The Next Great Solar Cell Material

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During Solar Power International 2010 (SPI 10), each afternoon there were more than eight concurrent conference sessions. It was not possible to attend them all, but the “The Next Great Solar Cell Material: What Technology Will Emerge Dominant on the Market?” session was particularly interesting. I briefly attended two other sessions. It was more than a little ironic that the “Greening the Solar Industry” session had fewer than 30 people. At the same time the “Where’s the Solar Money” and the “Cell Material” sessions were standing room only with more than 300 people each. Below is a brief summary of the two best presentations in the “Cell Material” session. Please forgive potential misreporting of technical information. The goal of this blog is to share the highlights about future technology trends in solar.

To prepare for his talk, Simone Arizzi, Global Technology Manager with DuPont Photovoltaic Solutions, conducted an informal poll of fellow scientists and found a wide variety of answers to the question of future solar cell technology. He chose one colleague’s response as the most representative: “I don’t have the foggiest idea, but I do know that it will be made by DuPont.”

Arizzi presented a very interesting comparison of how three other industries (automotive, electronics, and construction) handled growth over time. To compare the auto and solar industries, Arizzi displayed a photo of different automobiles: a Model T, a Prius, and a Los Angeles traffic jam. “Right now PV is at the similar stage as the Model T. There are only one or two mass manufacturers in the industry. Like the Model T, all modules look alike to the untrained eye.” The photo of a Prius and the traffic jam—filled with 18-wheelers, motorcycles, and SUVs—was to illustrate how in the future PV will likely introduce niche products to address specific needs. “The future of PV is to start designing for niche needs like specific geographies and climates,” Arizzi said. “In the future we will see design adapt to end use applications.”

Arizzi also said it is difficult to predict specific developments in future PV modules, but we do know that they will:

• have novel functionalities

• be thin and light in order to enhance cost efficiencies

• start introducing substituted materials such as new polymers to substitute for glass and metal

Solar can learn two things from the electronics industry

• miniaturization achieved through higher density of transistors (However, in PV there is a physical barrier for how small systems can get.)

• systems integration to optimize cost, performance, reliability (Current examples for PV include: 1) batch sheet optimization, 2) enhancements at the subcomponent level such as backside integration of circuitry and front side integration with functionalities.)

Arizzi discussed the parallels between solar and the construction industry. A priority for construction is to build things that last a long time. “The two factors that will define PV of tomorrow will be reliability and durability. In the future we will require solar systems to have 50, even 100-year life spans.”

Ryne Raffaelle, Director of NREL’s National Center for Photovoltaics, began his strong presentation with a look back to 1995 when solar first broke the 30 percent efficiency barrier. Just two weeks ago Spire Semiconductor announced a solar cell with 42.3 percent efficiency. Raffaelle was confident that it is a matter of when, not if, solar will exceed 50 percent efficiency.

To get there, Raffaelle says we will need

• metamorphic growth

• mechanical stacking

• spectrum splitting concentrators

• quantum mechanic approaches such as lower band gap middle junction

Some of the existing cutting edge technology concepts include:

• hot carrier extraction

• nanophotonics

Like Arizzi, Raffaelle was able to outline the characteristics of future successful technologies:

• thinner cells

• improved efficiencies

• lower costs

• flexible material (this is very attractive on building integration)

• greater than 50 percent efficiency

• new materials (that are earth abundant, nontoxic, noncontaminating)

• quantum confinement approaches

With respect to substitutes for silicon, Raffaelle outlined some of the breakthroughs that first began with copper in the 1970s. Although copper is not sufficiently stable, copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) cells have become permanent parts of solar technology. “The exploration of other materials will continue. The kesterites look good and sulfur is virtually the same crystal structure as CIGS. If you think about future potential materials, it leads you back to silicon. One thing about silicon is that it is bulletproof. You get 80 percent performance at 30 years.”

The session moderator asked panelists to identify which material will dominate when 20 percent of power comes from solar? And which will be the first to reach grid parity? All panelists agreed this is a nearly impossible question to answer. However, there seemed to be consensus that in the future the dominant solar cell will be a hybrid of the positive attributes of crystalline and thin film.

Arizzi said, “I am convinced crystalline, thin film, and CPV will make it. One of the strengths of the industry is that there are multiple technology platforms and options. PV overall will succeed. It is only a matter of how big one will be over the other.”

Chris Constantine, Director of New Technologies at Oerlikon Solar, concluded the session on a strong note by saying: “The sun is a diffuse, nonuniform, resource. CPV works really well in certain areas but not in others. Thin film is more appropriate for others. We are all talking about timing. Each individual company needs to know, when in the next three years, there is a market for its technology that is at least as good the competition.”

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