Thermoelectric properties of two stacking sequences of crystalline GST-225

Pseudobinary GeTe-Sb2Te3 compounds are widely used as phase-change optical materials for DVD-RAM.[3] Ge2Sb2Te5 (GST-225) is used for this propose but the stacking sequence of the stable crystal structure is motive of debate. Pseudobinary compounds there are claimed to be good thermoelectric materials due the large number of intrinsic structural vacancies.[4] Thermoelectric properties for two proposed stacking sequences of GST-225 are computed using DFT[5, 6] and Boltzmann transport equation in the constant relaxation time approximation. After phonon calculations, no dynamic instabilities were found in the Irreducible Brillouin Zone for either of the proposed stacking sequences. One of the stacking sequences shows semiconductor-like density of states (DOS) with a computed gap of 190 meV unlike the other stacking sequence which has a metallic-like DOS. Thermoelectric properties calculation reveals that semiconductor-like structure has the highest value of Seebeck coeffcient (SC)

Simulating Phase Change Materials : how crystal bonding features lead to the formation of a non-Zachariasen glass (Invited)

editorial reviewe

Metavalent Bonding : Characterization and Implications for Applications in Phase Change Materials, thermoelectric and Photovoltaic Compounds (Invited)

editorial reviewe

Tetrahedral clustering in molten lithium under pressure

A series of electronic and structural transitions are predicted in molten lithium from first principles. A new phase with tetrahedral local order characteristic of $sp^3$ bonded materials and poor electrical conductivity is found at pressures above 150 GPa and temperatures as high as 1000 K. Despite the lack of covalent bonding, weakly bound tetrahedral clusters with finite lifetimes are predicted to exist. The stabilization of this phase in lithium involves a unique mechanism of strong electron localization in interstitial regions and interactions among core electrons. The calculations provide evidence for anomalous melting above 20 GPa, with a melting temperature decreasing below 300 K, and point towards the existence of novel low-symmetry crystalline phases.Comment: 5 pages, 5 figure

Effect of hydrostatic pressure on the thermoelectric properties of Bi2 Te3

peer reviewedWe use first-principles calculations to understand the behavior of the Seebeck coefficient (S) in Bi2Te3 as a function of isotropic pressure. We perform calculations up to 5 GPa using density functional theory and with thermoelectric properties extracted using Boltzmann transport equations. We find that with the increase in pressure the system becomes more metallic, in agreement with previous calculations on Sb2Te3. For p-type doping the overall behavior is a decrease in S with an increase in pressure. At small values of hole doping (p=1.8×1018cm-3), we obtain an anomalous variation of S under 2 GPa, which is an indication of the electronic topological transition. For n-type doping, S slightly increases with pressure. © 2014 American Physical Society

Structure of liquid semiconductors

peer reviewe

Study of the opto-electronic properties of Cu2ZnXS4 (X=Sn,Ge,Si) kesterites as input data for solar cell efficiency modelling

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of 10^4 cm^{−1} are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results (absorption coefficient, refractive index and reflectivity) are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical model including non-radiative recombination via an external parameter defined as the in- ternal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.88 %, 19.94 % and 3.11 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using a realistic internal quantum efficiency which provides OC values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 (for an optimal thickness of 1.15 m). With this methodology we confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu2ZnGeS4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu2ZnSiS4 might be interesting for transparent photovoltaic windows

Relevance of Ge incorporation to control the physical behaviour of point defects in kesterite

To reduce the prominent VOC-deficit that limits kesterite-based solar cells efficiencies, Ge has been proposed over the recent years with encouraging results as the reduction of the non-radiative recombination rate is considered as a way to improve the well-known Sn-kesterite world record efficiency. To gain further insight into this mechanism, we investigate the physical behaviour of intrinsic point defects both upon Ge doping and alloying of Cu2ZnSnS4 kesterite. Using a first-principles approach, we confirm the p-type conductivity of both Cu2ZnSnS4 and Cu2ZnGeS4, attributed to the low formation energies of the VCu and CuZn acceptor defects within the whole stable phase diagram range. Via doping of the Sn-kesterite matrix, we report the lowest formation energy for the substitutional defect GeSn. We also confirm the detrimental role of the substitutional defects XZn (X=Sn,Ge) acting as recombination centres within the Sn-based, the Ge-doped and the Ge-based kesterite. Upon Ge incorporation, we highlight, along with the increase of the XZn (X=Sn,Ge) neutral defect formation energy, the reduction of the lattice distortion resulting in the reduction of the carrier capture cross section. Both of these elements leading to a decrease of the non-radiative recombination rate within the bulk material following the Sn substitution by Ge