Computationally-driven, high throughput identification of CaTe and Li3_\textrm{3}Sb as promising candidates for high mobility pp-type transparent conducting materials

High-performance $p$-type transparent conducting materials (TCMs) must exhibit a rare combination of properties including high mobility, transparency and $p$-type dopability. The development of high-mobility/conductivity $p$-type TCMs is necessary for many applications such as solar cells, or transparent electronic devices. Oxides have been traditionally considered as the most promising chemical space to dig out novel $p$-type TCMs. However, non-oxides might perform better than traditional $p$-type TCMs (oxides) in terms of mobility. We report on a high-throughput (HT) computational search for non-oxide $p$-type TCMs from a large dataset of more than 30,000 compounds which identified CaTe and Li$_\textrm{3}$Sb as very good candidates for high-mobility $p$-type TCMs. From our calculations, both compounds are expected to be $p$-type dopable: intrinsically for Li$_\textrm{3}$Sb while CaTe would require extrinsic doping. Using electron-phonon computations, we estimate hole mobilities at room-temperature to be about 20 and 70 cm$^2$/Vs for CaTe and Li$_\textrm{3}$Sb, respectively. The computed hole mobility for Li$_\textrm{3}$Sb is quite exceptional and comparable with the electron mobility in the best $n$-type TCMs.Comment: 10 pages, 5 figure

High-throughput calculations of charged point defect properties with semi-local density functional theory—performance benchmarks for materials screening applications

Calculations of point defect energetics with Density Functional Theory (DFT) can provide valuable insight into several optoelectronic, thermodynamic, and kinetic properties. These calculations commonly use methods ranging from semi-local functionals with a-posteriori corrections to more computationally intensive hybrid functional approaches. For applications of DFT-based high-throughput computation for data-driven materials discovery, point defect properties are of interest, yet are currently excluded from available materials databases. This work presents a benchmark analysis of automated, semi-local point defect calculations with a-posteriori corrections, compared to 245 “gold standard” hybrid calculations previously published. We consider three different a-posteriori correction sets implemented in an automated workflow, and evaluate the qualitative and quantitative differences among four different categories of defect information: thermodynamic transition levels, formation energies, Fermi levels, and dopability limits. We highlight qualitative information that can be extracted from high-throughput calculations based on semi-local DFT methods, while also demonstrating the limits of quantitative accuracy

Discovery of the Zintl-phosphide BaCd2_{2}P2_{2} as a long carrier lifetime and stable solar absorber

Thin-film photovoltaics offers a path to significantly decarbonize our energy production. Unfortunately, current materials commercialized or under development as thin-film solar cell absorbers are far from optimal as they show either low power conversion efficiency or issues with earth-abundance and stability. Entirely new and disruptive materials platforms are rarely discovered as the search for new solar absorbers is traditionally slow and serendipitous. Here, we use first principles high-throughput screening to accelerate this process. We identify new solar absorbers among known inorganic compounds using considerations on band gap, carrier transport, optical absorption but also on intrinsic defects which can strongly limit the carrier lifetime and ultimately the solar cell efficiency. Screening about 40,000 materials, we discover the Zintl-phosphide BaCd$_{2}$P$_{2}$ as a potential high-efficiency solar absorber. Follow-up experimental work confirms the predicted promises of BaCd$_{2}$P$_{2}$ highlighting an optimal band gap for visible absorption, bright photoluminescence, and long carrier lifetime of up to 30 ns even for unoptimized powder samples. Importantly, BaCd$_{2}$P$_{2}$ does not contain any critical elements and is highly stable in air and water. Our work opens an avenue for a new family of stable, earth-abundant, high-performance Zintl-based solar absorbers. It also demonstrates how recent advances in first principles computation can accelerate the search of photovoltaic materials by combining high-throughput screening with experiment

First-principles defect studies of photovoltaic materials : from understanding to high-throughput screening

The growing concerns about global warming combined with fossil fuel shortage call for developing green energy technologies. Even though solar light is one of the most promising renewable energy sources, harvesting it efficiently has its own inherent limitations. Two of the most critical components of solar cells in terms of efficiency are the transparent-conducting (TC) and absorber layers. This is particularly true in the case of thin-film cells that represent a potential low-cost path to scalable photovoltaic (PV) technology. Indeed, on the one hand, not all incident light passes through the TC layer and reaches the absorber layer, and, on the other hand, not all the generated electron-hole pairs are extracted and collected from the latter. In this thesis, we describe the impact of point defects on the electronic and optical properties of the materials used in PV applications. We discuss how ab initio calculations using hybrid functionals can help to explain the observed signatures from experimental studies. We demonstrate the importance of including such first-principles point-defect computations in the search for new efficient TC materials and PV absorbers. In addition, we examine the viability of high-throughput approaches for calculating point-defect properties based on semi-local DFT approaches. Finally, we extend the amount of performed computations from a small set of configurations to a dataset of thousands of compounds.(FSA - Sciences de l'ingénieur) -- UCL, 202

Defect compensation in the p-type transparent oxide Ba2BiTaO6

Ba2BiTaO6 is a transparent p-type oxide recently discovered and exhibiting attractive hole mobility but low carrier concentration. Using first-principles computations, we study how defects influence the carrier concentration in Ba2BiTaO6. The calculated defect formation energies confirm that K is an adequate p-type shallow extrinsic dopant but that high p-type doping is prevented by the presence of compensating, ‘‘hole-killing’’, intrinsic defects: O vacancies but also Ta on Bi anti-sites. Our work stresses the inherent difficulty in doping Ba2BiTaO6 to high carrier concentration and discusses a few avenues towards this goal

Disclosing the structural, phase transition, elastic and thermodynamic properties of CdSe1−xTex(x = 0.0, 0.25, 0.5, 0.75, 1.0) using LDA exchange correlation

The phase transition and structural, elastic, thermodynamic characteristics of CdSe1−xTex alloys for all compositions x (x = 0, 0.25, 0.5, 0.75, 1) in both hexagonal wurtzite (WZ) and cubic zinc-blende (ZB) are studied at zero K and zero pressure in emphasis of the Full Potential Linearized Augmented Plane Wave (FP-LAPW) approach, in accordance with the Density Functional Theory (DFT). This was inserted within the WIEN2k code, alongside a local density approximation (LDA) in order to consider the exchange-correlation functional. For all compositions the CdSe1−xTex alloys were found to be mechanically stable for both phases ZB and WZ, and the strongest material among all structures is CdSe. Our findings reveal that the relation between elastic constants and the Te concentrations is not linear. The induced phase transition from ZB to WZ is studied at zero K, and the corresponding volume collapses at the phase transition boundary are calculated for all compositions x (x = 0.0, 0.25, 0.50, 0.75). Our results show that for all compositions of the CdSe1−xTex alloys, the stable phase is zinc-blende. Furthermore, the elastic characteristics of ZB and WZ phases of CdSe1−xTex alloys, alongside elastic constants, bulk modulus and shear modulus were determined and assessed in comparison with other theoretical and experimental findings available. A positive relationship was observed. Keywords: Phase transition, Elastic properties, Thermodynamic properties, ZB and W

Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder

The controversial role of cation disorder in the extraordinarily low open-circuit voltage (VOC) of the Cu2ZnSnS4 (CZTS) kesterite absorber is examined through a statistical treatment of disorder within the cluster-expansion method. It is demonstrated that the extensive Cu–Zn disorder alone cannot be responsible for the large Urbach tails observed in many CZTS solar cells. While the band gap is reduced as a result of the Gaussian tails formed near the valence-band edge due to Cu clustering, band-gap fluctuations contribute only marginally to the VOC deficit, thereby excluding Cu–Zn disorder as the primary source of the low efficiency of CZTS devices. On the other hand, the extensive disorder stabilizes the formation of SnZn antisite and its defect complexes, which as nonradiative recombination and minority carrier trapping centers dominate the VOC loss in CZTS. Our analysis indicates that current CZTS devices might have already approached the maximum conversion efficiency (14%) given the limited growth conditions and the remnant cation disorder even after postannealing. In view of the improved efficiency achieved with CZTS-derived kesterite absorbers, the methodology presented in this work offers an avenue to understanding and optimizing these emerging kesterite solar devices towards higher efficiency

High-throughput computational search for high carrier lifetime, defect-tolerant solar absorbers

The solar absorber is a key component in a solar cell as it captures photons and converts them into electron–hole pairs. Its efficiency is driven by the carrier lifetime and the latter is controlled by Shockley–Read–Hall non-radiative processes, which involve defects. Here, we present an ab initio high-throughput screening approach to search for new high-efficiency photovoltaic absorbers taking into account carrier lifetime and recombination through defects. We first show that our methodology can distinguish poor and highly efficient solar absorbers. We then use our approach to identify a handful of defect-tolerant, high carrier lifetime, absorbers among more than 7000 Cu-based known materials. We highlight K3Cu3P2 and Na2CuP as they combine earth-abundance and the potential for high efficiency. Further analysis of our data articulates two challenges in discovering Cu-based solar absorbers: deep anti-site defects lowering the carrier lifetime and low formation-energy copper vacancies leading to metallic behavior. The alkali copper phosphides and pnictides offer unique chemistries that tackle these two issues

Giant thermoelectric figure of merit in multivalley high-complexity-factor LaSO

We report a giant thermoelectric figure of merit ZT (up to six at 1100 K) in n-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed from ab initio bands within Bloch-Boltzmann theory in an energy-, chemical potential-, and temperature-dependent relaxation time approximation. The lattice thermal conductivity is estimated from a model employing the ab initio phonon and Grüneisen-parameter spectrum. The main source of the large ZT is the significant power factor which correlates with a large band complexity factor. We also suggest a possible n-type dopant for the material based on ab initio calculations

Structural, elastic, mechanical and thermodynamic properties of Terbium oxide: First-principles investigations

First-principles investigations of the Terbium oxide TbO are performed on structural, elastic, mechanical and thermodynamic properties. The investigations are accomplished by employing full potential augmented plane wave FP-LAPW method framed within density functional theory DFT as implemented in the WIEN2k package. The exchange-correlation energy functional, a part of the total energy functional, is treated through Perdew Burke Ernzerhof scheme of the Generalized Gradient Approximation PBEGGA. The calculations of the ground state structural parameters, like lattice constants a0, bulk moduli B and their pressure derivative B′ values, are done for the rock-salt RS, zinc-blende ZB, cesium chloride CsCl, wurtzite WZ and nickel arsenide NiAs polymorphs of the TbO compound. The elastic constants (C11, C12, C13, C33, and C44) and mechanical properties (Young’s modulus Y, Shear modulus S, Poisson’s ratio σ, Anisotropic ratio A and compressibility β), were also calculated to comprehend its potential for valuable applications. From our calculations, the RS phase of TbO compound was found strongest one mechanically amongst the studied cubic structures whereas from hexagonal phases, the NiAs type structure was found stronger than WZ phase of the TbO. To analyze the ductility of the different structures of the TbO, Pugh’s rule (B/SH) and Cauchy pressure (C12–C44) approaches are used. It was found that ZB, CsCl and WZ type structures of the TbO were of ductile nature with the obvious dominance of the ionic bonding while RS and NiAs structures exhibited brittle nature with the covalent bonding dominance. Moreover, Debye temperature was calculated for both cubic and hexagonal structures of TbO in question by averaging the computed sound velocities. Keywords: DFT, TbO, Elastic properties, Thermodynamic propertie