We have studied the energetics, atomic, and electronic structure of Na and K point defects, as well as the (Na−Na), (K−K), and (Na−K) dumbbells in CuInSe 2 and CuIn 5 Se 8 solar cell materials by hybrid functional calculations. We found that although Na and K behaves somewhat similar; there is a qualitative difference between the inclusion of Na and K impurities. Namely, Na will be mostly incorporated into CuInSe 2 and CuIn 5 Se 8 either as an interstitial defect coordinated by cations, or two Na impurities will form (Na−Na) dumbbells in the Cu sublattice. In contrast to Na, K impurities are less likely to form interstitial defects. Instead, it is more preferable to accommodate K either as K Cu substitutional defect, or to form (K−K) dumbbells on Cu substitution positions. Our data show that all (Na−Na), (Na−K), and (K−K) dumbbells can form in both CuInSe 2 and CuIn 5 Se 8 . In the Cu-poor CuIn 5 Se 8 material the pristine Cu vacancies act as the most stable sites where Na and K can be inserted. The formation energy of Na-related defects is generally lower than the corresponding K-related defects, which would mean that if a defect site is already occupied by Na, then it is less likely that K is able to substitute Na during the postdeposition treatment. Regarding the electronic structure of the materials, Na and K point defects located in the Cu sublattice do not create deep defect levels in the gap, so they are not detrimental for the solar cell. In contrast, Se-related substitutional defects introduce defect levels in the gap, which act as charge traps, leading to severe degradation of the device efficiency. However, the formation energy of these Se-related defects are high so that they should have a low concentration in the material.

By using variational wave functions and quantum Monte Carlo techniques, we investigate the complete phase diagram of the Heisenberg model on the anisotropic triangular lattice, where two out of three bonds have super-exchange couplings J and the third one has instead J . This model interpolates between the square lattice and the isotropic triangular one, for J /J ≤ 1, and between the isotropic triangular lattice and a set of decoupled chains, for J/J ≤ 1. We consider all the fully-symmetric spin liquids that can be constructed with the fermionic projective-symmetry group classification [Y. Zhou and X.-G. Wen, arXiv:cond-mat/0210662] and we compare them with the spiral magnetic orders that can be accommodated on finite clusters. Our results show that, for J /J ≤ 1, the phase diagram is dominated by magnetic orderings, even though a spin-liquid state may be possible in a small parameter window, i.e., 0.7 J /J 0.8. In contrast, for J/J ≤ 1, a large spin-liquid region appears close to the limit of decoupled chains, i.e., for J/J 0.6, while magnetically ordered phases with spiral order are stabilized close to the isotropic point.

Using modified spin wave (MSW) method, we study the J1 − J2 Heisenberg model with first and second neighbor antiferromagnetic exchange interactions. For symmetric S = 1/2 model, with the same couplings for all the equivalent neighbors, we find three phase in terms of frustration parameter α = J2/J1: (1) a commensurate collinear ordering with staggered magnetization (Néel.I state) for 0 ≤ᾱ 0.207 , (2) a magnetically gapped disordered state for 0.207 ᾱ 0.369, preserving all the symmetries of the Hamiltonian and lattice, hence by definition is a quantum spin liquid (QSL) state and (3) a commensurate collinear ordering in which two out of three nearest neighbor magnetizations are antiparallel and the remaining pair are parallel (Néel.II state), for 0.396 ᾱ ≤ 1. We also explore the phase diagram of distorted J1 −J2 model with S = 1/2. Distortion is introduced as an inequality of one nearest neighbor coupling with the other two. This yields a richer phase diagram by the appearance of a new gapped QSL, a gapless QSL and also a valence bond crystal (VBC) phase in addition to the previously three phases found for undistorted model.

We have performed density functional theory calculations using the HSE06 hybrid functional to investigate the energetics, atomic, and electronic structure of intrinsic defects as well as Na and K impurities in the kesterite structure of the Cu 2 ZnSnSe 4 (CZTSe) solar cell material. We found that both Na and K atoms prefer to be incorporated into this material as substitutional defects in the Cu sublattice. At this site highly stable (Na−Na), (K−K), and (Na−K) dumbbells can form. While Na interstitial defects are stable in CZTSe, the formation of K interstitial defects is unlikely. In general, the calculated formation energies for Na-related defects are always lower compared to their K-related counterparts. On the basis of thermodynamic charge transition level calculations, we can conclude that the external defects are harmless except Na Sn and K Sn . These defects induce gap states that might be detrimental for the device performance.

We have employed first principles total energy calculations in the framework of density functional theory, with plane wave basis sets and screened exchange hybrid functionals to study the incorporation of intrinsic defects in bulk β-In2S3. The results are obtained for In-rich and S-rich experimental growth conditions. The charge transition level is discussed for all native defects, including VIn, VS, Ini, Si, SIn, and InS, and a comparison between the theoretically calculated charge transition levels and the available experimental findings is presented. The results imply that β-In2S3 shows n-type conductivity under both In-rich and S-rich growth conditions. The indium antiisite (InS), the indium interstitial (Ini), and the sulfur vacancy (VS′) are found to be the leading sources of sample's n-type conductivity. When going from the In-rich to the S-rich condition, the conductivity of the material decreases; however, the type of conductivity remains unchanged.

Creation of a partially filled intermediate band in a photovoltaic absorber material is an appealing concept for increasing the quantum efficiency of solar cells. Recently, we showed that formation of a partially filled intermediate band through doping a host semiconductor with a transition metal dopant is hindered by the strongly correlated nature of d-electrons and the antecedent Jahn–Teller distortion, as we have previously reported. In present work, we take a step forward and study the delocalization of a filled (valence-like) intermediate band throughout the lattice: a case study of Ti- and Nb-doped In2S3. By means of hybrid density functional calculations, we present extensive analysis on structural properties and interactions leading to electronic characteristics of Ti- and Nb-doped In2S3. We find that Nb creates an occupied doublet, which can become delocalized onto the crystal at high but feasible concentrations (around 2.5 at% and above). As a consequence, doping In2S3 with adequately high concentrations of Nb allows the subgap intermediate band to conduction band absorption, which leads to higher photocurrent densities compared to pure In2S3. Ti on the other hand forms an occupied singlet intermediate band, which remains strongly localized even at high concentration of 5 at%.

Earth-abundant and environmentally-friendly Cu 2 -II-IV-VI 4 (II=Sr, Ba; IV=Ge, Sn; VI=S,Se) are considered materials for the absorber layers in thin film solar cells. Attempts to understand and improve optoelectronic properties of these newly emerged absorbers resulted in an efficiency of 5.2% in less than two years. However, the energy band alignment at the buffer/absorber interface has not been studied yet; an information which is of crucial importance for designing high performance devices. Therefore, current study focuses on the band offsets between these materials and the CdS buffer. Using first-principles calculations, band discontinuities are calculated at the buffer/absorber interface. The results yield a type-II band alignment between all Cu 2 -II-IV-VI 4 absorbers and CdS, hence a negative DE c . Adoption of a negative DE c (cliff-like conduction band offset) at the buffer/ absorber interface, however, gives rise to low open circuit voltage and high interface-related recombinations. Therefore, it is necessary to search for an alternative buffer material that forms a type-I band alignment with these absorbers, where the conduction band minimum and the valence band maximum are both localized on the absorber side.

No abstract