Julien Vidal

Band-structure, optical properties, and defect physics of the photovoltaic semiconductor SnS

SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experiment to assess the PV relevant properties of SnS, we clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation. SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m0 perpendicular to the van der Waals layers to 0.2 m0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects. Potentially detrimental deep levels due to SnS antisite or S vacancy defects can be suppressed by suitable adjustment of the growth condition towards S-rich.

Defect Tolerant Semiconductors for Solar Energy Conversion.

Defect tolerance is the tendency of a semiconductor to keep its properties despite the presence of crystallographic defects. Scientific understanding of the origin of defect tolerance is currently missing. Here we show that semiconductors with antibonding states at the top of the valence band are likely to be tolerant to defects. Theoretical calculations demonstrate that Cu3N with antibonding valence band maximum has shallow intrinsic defects and no surface states, in contrast to GaN with bonding valence band maximum. Experimental measurements indicate shallow native donors and acceptors in Cu3N thin films, leading to 10(16)-10(17) cm(-3) doping with either electrons or holes depending on the growth conditions. The experimentally measured bipolar doping and the solar-matched optical absorption onset (1.4 eV) make Cu3N a promising candidate absorber for photovoltaic and photoelectrochemical solar cells, despite the calculated indirect fundamental band gap (1.0 eV). These conclusions can be extended to other materials with antibonding character of the valence band, defining a class of defect-tolerant semiconductors for solar energy conversion applications.

Density-based mixing parameter for hybrid functionals

A very popular ab initio scheme to calculate electronic properties in solids is the use of hybrid functionals in density functional theory (DFT) that mixes a portion of the Fock exchange with DFT functionals. In spite of its success, a major problem still remains, related to the use of one single mixing parameter for all materials. Guided by physical arguments that connect the mixing parameter to the dielectric properties of the solid, and ultimately to its band gap, we propose a method to calculate this parameter from the electronic density alone. This approach is able to cut significantly the error of traditional hybrid functionals for large and small gap materials, while retaining a good description of the structural properties. Moreover, its implementation is simple and leads to a negligible increase of the computational time.

Optimization of the derivative expansion in the nonperturbative renormalization group

We study the optimization of nonperturbative renormalization group equations truncated both in fields and derivatives. On the example of the Ising model in three dimensions, we show that the Principle of Minimal Sensitivity can be unambiguously implemented at order

Nonperturbative renormalization group approach to the Ising model: A derivative expansion at order ∂4

\partial^2

Equivalence of critical scaling laws for many-body entanglement in the Lipkin-Meshkov-Glick model

of the derivative expansion. This approach allows us to select optimized cut-off functions and to improve the accuracy of the critical exponents

Effects of electronic and lattice polarization on the band structure of delafossite transparent conductive oxides.

\nu

Aharonov-Bohm Cages in Two-Dimensional Structures

and

Continuous unitary transformations and finite-size scaling exponents in the Lipkin-Meshkov-Glick model

\eta

Thermodynamical limit of the Lipkin-Meshkov-Glick model.

. The convergence of the field expansion is also analyzed. We show in particular that its optimization does not coincide with optimization of the accuracy of the critical exponents.