Thin-film photovoltaic panels consist of individual solar cells which are monolithically interconnected in series. Today, these connections are commonly realized by mechanical methods. In order to increase the solar output, it is one approach to minimize the interconnection area (so called “dead area”). In this regard, recent advances in laser patterning are gaining increasing potential. However, especially high-impedance trenches realized via laser scribing generally suffer from insufficient shunt resistances. This is especially the case for the third structuring stage P3 of CIGS solar modules, which represents the isolation of nearby cells.
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.
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.
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 well tested and accepted ISO standard 11146 provides the
measurement procedures 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 11146-2 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 general astigmatic beams which are generated by propagating a simple astigmatic beam through a cylindrical lens which is tilted with respect to the symmetry axis of the beam. These generated general astigmatic beams are characterized according to the new ISO standard ISO/FDIS 11146-2. According to this standard the twist parameter is measured by acquiring two power density distributions in the focal plane of a cylindrical lens which is orientated in horizontally and vertically focusing direction. The twist parameter is given by the difference of these two mixed spatial moments divided by the focal length of the cylindrical lens. We present a possible improvement of this measurement technique by measuring in several planes behind the cylindrical lens. The measured data of the general astigmatic beams and their derived beam properties are presented with a detailed error analysis of the data.
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.
At LBOC6 meeting we presented an alternative approach for laser beam characterization, based on the decomposition of the electrical field distribution at certain cross section of the laser beam into a system of orthogonal functions. As such orthogonal function systems we selected "natural" laser eigenmodes of either GL or GH type. The looked for strength of the individual modal components then can be achieved by measuring the output signal of multi-channel correlation filters placed in a Fourier set-up, whereas the correlation filters themselves have been realized as DOEs by laser lithography.
In between different systems of such GL and GH correlation filters have been designed, manufactured and experimentally tested with miscellaneous laser beams. Achieved results demonstrate a very good conformity between optical experiment and computer simulation. First attempts to compare results of our method with results of "standard" beam characterization methods (ISO11146) indicated principal conformity, but illustrated the continuing demand for a sophisticated adjustment procedure for the filter during application.
At LBOC6 meeting we presented an alternative approach for laser beam characterization, based on the decomposition of the electrical field distribution at certain cross section of the laser beam into a system of orthogonal functions. As such orthogonal function systems we selected "natural" laser eigenmodes of either GL or GH type. The looked for strength of the individual modal components then can be achieved by measuring the output signal of multi-channel correlation filters placed in a Fourier set-up, whereas the correlation filters themselves have been realized as DOEs by laser lithography.
In between different systems of such GL and GH correlation filters have been designed, manufactured and experimentally tested with miscellaneous laser beams. Achieved results demonstrate a very good conformity between optical experiment and computer simulation. First attempts to compare results of our method with results of “standard” beam characterization methods (ISO11146) indicated principal conformity, but illustrated the continuing demand for a sophisticated adjustment procedure for the filter during application.
Within the EUREKA-project EU 2359 (Instruments and Standard Test Procedures for Laser Beam and Optics Characterization) a "Round-Robin"-experiment on reflectivity measurements at the wavelength λ = 1.06 μm has been carried out. The topic of this "Round-Robin"-experiment was the evaluation of the Working Draft ISO/WD 13697 and Committee Draft ISO/CD 15368 and the comparison to simple transmission and reflectivity measurements.
For the determination of transmittance and reflectance of laser components, commercially available spectrophotometers are used in industry to monitor the quality at a medium accuracy level. If a higher level of precision is required, especially to resolve the reflectivity of high-reflecting dielectric mirrors, the measurement set-up described in ISO/WD 13697 should be preferred. This standard procedure was elaborated in the framework of the CHOCLAB project.
This paper analyzes the results of the "Round-Robin"-experiment; the accuracy and comparability of measurements according to different methods will be given.
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