Epitaxial Metal Halide Perovskites by Inkjet‐Printing on Various Substrates

Metal‐halide‐perovskites revolutionized the field of thin‐film semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thin‐film devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of high‐end deposition chambers. Here epitaxial growth is enabled via a ≈1000 times cheaper device, a single nozzle inkjet printer. By printing, single‐crystal micro‐ and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The hetero‐epitaxial structures of methylammonium PbBr3 grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjet‐printing of metal‐halide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the high‐end semiconductor growth technologies.DFG, 404984854, Bleifreie Perovksite für die RöntgendetektionDFG, 399073171, GRK 2495: Energiekonvertierungssysteme: von Materialien zu Bauteile

Characterization of Inhomogeneities in CIGSSe Solar Cell Absorbers in the SEM

Inhomogenitäten in industriell mit SEL-RTP (Stacked Elemental Layer- Rapid Thermal Processing) hergestellten Absorbern für Solarzellen wurden mit Rasterelektronenmikroskopie und energiedispersiver Röntgenmikroanalyse (EDX) an Oberfläche und Querschnitt untersucht. Delaminieren der Absorber macht Rückseite der Absorber und Oberseite des Rückkontaktes zugänglich. Die vertikalen Ga- und S-Gradienten wurden über EDX-Messungen mit variierter Anregungsenergie abgeschätzt. Die Inhomogenitäten wurden drei verschiedenen Ursachen zugeordnet: Precursor, RTP oder Eindringen von Na aus dem Glassubstrat. Inhomogenitäten im Precursor werden durch die Entnetzung des In beim Sputtern mit getrennten In- und CuGa-Targets verursacht. Dies führt zu einer lateral inhomogenen Ga/In-Verteilung nahe dem Rückkontakt. Eine raue Precursoroberfläche wird in den Absorber übertragen. Ternäre Targets verbessern die laterale Ga/In-Homogenität. Während der RTP führen unausgeglichene Diffusionsströme von Ga und In zur Oberseite und von Leerstellen zur Unterseite zur Bildung von Poren am Rückkontakt (Kirkendall Effekt). Dies verursacht auch die Cu/Ga-reichen Rückstände, die beim Delaminieren auf dem Rückkontakt verbleiben. Vertikale Ga- und S-Gradienten werden durch RTP und Na-Gehalt im Precursor beeinflusst. Die gravierendsten Inhomogenitäten werden durch Eindringen von Na während der RTP durch Unterbrechungen in der Na-Diffusionsbarriere zwischen Glas und Precursor verursacht. Dies können Pinholes oder Risse in der Diffusionsbarriere sein, entstanden beim Schneiden der P1-Linie mit einem ns-Laser. Die beobachteten Veränderungen in Zusammensetzung und Morphologie erweiterten den Kenntnisstand über die Wirkungsweise von Na. Na aus dem Precursor ist von Beginn der Absorberbildung an anwesend und verbessert hauptsächlich die Versorgung mit Se. Na aus dem Glas dringt erst bei höheren Temperaturen ein, wenn Se abreagiert oder verdampft ist. Dadurch verbessert es sowohl die Versorgung mit Se als auch mit S. Dies ermöglicht die bevorzugte Reaktion des Ga mit S und verändert die Ga- und S-Gradienten im Absorber massiv. Korngröße, Anzahl der Poren und der Cu/Ga-reichen Reste sowie das Ausmaß der Mo(S,Se)2-Bildung werden beeinflusst. Risse können durch die Verwendung eines ps-lasers verhindert werden.Inhomogeneities in thin film CIGSSe solar cell absorbers industrially produced by SEL-RTP (Stacked Elemental Layer- Rapid Thermal Processing) were examined by SEM (Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray Spectroscopy) on surface and cross section. Back side of absorbers and surface of back contact were made accessible by delaminating the absorbers. Vertical Ga and S gradients in the absorber were assessed by EDX with varying acceleration energies. Inhomogeneities in the absorbers were assigned to three different causes: Precursor, RTP or intrusion of Na from the soda lime glass substrate. Inhomogeneities in the precursor are caused by dewetting of In during the sputter process, especially if separate In- and CuGa-targets were used. This leads to a laterally uneven distribution of Ga and In in the absorber close to the back contact. A rough surface of the precursor is transferred into the absorber. Ternary targets improve lateral Ga/In homogeneity. During RTP, the unbalanced diffusion of Cu and In to the absorber surface and of vacancies towards the back leads to the formation of pores at the back contact by the Kirkendall effect. This also causes the Cu/Ga-rich residues remaining on the surface of the back contact after delamination. Vertical Ga and S gradients are influenced by the RTP process and by the Na-content of the precursor. The most severe inhomogeneities are caused by Na intruding through discontinuities in the Na-diffusion barrier between glass and precursor during RTP. Discontinuities can be pinholes or cracks caused by the P1 laser cutting process. The observed changes in composition and morphology improved the knowledge about the influence of Na from the glass on absorber formation, as well as on Ga and S gradients through the layer. Na from the precursor is present from the beginning of the absorber formation process and enhances mostly Se offer. Na from glass intrudes at higher temperatures when Se is reacted and evaporated and enhances Se and S offer. This enables the preferred reaction of Ga with S and changes Ga- and S-gradients in the absorbers severely. Grain size, the amount of pores close to the back contact, as well as of Cu/Ga-rich residues and extent of Mo(S,Se)2 formation on the back contact are influenced as well. Cracks can be prevented by the use of a ps-laser instead of ns-laser

Influence of dislocation content on the quantitative determination of the doping level distribution in n-GaAs using absorption mapping

In an earlier paper [P.J. Wellmann, A. Albrecht, U. Künecke, B. Birkmann, G. Mueller, M. Jurisch, Eur. Phys. J. Appl. Phys. 27, 357 (2004)] an optical method based on whole wafer absorption measurements was presented to determine the charge carrier concentration and its lateral distribution in n-type (Si/Te) doped GaAs. The submitted results for Si-doped GaAs gave rise to questions concerning the interpretation of absorption mappings in wafers with high dislocation densities. GaAs substrates for optoelectronic devices are strongly affected by dislocations. Therefore further studies were conducted: absorption and Hall measurements were performed on GaAs:Si wafers with high and low dislocation densities. Absorption in Si-doped GaAs is far more complex than in Te-doped GaAs. It shows a co-dependency on charge carrier concentration and dislocation content which causes complications in the quantitative optical determination of the charge carrier concentration. Qualitatively, absorption mappings depict dislocations and variations of charge carrier concentration very well

Analysis of Compositional Gradients in Cu(In,Ga)(S,Se)2 Solar Cell Absorbers Using Energy Dispersive X-ray Analysis with Different Acceleration Energies

The efficiency of Cu(In,Ga)(S,Se)2 (CIGSSe) solar cell absorbers can be increased by the optimization of the Ga/In and S/Se gradients throughout the absorber. Analyzing such gradients is therefore an important method in tracking the effectiveness of process variations. To measure compositional gradients in CIGSSe, energy dispersive X-ray analysis (EDX) with different acceleration energies performed at both the front surface and the backside of delaminated absorbers was used. This procedure allows for the determination of compositional gradients at locations that are millimeters apart and distributed over the entire sample. The method is therefore representative for a large area and yields information about the lateral homogeneity in the millimeter range. The procedure is helpful if methods such as secondary ion-mass (SIMS), time-of-flight SIMS, or glow-discharge optical emission spectrometry (GDOES) are not available. Results of such EDX measurements are compared with GDOES, and they show good agreement. The procedure can also be used in a targeted manner to detect local changes of the gradients in inhomogeneities or points of interest in the µm range. As an example, a comparison between the compositional gradients in the regular absorber and above the laser cut separating the Mo back contact is shown

Tuning the emission colour by manipulating terbium-terbium interactions: Terbium doped aluminum nitride as an example system

Terbium-terbium interactions in terbium doped semiconductors and insulators may lead to the so-called cross-relaxation process, which increases the 5D4 (green) emission of the terbium ions at the cost of the 5D3 (blue) luminescence intensity. This effect can generally be reduced by increasing the distance between an excited ion and the nearest ion in the ground state. A straightforward measure is to use a specimen with a decreased terbium concentration. The alternative is to increase the intensity of the excitation (either by photons or electrons) and thereby to reduce the population of terbium ions in the ground state. This paper works this process out with the example of AlN:Tb on the basis of a model and respective experimental results. As will be seen, stronger excitation causes in essence more Tb ions to be excited, thus less ions in the ground state which increases the distance between an excited and the nearest ground state ions. This hinders energy transfer between the terbium ions and thus counteracts the cross-relaxation process. The advantage of changing the excitation intensity lies in the possibility to deliberately shift the apparent colour of the Tb luminescence from a single specimen between green and blue