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Harvesting the sun more efficiently

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Thin-film photovoltaic (PV) technologies are gaining ground despite their lower efficiency compared to silicon wafer-based technologies. This trend is mostly economy-driven and based on key advantages inherent to the technology.

The advantages of thin-film PV technologies are obvious: The production process for thin-film PV cells can be scaled up easily to streamlined, high-volume manufacturing, and the amount of absorber material needed (1-3 μm thick layer) is much lower than for crystalline silicon cells. This leads to dramatically lower fabrication costs per Watt peak power in high-volume production. Thin-film PV cells show a greater efficiency in diffuse weather conditions and high temperatures than other materials. The enormous potential of the thin-film PV market is best illustrated by the projected annual growth rate between 2009 and 2020, which is at a spectacular 24 %.1

Figure 1 Cross-section through an interconnect with the three scribes P1-P3 made between the three main process steps. The P1 scribe is applied on the molybdenum-coated sheet glass substrate. Then the CIGS layer is grown on top followed by the P2 scribe which removes the CIGS and exposes the moly back contact. In the third step, the front contact – a transparent conductive oxide (TCO) layer – is deposited and patterned in the P3 scribing process. Together, the three scribes form an electrical back-to-front contact as indicated by the dashed arrow. Graphic: A. Burn

Innovation in the thin-film solar industry
Despite these advantages, cost reduction remains a major challenge for companies trying to make the technology profitable. Let’s take the thin-film solar industry as a case in point. Continuous developments need to address ways of producing more efficient solar modules at lower cost with less energy. Besides the obvious – the optimization of the absorber material – there is also an enormous potential for improvement in module patterning, i.e. the scribing of thin films between production steps in order to build electrical interconnects (see «why interconnects are necessary»). Solneva Swiss Solar Tools SA in Aarberg is a young and highly innovative company, which is entering the global market for industrial laser scribing machines. Solneva's unique concept rests on their core competencies in machine and laser integration, controller development, and application know-how. Their machines are fast, reliable, energy efficient and have a small footprint. In the scribing process, the thin films are removed selectively along narrow lines on the panel. Three scribes per sub-cell strip are necessary and their arrangement allows a back-to-front electrical connection (see Figure 1). One industrial-sized solar module can contain hundreds of scribe meters. Therefore, it is necessary to develop highly reliable scribing processes. The zone between the three scribes is a non-productive area, also termed «deadzone », which has to be kept as small as possible. Laser scribing addresses these problems and is rapidly emerging as one of the most significant processes for photovoltaic elements production. It enables high-volume production of next-generation thin-film devices, surpassing mechanical scribing methods in quality, speed, and reliability.

Figure 2 Electron Micrograph of the scribing region on a functional mini-module. On this mini-module we demonstrated the feasibility of a dead-zone <200 μm. Photo: J. Zürcher

Laser scribing in CIGS solar modules
Solneva SA developed machines and processes for structuring amorphous and micromorph silicon (a-Si, μ-Si) thin-film cells and is now working on a solution for Copper Indium Gallium (di-)Selenide (CIGS) cells. CIGS absorberbased products are the fastest-growing branch in the thin-film family thanks to their favorable properties. But, there is one important drawback: CIGS is a particularly difficult material for laser-structuring and there is no industrial laser-based solution available for module patterning to date. Manufacturers fall back on mechanical needle scribing for the P2 and P3 process. Mechanical scribing produces up to 500 μm wide dead-zones mainly due to chipping of the thin-films, which broadens the scribe substantially. Calculations have shown that module efficiency can be increased by 4 % if the dead-zone width is reduced to below 200 μm. Scientific articles published in the past years gave rise to the assumption that high-quality CIGS laser-ablation could be achieved using picosecond laser pulses.

 

Why are interconnects necessary?
The most important performance determining factor beside the absorber material is the electrical connection of the solar cell. While the back contact can be a thin metal film, the front contact must be transparent to the sunlight. Often, transparent conductive oxides (TCO) layers like aluminumdoped zinc-oxide (Al:ZnO) are used for this purpose. A problem arises when the area of the solar cell is scaled up: the electric current scales with the cell area (L x W) but the TCO cross section scales with the width of the cell. Consequently the current density in the TCO increases and so do the Ohmic losses.

Increasing the TCO thickness is not an option as this increases optical transmission losses. A solution to the problem is the subdivision of the solar cell into strips (sub-cells) that are connected in series.

 

In thin-film solar modules, interconnects are typically formed from three scribes (lines where the films are selectively removed) that are made between thin film deposition process steps in industrial solar module production. An optimization process yields the best compromise between the reduction of Ohmic losses and the total active area loss caused by the non productive «dead zone» at the interconnects. This optimization process obviously profits from a reduction of the dead zone.

Research collaboration with the industry
In 2010, Solneva SA started collaboration with the ALPS institute at Bern University of Applied Sciences to benefit from our expertise in short- and ultrashort pulse material processing. The collaboration is funded by the Commission for Technology and Innovation (CTI). The first results of the study are very promising. We could prove the existence of stable process windows for all three process steps P1-P3 with picosecond laser sources. We further demonstrated that dead-zone widths smaller than 200 μm can be realized on functional mini-modules using these processes for structuring. An electron micrograph of the scribing zone on the finished mini-module is shown in Figure 2. These results have been presented to an expert audience at two international conferences, most recently at SPIE Photonics West in San Francisco. The ultimate goal of the project is to develop and build a working prototype of an industrial all-laser scribing machine that is adapted to the specific needs of the industry.