[Palo Alto, California USA]

SunPower Solar Cell Color Anomaly at the Photovoltaic Demonstration Project.
Just an aesthetic issue?

From Palo Alto Municipal Service Center PV Trackers

I was almost done taking photos of the Photovoltaic (PV) tracker arrays at the Palo Alto Municipal Service Center (MSC) (please see “Palo Alto Photovoltaic Showcase Project neglected”) before I noticed something odd. How could I have not seen this sooner?

Grey Eye
One back contact, monocrystalline silicon solar cell in the SunPower Corporation (NASDAQ:SPWRA) Model SPR-205-BLK-U photovoltaic module mounted upon the tracker I designate number three (#3) was a distinct shade of grey versus the other nominal black cells.

From Palo Alto Municipal Service Center PV Trackers

Rather than rushing to blog this, I took my time and sent photos to SunPower and a few PV experts to get their feedback on what could cause this Color Anomaly.

SunPower appreciated me bringing the issue to their attention. As I understand it, SunPower will work with their channel partner, SPG Solar, Inc., who will in turn work with their client, the City of Palo Alto, to swap out this module and return it to SunPower. I presume SunPower will rescreen the module using their outgoing Quality Assurance tests and proceed with further Failure Analysis depending on the result.

Does it even matter?
I had planned to post about this Tuesday evening. On occasion, sloth is rewarded, and I learned a new relevant wrinkle on Wednesday morning. Only eleven (11) of the twelve (12) SunPower modules per tracker are connected to produce power! So it is at least possible the SunPower module with the Color Anomaly I observed is not even connected to the string.

With the blessing of the City of Palo Alto, I had a productive discussion with SPG Solar Wednesday afternoon. SPG Solar was not certain if the module in question had the color issue at the time of installation; they seemed to think not. SPG Solar also was not certain if this module was indeed a dummy or connected to the string. However, there was mutual agreement the dummy module ought to be mounted somewhere on bottom row of each tracker.

SPG Solar says their role was only construction and installation based on a plan developed by an engineering firm, a PV expert, and an architect that was later approved by the City of Palo Alto after an iterative review process (I am certain to get this wrong if I try to summarize). Upon review of the tracker project design, SPG Solar advised the string length be reduced from twelve (12) to eleven (11) modules because of worst case cold and sunny day weather conditions.

What about the Color Anomaly?
The two PV experts I consulted on the Color Anomaly were split on their conclusions. Of course, there was a tacit assumption the module was “good” and had passed final test at the factory within datasheet specifications at the time I consulted them. As discussed above, this may or may not be true. Of course, each opinion is educated conjecture at the moment.

One expert thought the grey cell must have come from another batch or visual appearance bin but was otherwise within the operating specifications (has the same current as the black cells in the module) or this would have been caught in cell performance sorting or module final testing.

The second expert thought the grey cell would generate less power because of a difference in reflectivity. Since SunPower must be sorting cells for both power output and color, the reasoning pointed to localized cell degradation in its encapsulation as a plausible cause. This expert also observed greying along the edges of surrounding cells in the close up photo.

As the A-300 solar cell SUPPLY AGREEMENT between SunPower and Solon SE (ETR:SOO1) from the 2005 S-1 filing details, SunPower used a manual Visual and Binning Criteria based on reference “Rosetta Cells” back then. The section below explains solar cell color variations and classification categories including Off-Color Grey:

6.5.10. Color Variations The thickness and uniformity of ARC (SiN) and oxide (SiO2) coating on the cells leads to color variations. The ideal color is a deep and even dark blue. The simplest way to distinguish acceptable from unacceptable cell color is to place a production cell next to a “Rosetta Cell” against a black backsheet under halogen lighting. The color of the cell can be distinctly classified into 2 categories.

6.5.10.1. Dark Blue – A distinct and uniform dark navy blue color. When light falls on these cells, the color does not fade out into any other color, but instead appears dense throughout.

6.5.10.2. Off-Color

6.5.10.2.1. Light Blue – Light blue, with no detectable grey color. Under a halogen lamp, the cell carries a dark blue color with light blue streaks.

6.5.10.2.2. Grey – As light falls on the cell, the color fades out into a very thin layer of light blue color, which fades out into white towards the corners of the cell. In the absence of halogen light, the cell gives an appearance of a grey color with white corners and edges.

6.5.10.2.3. Other – Any other color of cell.

It is unclear from SunPower President and Chief Technical Officer Dr. Richard Swanson’s presentation, “Current Manufacturing Challenges from a Cell Maker Perspective”, at Intersolar North America if SunPower has automated the solar cell color binning process since 2005. I expect this must have been a priority to eliminate subjectivity and reduce manufacturing costs.

From Palo Alto Municipal Service Center PV Trackers [Photo by Ron Horii]

The above photo by Ron Horii shows the Color Anomaly was observable (very feint on the right tracker) back in February 2009. The photo appears in the Palo Alto Baylands Preserve with the caption, “Looking north: East Bayshore Road, the Bay Trail, solar panels, and sculpture by the Palo Alto Municipal Service Center, 2/7/09”.

I am curious to learn the result of SunPower’s Quality test and analysis of the Color Anomaly module if the effort still moves forward.

The Flickr photos Solar Panels Palo Alto and Yes, they turn (Image Source: Richard Masoner) illustrate tracker to tracker shading in the winter.

I commend the open government style of the City of Palo Alto. Everyone I have talked with has been courteous and helpful to a pesky Photovoltaic Blogger.