The optimization of laser scribing for the interconnection of CIGS solar cells is a current focus of laser process development. In addition to the geometry of the laser scribes the impact of the laser patterning to the electrical properties of the solar cells has to be optimized with regards to the scribing process and the laser sources. In-process measurements provide an approach for reliable evaluation of the electrical characteristics. In particular, the parallel resistanceRp that was calculated from the measured I–V curves was measured in dependence on the scribing parameters of a short-pulsed ns laser in comparison to a standard ps laser at a wavelength of 1.06 μm. With low pulse overlap of ∼20% a reduction of Rp to 2/3 of the initial value has been achieved for ns laser pulses. In comparison to ps laser slightly more defects were observed at the investigated parameter range.

Laser technology is of increasing interest for the improvement of photovoltaic generators also for mass production. Copper-indium-gallium-diselenide (CIGS) thin film solar cells on flexible polyimide (PI) foil have great potential for specific applications requiring mechanical flexibility. To reduce the dead area of solar modules especially that of the serial interconnection shingling of the solar cells is one encouraging approach. The former developed back side opening process is utilized for via preparation to realize the serial interconnection at the rear side of the solar cells. Arrays of square vias that were fabricated into the polyimide foil by UV laser ablation were filled with a silver-based conductive adhesive for realizing the electrical and mechanical interconnection. Contact resistance of the silver-based adhesive to the solar cell back contact of less than 0.2 Omega mm(2) has been measured. Shingled CIGS modules with a size of 100 cm(2) having an open circuit voltage of 2.3V and a short circuit current of more than 500mA demonstrate the great potential of this interconnection approach for flexible electronic applications. (C) 2013 Elsevier B.V. All rights reserved.

Laser scribing of functional thin-film stacks attracts increasing attention especially for applications of flexible electronics or photovoltaics. Laser can perform selective removal of the thin-film stacks that is essential for the isolation and interconnection of the solar cells. The optimization of the laser scribing process concerning the functional properties of the device requires customized characterization techniques minimizing side effects. The proposed and demonstrated nested circular laser scribing technique allows the in-process measurement of the electrical characteristics, e.g., the shunt formation due to laser scribing of the thin-film stack, minimizing secondary effects originating from aging, contacting, changing of sample characteristics, or alterations of the measurement conditions. This technique enables the identification of reliable and quick information on the changes of the solar cell characteristics by laser scribing as this is demonstrated in this work.

Long term stability is crucial to maturing any photovoltaic technology. We have studied the influence of sodium, which plays a key role in optimizing the performance of Cu(In,Ga)Se2 (CIGSe) solar cells, on the long-term stability of flexible CIGSe solar cells on polyimide foil. The standardized procedure of damp heat exposure (85% relative humidity at 85 °C) was used to simulate aging of the unencapsulated cells in multiple time steps while they were characterized by current-voltage analysis, capacitance-voltage profiling, as well as electroluminescence imaging. By comparing the aging process to cells that were exposed to heat only, it could be confirmed that moisture plays the key role in the degradation process. We found that cells with higher sodium content suffer from a more pronounced degradation. Furthermore, the experimental results indicate the superposition of an enhancing and a deteriorating mechanism during the aging process. We propose an explanation based on the corrosion of the planar contacts of the solar cell.

Laser scribing of thin film solar cells attracts increasing attention for performing integrated interconnection. For high efficiency solar modules, low damage laser scribing techniques are needed. The optimization of the laser scribing process concerns all functional properties of the device but in particular the efficiency. The recently demonstrated nested circular scribing technique allows the in-process measurement of the shunt formation due to laser scribing (P3) of the thin film solar cell. By using this technique, the influence of laser fluence and pulse overlap on the electrical shunt resistance formation at laser scribing of Cu(InGa)Se-2 solar cell with ultrashort laser pulses (tp = 10 ps, lambda = 1.06 mu m, v = 2000 mm/s) was investigated. The TCO layer is removed when the laser fluence is about 0.3 J/cm(2). The Cu(InGa)Se-2 is removed completely to expose molybdenum when the laser fluence is about 3.5 J/cm(2). For low defect scribing, a clear tendency of using a low pulse overlap and low laser fluences were found. (C) 2013 Elsevier B.V. All rights reserved.

This German joint project is directed towards the development of a flexible Cu(In, Ga)Se-2 (CIGSe) thin film solar cell technology on a polyimide (PI) substrate for space applications. A group of partners with an academic and/or industrial background in the field of chalcopyrite based thin film solar cells work together to ingrain space technology in production facilities for terrestrial PV. The three production technologies batch type multi-stage co-evaporation, in-line co-evaporation and roll-to-roll co-evaporation are investigated. So far, a maximum total area solar cell efficiency of 15.5 % has been achieved for lab scale devices (0.5 cm(2), AM 1.5, no AR). On a large area, standardized device an active area efficiency of 12.7 % has been achieved (25.9 cm(2), AM 1.5, no AR) based on an industrial roll-to-roll production process.

For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside. (C) 2009 Elsevier B. V. All rights reserved.

The presented joint project is directed towards the development and verification of a flexible, lightweight and highly efficient Cu(In,Ga)Se2 (CIGSe) thin film solar cell technology on a polyimide foil substrate for space applications. The German activities to harmonize the present resources are a unique approach in the field of chalcopyrite based thin film solar cells in order to enable a beneficial course of the project through the integration of space and terrestrial solar cell research activities. In addition to the project structure and objective, recent results of the back contact evaluation, the standardization of the thin film solar cell layout and CIGSe solar cell manufacturing on flexible polyimide foil are presented.