Hatim Haloui, application laboratory manager, and Joris Van Nunen, product marketing manager, Coherent
Shorter pulse lasers can deliver better products with improved yields, and dependable picosecond lasers provide thin film solar manufacturers with the ideal combination of high average power, extremely high repetition rate and 24/7 reliability.
Thin film is key to lower cost solar power
Solar power based on photovoltaic (PV) technology continues to advance in terms of semiconductor performance, module and system architecture, fabrication methods, and overall device efficiency. The end goal is lower cost, specifically with the target of eventually reaching parity with traditional grid power based on hydro, fossil or nuclear fuels. Until that time, however, efficiency improvements continue to be an important factor for widespread solar adoption.
In terms of technology, panels based on crystalline silicon still represent the largest market share and largely offer the highest efficiency. However, thin film PV offers several attractive features that will allow this technology to compete, namely lower material costs and the promise of deployment on curved or flexible surfaces. Currently, thin film devices are produced on a glass substrate, but there is a clear roadmap pointing to flexible substrates and the potential for low-cost, roll-to-roll production, with some pundits even talking about concepts such as “wrapped buildings”.
Patterning thin films
Thin film solar fabrication details vary between manufacturers, from the choice of semiconductor material to the size of production panels. Initially, silicon was used, but today these devices predominantly utilise cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). However, there are several critical fabrication steps that are common to all three types.
Each panel starts off as a sheet of glass with a typical thickness of 2–3 mm that can be more than a metre in the longest dimension. This is called a glass substrate, since sunlight will enter through this support glass. A multistep process of vapour deposition and scribing creates low-current active strips, typically only 5–10 mm wide, which are electrically connected in series in order to produce high power (typically a few hundred Watts).
The main fabrication steps are shown schematically in figure 1. The first is deposition of a continuous, uniform layer of transparent conductive oxide (TCO), with a typical thickness of a few hundred nanometres, which will form the front electrodes. This is followed by a scribe process called P1, which cuts through the entire layer thickness. The next step is vapour deposition of p- and n-type silicon with a total thickness of 2–3 µm, again followed by a scribing step called P2, which cuts through the semiconductor layer. The final deposition is the thin (sub-micron) metal (Al or Mo) layer, which forms the rear electrodes. These are patterned using a third scribe process called P3.
Figure 1: The fabrication of thin film solar panels involves three separate scribing steps which can all be performed by laser.
Picosecond laser tools improve scribes and process economics
These scribing steps are for particularly demanding micromachining applications requiring high cut quality. Microcracking, delamination or other forms of peripheral damage and debris must be avoided as these can lead to localised electrical shorts. Furthermore, scribe lines should be as narrow as possible to maximise the active area. As already noted, cost is the pre-eminent consideration in fabricating thin film solar panels. Compared with other mass-produced electronic components, such as logic chips and flat panel displays, solar cell value per unit area is quite low, making repair and other rework impractical. The solar industry faces a challenge: it needs processes that deliver the 24/7 reliability and high yield of traditional microelectronics, yet it can only sustain 1/10 of the cost.
While both mechanical and laser scribing are currently employed by various manufacturers, the technology roadmap points firmly in the direction of all-laser scribing. The question is, using what type of laser? In terms of material removal, the P1 scribe is the least challenging as only a few hundred nanometres of TCO are removed and no other layers are involved. Although this scribe is quite demanding on certain laser parameters, it can be performed using conventional techniques with the near-infrared output (1.06 µm) of a nanosecond Q-switched laser such as one from Coherent’s StarFiber series. However, the P2 and P3 scribes must remove a few microns thickness of semiconductor or the overlaying metal film, respectively. The PV industry is now looking at the picosecond laser for these critical processes.
There are two main reasons why several manufacturers are working with picosecond lasers. First, with laser micromachining, their shorter pulse width provides a colder, more precise process, which minimises thermal damage, eliminating the chance of electrical short circuits.
Second, picosecond lasers can operate at the high repetition rates necessary to achieve economically viable throughput, while simultaneously avoiding the high pulse energies that could cause thermal damage. Specifically, the Q-switching used to control nanosecond lasers is limited to a few 100 kHz, whereas picosecond lasers such as Coherent’s HyperRapid NX are capable of mode-locking, a pulsing technique that can easily deliver repetition rates at MHz frequencies. These lasers’ use of low pulse energy limits penetration depth, allowing for exceptional depth control, which is key to avoiding underlying damage and potential electrical shorts. At the same time, they have a high average power (100 W), ensuring advanced beam delivery of the total beam power and therefore the necessary scribe speeds for multiple parallel lines. Furthermore, these lasers are able to provide green and ultraviolet (UV) as well as infrared (IR) outputs to match the specifics of the semiconductor type and focusing/geometric requirements.
The availability of different wavelengths also enables optimisation of spallation, a type of laser lift-off, as an alternative scribing mechanism for both P2 and P3. Conventional ablation requires removal of the entire depth of the target layer/s, which has a somewhat linear dependence on the amount of laser energy deposited at each location. However, spallation offers much more efficient removal with less probability of residual unablated material, as shown in figure 2. These scribes are performed through the glass and the laser wavelength is chosen to be strongly absorbed at the start of the interface of the material to be removed. This vaporises a small amount of material at the film interface, removing the overlaying layers entirely in a microexplosive effect.
Figure 2: In spallation, 1) a laser beam passes through transparent layers, 2) it is focused on the interface with a layer that absorbs the laser wavelength so that rapid heating occurs in a very thin layer, 3) a shock wave expands out, and 4) the target layer is blown off.
Cutting glass modules
Picosecond lasers are also lowering the cost of packaging thin film solar modules, namely cutting the glass modules by a filamentation process called SmartCleave, which is well-established in other glass cutting applications, including cutting of strengthened glass. In simple terms, when a picosecond laser is correctly focused into glass, the high peak power causes an alternating focusing/defocusing effect that creates a stable filament, leaving a narrow microperforation extending over several millimetres in depth through the glass. In order to achieve a continuous cut, these laser-generated filaments are produced close to each other by way of a movement of the work piece with respect to the laser beam.
Filamentation cutting has several advantages for solar modules. First, it can cut tight curves and small holes such as the holes needed for electrical pass-through, as shown in figure 3. Second, it creates smooth edges with a Ra <0.5 µm, so very limited polishing, if any, is required. In addition, it leaves virtually no residual stress in the edge, unlike conventional glass cutting, making thin glass products considerably less vulnerable to breakage during handling, installation and use. Glass cracking nearly always propagates from outer edges due to residual microcracks.
Figure 3: Picosecond lasers enable the SmartCleave filamentation process, which can cut tight curves in glass, including pass-through holes.
Summary
The PV industry is on a relentless drive to rival the cost of grid power. This drive relies on continuous efficiency improvements while simultaneously lowering the manufacturing cost. The picosecond laser, which enables high-throughput precision processing, is well-suited to supporting these goals.
Coherent
www.coherent.com