Extensive research efforts are currently devoted to alternative materials for photovoltaic (PV) applications that can overcome issues of the currently dominating thin film solar cell technologies, namely, such as CdTe and Cu(In, Ga)Se2 (CIGS). In this context, the kesterite compound Cu2ZnSn(S,Se)4 (CZTS) containing earth-abundant and non-toxic elements, as well as an optimal direct bandgap at 1.5 eV, is a promising candidate for PV applications. However, the efficiency of CZTSSe of 12.6% achieved today is still below the ultimate goal of >20% efficiency. In particular, the synthesis of CZTS is rather challenging due to its relatively narrow phase region and good control of the composition is critical for obtaining high performance solar cells. In this paper, we will discuss the synthesis of the quaternary CZTS compound by pulsed laser deposition (PLD). We will present different approaches for the deposition of CZTS: 1) room-temperature deposition followed by post-annealing and 2) growth of CZTS at high temperature. In the former approach, a sulfur evaporation beam assisting the deposition of CZTS is used to compensate for the sulfur loss. Our findings reveal that during ablation of a multicomponent CZTS target, the stoichiometry of the films can vary dramatically from a fluence from 0.2 J/cm2 to 2 J/cm2. In particular, films deposited at a low fluence of 0.2 J/cm2 are Cu-free, and the Cu content in the films increases monotonically with increasing fluence. Interestingly, this effect is less pronounced for ablation of a single-phase CZTS target.
Ice made of ultrapure water or water doped with 1 % polymer (polyethylene glycol, "PEG") was irradiated by laser light
with fluences between 2 and 80 J/cm<sup>2</sup> in the ultraviolet (UV) regime at 355 nm and in the infrared (IR) regime at 1064
nm in vacuum. In the UV regime there is a threshold for plasma formation at 3.5 J/cm<sup>2</sup>, whereas the threshold is at 8.5
J/cm<sup>2</sup> in the IR regime. The ions from the plasma plume were studied by a Langmuir probe. The ion yield was much
higher for UV laser irradiation than for IR laser irradiation. The peak of the time-of-flight spectra comprises ions of
velocity from 60 to 110 km/s. Generally, the ion yield was slightly larger for ice samples doped with PEG than for pure
ones. The threshold behavior was much more pronounced in the IR regime than in the UV regime. These results indicate
that the behavior of the plasma current can be understood in terms of ionization breakdown at the ice surface.
The production of thin films by pulsed laser deposition (PLD) has become a standard method, even though many of the basic processes are not known in detail. The key quantitites are the ablation rate (yield) and the angular distribution of the ablated particles. The starting point for this study is the comparatively simple case of one-component metals rather than metal oxides which in the past have been comprehensively studied. The ablation rate depends primarily on the thermal properties of the metal, in such a way that a low cohesive energy leads to a high ablation rate. The angular distribution of the ablated atoms is important for the uniformity of the film thickness as well as the deposition rate on a substrate. However, if the ablation takes place in a background gas, the angular distribution of collected ablated atoms becomes comparatively broad. Combined diagnostic measurements of dposition rates and ion time-of-flight (TOF) signals have been used to study the dynamics of a laser albation plume in backgound gases. The angluar distribution and the TOF signals exhibit three separate regimes with increasing pressure, a vacuum-like regime, a transition regime with increasing plume broadening and splitting of the ion signal, and at the highest pressures a diffusion-like regime with a broad angular distribution.
The expansion of a plasma plume in a background gas is a key problem for film deposition and laser ablation studies. Combined diagnostic measurements of deposition rates and ion time-of-flight (TOF) signals have been used to study the dynamics of a laser ablation plume in an oxygen gas. This study is similar to our previous work on an argon background gas and shows essentially the same trend. At an enhanced gas pressure, the angular distribution of collected ablated atoms becomes comparatively broad, while the total collected yield decreases strongly. The total collected yield exhibits three separate regimes with increasing pressure, a vacuum-like regime, a transition regime with increasing plume broadening and splitting of the ion signal, and at the highest pressures a diffusion-like regime with a broad angular distribution. In the high pressure regime, the expansion can be described by a simple model based on diffusion from a confined plume.
It is possible to quantify cutting of plant stems by a laser beam in contrast to mechanical cutting. The biomass of the plants after a certain period under standard green house conditions was used to measure the effect of partial or complete cutting with a laser. Continuous laser irradiation at 10.6 micrometers of the plant stem turned out to be very efficient at values of the energy per width unit above 6 J/mm. The effect of laser irradiation at 355 nm or 1064 nm is less pronounced, but also at these wavelengths the re- growth or continuous growth are reduced. A monocotyledon- type, winter what (Triticum vulgare L), is substantially more resistant than a dicotyledon-type, charlock (Chenopodium album L.) against radiation. The exposure limits for laser light in living plants have been explored as well. The limit in terms of re-growth of the irradiated plants exceeds the MPE (maximum permissible exposure) of human skin by several orders of magnitude. The consequence is that very powerful (unfocused) lasers can be used in any environment without significant impact on living plants.