Issue



The problems and possibilities of thin film CIGS solar cells


01/01/2008







EXECUTIVE OVERVIEW

Among the most promising of thin-film solar cells are those made of copper indium gallium diselenide (CIGS), but controlling film composition is a big hurdle to commercialization. Some companies producing CIGS solar cells have had trouble getting “good” cells consistently in commercial volumes. Can this variability be solved through more precise control?

Part One: Composition control the key to CIGS efficiency

On paper, silicon doesn’t look like a particularly good photovoltaic material. It’s expensive; its optical properties are poor; and its coverage of the solar spectrum is limited. Yet bulk silicon solar cells, based on single crystal or multicrystalline wafers, dominate the market with more than 90% of sales.

That dominance may be changing, though. Rapid growth in the photovoltaic market has contributed to rising polysilicon prices and serious supply constraints. These in turn have sparked considerable interest in thin-film solar cells. While bulk silicon cells tile the module area with silicon wafers, thin film cells coat large substrates as single units. Not only do they offer substantially lower costs/m2 of cell area, but thin-film cells can potentially conform to curved or flexible substrates, allowing many more cell installation options.

One of the most promising of the thin film technologies is CIGS. The name encompasses a wide range of miscible quaternary and ternary alloys, with indium, gallium, and occasionally aluminum freely substituting for each other, and sulfur sometimes used instead of or in addition to selenium. The electrical properties depend on the exact composition, giving CIGS materials a bandgap anywhere from 1.0 to 3.5 eV [1].

The flexible bandgap is helpful for specialized installations, such as for indoor use, where the available spectrum can differ from that of normal sunlight. CIGS compounds can also create a stack of tandem cells, covering a wider fraction of the optical spectrum than a single junction can.

Unfortunately, while properties change dramatically with composition, the miscibility of CIGS alloys makes film composition very difficult to control. The most common manufacturing methods evaporate or sputter copper, indium, and gallium sequentially or simultaneously; reacting the resulting film with selenium vapor establishes the final film composition. Simultaneous deposition of the three elements results in a composition that varies with location in the process chamber; with sequential deposition, segregation and preferred Ga-Se reactions can lead to composition nonuniformity. The composition of the finished film depends on the thermal profile and diffusion from the substrate (usually Mo-coated glass), as well as on the initial deposition. Indeed, composition uniformity over large areas has so far been a major obstacle to CIGS commercialization.

Vapor deposition of CIGS thin films is undesirable for cost reasons as well. Vacuum chambers and multisource sputtering guns are expensive, especially for the large process areas desired for solar panels. Sputtering and evaporation have relatively poor materials utilization, coating the entire process chamber as well as the target substrate. Not only do wasted materials add cost, but CIGS films may face materials supply constraints once they achieve significant sales. Indium is already an important component of the transparent conducting oxide used in many flat-panel displays and photovoltaic cells. Research at the National Renewable Energy Laboratory estimates that indium availability is likely to constrain manufacturing once CIGS production reaches the tens of gigawatts level [2]. (Note that this is a long-term consideration; the current photovoltaic market produces <3GW/year, considering all cell technologies.)

An ideal CIGS deposition method would serve all three objectives: improved materials utilization, improved uniformity, and reduced process costs. One possible approach, being pursued by International Solar Electric Technology (Chatsworth, CA), Nanosolar (Palo Alto, CA), and others, uses printing processes to apply suspensions of metal oxide particles. In particle suspensions, the viscosity and other flow properties depend on particle size, particle concentration, and the suspension medium used. By varying these factors, manufacturers can adapt their “ink” to a wide variety of printing methods, from screen printing to ink jet deposition. Printing methods in general are adaptable to a wide variety of substrates, including metal foils, glass, and plastic. They achieve materials utilization rates in excess of 90%, while using far less expensive equipment than needed for vacuum processing.

Particle deposition leaves behind a porous film, which a thermal step densifies and reduces from oxide to metal. The high surface area of the initial powder layer allows the reduction reaction to proceed very quickly, much faster than either a bulk or a thin-film reaction could. Though the final composition still depends in part on the film’s thermal history, controlling the quantities of precursor powders gives manufacturers uniform initial compositions and nearly unlimited access to the full CIGS alloy system.

So far, non-vacuum CIGS processes have achieved very promising results. Nanosolar has demonstrated a cell efficiency of 14% [3], comparable to the 18.8% efficiency achieved by the best vapor-deposited CIGS on glass cells, and superior to the 9.8% achieved by commercial amorphous silicon cells [4]. Still, other CIGS commercialization efforts have run aground, or at least seen substantial delays, due to uniformity issues. Until someone actually produces high-efficiency CIGS cells in commercial quantities, the technology will remain one of many intriguing possibilities.

References

  1. Joseph D. Beach, Brian E. McCandless, “Materials Challenges for CdTe and CuInSe [2] Photovoltaics,” MRS Bulletin, Vol. 32, pp. 225-229, March, 2007.
  2. Rommel Noufi, Ken Zweibel, “High-efficiency CdTe and CIGS Thin-film Solar Cells: Highlights and Challenges,” Proc. 4th World Conf. On Photovoltaic Energy Conversion, Vol. 1, pp. 317-320, May 2006.
  3. J.K.J. van Duren, et al., “The Next Generation of Thin-film Photovoltaics,” Mater. Res. Soc. Symp. Proc., Vol. 1012, paper 1012-Y05-03, 2007.
  4. M.A. Green, et. al., “Solar Cell Efficiency Tables (version 30),” Prog. in Photovoltaics: Research and Applications, Vol. 15 (5), pp. 425-430, 2007.

Part Two: CIGS cells depend on web of conductors

For system users, photovoltaic efficiency is a single number showing what fraction of the photons impinging on the array emerges as usable current at the attached load. For scientists trying to build better devices, efficiency is a more complicated proposition. Absorbing light and generating free carriers isn’t sufficient. To perform useful work, the carriers must reach the cell electrodes. If the electron-hole pair recombines too quickly, then it can’t contribute to the photocurrent.

In crystalline silicon solar cells, getting carriers from the junction where they are generated to the cell electrodes is straightforward. At the junction, electron-hole pairs split, which each carrier traveling through the n-type or p-type material, as appropriate. That’s the way CIGS (copper indium gallium diselenide) solar cells are supposed to work, too. Though most carriers are excited in the CIGS layer, a CdS “window” layer actually defines the junction between n-type and p-type material.

Yet measurements on real cells reveal a more complex picture. On one hand, “good” cells occur over a broad range of CIGS compositions. On the other hand, most of the companies attempting to produce CIGS cells in commercial quantities have had trouble producing “good” cells consistently. Cells processed under seemingly identical conditions turn out to have widely varying properties.

Some researchers have attributed this variability to the complexity of the system, suggesting that the solution lies in more precise control of process conditions and film composition. In contrast, B.J. Stanbery, founder of HelioVolt Corp., argues that the problem, and its solution, derive from the basic structure of the material [1].

Though much of the work in this field has focused on the simpler CIS (gallium-free) material, gallium freely substitutes for indium, and the same conclusions apply. In the CIS system, the best cells lie in a two-phase region of the quaternary phase diagram. In this region, a CuInSe2 -phase and In-rich CuIn3Se5 β-phase coexist. The fraction of each phase varies with the overall film composition, but compositions of the individual phases do not.

Stanbery argues that the -phase is a p-type conductor, the β-phase is n-type, and that the cell performance depends on the intra-absorber junction (IAJ) between the two. In this model, the ideal cell structure is one in which the two phases form two continuous interpenetrating networks. Carriers generated too far away from an intra-absorber junction will recombine before they can reach a transport layer. Without this interpenetrating structure, cell efficiency is likely limited to 14% or less, suggests John Langdon, VP of marketing at HelioVolt. (The US National Renewable Energy Laboratory has set a target of 15.2% efficiency by 2010.)

The IAJ model appears to explain several puzzling aspects of CIGS cell behavior. As long as the minority phase forms a continuous path for carriers, the relative fractions of the - and β-phase, and thus the composition of the film, are relatively unimportant. On the other hand, the behavior of the film depends on nanoscale composition fluctuations-which drive phase segregation-and not on structural features such as grain boundaries. The - and β-phases are crystallographically coherent, creating no defects at the interface between the two regions. Thus, a film with completely uniform grain structure can still fail to have the desired phase structure, and vice-versa.

In CIGS, as opposed to CIS, the presence of gallium can help drive creation of the desired nanostructure. Gallium tends to segregate to the -phase, while indium segregates to the β-phase. However, the bandgap of each phase depends on its composition. If gallium concentration in the film as a whole or in a local area exceeds 35%, the band difference between the - and β-phase regions will become too large for a useful junction.

Actually creating the nanoscale phase structure in CIGS films turns out to be complicated. Most processes, both vacuum-based (PVD, evaporation) and non-vacuum (powder methods), use a final thermal annealing step to react the metal (or oxide) precursors with selenium. Yet the high temperature required for this step allows diffusion and phase segregation. The film structure after annealing will not necessarily be the same as the structure created by the metal precursors.

Given that the initial metal structure can be difficult to control, the widely varying results seen in practice are not surprising.

HelioVolt’s process coats two precursor “inks” onto two different substrates. Bringing them together and flash heating the sandwich forms a dense CIGS film with the desired nanostructure. Rapid heating minimizes diffusion, while separate preparation of two precursor films gives control over the structure. The company claims that its method gives superior cell efficiency, while the low thermal budget makes the process compatible with a wide range of substrates.

Reference

1. B.J. Stanbery, “The Intra-absorber Junction (IAJ) Model for the Device Physics of Copper Indium Selenide-Based Photovoltaics,” IEEE Photovoltaic Specialists Conference report, pp. 355-358, 2005.

Katherine Derbyshire is a contributing editor at Solid State Technology. She received her engineering degrees from the Massachusetts Institute of Technology and the U. of California, Santa Barbara. She is the founder of consulting firm Thin Film Manufacturing, PO Box 82441, Kenmore, WA 98028 United States; e-mail [email protected], http://www.thinfilmmfg.com.