The successful implementation of two new process steps into an existing Cu(In,Ga)(Se,S)(2) (CIS) production line was achieved. One, a newly developed back contact, aims for a better process control, as far as the transition of the metallic back contact to a selenide/metal bi-layer during CIS-formation is concerned. This was done by the introduction of a corrosion resistant barrier layer, which reliably stops chalcogenide diffusion from the top. By doing so, a back contact layer is obtained, with well defined properties in which the functionalities of the back electrode now is divided between two separated layers. The other development presented in this paper, tackles the complexity of CIS-module production and the interferences between the different processes required. By shifting the P1-scribing process after i-ZnO deposition, the process sequence for CIS is simplified and it will be shown that this new P1i exhibits superior properties as far as CIS morphology and groove quality is concerned. (C) 2015 The Japan Society of Applied Physics

In general, laser structuring of CIGS solar modules causes material modifications in the vicinity of the processed area. The irradiated region often reveals high densities of defect states or melt that affect the electrical properties dramatically. In particular, this is a problem for laser P3 scribing where trenches should isolate the front from the back electrode. In this paper, we demonstrate optimized P3 scribes using ultrashort-pulse laser processing in combination with thermal annealing yielding permanently increased solar cell performance. We further investigate this behavior with the help of a two-diode equivalent circuit model. The simulations correlate very well to our obtained experimental results. We show that basically two main effects are responsible for the performance increase after thermal annealing. Firstly, the recombination current within the p–n junction decreases over orders of magnitudes, and secondly, the overall shunt resistance rises significantly.

A thin-film photovoltaic module is made up of individual solar cells, which are monolithically connected in series via micro scribing. Depending on the semiconductor material used for electric energy production, the connection today can be made by lasers or mechanical tools. Of all thin-film technologies, it just happens to be CIGS technology with its high potential in terms of efficiency which is nowadays wasting power. This is because two of three structures are scribed with a mechanical tool thus loosing valuable module surface. This article presents first results of a completely laser-connected CIGS module from the BMBF-funded joint research program T4nPV as well as fundamental investigations of laser-based scribing processes.

In this study we present a new measurement technique to investigate the timescales of back side ablation of conductive films, using Molybdenum as an application example from photovoltaics. With ultrashort laser pulses at fluences below 0.6 J/cm(2), we ablate the Mo film in the shape of a fully intact Mo 'disc' from a transparent substrate. By monitoring the time-dependent current flow across a specifically developed test structure, we determine the time required for the lift-off of the disc. This value decreases with increasing laser fluence down to a minimum of 21 ± 2 ns. Furthermore, we record trajectories of the discs using a shadowgraphic setup. Ablated discs escape with a maximum velocity of 150 ± 5 m/s whereas droplets of Mo forming at the center of the disc can reach velocities up to 710 ± 11 m/s.

The CIGS (Copper Indium Gallium Selenide) solar panel industry is cautiously moving to adopt laser processes for the P2 and the P3 scribe steps that form the electrical interconnection between cells within a module [ 1]. In this work we study variants of these two laser processes and evaluate their relative performance. P2 scribes are applied with geometries that range from continuous scribes to discrete spots and we examine the relationship between scribe geometry and P2 contact resistance. Transmission line theory [ 2] is used to calculate P2 contact resistance as is common in the industry. The results are compared with two simple geometric models that predict relative contact resistance for different scribe geometries. We also apply different types of scribes for both P2 and P3 in the production of minimodules and evaluate the results. We find that not only is the optimal geometry for the P2 scribe a continuous line, high overlap of the laser spots yields an improvement in contact resistance not predicted by geometry alone. Finally we find that removing only the TCO (transparent conductive oxide) layer for the P3 scribe results in modules with good efficiency, however a P3 scribe that removes the TCO and CIGS layer yields better modules with about 1% higher absolute efficiency.

Ultrashort laser pulses are used to ablate a thin molybdenum layer from glass by irradiating the metal film through the transparent substrate. The trajectories of ablated molybdenum fragments are recorded using a shadowgraphic setup with a time resolution in the nanosecond range. In addition, the shape of collected molybdenum fragments is examined as a function of applied fluence. It is confirmed that in a fluence regime close to the ablation threshold one single disc is ablated as a whole and its velocity is determined in the order of 50 ms−1. In a second fluence regime, partial melting at the center of the disc is found and small melt droplets are recorded on their flight. Mo fragments ablated in this regime feature a ring-like structure with a brittle fracture at the outer and a molten appearance at the inner edge.

We present a novel laser beam measurement setup which allows the determination of the beam diameter for each single pulse of a pulsed laser beam at repetition rates of up to 200 kHz. This is useful for online process-parameter control e. g. in micromachining or for laser source characterization. Basically, the developed instrument combines spatial transmission filters specially designed for instantaneous optical determination of the second order moments of the lateral intensity distribution of the light beam and photodiodes coupled to customized electronics. The acquisition is computer-based, enabling real-time operation for online monitoring or control. It also allows data storage for a later analysis and visualization of the measurement results. The single-pulse resolved beam diameter can be measured and recorded without any interruption for an unlimited number of pulses. It is only limited by the capacity of the data storage means. In our setup a standard PC and hard-disk provided 2 hours uninterrupted operation and recording of varying beam diameters at 200 kHz. This is about three orders of magnitude faster than other systems. To calibrate our device we performed experiments in cw and pulsed regimes and the obtained results were compared to those obtained with a commercial camera based system. Only minor deviations of the beam diameter values between the two instruments were observed, proving the reliability of our approach.

The photovoltaic (PV) industry requires higher efficiencies at lower manufacturing costs to become competitive with other power generation techniques. There are several approaches to increase the efficiency of solar cells. For example enhancements of the way photons are absorbed and how they generate charge carriers with low losses. Today, the so called first generation of photovoltaic devices based on crystalline silicon wafers are produced on a multi-GW-level. However, in most production lines there is only one laser process used to electrically isolate front and rear side of the cell. Lasers are predestined to generate local structures which will be required to manufacture high efficient solar cells. As an example we will show results on the interaction of ultra short laser pulses with dielectric films on silicon. Second generation photovoltaic modules are based on thin films. These modules are monolithically interconnected by laser scribing of the films. Tools for amorphous silicon are well established, while there are a lot of challenges to scribe CIGS layers. Within this paper we will show new results on the temporal evolution of a laser induced "lift-off" process to scribe the molybdenum back electrode.

The well tested and accepted ISO standard 111461 provides the measurement procedure to characterize the propagation properties of stigmatic and simple astigmatic laser beams which are intrinsically symmetric. The beam diameters are defined by the second order moments of the power density distribution which can be measured e.g. with a CCD-camera. In this standard the second order moments are used since the knowledge of these second order moments allows the calculation of the beam properties behind aberration-free optical systems with the well known ABCD-matrices. The new ISO/FDIS 11146-22 provides a new measurement procedure to characterize general astigmatic beams which are characterized by ten independent second order moments of their Wigner distribution. We present experimental results of the characterization of a general astigmatic beam and compare these results with theoretically calculated values. In this experiment a well characterized simple astigmatic beam is propagated through a cylindrical lens which is tilted with respect to the symmetry axis of the beam so that the simple astigmatic beam is transformed into a general astigmatic beam. This general astigmatic beam is characterized according to the new ISO standard. The measured second order moments are in good agreement to the theoretically calculated beam properties.

The temporal evolution of the sawtooth crash in ASDEX Upgrade Ohmic and ECRH heated discharges is analysed using a heuristic reconnection model to simulate measured ECE signals. The method turns out to be well suited to detect islands of width above 0.1-0.2 times the mixing radius; for smaller island widths, we cannot distinguish between the resistive and the ideal mode. Thus, the nature of the precursor oscillation cannot be fully resolved. In contrast, we find that during the crash phase, reconnection is incomplete and a large island persists that fully reconnects on a slower time-scale after the crash. The observations are consistent with a recent sawtooth model.