A fully self-contained study of the thermodynamic and electronic properties of intrinsic point defects in the solar absorber materials CuInSe2 and CuGaSe2 based on screened-exchange hybrid density functional theory is presented. The results are partly at odds with data obtained within local density functional theory in former studies. GaCu electron traps as well as CuIn and CuGa hole traps are found to be the dominant intrinsic recombination centers. In contrast to the accepted view, complex formation of antisites with copper vacancies is not decisive for explaining the favorable properties of CuInSe2, since InCu is already a shallow defect by itself. The localization of holes is observed on CuIn and CuGa as well as on VIn and VGa when supercells of 216 atoms are used. Furthermore, the results raise doubts about the relevance of selenium vacancies and DX centers for experimentally observed metastabilities. Finally, a guide to the optimal preparation conditions in terms of the point defect physics of CuInSe2 and CuGaSe2 for their application as solar cell absorbers is provided.

Intrinsic point defects in CuInSe2and CuGaSe2as seen via screened-exchange hybrid density functional theory

https://publikationen.bibliothek.kit.edu/1000097712/40251361

On the oxidation mechanism of refractory high entropy alloys

Ceramic materials able to heal manufacture or damage induced microstructure defects might trigger a change in paradigm for design and application of load bearing ceramics. This work reviews thermodynamic and kinetic aspects governing the regeneration of solid contact able to transfer stress between disrupted crack surfaces in ceramics. Major crack healing processes include perturbation of crack-like pores followed by sintering of isolated pores, as well as reaction with an environmental atmosphere and filling of the crack space with an oxidation product. Since thermally activated solid state reactions require elevated temperatures which may exceed 1000 °C, processes able to trigger crack healing at lower temperatures are of particular interest for transferring into engineering applications. Generic principles of microstructure modifications able to facilitate crack repair at lower temperatures will be considered: (i) acceleration of material transport by grain boundary decoration and grain size reduction, and (ii) reduction of thermal activation barrier by repair filler activation. Examples demonstrating crack healing capability include oxidation reaction of low energy bonded intercalation metal from nano-laminate MAX phases and catalyzed surface nitridation of polymer derived ceramics containing repair fillers.

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Generic principles of crack-healing ceramics

Heterojunctions, such as ZnO/CdS/CuGaSe2, were fabricated for photovoltaic applications. Optimiza- tion of device structures based on monocrystalline CuGaSe2 led to the highest-to-date power conversion efficiencies for CuGaSe2 solar cells. At room temperature under 100 mW/cm 2 AM1.5 illumination a maximum cell efficiency of 9.7% was achieved, given by an open-circuit voltage of 946 mV, a short circuit current density of 15.5 mA/cm 2, and a fill factor of 66.5%. Preparation and performance of the optimum device are described. Current voltage characteristics dependent on illumination inten- sity and temperature, spectral response and electron- beam-induced current measurements were performed to determine the device parameters as well as to analyse the current transport and loss mechanisms. Tunneling, assist- ed by defect levels in the CdS layer, seems to play a major role. High injection effects are observed at forward bias of V > 0.5 V or an illumination level of P > 10 mW/cm 2. Under such conditions, as well as at low temperatures, the non-zero series resistance comes into play. Effects of the shunt resistance, however, are negligible in all cases.

CuGaSe2 solar cells with 9.7% power conversion

Author(s): Spindler, C; Babbe, F; Wolter, MH; Ehre, F; Santhosh, K; Hilgert, P; Werner, F; Siebentritt, S | Abstract: The electronic defects in any semiconductor play a decisive role for the usability of this material in an optoelectronic device. Electronic defects determine the doping level as well as the recombination centers of a solar cell absorber. Cu(In,Ga)Se2 is used in thin-film solar cells with high and stable efficiencies. The electronic defects in this class of materials have been studied experimentally by photoluminescence, admittance, and photocurrent spectroscopies for many decades now. The literature results are summarized and compared to new results by photoluminescence of deep defects. These observations are related to other experimental methods that investigate the physicochemical structure of defects. To finally assign the electronic defect signatures to actual physicochemical defects, a comparison with theoretical predictions is necessary. In recent years the accuracy of these calculations has greatly improved by the use of hybrid functionals. A comprehensive model of the electronic defects in Cu(In,Ga)Se2 is proposed based on experiments and theory. The consequences for solar cell efficiency are discussed.

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Electronic defects in Cu ( In , Ga ) S e 2 : Towards a comprehensive model

Recovery of mechanical strength by surface crack disappearance via thermal oxidation for nano-Ni/Al2O3 hybrid materials

Crack disappearance by high-temperature oxidation was studied in alumina (Al2O3) composites toughened by Ni nanoparticles. This process is performed in air at temperature ranging from 1000 to 1300°C for 1 to 48 h. The results showed that crack disappearance depends on both annealing temperature and time. Complete crack disappearance in this composite was confirmed at lower temperatures for long oxidation period, 1100oC for 48 h, and higher temperature for shorter time, 1300oC for 1 h in air. The crack disappearance mechanism was explained on the basis of the formation of NiAl2O4 spinel on sample surfaces produced by the oxidation reaction during the heat treatment.

Crack Disappearance by High-Temperature Oxidation of Alumina Toughened by Ni Nano-Particles

CuGaSe 2 (CGS) crystals (Cu/Ga=0.9–1.4) which have been expected as photovoltaic applications were successfully grown by a vertical gradient freezing method. All samples were of the chalcopyrite structures, nearly stoichiometric and showed p-type conductivities. The samples became poor crystallinity with increasing enclosed Cu/Ga ratios in the Cu-rich region because the values of full-width at half-maximum at (1 1 2) diffraction lines increased with increasing enclosed Cu/Ga ratios. Lattice constants of a and c axes in the Cu-rich regions were larger than those in the Cu-poor regions, indicating that Cu atoms played an important role with the lattice constants. The grown stoichiometric CGS sample was of high quality since the free-exciton emission line was clearly observed in photoluminescence (PL) spectrum at 77 K. The CGS samples turned out to have many donor- and acceptor-type defects because the donor–acceptor pair emission bands had considerable intensities in the PL spectra.

Photoluminescence spectra of CuGaSe2 crystals

Influences of Al2O3 grain size on high-temperature oxidation of nano-Ni/Al2O3 composites

The oxidation kinetics of 5 vol% nano-Ni dispersed Al 2 O 3 with Si-doping (nano-Ni/Al 2 O 3 -Si) was studied in the present paper. The starting powder mixture was prepared by drying aqueous slurry consisting of alumina with nickel nitrate and Si (OCH 3 ) 4. The powder mixture was reduced at 600°C for 12 h in a stream of Ar-1%H 2 gas mixture. Nano-Ni/Al 2 O 3 -Si was densified by pulsed electric current sintering. The oxidation test was conducted at 1200―1350°C for 1―14 d in air. Oxidized zone consisted of Al 2 O 3 matrix and NiAl 2 O 4 as the oxidation product. Growth of the oxidized zone followed a parabolic manner. A thin NiAl 2 O 4 layer was also observed on the sample surface. Si-doping decreases growth rate of oxidized zone effectively at lower temperatures such 1200°C.

Daisuke Maruoka

Papers

Recovery of mechanical strength by surface crack disappearance via thermal oxidation for nano-Ni/Al2O3 hybrid materials

Crack Disappearance by High-Temperature Oxidation of Alumina Toughened by Ni Nano-Particles

Photoluminescence spectra of CuGaSe2 crystals

Influences of Al2O3 grain size on high-temperature oxidation of nano-Ni/Al2O3 composites

Influence of Si-doping of nano-Ni dispersed Al2O3 composites on high-temperature oxidation