Commercializing Advent Solar’s Emitter Wrap Through (EWT) Technology.
Monolithic Module Assembly with Back Contact Silicon Solar Cells promises higher module efficiency, reliability, and automation all at lower cost and with better aesthetics.

Reporting from PHOTON’s 6th PV Production Equipment Conference (c-Si), “PV production pathways to 61 euro cents per W module costs in 2 years” by Shravan Kumar Chunduri for PHOTON International said:

US-based Applied Materials has developed a procedure for manufacturing modules with back-contact cells. According to Applied Solar’s president Charlie Gay, the company has developed a patterned copper foil with integrated electrical circuitry, which would form part of the laminate films and provide the cell’s back contact. Applied is planning to license the technology.

Upon request, PHOTON was kind enough to send me a distribution copy of the Applied Materials, Inc. (NASDAQ:AMAT) presentation, “High-efficiency back-contact modules”, by Applied Solar President Charlie Gay, Ph.D.

The Applied Materials c-Si (crystalline silicon) Cell Technology Roadmap details solar cell process improvements increasing PV (photovoltaic) module efficiency up and to the right towards Gen 2 back contact technologies including Emitter Wrap Through.

From PHOTON’s 6th PV Production Equipment Conference

EWT Cells
In the Emitter Wrap Through cell structure, a continuous emitter is diffused through thousands of laser drilled vias less than 100 µm (micrometer) in diameter taking current to the back of the cell. By eliminating the front contacts, EWT enhances light absorption and increases cell efficiency:

Improves short circuit current (ISC) by 8-9%
Increases voltage by reducing recombination over the total cell area
Increases fill factor by back contact pattern geometry

The AMAT Emitter Wrap Through process adds three steps that are supposed to be an Easy upgrade to the standard solar cell processing flow: Laser Hole Drill, Diffusion Barrier, and ILD Print. As a result, new required equipment includes a hole drilling laser, hole filling print and dry, and a back contact tester.

From PHOTON’s 6th PV Production Equipment Conference

As shown in the Emitter Wrap Through Results slide, the mc-Si (multicrystalline silicon) EWT cells have demonstrated Median 18.7% efficiency, short circuit current density (Jsc) of 39.5 mA/cm² (milliamps per square centimeter), and a Fill Factor (FF) of 75.7% with further improvements possible as shown in the EWT Cell Performance Trajectory slide. The EWT technology can also be used to produce monocrystalline silicon solar cells.

From PHOTON’s 6th PV Production Equipment Conference

From PHOTON’s 6th PV Production Equipment Conference

Monolithic Module Assembly
In Monolithic Module Assembly (MMA), back contact silicon solar cells are positioned using a pick and place process onto laminate films with a pre-manufactured large area flexible (flex) circuit or copper (Cu) foil to interconnect the cells. The EWT solar cell metal contacts connect to the copper foil via electrically conductive adhesive (ECA) bonds.

From PHOTON’s 6th PV Production Equipment Conference

Of course, it is not quite that simple. Presented at the 25th EU PVSEC (European Photovoltaic Solar Energy Conference and Exhibition) / WCPEC-5, in Valencia, Spain, during September 2010, the paper:

D.H. Meakin, J.M. Gee, J. Telle, W. Bottenberg, Development and Qualification of Monolithically Assembled Module for Back Contact PV Cells

is invaluable in understanding the April 2011 presentation. I am still grumpy though about paying 10 Euros to download the paper!

“The PV module backsheet layers (e.g., Tedlar^TM or equivalent, moisture-barrier or dielectric standoff layers, cut-resistance layer, etc.)” are integrated with the flexible circuit layer. Instead of EVA (ethylene vinyl acetate), a thermoplastic type encapsulation material is utilized. At this point, the ECA “can be applied to the backsheet” by stencil printing onto the flexible circuit. A “layer of encapsulant with vias formed to match the contact points is applied to the backsheet” and is shown as a dielectric per the cross section below.

From PHOTON’s 6th PV Production Equipment Conference

Next egress bussing from the flex circuit is applied for a junction box or external connectors. Finally, the pick and place of the solar cells onto the module stack is done followed by a second encapsulant layer and the front glass.

After localized heat liquefies the encapsulants over the solar cells to hold them in place, the module stack is flipped with the glass side down and put into a vacuum press module laminator. The lamination process softens and allows the thermoplastic encapsulant “to flow and fill voids within the package” at lower temperatures than the curing point of the ECA or interconnect material.

New MMA Tooling
Monolithic Module Assembly eliminates the need for tabbers and stringers and lay-up stations used in conventional PV module assembly. However, new replacement tools include an automated ECA screen printer, contact via punch and lay-up of the encapsulant layer, solar cell pick and place, and final lay-up of the top encapsulant layer and front glass.

MMA Materials and Certification
AMAT evaluated over 50 Monolithic Module Assembly designs with different construction materials such as ECAs, copper circuit finishes, and encapsulants, using single cell or 3×4 cell mini modules and test coupons (postage stamps?) before attempting to build standard sized PV modules with sixty (60) 156 millimeter cells. The integrated nature of the back contact cell and module design is illustrated by the “tooling development and identification of large-format production tools” for flexible circuit manufacturing wider than the 24 inch (~61 cm) format common in the printed circuit board industry.

Applied Materials shared pilot production results for the 60 cell PV modules at the 25th EU PVSEC and has since obtained IEC Quality and Safety certifications from TÜV Rheinland. Internal AMAT PV module qualification tests are typically run two times the IEC certification requirements.

MMA Advantages
Besides highlighting lower cost and greater efficiency, reliability, and aesthetics, Applied Materials claimed the following advantages for Monolithic Module Assembly:

Pre-fabricated back sheets with integrated circuits allow for faster production flow, higher capacity per unit area of factory floor space and rapid scaling capabilities.

Planar assembly with pick and place allows for high through put and low yield impact.

Ability to process very thin wafers without impacting yield with increased wafer breakage

Robust interconnects through distribution of current collection points on the cell.

Applicability to all back contact technologies.
– EWT, MWT, IBC

Increased module performance through higher packing factors and lower encapsulation loss.
– Encapsulation losses, the difference between individual cell performance and encapsulated module performance are minimized.
– Reduced variance in the module performance for a given bin of cells.

Highly Automated Quality and reduced requirement on manual labor

And, yes, even multicrystalline silicon (mc-Si) blue back contact solar cells look aesthetic.

Even with a crystalline silicon lineage, the new MMA process and material set will at least have some bankability hurdles to jump before tackling larger debt financed PV installations.

Schedule
Applied Materials told me the schedule or specific timeline for EWT and MMA market introduction had not been discussed in the presentation. AMAT is certain to be updating their solar equipment customers on their progress under NDA (Non-Disclosure Agreement). In November 2009, AMAT Chairman and CEO Michael Splinter said the technology assets acquired from Advent Solar were “not expected to generate sales until sometime in 2011”. With Applied’s return to PHOTON’s Solar Terawatt-hours Conference Series in Europe after last year’s absence, I’ll take a guess the technology will be launched no later than the 26th EU PVSEC in Hamburg, Germany.

The new EWT cell and MMA tools and novel materials such as the prefabricated back sheet with the flex circuit are interdependent and require lock step coordination before the first Applied Solar customer can enter production. DuPont (E.I. du Pont de Nemours & Company, NYSE:DD) is a prime suspect as an initial supplier of the integrated back sheet. I wonder if Applied Material’s technology licensing strategy also extends to traditional module manufacturing equipment suppliers in order to accelerate time to market and customer production?

End customer capital investments in these new technologies will be subject to the interplay between PV overcapacity and oversupply that will reach a near term denouement in the second half of 2011 on top of recent industry consolidation by Meyer Burger Technology AG (SWF:MBTN) and Applied Materials.

The latest AMAT presentation has uncanny similarities to “Maximizing Cell and Module Performance” presented by then Advent Solar, Inc. Chief Scientist James Gee at an IEEE/AMAT Symposium thirteen months before “Applied Materials buys assets of Advent Solar: Emitter-Wrap Through technology provider”.

Further information
I found a second 25th EU PVSEC paper available for free download:

J.M. Gee, P. Kumar, J. Howarth, T. Schroeder, J. Franklin, J. Dominguez, D. Tanner, “Development of industrial high-efficiency back-contact czochralski-silicon solar cells”, 25th EU PVSEC / WCPEC-5, Valencia, Spain (September 2010).

Four (4) key AMAT EWT and MMA related patent applications have been published at the World Intellectual Property Organization (WIPO):

(WO/2011/011091) MONOLITHIC MODULE ASSEMBLY USING BACK CONTACT SOLAR CELLS AND METAL RIBBON

(WO/2011/035153) LASER DRILLING OF VIAS IN BACK CONTACT SOLAR CELLS

(WO/2011/035268) THRESHOLD ADJUSTMENT IMPLANTS FOR REDUCING SURFACE RECOMBINATION IN SOLAR CELLS

(WO/2010/057216) INTEGRATED BYPASS DIODE ASSEMBLIES FOR BACK CONTACT SOLAR CELLS AND MODULES

Photo credit: Rolf Schulten / photon-pictures.com from PHOTON’s Solar Terawatt-hours Conference Series.