, and photoluminescence.Therefore, passivation of defects in BaSi2 is particularly important for their use in solar cell applications.In this doctoral thesis, we first investigate the effect of atomic hydrogen (H) supply on the photoresponsivity enhancement of undoped and boron (B)-doped BaSi2 films.Then, we explore H states and position in undoped BaSi2 using muon spin rotation.In chapter 3, we demonstrate the effect of H passivation on both the photoresponsivity and minority-carrier lifetime of BaSi2 epitaxial films.Atomic H is applied to BaSi2 films at 580 ºC and change H supply duration in the range of 1-30 min, followed by capping with an amorphous Si layer.The photoresponsivity of the films changes considerably depending on H supply duration and reach a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for H supply duration of 15 min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode.This value is approximately one order of magnitude higher than the highest value previously reported.Microwave photoconductivity decay measurement reveals that the minority-carrier lifetime of the BaSi 2 film with the highest photoresponsivity is 14 μs.We also calculate the total density of states of BaSi2 by Ab initio studies when one Si vacancy exists in a unit cell, and one, two, and three H atoms occupy Si vacancy or interstitial sites.In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration.From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for BaSi2 solar cells.In chapter 4, we investigate the H effect on the photoresponsivity of 500-nm-thick B-doped p-BaSi2 films.The photoresponsivity reaches ~4 A/W of H passivated lightly B-doped BaSi2, which is about twice the highest value reported for H passivated undoped BaSi2 films, because of an increased minority-carrier lifetime, as measured by the microwave-detected photoconductivity decay.Furthermore, Ab initio studies are used to interpret and understand experimental observations by analyzing states in the gap region, which can act as traps, in B-doped p-BaSi2 with H incorporation.Moreover, the effect that atomic H had on the performance of B-doped p-BaSi2/n-Si heterojunction solar cells is also studied.The saturation current density is found to decrease by three orders of magnitude with the atomic hydrogen supply, and the η is increased up to 6.2%.Deep-level transient spectroscopy revealed a reduction of defect densities induced by the atomic hydrogen.Chapter 5, we explore H states and position by an advanced technique, which is muon spin rotation.An implanted muon beam binds electrons to form muonium. Their response to thermal activation shows that muonium accompanies a shallow energy level of approximately 31 meV below the conduction band minimum, indicating that atomic H also serves an electronically active donor impurity in BaSi2.This result is in good agreement with Ab initio studies showing that a localized half-filled peak appears approximately 40 meV below the conduction band minimum if the first neighbors of the H atom are one Si atom and one Ba atom.

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