Efficient, reliable, and affordable solar panels made of copper indium gallium selenide

Harnessing solar energy has increased tremendously in recent years as the importance of renewable energy has moved to the forefront of social consciousness. Thin-film photovoltaic (PV) technologies have attracted much attention, because they offer a distinct cost advantage. Copper indium gallium selenide (CIGS) is the most promising material for such applications, because it has the highest energy-conversion efficiency of any thin film: 20.3% for cells¹ and 15.1% for modules.² However, it has been challenging to reliably and rapidly produce CIGS films on scale. To address this problem, we developed a reactive transfer process that benefits from good compositional control and fast, high-quality reaction over large areas. We also developed packaging technologies that provide high environmental stability.

Our reactive transfer process can form CIGS films on a variety of materials, including glass and various metals and plastics. Our monolithic interconnection scheme involves three main patterning steps, including laser and mechanical scribing. After initial sputtering of a molybdenum back-contact film and laser patterning, a two-stage reactive transfer process forms a high-performance, thin-film CIGS absorber layer (see Figure 1).³ In the first stage, copper, indium, gallium, and selenium-based precursor films are deposited onto the substrate and cover plate. This film stack allows precise control of the composition and crystalline structure of the resulting CIGS film. The film stack also permits different modes of processing, for example, physical vapor deposition and liquid-precursor spray printing.⁴ Furthermore, precursors can be deposited at low substrate temperatures, thus lowering cost and increasing throughput. In the second stage, the precursors rapidly react under pressure, with the plates held in close proximity and the precursor-film stacks facing each other. In the vapor phase, a controlled amount of material transfers from the cover plate—a reusable ‘source’ plate—to the substrate. This conditions both the bulk and surface of the resulting positive-type CIGS film. By pulse heating the films (or by rapid thermal processing), the overall thermal budget is significantly reduced. The lower heat requirement also allows use of low-cost substrate materials of reduced thermal stability such as polyimide. We achieved increased incorporation of selenium by applying significant pressure, thus preventing loss of this element from the reaction zone. Our reactive transfer process synthesized CIGS films in a matter of seconds.

Figure 1. Two-stage reactive transfer process for producing photovoltaic copper indium gallium selenide (CIGS) film. Cu: Copper. In: Indium. Ga: Gallium. Se: Selenium.

After CIGS formation, we deposited an optimized negative-type thin buffer layer of cadmium sulfide using chemical-bath deposition. Following mechanically scribing the active materials, we sputtered a layer of transparent conducting aluminum zinc oxide onto the structure. We then performed the final isolation scribe, which involved mechanical scratching to isolate the front contacts of the different cells of the module. We packaged the modules using bus bar and tab connections, applied edge sealant, laminated the substrate and low-iron superstrate using an encapsulant sheet, and attached the junction box. Our unique edge-sealant solution—whose composition is a trade secret—optimized moisture resistance and provides a minimum 25yr lifetime, even in humid environments. Use of low-iron encapsulant and superstrate endowed high optical transparency to the modules and improved photon transmission.

We subsequently characterized our modules. Tilted cross-sectional scanning-electron micrographs revealed that large grains were produced: see Figure 2(a). The films also bore a nano-faceted surface—see Figure 2(b)—which renders them black and provides the ability to effectively capture a broad range of the optical spectrum from a wide range of angles under a variety of lighting conditions. CIGS solar cells fabricated with our reactive transfer process exhibited cell efficiencies of 14–15%.³Based on these promising characteristics and the low cost of manufacture, we have begun production of large CIGS modules with a 120×60cm² form factor (see Figure 3). Our first production line of 20MW annual capacity achieved approximately 12% efficiency. The National Renewable Energy Laboratory independently verified the module efficiency as 11.8±0.6% (see Figure 4), which corresponds to an output power of 75.64W.

Figure 2. Scanning-electron micrographs showing (a) cross-sectional and (b) top views of a CIGS layer formed by reactive transfer processing on molybdenum contact film. WD: Working distance between sample surface and lens.

Figure 3. A 120×60cm²module produced at our factory. (Inset) Our automated manufacturing line.

Figure 4. Light current-voltage (IV) curve measured by the National Renewable Energy Laboratory (NREL) on a 120×60cm² CIGS module. The efficiency was confirmed at 11.8±0.6%. LACSS: Large-area continuous solar simulator. ASTM G173: Standard set by the American Society for Testing and Materials for reference solar spectral irradiance. V_oc: Open-circuit voltage. I_sc: Short-circuit current. V_max: Maximum voltage. I_max: Maximum current. P_max: Maximum power.

Our production-line modules have been tested for durability and operation under different conditions. For example, the results of 1000h damp heat (85°C/85% relative humidity) and humidity freeze (−40°C to 85°C/85% relative humidity cycling) tests showed virtually no degradation.³ We also field tested these modules under a range of weather conditions including extreme high and low temperatures, high humidity, torrential rain, snow, hail, high winds, and tornado-like conditions. In 15 months, the modules did not fail and exhibited a degradation rate two orders of magnitude lower than that permitted. Based on these results, our modules easily conform to an output power decrease of less than 20% over 25 years. They are currently undergoing certification testing (UL-1703, IEC-61646, and IEC-61730).³

In summary, our reactive transfer process produces high-quality CIGS films with high uniformity over large areas. We used this method to produce solar cells and 120×60cm² modules with 14–15 and 12% efficiency, respectively. Our advanced packaging technologies provided modules that passed reliability tests with a wide margin. In addition, the monolithic nature of our modules drives manufacturing costs down, because no cell cutting, testing, sorting, tabbing, stringing, or mounting is involved. We are currently commercializing this reactive transfer process on a production line with 20MW capacity, and are planning our next giga-Watt-scale factory.