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Soft pretzels and CIGS: Even more from the IEEE PVSC/PV America event

15 June 2009 | By Tom Cheyney | Chip Shots

soft_pretzel3 PHILADELPHIA—No one was jonesing for CIGS at last week's IEEE Photovoltaic Specialists Conference—copper-indium-gallium-(di)selenide thin film-related programming, that is. Several oral and poster sessions were dedicated to the technology (or, in some cases, its copper-indium-sulfide cousin), while CIGS-specific presentations on window and contact layers as well as characterization and analysis approaches were also prevalent in other sessions. For a PV food group with less than 1% market share of the installed system base, CIGS had a mighty presence at the annual event.

Over at PV America, no CIGS producers deigned to exhibit, but at least one example of their products could be found there. A Solyndra tubular-module rooftop panel was a big curiosity draw at the Carlisle Energy Services booth (a Solyndra customer). Still, CIGS was on people's minds among a good chunk of the exhibiting process and analytical tool suppliers trying to get a piece of this nascent—and growing--TFPV production market.

Ric Borges of Synopsys told me his team has done some work lately on applying the company's TCAD package to the design of CIGS devices. It seems that people are curious to know why certain layer thicknesses work better than others and want to use the Synopsys software to simulate and model the effects, so they can then optimize their devices and get more efficiency out of them.

Evans Analytical Group--with its alphabet soup of AES, SIMS, FTIR, TEM, SEM, XPS, and other analytical and characterization tools--has been busy examining rigid and flexible CIGS cells and materials for a variety of clientele looking to improve film uniformities and performance reliability. Gary Mount pointed to the formation of an EAG internal task force, which focuses specifically on the thin-film PV compound.

After decades of working in other semiconductor and other thin-film industries, Nanometrics has expertise that it believes can repurposed to assist solar manufacturers—including CIGS producers--in their process control efforts. Linda Heywood said the company has dedicated more engineering and applications resources to the space, with CIGS and monocrystalline-silicon developers and producers showing the biggest interest in Nano's monitoring and metrology tools.

Citing in situ film thickness metrology as an example, she said that systems had to offer low-impact, fast, on-the-fly capabilities, all while staying relatively cheap in price to be attractive to the PV crowd. For a flexible R2R process, she explained that varying numbers of optical sensors, depending on the width of the web and the budget of the customer, could be mounted inside deposition chambers to quickly acquire process data as the material rolls by.

Back at the PVSC, the attendance rolls featured a posse of CIGS researchers from the likes of NREL and other global institutions as well as commercial purveyors of the direct-bandgap material, including honchos from Ascent Solar, Heliovolt, Nanosolar, ISET, and SoloPower.

One of those honchos, ISET's Vijay Kapur, a CIGS pioneer and PV industry veteran, coauthored a poster paper about his company’s method of making ink-based, high-efficiency CIGS cells on titanium foils using a low-cost, roll-to-roll production approach. A key element of the ISET scheme is a proprietary process that covers the 40-µm-thick titanium foil with a 2-10-µm-thin protective insulating layer that does not compromise the substrate's flexibility and may help enable the cost-effective fabrication of monolithically integrated CIGS modules.

solopower_flex Bulent Basol, a cofounder of ISET with Kapur back in the day, now plies his considerable TFPV knowledge as CTO/executive VP (and yes, cofounder) of the venture-backed CIGS outfit, SoloPower, which employs an electroplating-based approach to make its flexible chalcopyrite devices. He presented a quasi-comprehensive invited paper updating the status of SoloPower's technology development efforts.

After a brief overview of the company's history and the scene in general, Basol reiterated that CIGS, however it's made, needs excellent composition and microstructure characteristics. He then launched into the heart of his talk, describing SoloPower's own electroplating technology, which can consistently maintain compositional control of the CIGS absorber layers and achieved champion cell efficiencies near 14% in tests conducted at NREL.

In a none-too-subtle jab at those other CIGS players that need what he called "preformed expensive materials" such as sputtering materials and nanoparticles, he pointed to the low cost of hardware and the nearly 100% material utilization (they only need to change the plating baths every eight or nine months, he said) inherent in his company's electrodeposition approach.

solopower_tool Issues of composition, film thickness, uniformity, and morphology have been the main focus of work at SoloPower, according to Basol, with surface roughness and defectivity as two parameters of special concern. Over the past few quarters, the efforts have resulted in steady and continuous improvement, with many problems eliminated, he claimed.

SoloPower runs 50-µm-thick, 13.5-in.-wide flexible stainless-steel foil substrates, which come in rolls as long as 2500 ft (~760 m), at its 20-MW production facility in San Jose. After plating a CIGS-containing precursor film on the foil, they put the material through a rapid thermal processing step, which gives it a good grain structure, Basol said.

In one experiment, they tweaked the Cu/(In+Ga) ratio about halfway through the run of a 240-meter-long roll, while keeping the Ga/(In+Ga) ratio constant. By independently controlling the two metals ratios, they were able to demonstrate excellent compositional control within the established process window and achieve a tighter distribution cluster of high-efficiency cells. From center to edge, the CIGS maven posits very good cross-web uniformity with few disparities.

Once they cut the individual cells from the flexible foil rolls, and the cells are tested, sorted, and binned, the devices can either be strung into traditional glass modules or be integrated as flexible modules. SoloPower, which runs a raft of reliability and performance evaluations on its panels, has hit 10% efficiencies and a total power output of 107.5 W for a 1.07 m² aperture-area module in tests conducted at NREL.

solopower_rooftop The company has a few kilowatts of glass panels under sun on its own rooftop test facility (see photo). Basol mentioned that SoloPower showed some modules for the first time at the recent Intersolar show in Munich and told me after his talk that they are shipping samples to customers.

But the productization of a flexible module remains, for SoloPower as well as other CIGS cell makers, a more difficult nut to crack. CIGS' moisture sensitivity, especially in high temperatures, means there needs to be rugged, hermetic encapsulation packaging—and smart module design enhancements too—for the absorber layers within a flexible panel to be protected from the elements.

SoloPower has some excellent data for proof-of-concept transparent barrier films, said Basol, and he expects the protective materials to be in production by sometime in 2010. As part of bringing a flexible module to market, he cited the need for special end-seal materials and moisture-blocking backsheets, adding that there were "adequate" materials currently available.

When I complimented him on the level of detail in his presentation, Basol said that "we don’t talk unless we have something to say." Now that they are talking, perhaps the project integrators and developers are starting to pay attention and perhaps SoloPower and their electroplated CIGS might finally move later this year from development to deployment.

Perhaps.