A structural perception about intrinsic point defects in nonstoichiometric compound semiconductors

Photovoltaics (PV), the direct conversion of light into electricity, is dominated by crystalline silicon (c-Si) technology nowadays. Thin film PV constitute an emerging alternative because of short energy payback time and minimum use of higt purity materials, addressing the urgent need for cost-competitive renewable energy technologies [1]. Compound semiconductors, like chalcopyrite type Cu(In,Ga)(Se,S)2 (CIGSe) are the most advanced and most efficient absorber materials. CIGSe-based solar cells are very well positioned in the field of PV technologies with present record efficiencies for small cells of 22.3% (production size modules 16.5% ) [1]. One reason for their success is related to the high flexibility of the chalcopyrite crystal structure, accepting strong deviations from stoichiometry forming native point defects, such as vacancies, interstitials and antisites [2]. But CIGSe relies on the scarce elements In and Ga, which may severely limit the mass deployment of this PV technology. Please click Additional Files below to see the full abstract

Zinc germanium nitrides and oxide nitrides: the influence of oxygen on electronic and structural properties

Zinc containing ternary nitrides, in particular ZnSnN2 and ZnGeN2, have great potential as earth-abundant and low toxicity light-absorbing materials. The incorporation of oxygen in this system – may it be intentional or unintentional – affects the crystal structure of the materials as well as their optical band gaps. Herein, we explore the origins of structural changes between the wurtzite type and its hettotype, the β-NaFeO2 type, and highlight the effect of oxygen. Furthermore, we study the electronic structure and bonding in order to understand the reason for the narrower band gap of zinc germanium oxide nitrides as opposed to pure zinc germanium nitride

On the thermal expansion of the tetragonal phase of MAPbI3 and MAPbBr3

Based on previously published research, the structural response of the tetragonal hybrid perovskite crystal structure of MAPbX3 [MA: [CH3NH3]+, methylammonium; X = I, Br] to thermal expansion is reviewed here. From an averaged crystal structure perspective, the tetragonal perovskite structure of MAPbI3 and MAPbBr3, based on diffraction data, shows apparent Pb-X bond length shortening and apparent shrinkage of the [PbX6] octahedra with increasing temperature. At the same time, these apparent observations, and hence the thermal expansion, are related to the progressive phase transformation towards the cubic structure, as the lattice parameters respond to a shear stress that couples to the order parameters, and this coupling is predicted by group theory and thus aims to explain precisely the apparent negative thermal expansion-like effects. A different picture emerges for the thermal expansion when considering the very localized structure, since neither a shortening of the Pb-X bond lengths nor a shrinking of the [PbX6] octahedra is observed with pair distribution function analysis, and the presence of orthorhombic short-range order in the tetragonal and cubic perovskite structures is assumed in published studies. The compared extended X-ray absorption fine structure studies, which also map the local structure and provide the “true” bond distance, show no lead-halide bond length shortening with temperature. The perpendicular mean square relative displacement has been determined. Therefore, a comparison of the tension and bond expansion effects in both perovskites can be made. In the orthorhombic phase of MAPbI3 and MAPbBr3, positive expansion and negative tension of the lead-halide bond are almost balanced. After transitioning to the tetragonal phase, the equilibrium shifts toward negative tension. This suggests that both hybrid perovskites have tighter lead-halide bonds and less rigid [PbX6] octahedra in the tetragonal phase than in the low temperature perovskite crystal structure

A thorough investigation of the crystal structure of willemite-type Zn2GeO4

The thermodynamically stable phase of Zn2GeO4 contains tetrahedrally coordinated cations only and crystallizes isostructurally to Zn2SiO4 (willemite, space group R3? , no. 148). While this material is considered for a plethora of energy-related applications, such as transparent conducting oxide, battery material and photocatalyst, cation ordering in the crystal structure has not been investigated thoroughly. We have therefore re-determined the crystal structure of Zn2GeO4 using a combination of X-ray and neutron powder diffraction. The additional neutron diffraction study helps to distinguish between the isoelectronic Zn2+ and Ge4+ cations and yields valuable information about a partial or complete cation permutation in this material. The experimental study is supported by first-principles calculations on the structural properties of Zn2GeO4 utilizing a standard generalized gradient approximation, and the more accurate hybrid functional HSE06. In order to better understand cation permutations, additional calculations including defective Zn2GeO4 have been performed based on a supercell approach. Our results show that, with the preparation conditions applied, cation permutation is unlikely to occur in our samples

The big bang of halide perovskites: The starting point of crystallization

Hybrid halide perovskites (HHPs) are very promising absorber materials for solar cells due to their high power conversion efficiency and the low-cost solution-based processing methods. We applied small angle X-ray scattering to MAPbI3, FAPbI3 and MAPbBr3 precursor solutions in different solvents (GBL, DMF, and mixtures) to gain a deeper understanding of the building blocks during the early stage of HHP formation. We present a core–shell model where the core is formed by [PbX6] octahedra surrounded by a shell of solvent molecules, which explains the arrangement of the precursors in solution and how the solvent and the halide influence such arrangement

The kesterite–stannite structural transition as a way to avoid Cu/Zn disorder in kesterites: the exemplary case of the Cu2(Zn,Mn)SnSe4

The solid solution series between Cu2ZnSnSe4, crystallizing in the kesterite type structure, and Cu2MnSnSe4, adopting the stannite type structure, i.e. Cu2(Zn1−xMnx)SnSe4, was studied by a combination of neutron and X-ray powder diffraction. Powder samples with 0 ≤ x ≤ 1 were synthesized by the solid state reaction of the pure elements and it was confirmed by wavelength-dispersive X-ray spectroscopy that each contained a homogeneous, off-stoichiometric quaternary phase. The lattice parameters and cation site occupancy factors were determined simultaneously by the Rietveld analysis of the neutron and X-ray powder diffraction data. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in the crystal structure of the Cu2(Zn1−xMnx)SnSe4 mixed crystals, from which a cation distribution model was derived. For the end member Cu2ZnSnSe4, the disordered kesterite structure was confirmed and for Cu2MnSnSe4, the stannite structure was confirmed. The cross-over from the kesterite to stannite type structure by Zn2+ ↔ Mn2+ substitution in the Cu2Zn1−xMnxSnSe4 solid solution can be seen as a cation re-distribution process among the positions (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu+, Zn2+ and Mn2+. The Sn4+ cation does not take part in this process and remains on the 2b site. Moreover, the cross-over is also visible in the ratio of the lattice parameters c/(2a), showing a characteristic dependence on the chemical composition. The order parameter Q, the quantitative measure of Cu/BII disorder (BII = Zn and Mn), shows a distinct dependence on the Mn/(Mn + Zn) ratio. In Zn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals, the order parameter Q ∼ 0.7 and drops to Q ∼ 0 (complete Cu/BII disorder) in the compositional region 0.3 ≥ x ≥ 0.7. In Mn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals, adopting the stannite type structure, the order parameter reaches almost Q ∼ 1 (order). Thus, it can be concluded that only Mn-rich Cu2(Zn1−xMnx)SnSe4 mixed crystals do not show Cu/BII disorder. A similar trend of the dependence on the chemical composition of both Cu/BII-disorder and the band gap energy Eg in Cu2(Zn1−xMnx)SnSe4 mixed crystals was observed

The use of anomalous x ray diffraction as a tool for the analysis of compound semiconductors

We provide a review about the current and previous use of anomalous diffraction of x rays in the analysis of compound semiconductors. Among the large number of available techniques, those that have been used in successful experiments on this class of compounds are identified. An exhaustive overview of the compound semiconductor systems that have been studied successfully is provided and the kind of results derived from experiments is discusse

Uncovering cation disorder in ternary Zn1+xGe1−x(N1−xOx)2 and its effect on the optoelectronic properties

Ternary nitride materials, such as ZnGeN2, have been considered as hopeful optoelectronic materials with an emphasis on sustainability. Their nature as ternary materials has been ground to speculation of cation order/disorder as a mechanism to tune their bandgap. We herein studied the model system Zn1+xGe1−x(N1−xOx)2 including oxygen – which is often a contaminant in nitride materials – using a combination of X-ray and neutron diffraction combined with elemental analyses to provide direct experimental evidence for the existence of cation swapping in this class of materials. In addition, we combine our results with UV-VIS spectroscopy to highlight the influence of disorder on the optical bandgap

Synthesis of Cu2ZnxSnySe1+x+2y nanocrystals with wurtzite-derived structure

The most reported stable crystal structure of Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) is kesterite, which is derived from the ternary chalcopyrite structure. However, by controlling the reaction conditions, we found that the structure and composition of the CZTSe nanocrystals (NCs) can be tuned. This can be achieved by using a simple hot injection approach. The structural properties of the CZTSe NCs were characterized by powder X-ray diffraction (PXRD), Raman spectroscopy and transmission electron microscopy. The energy dispersive X-ray spectroscopy confirms the stoichiometry of CZTSe NCs. The optical band gap of the NCs is found to be around 1.38 eV, as estimated from UV-Vis absorption spectroscopy. PXRD studies show that the obtained CZTSe NCs occurring in three structurally different phases (tetragonal kesterite type, hexagonal wurtzite type and orthorhombic wurtz-stannite type) are converted to the kesterite structure by annealing at 540 °C for 30 min under an Se-vapour atmosphere