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Solar cell makers must improve cell efficiency for more power/sq.m. of surface area and increase productivity without correspondingly higher investment. The science of fine-pitch screen printing deployed in SMT assembly is translating to metallization advances. Next-gen solar cell manufacturing challenges are not unfamiliar to those cognizant of parallel sectors, like electronics manufacturing.
BY Darren Brown, DEK International
While many factors impact on manufacturing productivity and cost effectiveness on the road to a completed solar cell assembly, the metallization process arguably offers the most scope to deliver improvements that manufactures seek. Metallization of silicon wafer substrates used in crystalline silicon (cSi) solar cells is a critical connectivity process. It is imperative to collect the electrical current produced by the cell and also directly affects the cell’s energy exchange efficiency. Screen printing is the optimum process to deliver metallization; it is the process of choice for progressive solar cell producers (Figure 1).
Screen printing is a precision science deployed in a myriad of innovative industrial applications from product labels to embedded passive electronic components and conductive inks. The latter is closely related to the production of cSi solar cells. Crystalline silicon solar cell production has been around for decades. In the distant, and in some cases not-so-distant, past of this now-burgeoning sector, the challenges for screen printing consisted mostly of producing fine conductors accurately placed and doing this repeatedly with a negligible level of variance. There also were issues of material characteristics to consider, like thixotropic properties and rheology.
Figure 1. Automated in-line screen printing of crystalline silicon solar cell wafers (c-Si).
Today, the science of screen printing in market sectors closely allied to solar cell processing has driven the accuracy and repeatability capabilities well beyond that normally required to print conductive patterns on doped silicon wafers. Surface mount electronics circuit board assembly, for example, regularly demands ultra-fine-pitch printing of rows of solder paste on 0.3-mm pads at spacings of 0.3 mm or less. Solder paste is a more demanding substance than the chemistries used for screen printing solar cell metallization. Solder paste chemistry originally comprised microscopic tin and lead spheres in a flux suspension and is now predominantly lead-free with elements like copper, silver, and indium included in the formulation. Solder paste is typically printed onto conductive copper pads on bare circuit boards, whereafter an electronic device or component with a surface mount form factor (i.e. conductor leads or legs) is accurately placed on top of the printed pads. The board then travels through an oven where the solder paste reflows to form robust electrical and mechanical connections.
While some developments investigate alternative metallization processes and materials, such as hot melt, the majority of manufacturers have settled on screen printing as the strategy of choice with a very narrow range of materials options comprising silver pastes for front-side fingers and bus bar metallization and aluminum pastes for back-side field coating (Figure 2).
The Manufacturer’s Challenge
Next-generation solar cell manufacturing requirements bring a new set of challenges to bear on every part of the process. These are not unfamiliar challenges to businesses cognizant of parallel industry sectors, like electronics subassembly manufacture.
Key among the needs of progressive solar cell production companies are two distinct metallization demands: improvement in hardware and equipment performance; and a drive to enhance process developments to make the solar cell itself more efficient. The former relates to familiar factors like increased throughput and greater yields – achieved by minimizing breakages and controlling the process so that every wafer off the end of a line is good. The latter demand raises the bar for equipment vendors beyond simply delivering effective production machinery. Equipment designers must get involved with manufacturers at application and process technology levels to develop new techniques that maintain high yields and throughputs and also deliver a better solar cell.
Figure 2. Front side of a polycrystalline wafer showing screen printed conductive fingers and busbars.
Productivity drivers call for throughput exceeding 2,400 wafers per hour (WPH) on each metallization line. Current lines typically deliver 1,200 to 1,400 WPH, so doubling the output might be seen as a tall order, especially when the transport and conveying requirements for the fragile substrates will have to be correspondingly faster without increasing breakages. Manufacturers also want more throughput from less floor space, and better cost of ownership (COO).
A necessary screen printing development is a metallization line that exceeds the throughput target and satisfies the real estate concerns. One technology developed for this purpose has throughput of 3,000 wafers per hour from a line that’s the same length as its 1,200-WPH product, and is 25% wider than the conventional line. This technology deploys multiple print heads operating in parallel, which also brings a positive impact on productivity should there be a stoppage on the metallization line. If a conventional 1,200 WPH line halts, output stops completely. A 5 minute stoppage means 100 fewer wafers are produced, which amounts to a productivity loss of 8.3% for that hour. If a conventional line were able to process 3,000 wafers per hour, an identical stoppage would cost 250 wafers. With the technology devised for 3,000 WPH processing, however, only one print head would cease to function so a 5 minute stoppage would deplete the throughput by about 84 wafers. The other metallization print heads continue to be productive. The net effect in that hour therefore would be an output of 2,916 wafers, representing a more palatable productivity loss of just 2.8%.
Equipment footprint is now a very real issue with manufacturers. It has long been so in other assembly sectors for the same reason: cost of ownership. Maximizing factory floor space utilization contributes to faster return on investment (ROI), and aids manufacturers that want to avoid the costs of building new premises just to increase productivity. These needs are ushering in a new generation of compact metallization solutions designed to be modular and scalable – to grow quickly and easily with escalating market demand – and compact to occupy a minimal footprint.
Materials in Process
The materials side of the metallization process seems to be simplifying. Interest in hot melt has waned, so the spotlight is on a simple set of conductive chemistry formulations. That allows vendors with process expertise to focus on development to improve the efficiency of the solar cell by enhancing the metallization process itself without being forced to accommodate a fragmented materials supply.
A key challenge for metallization process experts is creating enough conductive fingers or patterns on the front side of the wafer to conduct the electrical current from the silicon, without shadowing too much of the wafer surface. Any part of the wafer surface obscured from receiving light – including those parts over-printed with conductive tracks – does not produce current, reducing the maximum possible cell efficiency. Contrarily, printing fewer conductive tracks also reduces the efficiency due to the impedance of the natural surface of the silicon, which is a semiconductor. In this scenario, more current is actually produced at the wafer surface, but less is collected.
The optimum balance can be improved in favor of extra energy-conversion efficiency by printing finer conductive tracks to cover less surface area, but making them vertically thicker to carry the current effectively. The goal is to produce conductors of a higher aspect ratio, typically 50-µm wide but 22-µm high. This is receiving much attention. Other factors come into play, such as the precision of the emulsion screens used in print. In the development mix today are new stencil technologies, potentially comprising multiple layer and electroformed compositions, as well as specialized hybrid screens.
Screen printing equipment vendors with in-house screen and stencil manufacturing operations see three levels of activity in metallization process development:
- benchmarking standard emulsion screens and optimizing the print process with a range of printable material formulations;
- deploying the latest emulsion screen technologies featuring stronger alloys to give a narrower wire profile in the mesh, coupled with new photoimaging techniques;
- and generations of precision hybrid stencils.
With the production of finer conductor tracks, part of the challenge becomes to avoid introducing defects in the printing process. Already narrow conductor lines must be printed perfectly; further narrowing, either in width or depth, will introduce electrical impedance to the current flow, which will reduce efficiency. Of course, a break in the conductor arising from poor printing, however small, will be an open circuit that renders that area of the solar cell surface useless. This challenge also can be addressed borrowing knowledge gained in the solder joint reliability work of SMT assemblers.
The holy grail of productivity for manufacturers is optimization of screens and materials technology against the throughput demands and handling capabilities of the metallization line to avoid breakages and stoppages. This all needs to be kept in a small footprint. Solutions for more effective and efficient solar cells and ultimately solar modules and arrays are in the pipeline. SMT
Darren Brown, alternative energy development manager, DEK International, may be contacted at firstname.lastname@example.org.