Materials Developments to Enable Roll-to-Roll Printing of CIGS Photovoltaics
Published: June 01, 2009 Category: Advanced Materials Renewable Energy

Significant efforts are underway to enable the use of roll-to-roll printing to manufacture CIGS PV cells. NanoMarkets believes that these will result in significant new business revenues. Our research suggests that inks for the CIGS absorber layer will grow from $8.4 million in 2011, to close to $189 million in 2016, about 18 percent of total revenues for the CIGS absorber layer materials.

Because the choice of manufacturing process goes hand in hand with the type of materials that can be used, these efforts to develop printed CIGS center around materials developments. In particular, several firms and research institutions are developing nanoparticle-based ink formulations and flexible substrates to allow for roll-to-roll printing of the absorber layer and the electrodes.

CIGS PV is coming to be known as the rising star of the TFPV world. It has achieved the highest efficiency of any thin-film PV technology--20.0 percent efficient champion cell created in its lab. Although this falls below the efficiency of crystalline silicon solar cells (barely below that of multicrystalline cells at 20.3 percent efficiency), CIGS cells can be substantially cheaper because of their reduced material usage and the ability to manufacture them using low-cost processes.

However, most of the current CIGS PV cells are manufactured using costly evaporation processes. This, in part, could hinder the ramp up of CIGS PV once the economy turns around. As such, firms including HelioVolt, International Solar Electric Technology (ISET), and Nanosolar are developing nanoparticle-based inks for high-volume manufacture of CIGS PV. NanoMarkets believes that ink-based processes, while not significant in 2009, will grow to account for more than 25 percent of CIGS volume by 2016. As the pioneering companies demonstrate inks in high-volume manufacturing, we expect more companies to enter with outsourced nanoparticles and inks.

Printing Absorber Layer

The inks currently being used for the CIGS absorber layer consist of oxide or selenide nanoparticles of the metals copper, indium, and gallium, dispersed as a colloidal suspension in a solvent. The nanoparticles are intended to form a solid layer of CIGS upon heat treatment, similar in function to layers applied by the conventional methods.

ISET is one example of a company using nanoparticle oxides for its CIGS ink formulation. HelioVolt and Nanosolar, on the other hand, offer metal selenide nanoparticles. The companies claim that selenides avoid the need for reduction of the oxides and the additional thermal processing step that the reduction requires. The use of these selenide nanoparticles is made possible by the low melting points of copper selenide and indium selenide. At the annealing temperature, these compounds melt and help form the new CIGS phase and grain structure.

Nanosolar has developed a method to embed CIGS into thin polymer films using a roll-to-roll process. To sinter the ink coating, the company uses rapid thermal processing (RTP) to flash heat a thin layer for several picoseconds, leaving the rest of the material untouched. Not only does this process reduce materials costs, but it also reduces energy consumption compared with other approaches, according to the company.

HelioVolt's FASST process creates the CIGS absorber layer in two steps. The first step deposits two precursor layers that form the chemical basis of the CIGS layer. The second step synthesizes those materials into an actual CIGS layer: rapid heating induces a chemical reaction between the two precursor films, a process that is said to be similar to anodic wafer bonding.

Currently, it appears that CIGS manufacturers prefer to develop and produce their own nanoparticles and ink formulations. However, there are several firms, including American Elements and Nanoco Technologies, that offer nanoparticles of CIGS and its precursors, and new CIGS PV entrants may choose to outsource their nanoparticle and ink production.

This printing concept is not necessarily limited to the CIGS layer. Printing of the zinc oxide top electrode, though not yet feasible for CIGS PV, could result in additional cost savings.

Flexible Substrates

The full potential of manufacturing CIGS PV cells via printing will not be obtained without the use of flexible substrates, which will allow for roll-to-roll processing. But changing the substrate has proven to be a non-trivial exercise. The following are some of the issues that arise from using a flexible substrate: Undesirable impurities may be introduced into the absorber layer; sodium, which diffuses from glass in the standard process but is not necessarily present with flexible substrates, actually improves the absorber layer film quality by increasing its charge carrier density; the allowed processing temperatures may not be sufficient for high film quality, and additional films may become necessary, such as an insulating layer to allow monolithic interconnection on conductive substrates. Despite the challenges, Global Solar, Odersun, and Solarion have commercially produced CIGS PV modules on flexible substrates in 2008, and more companies are eager to do so in 2009.

Soda lime (aka, glass) is the standard material currently used as the CIGS PV substrate. For flexibility, firms are using and developing polymer and metal foil substrates. Metal foils are already in widespread use in a variety of applications. While CIGS PV manufacturers demand specific alloy formulations and dimensions, such requests are common for metal foil providers, and the CIGS PV demand is expected to simply increase the volume of business done by the metal foil suppliers.

Polymer films, especially polyimide, are expected to grow significantly in volume due to demand from CIGS PV and other flexible electronics. Only a small percentage of CIGS PV production currently uses polymer films, but several firms are interested in developing cells on them. With polyimide film substrates expected to grow to nearly 20 percent of CIGS PV volume within the next eight years, nearly four million square meters of polyimide film will be required, a significant chunk of global polyimide film production. NanoMarkets believes polyimide will be the fastest-growing substrate material until flexible glasses and composites are demonstrated and quickly outpace it.

Based on all of the above NanoMarkets believes printing CIGS has a lot of potential because it offers a way to produce CIGS PV cells using high-speed, roll-to-roll processing--a likely path to low-cost manufacturing. But before this can happen in any big way, producers will need to demonstrate that they can achieve high efficiencies using high-speed printing. This they still have to prove.