Lower Cost, Greater Efficiency Drive Back-contact Solar Module Dev

While the average growth in PV cell production has surpassed 40% over the last ten years, solar still only accounts for a very small percentage (<1%) of the world's electricity generation [1]. And, while adoption of the technology has dramatically increased, much of this has been spurred by government subsidies and investment backing. So, what happens to the industry when it has to be self-supporting? The truth is that PV technology must be cost-competitive - and soon.

This stark reality is, in large part, driving much of today's cost-down initiatives, which include novel cell architectures, lower cost high-volume cell production methodologies, new module assembly techniques, and high reliability materials. Significant advances have been made in all areas of PV production, though there continues to be a fine balance between greater cell efficiency and reduced production costs. Thinner, larger wafers, rear-side designs and highly automated, high-throughput processes are all contributing to PV's cost competitiveness and decreased reliance on financial subsidies. However, while greater than the average 15%-18% crystalline-silicon (c-Si) cell efficiencies have been realized, these have generally been accomplished with more complex and/or higher cost materials and methodologies.

To be sure, c-Si cells still dominate the PV market (comprising ~80% of cell production in 2009, according to Prismark [2]) and will represent a significant percentage of future cell production. As such, the authors will focus here on conversion and cost efficiency progress as it relates to c-Si technology.

Figure 1. Current c-Si cell connection methods use tabbing ribbons to electrically join adjacent cells laterally.

Why alternative cells, methods?

Though alternative cell structures and module assembly techniques are making headway, incorporation of these methodologies into final, installed solar modules has not yet occurred. In fact, the largest percentage of commercially marketed c-Si modules produced in 2010 were manufactured with traditional cell structures that incorporate front side collector fingers/busbars (H-pattern) and rear-side silver or silver-aluminum fired lines. With these cells, the front-side busbar parallels of one cell are connected to the backside conductive material of the adjacent cell via a solder-coated copper tabbing ribbon in what is known as the stringing process. This electrically connects the cells that are joined in a series to form a solar module (Fig.1).

Figure 2. Cross section of a back contact MWT module shows the build up layers [3]. Image courtesy of ECN.

While this method has produced the current 15%−18% on average c-Si cell efficiency (which translates to ~12-15% module efficiency) previously mentioned, this energy conversion rate will not be enough to spur massive adoption, as the production costs still remain relatively high by comparison. Adding more collector fingers to the front side of the cell would, in theory, allow more current collection. The downside is that the additional fingers mean more shading and, therefore, the front side design has to be optimized to balance shading losses with resistive losses [3]. What's more, when the cells are then interconnected via the tabbing ribbons, there are also associated resistive losses that can account for several percentage points of efficiency [3].

Improved efficiency, lower production costs

Cells that reduce cost with thinner wafers and improved efficiency (greater currents), however, pose challenges to the cell connection process - particularly when using traditional solder interconnects. Solder materials and the high temperatures they require for interconnect formation add stresses to the cells that may be catastrophic. The necessary move to thinner wafers/cells will require alternative interconnect materials that can deliver comparable (or better) performance to that of solder. This will be a crucial element of the low cost/high collection capability equation that is imperative if solar power is to become mainstream.

Efficiency and cost drivers

The need to improve efficiency while simultaneously reducing production costs has driven solar specialists to develop new cell designs and module assembly processes that will put grid parity within closer reach. At the c-Si cell level, once such innovation has centered on back-contact cell designs that offer many opportunities for improvements over current designs. The two back contact design types that have emerged as the arguable go-forward technologies are metal wrap-through (MWT) and emitter wrap-through (EWT) cells. With these cell architectures, energy is moved through an individual cell (from front to back) instead of achieving this laterally by connecting the top of one cell to the bottom of the adjacent cell. Back-contact cells and modules can enable greater efficiency through reduced shadowing losses, enlarged effective cell areas and tighter packing densities of individual cells. As compared to traditional c-Si module production methods, costs for back-contact modules may also prove to be lower. The ability to incorporate proven high-volume, low-cost surface-mount technology (SMT) assembly methods, eliminate certain process steps, improve yield and use thinner, larger silicon wafers all contribute to overall cost reductions. Add to this the greater efficiency that is possible with back contact technology and its future is even more promising. As noted in a recent publication by the Energy Research Centre of the Netherlands (ECN), comparisons of traditional H-pattern wafers, cells and modules and certain MWT back-contact designs, the MWT modules have been shown to consistently produce 2% more output current [3].

Making the connection

Not only have back contact cell designs shown improved energy conversion efficiency, but the module assembly process for these types of cells is also exponentially more streamlined compared to current methods. Though there are competing connection methodologies that include back contact sheets or back contact ribbons, the back contact sheet process may provide cost and reliability advantages. In fact, the technique is quite similar to that of SMT processes that are the benchmark for high-volume, low-cost electronics assembly. Here, we describe the back contact process utilizing MWT cells and a back contact sheet technique.

Figure 3. An MWT back contact module assembled using ECAs has shown no significant degradation in efficiency over a period of 3 ½ years [4]. Data courtesy of ECN.

Basic process steps of MWT back-contact module assembly include:

• The patterned back contact sheet is printed with conductive adhesive material using standard printing techniques;
• The encapsulant (EVA) layer is placed on top of the back contact sheet;
• Back contact cells are picked off the stack via an automated placement system and placed so that the contact points on the cell are centered on the conductive adhesive deposits; automation of this process limits cell damage and allows for the use of thinner cells;
• The second and final layer of EVA is placed on top of the entire module;
• Module is laminated.

A cross-section of a finished MWT module can be seen in Fig. 2.

Central to the long-term reliability of the completed module is the interconnect material used to attach the individual cells to the back contact sheet. Though solder interconnects can be and are used, there are several shortcomings: 1) as the silicon wafers become thinner and larger, the brittle nature of solder presents stresses that can cause wafer damage; 2) the higher temperatures required to reflow solder materials - particularly as the industry moves toward lead-free materials, which reflow at 250° to 260°C - may also result in cell damage. Therefore, as module manufacturers evaluate the manufacturing infrastructure required for back contact assembly techniques, the lower modulus and resultant flexibility offered by electrically conductive adhesives (ECAs) are proving advantageous for the long-term.

Not only do ECAs present a more adaptable interconnect, but these materials also enable greater throughput and reduced capital equipment costs as compared to solder-based processes for back-contact module assembly. Because some ECAs can be cured during the module lamination step, there is no requirement for a secondary cure process. With solder, on the other hand, a reflow step is required to form the interconnects. Not only does this add time to the process, but the high temperatures required to reflow the solder can also be detrimental to the thin cell. ECAs have proven to be just as reliable as solder materials and, given their clear throughput, limited capital investment requirement and lower modulus advantage, are likely to emerge as the more robust option going forward.

When evaluating ECAs for back-contact module assembly, there are several materials characteristics that must be considered. First, the adhesive must deliver a highly reliable interconnect that ultimately enables module compliance with IEC standards. The ECA must be able to reliably cope with the stresses induced by -40°C to 85°C thermal cycling tests, as well as show robust performance when subjected to 85°C/85% relative humidity testing. Recent testing results provided by ECN have shown that conductive adhesives can meet these stringent requirements [4]. Processabilty is also key in determining an ECA's viability for back contact techniques. The material's ability to cure within the lamination process is one of its central advantages, so ensuring compatibility with the EVA cure profile both in terms of time (anywhere from 5−20min) and temperature (typically between 140°C and 150°C) is critical. Rheology control and printability are other important considerations when analyzing an adhesive's processability. Module assembly specialists must be able to use a standard printing process to ensure good, consistent adhesive dot height and diameter required to enable robust interconnects.

Materials advances and future requirements

While today's ECAs must deliver good adhesion on silver to silver and/or silver to copper metallizations, there are technology roadmaps that outline a path to even lower-cost modules through the use of alternative metals. Currently, development work is underway on novel aluminum-based connection designs both on the cell backside and for the back contact sheet. As solar technology progresses toward lower-cost metal alternatives, ECA development must follow suit. ECA formulations that comply with these forward-looking, cost-reducing solar modules are already being developed and tested, with very promising early results.

What's clear is that the technology to reduce costs and raise conversion efficiency exists today in the form of back-contact solar modules. And all indications are that current development work on even lower cost materials and production methodologies are within very close reach. Cell developers have done their part; now module manufacturers must decide the production path forward so that the industry can lessen its dependence on government incentives and consumers can more quickly move off the grid.

Acknowledgment

The authors would like to thank ECN for its valuable input for this article.

References

1. T. Saga, "Advances in Crystalline Silicon Solar Cell Technology for Industrial Mass Production," NPG Asia Materials, July 2010.

2. Photovoltaic: Market Update, Prismark Partners, LLC, June 2010.

3. P. de Jong, "Achievements and Challenges in Crystalline Silicon Back-Contact Module Technology," Photovoltaics International, Edition 7.

4. I. Bennett, "Climate Chamber Testing and IEC Certification of Full-size MWT Modules," MWT Solar Cell and Module Technology Workshop, Nov. 2010, Amsterdam, The Netherlands.

Tom Adcock received his BS in chemical engineering from Lehigh U. and is a Marketing Manager at Henkel Electronic Materials, LLC., Assembly Electronics Group, 14000 Jamboree Rd., Irvine, CA 92606 USA; email tom.adcock@us.henkel.com.

Anja Henckens received her PhD in polymer/organic chemistry from the U. of Hasselt, Belgium and is a Development Scientist at Henkel Electronics Materials (Belgium) N. V.