Christiane Stephan

Comprehensive insights into point defect and defect cluster formation in CuInSe2

The concentration of native point defects in CuInSe2 powder material as a function of stoichiometry has been experimentally determined by neutron powder diffraction. A correlation between the Cu/In ratio and the density of VCu as well as InCu has been established and their concentrations are quantified. It is demonstrated, that assuming the spontaneous formation of defect pairs, the density of native point defects is reduced significantly by an order of magnitude. The functionality of a solar device, assuming same conditions like in the analyzed material, may be explained by a neutralization due to the formation of electrically inactive defect complexes.

Cationic point defects in CuGaSe2 from a structural perspective

The chalcopyrite semiconductors show large tolerances to deviations from stoichiometry by keeping the crystal structure. Such deviations always cause structural inhomogeneities and charge mismatches which influence the material properties. We studied the type and concentration of cationic point defects on Cu-poor CuGaSe2 powders by the complementary use of neutrons and photons. It is demonstrated that the main existing defects in this Cu-poor wide gap semiconductor are copper-vacancies (VCu) and gallium on interstitial sites (Gai). The latter may explain why tailoring a highly efficient CuGaSe2 solar cell is an even more challenging task than previously expected.

Atomic-scale structure, cation distribution, and bandgap bowing in Cu(In,Ga)S2 and Cu(In,Ga)Se2

Mixed chalcopyrite semiconductors like Cu(In,Ga)S2 and Cu(In,Ga)Se2 are characterized by the coexistence of different local atomic arrangements around the S or Se anion. The resulting anion displacement strongly influences the material bandgap. We studied the atomic-scale structure of Cu(In,Ga)S2 as a function of composition using x-ray absorption spectroscopy and valence force field simulations. Applying a specially developed model for not fully random cation distributions, we find that structural relaxation of the anion with respect to In and Ga contributes significantly more to the bandgap bowing observed for Cu(In,Ga)S2 and Cu(In,Ga)Se2 than relaxation with respect to Cu and group-III atoms.

New Structural Investigations in the Cu 2 Se(S)-In 2 Se 3 (S)/Cu 2 Se(S)-Ga 2 Se 3 (S) Phase Diagrams

Non-stoichiometry is a characteristic feature of ternary chalcopyrites like Cu-III-VI 2 (III=In,Ga; VI=S,Se). The results of a comparative study of structural trends within the homogeneity region of the chalcopyrite type α-phase of the Cu 2 Se(S)-In 2 Se 3 (S) and Cu 2 Se(S)-Ga 2 Se 3 (S) quasibinary phase diagrams are presented. Powder samples of Cu-rich and Cu-poor [Cu 2 Se(S)] 1-y -[In 2 Se 3 (S)] y as well as [Cu 2 Se(S)] 1-y -[Ga 2 Se 3 (S)] y alloys were prepared (0.4 1-x III 1+x/3 VI 2 samples. In Cu-rich samples the Cu-content is in all cases the driving force for the formation of the homogeneous microstructure observed.

Aspects for the optimization of CIGSe growth at low temperatures for application in thin film solar cells on polyimide foil

Cu(In,Ga)Se2 (CIGSe) thin film solar cells are the most efficient thin film photovoltaic technology available. Deposited onto the appropriate substrate they are potentially flexible, very light, robust and low cost. Due to their excellent radiation hardness and potentially high specific power, they have also attracted interest for use in space applications. Highest quality CIGSe absorber layers are usually grown at temperatures well above 500°C. So far only metal foils are a suitable choice as flexible substrate material in this temperature range. However, as those are conductive, the use of monolithic integration for solar cell interconnection requires an electrically insulating barrier between substrate and solar cell back contact. A non-conductive alternative to metal is polyimide foil. Commercially available polyimide foils are only tolerant to temperatures of up to around 400°C. It is therefore necessary to identify and understand the influence of main process parameters in order to achieve growth of high quality absorber material at these low temperatures. Former work has already highlighted that the amount of sodium present during film growth is a key parameter regarding optimum growth results. The work that is presented here summarizes previous work and investigates the complex relationship between the growth temperature and the effect of Na on the compositional, structural and electronic properties of CIGSe thin films.