In 2011, Reel Solar, a stealthy thin film startup, raised $15 million from Mayfield Fund, CalCEF, CMEA Capital, and Pangaea Ventures.

Today, the firm raised another $4 million, according to a regulatory form spotted by Lindsay Riddell at the San Francisco Business Times. Riddell notes that Reel Solar's board of directors includes former CEO of California utility PG&E Peter Darbee.

The firm is in stealth mode and the CEO, Ed Grady, has not responded to inquiries from GTM. Grady has been Chairman at Evergreen Solar and Molecular Imprints, according to his LinkedIn profile.

The startup is working in the cadmium telluride (CdTe) materials system -- the same materials used by thin film leader, First Solar, as well as General Electric and now-defunct DOE loan recipient Abound Solar.

When it comes to volume shipments of thin film solar -- there's First Solar with its cadmium telluride product, and to a lesser extent Solar Frontier with its CIGS materials. After those two players, it's difficult to pick any other likely contenders although the field still remains broad with MiaSolé (now Hanergy), Calyxo, NuvoSun (now Dow), Solibro (now Hanergy), Stion, SoloPower, Nanosolar, Encore, and a number of other aspirants.

PrimeStar Solar was acquired by GE in April 2011. PrimeStar/GE uses (as did Abound) a close space sublimation (CSS) process for CdTe manufacturing, while First Solar uses a vapor transport deposition (VTD) process. General Electric had announced plans to go into production in Colorado with 13 percent panels, and then backed off those plans in July 2012. 

According to sources close to the company, Reel Solar is working in CdTe and using what is known as the BP process. There was an attempt by Reel to identify First Solar's production weaknesses and attack the weakest point, according to our sources. Many of the folks on Reel Solar's technical team have come from Intermolecular, the combinatorial research firm that went public last year. Intermolecular is currently working with First Solar.

The BP process uses an electrodeposition technology "inherited from Monosolar." BP dropped the CdTe technology a decade ago and has since completely retreated from the solar business.  

Efficiency and cost are what counts in solar panels. First Solar's average production module efficiency was 12.9 percent in the fourth quarter of 2012, while its "lead line" was producing modules with 13.1 percent efficiency during the fourth quarter at a cost of below $0.70 per watt. Silicon solar panels have efficiencies in the mid- to high-teens while efficiency leader SunPower ships c-Si modules with greater than 20 percent efficiency. Leading Chinese solar module manufacturer cost is below $0.60 per watt.

First Solar's CTO Raffi Garabedian told GTM, "CdTe already provides the best combination of manufacturability and performance, making it the most cost-effective technology for utility-grade PV. Further efficiency improvements at the manufacturing scale will solidify this position."

If Reel Solar or our sources start to share more information -- you'll be the first to know.