The Next Wave of PV Technologies: Silicon PV Evolution, Not a Revolution

The Next Wave of PV Technologies: Silicon PV Evolution, Not a Revolution

By James Montgomery, SEMI

The solar PV industry rose up over the past seven-plus years largely due to massive capacity expansions, but scale alone won't pave the way for solar PV in the coming years. What's also needed are new technologies and methods to improve the energy performance that devices put out, and the efficiency of the systems around them, all ultimately helping to lower costs upstream to downstream for solar PV.

"We're getting close to the cost of coal," noted Frank van Mierlo, CEO of 1366 Technologies, pointing to Austin Energy's recent power-purchase deal for less than 5¢/kWh. Yet there's no time to "order champagne and celebrate how far we've come," he urged. "We need to make further cost reductions everywhere."

As PV makers push ahead with gigawatt-scale expansions requiring massive capital investments, they are increasingly eager to embrace technology improvements that increase efficiency without increasing production costs. Equipment vendors "have wrung huge costs out of their equipment to deliver current processes," acknowledged John Benner, executive director for the Bay Area PV Consortium (BAPVC). Now the challenge and interest among engineers is developing new processes that can be implemented in today's production lines, including next-generation tools, to produce PV devices for much lower capex, especially up-front investments. How these new solar PV technology innovations will overlay with PV manufacturing operations over the next couple of years is the topic being tackled by a panel of experts, including Benner and van Mierlo, at the July 7th SEMI PV Advanced Manufacturing Forum at Intersolar North America in San Francisco, Calif.

Most of the technology innovations in silicon-based solar PV will build on the existing infrastructure in place, a trend confirmed by the latest version of the International Technology Roadmap for Photovoltaic (ITRPV). Manufacturers will increasingly embrace n-type silicon. Diamond wire saws are seen replacing steel wire to slice wafers. Anti-reflective films on solar cell surfaces can enable higher efficiencies without investing a lot of capital, noted Fatima Toor, research analyst at Lux Research. Other specific sweetspots of technology/process improvements include high-performance encapsulants, backsheets, and metallization pastes, which can improve modules' absolute efficiency by one to more than three percent, respectively. Focusing on PV improvements via exotic new materials "is a fool's errand," suggested van Mierlo; after billions of spending on development, "at some point you look at the data and move on."

Silver consumption is another popular target. The industry has long sought ways to use less of the expensive material, and vendors have responded by sharply reducing silver content by as much two-thirds from just a few years ago. Copper could replace much of silver's functionality at a fraction of the material cost; it also introduces some integration challenges, and a wholesale swap is debatable, though van Mierlo suggests "you just need good hard work by brilliant R&D teams."

Silicon itself remains a prime target of cost-cutting efforts. The industry has "done a fabulous job" of improving silicon's quality and reducing costs over time, van Mierlo said, but there are "lots of ways to still do better," from the emergence of high-performance back-contact devices to high-quality texturing, to further streamlining the wafer manufacturing process. That's also the area of focus for 1366 Technologies and fellow Intersolar panelist Crystal Solar: ultrathin silicon wafers just tens of microns thick, which require far less starting material and fewer associated costs such as energy, consumables, and subsequent waste post-processing. BAPVC members are also working on ways to get light into those thinner structures more effectively, Benner added.

Several of these panelists agree on the need for diligence on these incremental improvements in silicon PV rather than new leaps ahead. Driven by the broader goals of competing with the cost of coal, and the DOE's $1/W SunShot targets for total installed solar costs, it can be difficult to appreciate the individual contributions from improvements in silicon, cell efficiencies, balance-of-system technologies, etc. "People tend to underestimate incremental change," mused Marcie Black, president and co-founder of Bandgap Engineering, whose nanowires can boost solar cells' efficiencies by roughly 1% absolute, swapping out only the surface texturing step of the cell manufacturing process.

Relative investment per MWp for a c-Si cell production line. (Credit: ITRPV)

PV companies won't have to solve these cost and reliability issues alone. Industry and university researchers "have different perspectives on the problems," Benner noted, and bringing those two groups together can "spark some great innovation." That's the approach used by BAPVC, a U.S. DOE-backed group led by Stanford University and University of California/Berkeley. BAPVC helps support and connect university research with industry needs,  focusing on higher-performance cells, photon management, absorbers and cells (both silicon and thin film), encapsulation and reliability — all targeting a three- to five-year window to transfer to industry. The focus is not necessarily “drop-in ready” but on allowing companies to blend and tweak the technology to their own needs.

That three- to five-year timeframe by definition emphasizes silicon PV technology and "rules out some of the exotic stuff," but Benner is quick to point out that research work can move quickly when its impact is identified. He pointed out that in the early 1990s, the low resistivity and poor performance of p-type silicon wafers was little understood, until the realization of better incorporating iron led to a quick adjustment to handling wafers and higher p-type doping — a "very fundamental change" that the industry "implemented almost instantaneously." And it's very early, but he also pointed to the rapidly evolving work around perovskites which show excellent promise in performance and simplicity of process.

The key is to remain vigilant "in places where robust engineering can steadily advance that learning curve," Benner said. "A little change in silicon results in a big change in PV."

Speakers from National Renewable Energy Labs (NREL), IMEC, 1366 Technologies, Crystal Solar, Bandgap Engineering, and the Bay Area PV Consortium all will discuss the next wave of “PV Technology Innovation” at the SEMI PV Advanced Manufacturing Forum at Intersolar North America (co-located with SEMICON West 2014), on Monday afternoon, July 7, in San Francisco, Calif.  Registration is open.

June 30, 2014