Assessment of the ps-lasers for CIGS solar cell scribing / Validation of high-throughput thin-film scribing processes

Integration of new lasers, polygon scanner with on-line monitoring tools into laser system for thin film scribing assessment and validation with the integrated system of the scribing performance in CIGS solar cells was performed in WP2 at FTMC.
ELAS has finished upgrading its DuoMaster laser processing system by changing the main optical path. They finalised installation of its embedded Roll-to-Roll unit to the DuoMaster laser system at FTMC for validation of the laser scribing processes of CIGS solar cells on flexible polymer foils.
The used web width for PET was 150 mm, for Kapton – 250 mm. Flat working field for laser processing on the web is 300x300mm. The web could be moved into both directions: forth and back. Adjustable film tension and constant speed can be maintained during the laser process and in the stop mode. The incremental encoder is linked to the roll which is the nearest to the laser working field. The additional output of the encoder can be used for laser pulse synchronisation and beam position correction. 
The on-line monitoring tools from LUT and AMSYS were validated in laser scribing experiments at FTMC enabling control of the process with a scanning speed of above 1 m/s and recognise defects in the ablation trench less than 50 µm at the scan speed of 50 m/s through optics of the polygon scanner LSE170.
The laser scribing of the CIGS samples without front contact (P2) was performed using pulse energy of 12.2 µJ and varying the number of scans per scribe. The best P2 scribe was got when 50 scans were applied. The average reflected the signal from such scribe was about 1 V. When the number of scans was increased further, the damage of polyimide layer took place and the signal drop was observed. These results show that it is possible to monitor online how the P2 scribe is being produced at the speed corresponding to the real processing conditions.
Fundamental harmonics of the 1.34 µm picosecond laser from Ekspla was utilised in the P2 and P3 processes in CIGS and CZTSe thin-film solar cells. The investigation was mainly focused on the P2 laser welding. Additionally, the laser-induced P3 lift-off process by exposing a molybdenum layer was investigated in thin-film CIGS solar cells deposited on flexible stainless steel substrates. In the case of CZTSe absorber, the P2 results were very promising. At the optimal laser processing conditions, a significant increase in the interconnect conductivity was achieved. By reducing scanning speed and increasing laser irradiation dose resulted in the conductivity increase to up to 3.83 S/m. Further increase of irradiation dose could lead to further increase of the P2 interconnect conductivity.

Combining of high-performance laser scribing with on-line monitoring tools makes it feasible optimisation of new laser processes development.


Validation of high-throughput thin-film scribing processes

In APPOLO WP2, the assessment of optimized high-throughput scribing processes for the industrial patterning of CIGS solar cells and suitable tailored laser systems are in the focus. A strong cluster of laser provider (Onefive), end-user of scribing processes (Flisom), R&D sample and thin-film knowledge provider (EMPA) and APPOLO application lab (BUAS) was formed. The common goal is the elaboration of the assessment experiments and standards for laser-scribing processes and laser equipment. The phenomenon of heat accumulation occurs also for ultrashort pulses and depends mainly on the time available for thermal relaxation between two consecutive pulses. Based on this property, the scaling approach followed at the BUAS application lab seeks to maximize time spans available for said thermal relaxation by increasing the area processed per pulse.

By increasing the spot size and the laser pulse energy the area processed per pulse can be increased using only one beam path. If the focal spot size is increased only in direction of scribing, a linear focus results. An additional optical Gaussian to top hat beam profile converter is used which produces a homogenous intensity distribution in direction of the line.

The lab scale scribing machine available at BUAS application lab is a highly versatile setup consisting of an industrial grade direct drive motion system and an optics platform. The latter provides a 4.2 x 0.6 m2 optical experimentation area with laminar flow boxes installed for dust protection. The machine has been significantly expanded. The vision feature allows automated mapping of a large sample area and automated defect recognition.

Several types of validation experiments were made using APPOLO laser sources and reference lasers. High throughput scribing processes developed in the frame of APPOLO were validated against the BUAS reference processes. All validation scribing experiments shown here were made on samples produced at EMPA. The layer stack Mo/Cu(In,Ga)Se2/CdS/ZnO was grown on float glass substrates of 50 x 50 mm2 size. All presented scribing processes lead to very good results, complete or nearly complete exposure of the back contact, regular and smooth scribe borders and no cracks in the CIGS outside the processed area. Most important, there is no damage and no thinning of the back contact layer observed for the high-throughput processes. Electrical performance of modules, processed using the optimized APPOLO high throughput P2 scribing process at 1720 mm/s and 325 mm/s, was comparable to the performance of the reference sample processed with a round focal spot at 90 mm/s. All three have shown conversion efficiency around 14 percent after annealing and without anti reflection coating.

In the following, the final phase of the APPOLO assessment is the high through-put scribing processes and Onefive optimized laser sources. A series of assessment experiments are conducted at the facilities of industrial end-user Flisom. A tight collaboration between Flisom, BUAS, EMPA and Onefive makes it possible to assess the optimized scribing laser source in combination with optimized high-throughput scribing processes on industrially produced CIGS grown on flexible substrate at Flisom. Several series of functional modules on flexible substrate are produced and compared directly with Flisom’s own reference modules.