Stephan Brunken

The mechanism of nickel sulfide induced rapid crystallization of highly textured tungsten disulfide (WS2) thin films : An in situ real-time diffraction study

Highly (001)-textured, photoactive tungsten disulfide (WS2) thin films have been grown by nickel sulfide induced rapid crystallization of amorphous sulfur rich tungsten sulfide (WS3+x) films. This paper focuses on the mechanism of the crystal growth. The rapid crystallization was monitored by real-time in situ energy dispersive x-ray diffraction. Provided that a thin nickel film is deposited prior to the deposition of WS3+x, the films crystallize rapidly (>20nm∕s). The crystallization takes place at a temperature of about 650°C, i.e., slightly above the Ni–S eutectic temperature of 637°C. After crystallization, isolated hexagonal NiSx crystallites are located on the surface of the WS2 layer, which was observed by scanning and transmission electron microscopy. These results lead to the model that the rapid crystallization occurs by liquid-phase crystal growth from NiSx droplets floating on top of the crystallized WS2 films. Nickel sulfide induced crystallized WS2 layers exhibit a pronounced (001) orientati...

Effect of Na presence during CuInSe2 growth on stacking fault annihilation and electronic properties

While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply—particularly at low growth temperatures—are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth.

Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron–hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of – partially electronically active – planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu–Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu–Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

Analysis of the early stages of the rapid, nickel-assisted crystallization of WS2 films

The nickel-sulfide-assisted rapid crystallization process was investigated in detail, and the understanding of the process of crystallization was improved. We found that nickel-sulfide-assisted crystal growth of WS2 films starts at the temperatures already below the eutectic temperature, leading to films with metallic behavior. This metallic behavior is due to a nickel-containing phase at the grain boundaries of the WS2-crystallites. Only annealing temperatures above the Ni-S-eutectic temperature lead to highly (001) textured WS2 films due to recrystallization induced by liquid nickel sulfide droplets. In these films, the WS2 crystallites have lateral sizes of several micrometers. Across the film NiSx crystallites are distributed. They are isolated from each other and generally exhibit a connection to the substrate surface.

Co-evaporated CuInSe 2 : Influence of growth temperature and Na on solar cell performance

CuInSe2 films were prepared by a 3-stage-co-evaporation process. For Na-free absorbers we do not observe any influence of the preparation temperature between 330 °C and 510 °C on the solar cell performance with typical efficiencies around 9 %. NaF-PDT leads to an increase of the open-circuit voltage and the efficiency for samples prepared at a growth temperature of 450 °C and 510 °C (around 13 % for such samples). For samples prepared at 330 °C NaF-PDT has no influence on the open circuit voltage and the efficiency although the treatment increases the charge carrier concentration. At higher preparation temperatures, absorbers with Na present during growth (NaF-precursor samples) lead to similar IV parameters as NaF-PDT absorbers. NaF-precursor samples grown at temperatures at 330 °C, show an X-ray diffraction reflex, which is characteristic for the presence of twin stacking faults within the growing thin films. Solar cells with these absorbers show bad performances. In contrast, Na-free grown absorbers do not show twin stacking faults after the absorber growth process independently of the preparation temperature. Capacitance-voltage measurements give charge carrier concentrations of around 3×1016 cm-3 (NaF-precursor samples), 5×1015 cm-3 (NaF-PDT samples) and 6×1014 cm-3 (Na-free samples). From scanning electron microscopy it is deduced that Na, which is present during the growth, leads to films with smaller grain sizes.