Heinz P. Huber

Energetics of the primary electron transfer reaction revealed by ultrafast spectroscopy on modified bacterial reaction centers

The modification of reaction centers from Rhodobacter sphaeroides by the introduction of pheophytins instead of bacteriopheophytins leads to interesting changes in the primary photosynthetic reaction: long-living populations of the excited electronic state of the special pair P* and the bacteriochlorophyll anion B−A show up. The data allow the determination of the energetics in the reaction center. The free energy of the first intermediate P+B−A, where the electron has reached the accessory bacteriochlorophyll BA lies ≈ 450 cm−1 below the initially excited special pair P*.

Ultrafast pump-probe microscopy with high temporal dynamic range

Ultrafast pump-probe microscopy is a common method for time and space resolved imaging of short and ultra-short pulse laser ablation. The temporal delay between the ablating pump pulse and the illuminating probe pulse is tuned either by an optical delay, resulting in several hundred femtoseconds temporal resolution for delay times up to a few ns, or by an electronic delay, resulting in several nanoseconds resolution for longer delay times. In this work we combine both delay types for temporally high resolved observations of complete ablation processes ranging from femtoseconds to microseconds, while ablation is initiated by an ultrafast 660 fs laser pump pulse. For this purpose, we also demonstrate the calibration of the delay time zero point, the synchronization of both probe sources, as well as a method for image quality enhancing. In addition, we present for the first time to our knowledge pump-probe microscopy investigations of the complete substrate side selective ablation process of molybdenum films on glass. The initiation of mechanical film deformation is observed at about 400 ps, continues until approximately 15 ns, whereupon a Mo disk is sheared off free from thermal effects due to a directly induced laser lift-off ablation process.

Laser lift-off initiated by direct induced ablation of different metal thin films with ultra-short laser pulses

Molybdenum thin films on glass substrates play an important role as contact layer for thin film solar cells. They can be ablated by picosecond laser pulses irradiated from the substrate side at low laser fluences of less than 1 J cm−2, while structured trenches remain free from thermal damage and residues. The fluence for that so-called direct induced ablation from the substrate side is in contrast to metal side ablation reduced by approximately one order of magnitude and is far below the thermodynamic limit for heating, melting and evaporating the complete layer. For an extended investigation of the direct induced laser ablation and the underlying mechanism, further thin film materials, chromium, titanium and platinum, with thicknesses between 200 nm and 1 µm were examined. Finally, a simple thermo-dynamical model is able to connect the observed ablation energetics with the mechanical ductility and stress limit of the metal thin films.

TEMPERATURE DEPENDENCE OF THE PRIMARY ELECTRON TRANSFER REACTION IN PIGMENT-MODIFIED BACTERIAL REACTION CENTERS

The initial electron transfer steps in pigment modified reaction centers, where bacteriopheophytin is replaced by plant pheophytin (R26.Phe-a RCs) have been investigated over a wide temperature range by femtosecond time-resolved spectroscopy. The experimental data obtained in the maximum of the bacteriochlorophyll anion band at 1020 nm show the existence of a high and long-lived population of the primary acceptor P+BA− even at 10 K. The data suggest a stepwise electron transfer mechanism with BA as primary acceptor also in the low temperature domain. A detailed data analysis suggests that the pigment modification leads to a situation with almost isoenergetic primary and secondary acceptor levels, approximately 450 cm−1 below P*. A Gaussian distribution (with σ = 400 cm −1) of the ΔG values has to be assumed to account for the strong dispersive character of the kinetics in this sample. Based on these assumptions, a model is presented that reproduces the observed kinetics, heterogeneity and temperature dependence.

Selective structuring of thin-film solar cells by ultrafast laser ablation

Thin film solar cells have shown a big potential to decrease cost of manufacturing for photovoltaic power generation. Despite of all research attempts to optimize materials and efficiency the mass production of thin film solar cells is still employing some mechanical steps of structuring, where thin films with a thickness of approximately 1 μm are selectively separated to be galvanic isolated. Here we show the structuring of CIS (CuInSe2) thin films solar cells by picosecond laser ablation. We show a selective removal of single layers on sample substrates and on real solar cells. We utilized high repetition rate lasers in order to maximize process speed enabling an application of ultrafast laser structuring for mass production.

Picosecond laser structuring of thin film platinum layers covered with tantalum pentoxide isolation

A thin film layer system consisting of platinum (Pt) as conductive layer on a glass substrate and tantalum pentoxide as isolating layer on top of the platinum is attractive for designing biocompatible conductor paths and contact pads for bio sensor chips. For the flexible and rapid patterning of the conductive and the isolating layers, both, the complete removal and the selective ablation of the individual thin films were investigated using ultra-short laser pulses with about 10 ps pulse duration and 1064 nm wavelength at low laser fluences. A platinum film covered with tantalum pentoxide shows a significantly lower ablation threshold than a single Pt film on glass alone when illuminated from the front side. Furthermore, we explored that the tantalum pentoxide film can be removed by glass side illumination from the Pt film, without affecting the Pt film and leaving the Pt film on the glass substrate intact. Those ablation phenomena occur at laser fluences of about 0.2 J/cm2, far below the evaporation limi...

Directly induced ablation of metal thin films by ultrashort laser pulses

Molybdenum films on a glass substrate are ablated from the glass side by picosecond laser pulses at fluences below 1 J/cm2, without damage. Thin films of chromium, titanium and platinum with thicknesses between 200 nm and 1 μm were examined to investigate the underlying ablation mechanisms. For molybdenum an influence of the intermediate buffer layer was observed. Ablation from the glass side clearly has higher ablation efficiency and a better structural quality in contrast to metal side patterning. A model will be presented, in which the ablation characteristics are connected with the mechanical ductility of the metal.

Monolithic interconnection of CIGSSe solar cells by picosecond laser structuring

We report on the selective structuring of CIS (Cu(In,Ga)(S,Se)2) thin film solar cells applying picosecond lasers at 1064 nm. For a monolithic serial interconnection the thin layers are selectively separated by so called laser patterns 1, 2 and 3 (P1, P2 and P3). We demonstrate that the half micron thick molybdenum back electrode can be structured with a P1 process speed of more than 4 m/s without detectable residues and damages by direct induced laser ablation from the back side. A CIS layer (~2 μm thickness) is structured by standard direct laser ablation at higher energy densities and a process speed up to 200 mm/s. A 1.5 μm thick ZnO front electrode layer can be line separated with P3 speed up to several 1000 mm/s by indirect induced laser ablation. We demonstrate that direct induced (P1) and indirect induced (P3) picosecond laser ablation are not purely thermal processes working at energy densities far below the evaporation enthalpy. To increase the scribing speed elliptical and rectangular beam profiles were investigated. Validation of the processes for functionality within a CIS solar cell will be presented.

Selective removal of transparent conductive oxide layers with ultrashort laser pulses: Front- vs. back-side ablation

High average power, high repetition rate ultrashort pulse (pulse duration < 10 ps) laser systems with µJ pulse energies [1, 2] are increasingly used for bio-medical and material processing applications. Interesting applications for the ultrashort pulse laser systems are in the field of selective structuring of thin-films. Thin-film solar cells have shown a big potential to decrease cost of manufacturing for photovoltaic generation. Despite many research attempts to optimize materials the mass production of thin-film solar cells is still looking for versatile tools for the structuring of the thin-film coated area, where thin films with a thickness of ca. 1 µm have to be line structured with galvanic separation without damaging the substrate or any other layers.In this paper we report on recent results on the selective ablation of transparent conductive oxide (TCO) thin film, i.e. Boron-doped ZnO. The multi-pulse process thresholds were determined for direct and induced (lift off) laser-processing. From comparison of the process thresholds for both methods we can conclude that the induced ablation is more suitable for the structuring of the TCO layer. On the other hand, structuring of the TCO layers by induced ablation is more complicated due to its strong dependence on the quality of the glass substrate.High average power, high repetition rate ultrashort pulse (pulse duration < 10 ps) laser systems with µJ pulse energies [1, 2] are increasingly used for bio-medical and material processing applications. Interesting applications for the ultrashort pulse laser systems are in the field of selective structuring of thin-films. Thin-film solar cells have shown a big potential to decrease cost of manufacturing for photovoltaic generation. Despite many research attempts to optimize materials the mass production of thin-film solar cells is still looking for versatile tools for the structuring of the thin-film coated area, where thin films with a thickness of ca. 1 µm have to be line structured with galvanic separation without damaging the substrate or any other layers.In this paper we report on recent results on the selective ablation of transparent conductive oxide (TCO) thin film, i.e. Boron-doped ZnO. The multi-pulse process thresholds were determined for direct and induced (lift off) laser-processing. From com...

Transient temperature modeling and shock wave observation in confined laser ablation of thin molybdenum films

The transient behavior of the laser lift-off of thin molybdenum films, initiated by glass substrate side irradiation with a 660 fs laser pulse, is investigated in the picosecond range. For this purpose, a pump-probe microscopy setup is utilized to measure the transient relative reflectivity change in the center of the irradiated spot at the molybdenum/glass interface, which enables an interferometric observation of the shock wave propagation in the glass. In addition, a transient simulation of the electron and lattice temperature was performed. The results suggest that ultrafast heating initiates a shock wave in the molybdenum and the glass when the laser pulse has reached maximum intensity. At 10 ps, a confined phase explosion adds further momentum, and the Mo layer is caused to bulge.