Alireza Faghaninia

Enhanced thermoelectric efficiency via orthogonal electrical and thermal conductances in phosphorene.

Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a devices electrical conductance to its thermal conductance. In this paper, a recently fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal not only that phosphorene possesses a spatially anisotropic electrical conductance, but that its lattice thermal conductance exhibits a pronounced spatial-anisotropy as well. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT may reach the criterion for commercial deployment along the armchair direction of phosphorene at T = 500 K and is close to 1 even at room temperature given moderate doping (∼2 × 10(16) m(-2) or 2 × 10(12) cm(-2)). Ultimately, phosphorene hopefully stands out as an environmentally sound thermoelectric material with unprecedented qualities. Intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (∼300 K), one whose performance does not require any sophisticated engineering techniques.

Ab initio electronic transport model with explicit solution to the linearized Boltzmann transport equation

Author(s): Faghaninia, A; Ager, JW; Lo, CS | Abstract:

First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, β-Zn(4)Sb(3), is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in β-Zn(4)Sb(3). To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in β-Zn(4)Sb(3) density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in β-Zn(4)Sb(3), as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn(4)Sb(3) has a partially occupied topmost valence band, making this defect an acceptor so that Li:Zn(4)Sb(3) is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li:Zn(4)Sb(3) is more stable and better performing as a thermoelectric material than undoped β-Zn(4)Sb(3).

Improving the Carrier Lifetime of Tin Sulfide via Prediction and Mitigation of Harmful Point Defects

Tin monosulfide (SnS) is an emerging thin-film absorber material for photovoltaics. An outstanding challenge is to improve carrier lifetimes to >1 ns, which should enable >10% device efficiencies. However, reported results to date have only demonstrated lifetimes at or below 100 ps. In this study, we employ defect modeling to identify the sulfur vacancy and defects from Fe, Co, and Mo as most recombination-active. We attempt to minimize these defects in crystalline samples through high-purity, sulfur-rich growth and experimentally improve lifetimes to >3 ns, thus achieving our 1 ns goal. This framework may prove effective for unlocking the lifetime potential in other emerging thin-film materials by rapidly identifying and mitigating lifetime-limiting point defects.

Prediction and control of harmful point defects in thin film photovoltaics: Improving the performance of tin sulfide

Many thin-film photovoltaic materials show promising optical properties, but are limited by low minority-carrier lifetimes. Many factors can lower the carrier lifetime, including structural defects, intrinsic point defects, and extrinsic point defects from impurities. We focus on the case of tin (II) sulfide (SnS), in which point defects in particular may be lifetime-limiting. We model the impact of intrinsic and extrinsic point defects to narrow down the most likely lifetime-limiting defects to the extrinsic iron- and cobalt-on-tin substitutional defects and the sulfur vacancy. We grow material to eliminate these defects and observe over 1 order of magnitude increases in photoluminescence decay time, which is reflective of minority carrier lifetime. We believe that this kind of targeted approach towards point defect management could be applied to new thin film materials.

Alloying ZnS to create transparent conducting materials

In the exploration and design of new transparent conducting materials (TCM), alloyed ZnS has shown great promise. Particularly, Cu:ZnS is particularly intriguing as a p-type TCM, which, when combined with n-doped ZnS, could find eventual applications in photovoltaics and optoelectronics. We desire to identify the most promising materials with the optimal combination of physical stability, transparency, and electrical conductivity. In this study, we employ hybrid density functional theory and a new carrier transport model, aMoBT, developed within the Boltzmann transport framework, to analyze the defect physics of different cation and anion alloyed ZnS. We obtain formation energies and correct band gaps for ZnS doped with B, Al, Ga, In, Tl, F, Cl, Br and I. Furthermore, we calculate the effective mass and electrical mobility of these compounds at various compositions, temperatures and electron concentrations, to identify the best-performing n-doped ZnS TCMs, without need for costly experimental trial and error. Our results show that among the doping candidates, Al:ZnS is the most promising with the highest solubility, the smallest reduction in the band gap, and the highest conductivity of 41 S · cm-1 at 300 K (9.375% Al, n = 2.32 × 1018 cm-3). Our calculations predict that the conductivity may be as high as 881 S · cm-1 (at 300 K) at a high concentration of n = 1.00×1020 cm-3. Furthermore, our ab initio electronic and thermodynamics calculations provide significant insight on phase stability and the underlying electronic interactions that result in optimal transparent conducting behavior.

ab initio electronic transport model for photovoltaics

Accurate ab initio electronic transport models facilitate the rapid development in design of new photovoltaic (PV) materials. In order to correctly predict low-field electronic drift mobility and conductivity of semiconductors, we present here an ab initio transport Model in the Boltzmann Transport (aMoBT) framework. Using the relevant inputs from ab initio band structure and density of states of the semiconductor, we calculate electron group velocity, effective mass, orbital hybridization, and phonon frequencies. We then explicitly solve the linearized Boltzmann transport equation (BTE) via Rodes iterative method, to calculate the perturbation to a small electric field and, thus, drift mobility, without the need for experimental data and fitting. We have validated the calculated mobility of GaAs and InN against experimental data and find that the agreement is satisfactory in both the qualitative prediction of changes of mobility with temperature and carrier concentration, as well as the quantitatively predicted values. We believe that this tool facilitates high- throughput density functional theory calculations in search of new PV materials. Furthermore, it offers insight on physical limitations to mobility, including elastic and inelastic scattering mechanisms inside these materials.

Computational design of p-type transparent conductors for photovoltaic applications

Ternary oxides are particularly attractive for applications ranging from transparent electronics to spintronics to photoelectrocatalysis, yet difficulties in achieving p-type conductivities have hindered progress in these fields. A common means of searching for ternary oxides has been to sample the triangular phase space and determine whether the most favorable structures are thermodynamically stable. We believe, however, that computational techniques must be improved to enable accurate property predictions and facilitate the search for these new materials. As an example, we propose that late transition metal zinc oxide spinel compounds, as identified by the Materials Project, may exhibit p-type behavior and good optical properties for use as transparent conducting oxides. We use density functional theory calculations and the Boltzmann transport equation to calculate electrical transport and optical properties of the candidate materials. We employ several improvements to the theory, namely: 1. Hybrid density functionals to account for the incorrect treatment of electron exchange in DFT, and additional consideration of the scissor operator to the band energies to match only experimental band gaps without correcting for valence band edge curvature, and 2. Momentum-matrix method to calculate electron group velocities. We find that the copper zinc oxide spinel is a promising dilute magnetic semiconductor, while the nickel zinc oxide spinel is a promising transparent conductor. We also compare our results on the spinel compounds to the corresponding doped zinc oxides. Thus, we have proposed improvements in materials theory that enable the discovery of new materials for photovoltaic applications.