Lakshmi Krishna

Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Inorganic type II clathrates are low density, semiconducting allotropes of group IV elements with the potential for optoelectronic applications. This class of materials is predicted to have direct or nearly-direct band gaps, and, when Si and Ge are alloyed in the clathrate structure, the band gap is tunable in the range of 0.8–1.8 eV. In this work, we demonstrate for the first time the synthesis of alloyed Si–Ge type II clathrates. Within this alloy system, we find an amorphous region which is likely due to a miscibility gap. The optical absorptance spectra of the crystalline clathrate samples show the predicted band gap tuning with Ge content, and calculations find that the Si type II clathrate has a strong absorption coefficient for the direct interband transition. The findings in this work lay the foundation for the future use of type II clathrates in optoelectronic applications.

Synthesis of Group IV Clathrates for Photovoltaics

Although Si dominates the photovoltaics market, only two forms of Si have been thoroughly considered: amorphous Si and Si in the diamond structure ( d-Si). Silicon can also form in other allotropes, including clathrate structures. Silicon clathrates are inclusion compounds, which consist of an Si framework surrounding templating guest atoms (e.g., Na). After formation of the type II Na 24Si136 clathrate, the guest atoms can be removed (Si136), and the material transitions from degenerate to semiconducting behavior with a 1.9 eV direct band gap. This band gap is tunable in the range of 1.9-0.6 eV by alloying the host framework with Ge, enabling a variety of photovoltaic applications that include thin-film single-junction devices, Si136 top cells on d-Si for all-Si tandem cells, and multijunction cells with varying Si/Ge ratios. In this study, we present electronic structure calculations that show the evolution of the direct transition as a function of Si/Ge ratio across the alloy range. We demonstrate the synthesis of type II Si/Ge clathrates spanning the whole alloy range. We also demonstrate a technique for forming Si clathrate films on d-Si wafers and sapphire substrates.

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

Phase selective synthesis of type II silicon clathrates from thermal decomposition of NaSi has previously been limited to small quantities due to the simultaneous formation of competing phases. In this work we show that the local sodium vapor pressure during the NaSi precursor decomposition is a critical parameter for controlling phase selection. We demonstrate synthesis techniques that allow us to tune the local Na vapor pressure, yielding type I or II clathrate products that are ≥90 wt.% phase pure. The “cold plate” reactor design discussed in this work maintains low Na vapor pressure during thermal decomposition of NaSi, thus yielding large scale, phase selective synthesis of type II silicon clathrates. The low Na vapor pressure maintained in this reactor is also shown to efficiently produce low Na (x ~ 1; NaxSi136) Si clathrate through Na sublimation. To further reduce sodium occupancy (x < 1), we demonstrate etching of NaxSi136 in HF/HNO3 solutions, which rapidly yields a clathrate product with reduced x. 29Si NMR and electron spin resonance (ESR) characterization validate the low Na occupancy of Si clathrate synthesized. The acid etch also selectively dissolves the type I silicon clathrate impurity phase, thereby enabling the synthesis of large quantities of phase pure type II silicon clathrate with low Na content.

Potential for high thermoelectric performance in n-type Zintl compounds: a case study of Ba doped KAlSb4

High-throughput calculations (first-principles density functional theory and semi-empirical transport models) have the potential to guide the discovery of new thermoelectric materials. Herein we have computationally assessed the potential for thermoelectric performance of 145 complex Zintl pnictides. Of the 145 Zintl compounds assessed, 17% show promising n-type transport properties, compared with only 6% showing promising p-type transport. We predict that n-type Zintl compounds should exhibit high mobility μn while maintaining the low thermal conductivity κL typical of Zintl phases. Thus, not only do candidate n-type Zintls outnumber their p-type counterparts, but they may also exhibit improved thermoelectric performance. From the computational search, we have selected n-type KAlSb4 as a promising thermoelectric material. Synthesis and characterization of polycrystalline KAlSb4 reveals non-degenerate n-type transport. With Ba substitution, the carrier concentration is tuned between 1018 and 1019 e− cm−3 with a maximum Ba solubility of 0.7% on the K site. High temperature transport measurements confirm a high μn (50 cm2 V−1 s−1) coupled with a near minimum κL (0.5 W m−1 K−1) at 370 °C. Together, these properties yield a zT of 0.7 at 370 °C for the composition K0.99Ba0.01AlSb4. Based on the theoretical predictions and subsequent experimental validation, we find significant motivation for the exploration of n-type thermoelectric performance in other Zintl pnictides.

Development of ZnSiP

A major technological challenge in photovoltaics is the implementation of a lattice matched optically efficient material to be used in conjunction with silicon for tandem photovoltaics. Detailed balance calculations predict an increase in efficiency of up to 12 percentage points for a tandem cell compared with single junction silicon. Given that the III-V materials currently hold world record efficiencies, both for single and multijunction cells, it would be transformative to develop a material that has similar properties to the III-Vs which is also lattice matched to silicon. The II-IV-V2 chalcopyrites are a promising class of materials that could satisfy these criteria. ZnSiP2 in particular is known to have a bandgap of ~2 eV, a lattice mismatch with silicon of 0.5%, and is earth abundant. Its direct bandgap is symmetry-forbidden. We have grown single crystals of ZnSiP2 by a flux growth technique. Structure and phase purity have been confirmed by X-ray diffraction and transmission electron microscopy. Optical measurements, along with a calculation of the absorption spectrum, confirm the ~2 eV bandgap. Because of its structural similarity to both crystalline silicon and the III-Vs, ZnSiP2 is expected to have good optoelectronic performance.

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The pressure-dependent phase behavior of semiconducting chalcopyrite ZnSiP2 was studied up to 30 GPa using in situ X-ray diffraction and Raman spectroscopy in a diamond-anvil cell. A structural phase transition to the rock salt type structure was observed between 27 and 30 GPa, which is accompanied by soft phonon mode behavior and simultaneous loss of Raman signal and optical transmission through the sample. The high-pressure rock salt type phase possesses cationic disorder as evident from broad features in the X-ray diffraction patterns. The behavior of the low-frequency Raman modes during compression establishes a two-stage, order-disorder phase transition mechanism. The phase transition is partially reversible, and the parent chalcopyrite structure coexists with an amorphous phase upon slow decompression to ambient conditions.

for Si-Based Tandem Solar Cells

Group IV clathrates are a unique class of guest/framework type compounds that are considered potential candidates for a wide range of applications (superconductors to semiconductors). To date, most of the research on group IV clathrates has focused heavily on thermoelectric applications. Recently, these materials have attracted attention as a result of their direct, wide band gaps for possible use in photovoltaic applications. Additionally, framework alloying has been shown to result in tunable band gaps. In this review, we discuss the current work and future opportunities concerning the synthesis and optical characterization of group IV clathrates for optoelectronics applications.

Pressure-induced structural transition in chalcopyrite ZnSiP2

Type I silicon clathrates based on Ba8AlySi46-y (8 < y < 12) have been studied as potential anodes for lithium-ion batteries and display electrochemical properties that are distinct from those found in conventional silicon anodes. Processing steps such as ball-milling (typically used to reduce the particle size) and acid/base treatment (used to remove nonclathrate impurities) may modify the clathrate surface structure or introduce defects, which could affect the observed electrochemical properties. In this work, we perform a systematic investigation of Ba8AlySi46-y clathrates with y ≈ 16, i.e, having a composition near Ba8Al16Si30, which perfectly satisfies the Zintl condition. The roles of ball-milling and acid/base treatment were investigated using electrochemical, X-ray diffraction, electron microscopy, X-ray photoelectron and Raman spectroscopy analysis. The results showed that acid/base treatment removed impurities from the synthesis, but also led to formation of a surface oxide layer that inhibited lithiation. Ball-milling could remove the surface oxide and result in the formation of an amorphous surface layer, with the observed charge storage capacity correlated with the thickness of this amorphous layer. According to the XRD and electrochemical analysis, all lithiation/delithiation processes are proposed to occur in single phase reactions at the surface with no discernible changes to the crystal structure in the bulk. Electrochemical impedance spectroscopy results suggest that the mechanism of lithiation is through surface-dominated, Faradaic processes. This suggests that for off-stoichiometric clathrates, as we studied in our previous work, Li+ insertion at defects or vacancies on the framework may be the origin of reversible Li cycling. However, for clathrates Ba8AlySi46-y with y ≈ 16, Li insertion in the structure is unfavorable and low capacities are observed unless amorphous surface layers are introduced by ball-milling.

Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications

ZnSiP2 demonstrates promising potential as an optically active material on silicon. There has been a longstanding need for wide band gap materials that can be integrated with Si for tandem photovoltaics and other optoelectronic applications. ZnSiP2 is an inexpensive, earth abundant, wide band gap material that is stable and lattice matched with silicon. This conference proceeding summarizes our PV-relevant work on bulk single crystal ZnSiP2, highlighting the key findings and laying the ground work for integration into Si-based tandem devices.