Silicon Clathrate Research Articles

Influence of Sodium Concentration on the Optoelectronic Properties of Silicon Clathrate Films

Influence of Sodium Concentration on the Optoelectronic Properties of Silicon Clathrate Films

Tunability of silicon clathrate film properties by controlled guest-occupation of their cages.

Type I and type II silicon clathrates are guest-host structures made of silicon polyhedral cages large enough to contain atoms that can be either inserted or evacuated with only a slight volume change of the structure. This feature is of interest not only for batteries or storage applications but also for tuning the properties of the silicon clathrate films. The thermal decomposition process can be tuned to obtain Na8Si46 and Na2<x<10Si136 silicon clathrate films on intrinsic and p-type c-Si (001) wafer. Here, from a unique synthesized NaxSi136 film, a range of resistivity of minimum four order of magnitude is possible by using post-synthesis treatments, switching from metallic to semiconductor behavior as the Na content is lowered. Extended exposition to sodium vapor allows us to obtain fully occupied Na24Si136 metallic films, and annealing under iodine vapor is a way to reach the guest-free Si136, a semiconducting metastable form of silicon with a 1.9eV direct bandgap. Electrical measurements and resistance vs temperature measurements of the silicon clathrate films further discriminate the behavior of the various materials as the Na concentration is changing, additionally shouldered by density functional theory calculations for various guest occupations, further motivating the urge of an innovative pathway toward true guest-free type I and type II silicon clathrates.

Formation of Type II Silicon Clathrate with Lithium Guests through Thermal Diffusion.

At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si-Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming "molecular-like" pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.

Electrochemical Flux Synthesis of Type-I Na8Si46 Clathrates: Particle Size Control Using a Solid or Molten Na–Sn Mass Transport Mediator

Sodium-filled silicon clathrates have a host of interesting properties for thermoelectric, photovoltaic, and battery applications. However, the metastability of the clathrates has made it difficult to synthesize them with the desired morphology and crystallite size. Herein, we demonstrate an electrochemical method whereby Na4Si4 dissolved in a Sn-based flux is converted to the Na8Si46 type-I clathrate using galvanostatic (constant current) oxidation. The temperature has a large influence on the products, with the reactions at 485 °C resulting in clathrates with small particle sizes (1-2 μm), while larger single crystals are obtained at 538 °C. The difference in microstructure is attributed to the solid vs liquid state of the Na-Sn phase at the reaction temperature, which is supported by the observed voltage profiles. The demonstrated method is promising for the tunable growth of Si clathrates and could be applicable to a broad range of intermetallic compounds.

Structural Dynamics, Phonon Spectra and Thermal Transport in the Silicon Clathrates.

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in “phonon-glass electron-crystal” materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.

Thermal and mechanical properties of the clathrate-II Na24Si136

Thermal expansion, lattice dynamics, heat capacity, compressibility, and pressure stability of the intermetallic clathrate ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ have been investigated by a combination of first-principles calculations and experimentation. Direct comparison of the properties of ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ with those of the low-density elemental modification ${\mathrm{Si}}_{136}$ provide insight into the effects of filling the silicon clathrate framework cages with Na on these properties. Calculations of the phonon dispersion only yield sensible results if the Na atoms in the large cages of the structure are displaced from the cage centers, but the exact nature of off-centering is difficult to elucidate conclusively. Pronounced peaks in the calculated phonon density of states for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, absent for ${\mathrm{Si}}_{136}$, reflect the presence of low-energy vibrational modes associated with the guest atoms, in agreement with prior inelastic neutron-scattering experiments and reflected in marked temperature dependence of the guest atom atomic displacement parameters determined by single-crystal x-ray diffraction. The bulk modulus is only weakly influenced by filling the Si framework cages with Na, whereas the phase stability under pressure is significantly enhanced. The room-temperature linear coefficient of thermal expansion (CTE) is nearly a factor of 3 greater for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ compared to ${\mathrm{Si}}_{136}$. Negative thermal expansion (NTE), observed in ${\mathrm{Si}}_{136}$ below 100 K, is noticeably absent in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$. In contrast to ${\mathrm{Si}}_{136}$, the thermal expansion behavior in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ is relatively well described by the conventional Gr\"uneisen-Debye model in the temperature range of 10--700 K. First-principles calculations in the quasiharmonic approximation correctly predict an increase in high-temperature CTE with Na loading, although the increase is less than observed in experiment. The calculations also fail to capture the absence of NTE in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, perhaps due to anharmonic effects and/or inadequateness of the ordered structural model.

Stability of mixed carbon–silicon clathrates

Stability of mixed carbon–silicon clathrates

Synthesis and characterization of phase-pure clathrate-II Rb12.9Si136

The synthesis and processing of different compositions and structure types is of fundamental importance, although challenging in many cases with inorganic clathrate-II compositions representing one such example. Herein we report on the synthesis and structural properties investigation of phase pure clathrate-II (Fd3¯m) Rb12.9Si136. Temperature-dependent structural data and analyses revealed lattice anharmonicity in this inorganic clathrate. Moreover, the selective synthesis of either clathrate-I (Rb6.8Si46, Pm3¯n) or clathrate-II (Rb12.9Si136) was achieved by varying the processing temperature. Our results are compared to that of other silicon clathrates.

Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition

Type I (Na8Si46) or type II (Nax≤24Si136) silicon clathrates films with a large 15 × 45 mm2 surface have been synthesized from p-type and intrinsic c-Si (001) wafers using a two-step process without the need of any glove box. Conditions to selectively obtain either type I or type II silicon clathrates phase have been finely tuned. Optical absorption coefficients are found much larger in the Si clathrates than in diamond silicon in the visible light range. Type II films provide a direct bandgap of around 1.9 eV which is supporting the high absorption coefficient observed. Photovoltaic response of the films has been confirmed using Surface Photovoltage. As prepared type II films show many surface defects, cracks and inhomogeneities which have been drastically reduced thanks to a pressure annealing treatment.

Isolated alternative crystalline silicon phase could lead to improved solar cell performance

Method to isolate type II silicon clathrate films with exciting optoelectronic properties reported.

Synthesis and characterization of type II silicon clathrate films with low Na concentration

A two-step process for the synthesis of the silicon clathrate film on a diamond silicon wafer is explored in detail. Key factors impacting the film quality are uncovered. We find that the optical properties of the films are strongly influenced by inhomogeneities and defect phases that dominate the top surface and grain boundaries of the material. For the first time, we systematically develop two approaches for minimizing the effects of defective structures and allow intrinsic properties of the clathrate material to be probed. One is separating the film surface from the Si substrate to expose the buried high-quality interface, and the other one is wet or dry etching of the clathrate film to remove the disordered material which is more heavily concentrated at the top surface. With high-quality clathrate surfaces and films produced, more reliable optical measurements are taken and interpreted. Techniques in this work provide a pathway for Si clathrate thin film toward an optically efficient alternative crystalline form of Si that can transform Si-based applications in optoelectronics.

Understanding the Li and Na Intercalation in Si Clathrate Frameworks

Tetrel (Si, Ge, Sn) clathrates are host-guest structures that display novel electrochemical reactions with Li and Na due to their unique cage structure. One area of interest for Tetrel clathrates is the possibility of topotactic Li/Na insertion into the clathrate framework due to the open cage structure. The type II clathrate Na24-x(Si,Ge)136 is unique because the structure can be obtained without guest atoms in the clathrate framework. Figure 1 shows a crystal model schematic of the two unique polyhedral cages that comprise the type II Si clathrate structure: the dodecahedra (Si20, grey) and the hexakaidecahedra (Si28, blue). The structure usually forms with Na guest atoms (yellow) in the center of these cages, but when heated (300 – 500 °C ) under vacuum, the Na atoms diffuse and evaporate out of the structure resulting in a guest free clathrate, Si136.1 Once Na is removed the structure, Li intercalation into the vacant cages becomes possible.2 During the lithiation of the guest-free type II Si clathrate (Na1Si136), there is plateau at 0.30 V vs Li/Li+ which corresponds to Li intercalation into the vacant cages of the clathrate structure.2,3 With synchrotron powder X-ray diffraction, we confirm that Li intercalation is occurring in a topotactic fashion into the framework and identify the Li positions in the two types of cages. In the Si20 cage, Li is found near the center of the cages with a slightly split position, while Li in the Si28 cage is found very off-center, coordinated off the hexagonal faces. Density functional theory (DFT) calculations corroborate the Li positions in both cages. Next, we demonstrate that by using a voltage cutoff of 0.26 V, reversible Li insertion is possible as evidenced by the reproducible voltage profile. Due to the very low volume expansion (0.23 %) and reasonable capacity and voltage (230 mAh/g at 0.30 V), the type II Si clathrate has possible applications as an anode for Li-ion rechargeable batteries.In contrast to the room temperature intercalation of Li, we find that electrochemical deintercalation of Na from the type II structure requires high temperatures (350-450 °C) and overpotentials to achieve. We demonstrate electrochemical desodiation using a novel high temperature approach using a Na beta-alumina solid electrolyte.4 By using DFT to calculate the migration barriers for Na and Li, we demonstrate that the large differences between Li and Na intercalation originates from the much higher migration barriers for Na (1.0 - 2.0 eV) in the type II structure when compared to Li (0.2 eV). Based on these results, design rules for intercalation into Tetrel frameworks will be discussed and other promising open Tetrel frameworks highlighted. Figure 1: Crystal model schematic of the two types of polyhedral in the type II Si clathrate structure: the dodecahedra (grey) and the hexakaidecahedra (blue). Si atoms are orange and Na atoms are yellow. References (1) Krishna, L.; Baranowski, L. L.; Martinez, A. D.; Koh, C. A.; Taylor, P. C.; Tamboli, A. C.; Toberer, E. S. Efficient Route to Phase Selective Synthesis of Type II Silicon Clathrates with Low Sodium Occupancy. CrystEngComm 2014, 16, 3940–3949.(2) Langer, T.; Dupke, S.; Trill, H.; Passerini, S.; Eckert, H.; Pöttgen, R.; Winter, M. Electrochemical Lithiation of Silicon Clathrate-II. J. Electrochem. Soc. 2012, 159, A1318–A1322.(3) Dopilka, A.; Weller, J. M.; Ovchinnikov, A.; Childs, A.; Bobev, S.; Peng, X.; Chan, C. K. Structural Origin of Reversible Li Insertion in Guest‐Free, Type II Silicon Clathrates. Adv. Energy Sustain. Res. 2021, 2000114.(4) Dopilka, A.; Childs, A.; Bobev, S.; Chan, C. K. Solid-State Electrochemical Synthesis of Silicon Clathrates Using a Sodium-Sulfur Battery Inspired Approach. J. Electrochem. Soc. 2021, 168, 020516. Figure 1

Transformations of silicon clathrate Si136 under high hydrogen pressure up to 11 GPa

Phase transformations of silicon clathrate Si136 under high hydrogen pressure up to 11.5 GPa were studied by in situ Raman spectroscopy and X-ray diffraction at room temperature in a diamond anvil cell. The pressure dependencies of the vibrational mode frequencies of Si136 agree well with the ab initio calculation results. In addition to the “rigid” one-phonon modes a series of “soft” modes was found in the 300 ÷ 400 cm−1 frequency range, which results from the two-phonon Raman scattering. At the pressure of 10 GPa the silicon clathrate Si136 transformed into a mixture of the Si-II (β-Sn-type) and Si-III (BC8-type) phases. During the subsequent decompression down to 9 GPa the Si-II phase transformed to Si-III, which was then recovered to ambient pressure. No interaction between hydrogen and silicon was observed for Si136 upon compression and for the Si-II and Si-III phases upon decompression.

Isostructural phase transition by point defect reorganization in the binary type-I clathrate Ba7.5Si45

Competition between microscopic point defect (vacancy and interstitial) configurations is inherent to crystalline phases of increased structural complexity. Phase transitions that preserve symmetry between them belong to a specific class of isostructural transitions. Type-I silicon clathrates are representatives of such structurally complex crystalline phases showing an intriguing structural transition at high pressure associated with an abrupt reduction of volume with no indication for any breakage of symmetry. Using isothermal high-pressure X-ray diffraction performed on a single crystal of the simplest representative type-I silicon clathrates, binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure. An analysis based on the Landau theory gives a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.

Solid-State Electrochemical Synthesis of Silicon Clathrates Using a Sodium-Sulfur Battery Inspired Approach

Clathrates of Tetrel elements (Si, Ge, Sn) have attracted interest for their potential use in batteries and other applications. Sodium-filled silicon clathrates are conventionally synthesized through thermal decomposition of the Zintl precursor Na4Si4, but phase selectivity of the product is often difficult to achieve. Herein, we report the selective formation of the type I clathrate Na8Si46 using electrochemical oxidation at 450 °C and 550 °C. A two-electrode cell design inspired by high-temperature sodium-sulfur batteries is employed, using Na4Si4 as working electrode, Na β″-alumina solid electrolyte, and counter electrode consisting of molten Na or Sn. Galvanostatic intermittent titration is implemented to observe the oxidation characteristics and reveals a relatively constant cell potential under quasi-equilibrium conditions, indicating a two-phase reaction between Na4Si4 and Na8Si46. We further demonstrate that the product selection and morphology can be altered by tuning the reaction temperature and Na vapor pressure. Room temperature lithiation of the synthesized Na8Si46 is evaluated for the first time, showing similar electrochemical characteristics to those in the type II clathrate Na24Si136. The results show that solid-state electrochemical oxidation of Zintl phases at high temperatures can lead to opportunities for more controlled crystal growth and a deeper understanding of the formation processes of intermetallic clathrates.

Isostructural Phase Transition by Point Defect Reorganization in the Binary Type-I Clathrate Ba &lt;sub&gt;7.5&lt;/sub&gt;Si &lt;sub&gt;45&lt;/sub&gt;

A competition of point defect configurations is inherent to crystalline phases of increased structural complexity. Symmetry preserving phase transitions between them belong to the special class of isostructural transitions. Type-I silicon clathrates are crystalline complex phases whose unit cell containing a fifties of atoms consists of a 3D covalent silicon framework of face-sharing polyhedral cages encapsulating guest cations. At high pressure, an intriguing structural transition is associated to an abrupt volume reduction with no indication for any symmetry breaking. By means of isothermal high-pressure X-ray diffraction performed on single crystal of the simplest representative type-I silicon clathrates, the binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a defect rearrangement initially present in the structure. An analysis based on the Landau theory allows to give a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.

Electrochemical Synthesis of Type I Na8Si46 Clathrate with a Na Β''-Alumina Solid Electrolyte

Intermetallic clathrates are a class of materials comprising face-sharing polyhedral cages of Tetrel (Si, Ge Sn) elements that encapsulate alkali or alkaline earth metal guest atoms. With their unique structures and tunable cage sizes, clathrates have received much interest for their thermoelectric, superconducting, optoelectronic, and electrochemical properties. Due to the large interest in Tetrel elements for Li-ion anodes, our group has been investigating the electrochemical properties of clathrates for Li-ion battery applications1–4. We recently studied the guest free Type I clathrates (i.e. Si46) with ab initio calculations and found that Li insertion is favorable into the empty cages and the migration barriers are very low (0.15-0.3 eV) suggesting possible applications as anodes. However, obtaining pure phase selection of the Si clathrates from the desodiation of the Na4Si4 Zintl phase has been difficult due to the lack of kinetic control during the reaction. To address this challenge, we demonstrate a proof-of-concept electrochemical cell which desodiates Na4Si4 at 450-550 ºC to phase selectively form the type I Na8Si46 clathrate phase.The electrochemical cell operates at 450-550 ºC and is comprised of a Na4Si4 working electrode, Na β''-alumina solid electrolyte, and a Na or Sn counter electrode (Figure 1). The electrochemical cell is operated at a constant current using the galvanostatic intermittent titration technique (GITT) which allows for a better understanding of the reaction processes by observing the voltage under open circuit conditions. We find that desodiation of Na4Si4 with a 1-2 mA/cm2 current density at 450 ºC results in the phase selective formation of the type I Na8Si46 clathrate via a two-phase reaction mechanism. By altering the temperature and the Na vapor pressure (by choice of counter electrode), we demonstrate further control over the phase selection and product morphology.These results demonstrate that the bulk conversion of Na4Si4 via electrochemical oxidation to Na8Si46 is possible and we believe that this system can be applied to other Zintl compound precursors. We preliminary show that type II Ge clathrate can be prepared from Na4Ge4 at 350 ºC using the same approach. Compared to other oxidation pathways to obtain intermetallic clathrates, the electrochemical method has distinct advantages. For instance, the rate of oxidation can be controlled independently of the temperature of reaction through tuning of the current density, which could lead to greater control over the reaction products. Due to the flexible nature of the electrochemical synthesis, we expect these results to have important implications for the solid-state synthesis of clathrates for Li-ion battery applications. Figure 1 Schematics of the (a) electrochemical cell and (b) proposed electrochemical reactions. Na4Si4 is oxidized to Na8Si46 at the interface with the electrolyte via removal of Na+ and e-; the Na+ ions are reduced at the liquid metal counter electrode to form either Na metal or NaxSn.(1) Li, Y.; Raghavan, R.; Wagner, N. a.; Davidowski, S. K.; Baggetto, L.; Zhao, R.; Cheng, Q.; Yarger, J. L.; Veith, G. M.; Ellis-Terrell, C.; Miller, M. a.; Chan, K. S.; Chan, C. K. Type I Clathrates as Novel Silicon Anodes: An Electrochemical and Structural Investigation. Adv. Sci. 2015, 2 (6), 1500057.(2) Zhao, R.; Bobev, S.; Krishna, L.; Yang, T.; Weller, J. M.; Jing, H.; Chan, C. K. Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba 8 Al 16 Si 30 - Role of Processing on Surface Properties and Electrochemical Behavior. ACS Appl. Mater. Interfaces 2017, 9 (47), 41246–41257.(3) Dopilka, A.; Zhao, R.; Weller, J. M.; Bobev, S.; Peng, X.; Chan, C. K. Experimental and Computational Study of the Lithiation of Ba8AlyGe46- y Based Type I Germanium Clathrates. ACS Appl. Mater. Interfaces 2018, 10 (44), 37981–37993.(4) Dopilka, A.; Peng, X.; Chan, C. K. Ab Initio Investigation of Li and Na Migration in Guest-Free, Type I Clathrates. J. Phys. Chem. C 2019, 123, 22812–22822. Figure 1

Silicon Clathrate Films for Photovoltaic Applications

Silicon clathrates are allotropes of silicon that show great promises for optoelectronics and batteries. However silicon clathrates in the form of films are relatively new and devices based on this...

Electron paramagnetic resonance study of type-II silicon clathrate with low sodium guest concentration

An electron paramagnetic resonance (EPR) study of type-II silicon (Si) clathrate with low sodium (Na) guest concentration provides insights into the properties of this cagelike Si allotrope and into Na as an $n$-type dopant. An EPR line arising from unterminated Si bonds in a disordered Si phase is identified in the system. The presence of this amorphouslike component helps explain the thermally activated, hopping-related conductivity that has long been observed in the system but which has been poorly understood. This study also identifies ``superhyperfine'' lines surrounding each Na hyperfine line due to interactions of the donor electron with a $^{29}\mathrm{Si}$ nucleus on the cage and confirms the identification of hyperfine lines due to interactions of two neighboring Na nuclei. Analysis of the strength of these interactions shows that the electron wave function associated with the Na donor extends onto surrounding cages consistent with identification of Na as a shallow donor, and provides quantitative evidence of Na clustering.

Numerical simulation and optimization of a silicon clathrate-based solar cell n-Si136/p-Si2 using SCAPS-1D program

The aim of this work is to optimize a prototype solar cell based on a real model with type-II silicon clathrate Si136 active layer. For that purpose, a simulation of a ITO/Ag/n-Si136/p-Si/Al solar cell was carried out using the SCAPS-1D software. We have systematically investigated the effects of the thickness of n-Si136 layer and the series resistance RS on the main photovoltaic parameters including: open-circuit voltage (Voc), short-circuit current density (Jsc), conversion efficiency (η), and quantum efficiency QE%. Initial simulation of the structure built from the fabricated model revealed primary weak PV properties with Jsc = 9.621 nA/cm2, Voc = 0.225 V, efficiency of η~10−5% and a maximum output power in the range of PM ~ nW with undetected values for FF. After increasing the thickness of the Si136 clathrate layer, (thickness of Si136 = 100 μm and neglected series resistance RS~0), the optimized device architecture revealed better photovoltaic parameters under AM1.5G spectrum and T = 300 K as: Jsc = 22.65 mA/cm2, Voc = 0.92 V, FF = 67.74% and η = 4.03%, maximum output power of PM = 14.11 mW and quantum efficiency of QE = 48% at 980 nm wavelength. Despite the low conversion efficiency of this prototype solar cell, by this work we aim to shed some light on the possible integration of clathrate phases into photovoltaic devices.