There has been a recent upsurge in developments for building-integrated phototovoltaics (BiPV) roof top materials based on CIGS. Several new companies have increased their presence and are looking to bring products to market for this application in 2011. For roof-top application, there are significant key requirements beyond just having good conversion efficiency. Other attributes include lightweight, as well as moisture-proof, and fully functionally reliable. The companies bringing these new BiPV products need to ensure functionality with a rigorous series of tests, and have an extensive set of "torture" tests to validate the capability. There is a convergence of form, aesthetics, and physics to ensure that the CIGS BiPV deliver on their promises. This article will cover the developments in this segment of the BiPV market and delve into the specific tests and measurements needed to characterize the products.

The most familiar view of solar power is the PV panel tilted into the sun. Very recently there has been a strong surge of new incarnations of PV that are showing promise to change that iconic view for residential and commercial rooftops. In particular, there have been recent announcements and products such as flexible PV rolls that can be applied to rooftops and have a low profile and are more aesthetic in appearance. The technology and market for this application of flexible substrates has been a long time in the making and certainly the leader - UniSolar - has been key in developing this particular segment. Now with the advent of higher efficiency thin films, in particular CIGS, this application appears poised to become a significant part of the overall solar PV picture show.

Because flex PV modules are part of the roof membrane, they are required to endure weathering elements. To demonstrate the product capability, they must pass a series of "torture tests" showing that they can withstand a barrage of temperature, humidity, and mechanical extremes, and do so for 25 years. A number of suppliers along with the National Renewable Energy Laboratory have been working to demonstrate the necessary reliability [1-5]. Dr. Rommel Noufi of NREL states that "reliability testing at both the cell and module level are key points of focus at NREL, and there is extensive activity for this, especially with CIGS." [6].

Because encapsulation coatings are used instead of glass, resistance to moisture ingress and UV degradation are the primary concerns. Since lightweight modules utilize flexible cells, this configuration leads to additional failure modes specific to flexible substrates.

The "back story" on flex panels

There are now several companies targeting the commercial and industrial flat-roof market with a lightweight, non-penetrating flexible solar module element.These "panels" can be rolled up, laid out across rooftops with minimal effort, and walked upon. Another appealing aspect is that they can provide full coverage of the roof by accommodating steps in the roof surface, and other protrusions that dot the tops. Architects want four key attributes: good conversion efficiency, light weight, good aesthetics, and reasonable cost. With the recent developments from various CIGS companies targeting this sector, architects will be getting what they want.

Uni-Solar was first in the market, but stepping into the mix are companies such as SoloPower, Global Solar and Ascent Solar (see Table). However here is the conundrum: Uni-Solar has established its market presence with recognized reliability from years of activity. The newcomers have a burden of proof to demonstrate reliability. The testing helps with the proof of reliability and establishes the credentials

The recent opening of this market is due to two key elements. As succinctly stated by Jean-Noel Poirier, VP of Business Development at Global Solar, these elements are: "1) CIGS cell technology is now reaching high efficiency, at production scale; and 2) Encapsulation technologies and materials are now available allowing packaging the CIGS cells into flexible modules." The company insures reliability via material specifications from large chemical manufacturers (vapor barriers, etc.) and from tough accelerated testing, which allows the company to warranty power output for 25 years [7].

Weathering or accelerated lifetime testing

The main difference with the flex PV is that instead of glass, a polymer coating is used. Especially for CIGS, the critical concern is moisture-induced degradation. The path to demonstrating its viability as a rooftop flexible product comes down to the various "torture tests" and to collecting data - lots of data. In the short term enough reliability data must be obtained to get into the market and a track record established.

The main tests are similar to those of standard glass-based PV and are based on accelerated stress exposures under extreme conditions. The principal tests are:

 

  • Damp heat 85°C/85% RH (DH) and modified DH with acetic acid vapor; and
  • Humidity freeze and accelerated temperature cycle testing (-40°C /90°C).

 

Additional tests specific to the flex PV include:

 

  • Adhesion testing,
  • UV exposure, and
  • Flexibility tolerance.

 

The main certifications include: IEC 61646 and UL 61730.

Key CIGS-related stress testing

CIGS structures are built out of several critical layers using various fabrication methods. Manufacturing deviations and deposition method limitations can result in imperfections that cause cell and module performance deterioration.
Table. Comparison of different PV technologies.

The first problem that is encountered in the fabrication of CIGS solar cells is the preparation of a Mo layer due to the large thermal and lattice mismatch between Mo, the substrate, and between the CIGS material and Mo. Similarly, Ag-front grid lines over the top TCO layer also can be adversely affected by the thermocycling, which causes deterioration of the material.

Other factors that can affect flexible CIGS performance include:

Cu diffusion through the junction irreversibly damaging the cell: The necessity to avoid the formation of an unstable CuxSe phase at the surface is addressed by process optimization.

Deviation from the right stoichiometry in some zones of the absorber layer: This phenomenon is more pronounced in large area size products over time that can negatively influence working cells' energy output. The metastable effects in CIGS-based thin-film solar cells are responsible for long-term stability under illumination at elevated temperature. Accumulation of negative charges at the heterointerface due to the net doping density variations leads to a change of the fill factor and VOC (the cell's open circuit voltage). The position of quasi Fermi levels in the space charge region is to blame for the light soak effect in CIGS cells.

Moisture ingress that is prominent due to the granular structure of the multilayer film stack of the cell: this leads to the substantial changes in sheet and shunt resistance lowering power conversion efficiency of the device.
Figure 1. Novel moisture barrier. SOURCES: DuPont and University of Delaware [8]

UV-degradation of encapsulating layer: this contributes to the overall degree of moisture intake.

The flexible encapsulating layer plays a major role, and is the subject of numerous cycling tests. Representative data showing an approach using ALD is shown in Fig. 1 from DuPont and the University of Delaware [8], and other work is ongoing [9,10].

UV damage to the organic material is an obvious concern, therefore, the encapsulating and bonding materials used in flexible CIGS modules are tested in various UV and sunlight irradiation to verify their stability to bimodal failure type. The probability of disintegration of commonly used polymers is tested under combined alternation of UV-DH exposure and the results indicate the likelihood of a lifetime of 25 years.

Transmission spectra are often measured by using broadband spectrophotometry for samples exposed to UVB fluorescent lamps for three weeks in an arrangement that complies with IEC 61646. The dosages are calculated to be equivalent to 15kW-hr/m2 in wavelengths between 280 and 385nm, and 5kW-hr/m2 in wavelengths between 280 and 320nm, respectively.

Representative data elements for flex PV products

There is a clear recognition by flex PV suppliers that they need to solidly demonstrate the reliability of their products. Dr. Mustafa Pinarbasi of SoloPower affirms this need and has developed testing procedures to exceed the standard test requirements. The company's data is representative of the high level of testing to ensure reliability.
Figure 2. Damp heat test of flexible modules for normalized Pmax. SOURCE: SoloPower

Damp heat is critical and SoloPower's tests conducted on product modules showed no significant change in power even beyond the 1000 hours standard as shown in Fig. 2. These panels were previously exposed to 1 week of outdoor light exposure and 10 humidity/freeze cycles (-40°C to 85°C) prior to damp heat exposure. Flexible panels were also subjected to temperature cycle testing (-40°C /90°C). Results shown in Fig. 3 demonstrate that very small or no power degradation is observed after 550 cycles.

In addition, a mechanical flexibility test was developed to determine the reliability of the modules. The test equipment coils and uncoils the panels over a 20-inch diameter drum. Each cycle of the mechanical test consists of coiling and uncoiling the panel both under tensile and compressive stress conditions. For installation, 25 coiling steps are needed. Figure 4 shows that even after 300 cycles of testing, no significant change in power is measured. With this data, SoloPower is able to address and mitigate concerns about product capability.
Figure 3. Temperature cycle test of rigid panels at -40°C / 90°C . SOURCE: SoloPower

A similar philosophy to not just meet, but to exceed the test a requirement is in place at Ascent solar. According to Dan Tomlinson, in the business development group, Ascent will even use an autoclave (essentially a large pressure cooker) to force the moisture issue. The company recognizes that this test is dramatically more demanding and can cause failures not actually present in the real world.

In addition, Tomlinson indicates that adhesion testing is an important point of focus to ascertain how well the modules will stick to the roof. For these pull tests, the team notes how much force is needed to pull a laminate to the point of failure.

Conclusion

There has been a significant uptick in the activity for flexible CIGS thin-film rooftop laminates in the last six months. This market has the potential for becoming a major application and this development reflects the maturing aspect of PV in that different products with application specific characteristics are evolving to meet market needs. In the case of rooftops, there are now a number of roofing companies that are partnering with PV companies to provide lightweight, flexible and aesthetic products. As Poirier says, "Roofing companies are looking for PV products that respect roofs (no penetration) and the building integrity (light weight, low wind load, aesthetics). Flexible modules perfectly answer these needs," , observes Poirier. "Furthermore, flexible modules can integrate into their own roofing products, allowing for synergetic systems and solutions. New CIGS flex modules provide the same benefits, but with higher efficiency. So the roofing industry is very excited by the arrival of high efficiency flexible modules, and is currently testing the product according to their stringent standard testing process."
Figure 4. SoloPower's mechanical flexibility test. SOURCE: SoloPower

These "torture tests" are necessary because they are the first step to provide an entry for installation. The testing mitigates concerns about field reliability. In the end, it will be the "real-world" rooftops that have been functioning for 20 years that provide the ultimate proof. In the mean time, extensive effort is being applied to demonstrate that the rooftop show (and performance) will be a feature presentation with enduring qualities and even multiple sequels.

References

1. A. A. Dameron, M.O. Reese, T.J. Moricone, M.D. Kempe, "Understanding Moisture Ingress and Packaging Requirements for Photovoltaic Modules," Photovoltaics International 13, 121-130 (2009).

2. P. Hacke, D. Trudell, K. Terwilliger, N. Bosco, E. Gelak, S. Kurtz, "Application of the NREL Test-to-Failure Protocol for PV Modules," 19th Crystalline Silicon Workshop, Vail, CO, (2009).

3. S. Kurtz, J. Granata, M. Quintana, "Photovoltaic-Reliabilty R&D Toward a Solar-Powered World," Proc. SPIE Optics and Photonics (2009), San Diego.

4. J. Pern, R. Noufi, X. Li, C. DeHart, B. To, "Damp Heat-induced Degradation of Transparent Conducting Oxides for Thin Film Solar Cells," NREL Report No. PR-520-43256 (2008).

5. NREL PV Reliability Workshop http://www1.eere.energy.gov/solar/pv_module_reliability_workshop_2010.html#thin

6. Private communication with Dr. Noufi to author.

7. Private communication with author and J-N. P.

8. P. Carcia, R. S. McLean, S. Hegedus, Presented at the NREL PV Reliability Workshop Feb 18, 2010; http://www1.eere.energy.gov/solar/pdfs/pvrw2010_poster_carcia.pdf

9. S. Hegedus, P.F. Carcia,R. S. McLean, B. Culver, "Encapsulation of Cu(InGa)Se2 Solar Cells with ALD Al2O3 Flexible Thin-film Moisture Barrier: Stability under 1000 hour Damp Heat and UV Exposure," Conf. Proc. of thePhotovoltaic Specialists Conference (PVSC), 2010 35th IEEE, June 20th 2010.

10. J. Kapur, K. Proost, C. A. Smith, "Determination of Moisture Ingress through Various Encapsulants in Glass/Glass Laminates," http://www2.dupont.com/Photovoltaics/en_US/assets/downloads/pdf/Determination_of_Moisture_Ingress.pdf

Andy Skumanich received his PhD in physics from UC, Berkeley and is Founder and CEO of SolarVision Co, 412 Los Gatos Almaden Rd. Suite 7, Los Gatos, CA 95032; ph.: 408-377-0545 email askumanich@solarvisionco.com.