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Advanced laser ablation barrier films for organic and large area electronic devices

Periodic Reporting for period 2 - ALABO (Advanced laser ablation barrier films for organic and large area electronic devices)

Reporting period: 2016-01-01 to 2016-12-31

The overall approach of the project is developing organic electronic building elements on flexible substrates with monolithically integrated barrier foils as substrate. The barrier acts as inevitable protection against atmospheric gases as water vapor and oxygen as the most crucial agents for unwanted material degradation processes. This topic is one of the keys for enhancing both performance of TOLAE components and addresses some of the main technology barriers of TOLAE: lifetime and cost-performance ratio.

Current state of the art in flexible OPV is the encapsulation in between additional ultra-barrier foils to achieve relevant lifetimes. So far, these layers are delivered as coatings on dedicated polymer foils, such as PET. There is a small number of suppliers and low production volumes. As a result, the ultra-barriers are among the main cost drivers of OPV technology. By using two different kinds of foils, one for the substrate and the transparent electrode and the other one coated with an ultra-barrier film, there is no interference between laser structuring on the barrier, since the barrier foil is attached after all OPV deposition and scribing steps.
Motivation of the ALABO project is the simplification of devices by reducing the number of production steps and needed materials. The ultimate goal will be the replacement of the four polymer layer system of OPV devices by just two or in wider future even only by one polymer foil.

As reference system for such an integrated device organic photovoltaic modules have been chosen as test objects for a scalable and general approach suitable also for other TOLAE devices. In case of the Heliatek produced solar modules a number of laser processes is used to realize the interconnection of small solar cells to a full device in line. The ALABO project will develop laser processes for the interconnection without damaging the barrier and suitable adaption of the barrier system itself to higher the process ability. Further research will be done on the inline controlling of these laser processes by roll to roll processing.

Inside ALABO three different barrier measurement methods, namely Ca-testing, He-transmission rate measurement and Water Vapor Transmission Rate measurement (WVTR) have been and are characterized regarding their ability to detect a possible reduction of the barrier performance due to local defects. As of now, the performance and capability of these methods have been proven valuable for ultra-barrier characterization.
Because the ALABO project handles with a number of process development steps to realize electrical interconnection without damaging the barrier, groundwork of the research project was the properties definition of the barrier as well as of the organic stack. As barrier system a development from Holst Centre Eindhoven is used in the project which is covered by a transparent front contact. The development of the laser structuring of this layer (so called P1 scribing) was and is also done by Holst Center. As TCO layer the focus is on DMD as well established electrode system with ITO and AgNW as backup. On top of this, Heliatek applies his organic stack and the back electrode to build up the solar device. This is done in two ways: firstly as a structured shadow mask process to evaluate the possible performance of the stack system and secondly as full area deposition to characterize the electrical performance of the single laser process steps.

The laser ablation of the organic stack (P2) and the back electrode (P3) is investigated by Fraunhofer IWS. For this a mount of pulsed laser systems with pulse durations from ns down to fs was and is tested and characterized regarding their ability to structure without barrier influence. During the reporting period most trustable settings have been gathered, a suggestion for an industrial suitable laser setup and processing strategy was worked out.

For evaluating the possibly caused damage to the barrier layer three different measurement methods are used inside ALABO. Objectives are evaluation of the performance of the laser process itself and calibration of these three barrier performance evaluation methods against each other. The first method is the Ca-test from Holst Centre which opens the opportunity of localized defect detection. The method has been proven a very valuable tool. This method gets compete with the WVTR measurement from Fraunhofer IWS and the HeTR measurement from CEA in France. During the reporting period the barrier performance was tested with the named measurement systems and confirmed by all three methods.

The key development of suitable laser processes without damaging the barrier is accompanied by the investigation of possible in-line process monitoring through laser induced breakdown spectroscopy (LIBS) by analyzing the emission spectra of the laser ablation. This work was and is done by CNRS. During the second year a workflow was worked out to identify the laser affected layer by sensing their respective characteristic spectral lines.

An important issue is the demonstration of the up-scale-ability of the developed P1, P2 and P3 processes in a R2R machine with an addressed web width of up to 1200 mm. Further research was performed to improve the recognition of scribes and the accuracy of scribe positioning. This was and is done by the companies 3D-Micromac and Sorter. In the second year, significant work was performed on pattern recognition under difficult lighting conditions. A sophisticated solution was found and patented.
LKTM-design sample cells during P2 laser processing in vacuum chamber