Solar Cell Efficiency – Approaching a Tipping Point to Widespread Adoption

Thu, 09/03/2017 - 11:08


Compared to other renewable energy sources, solar has always enjoyed a relatively positive reputation. Solar panels do not present the visual intrusion of a wind turbine, nor the environmental impact of a nuclear power plant. The real obstacle to broader adoption lies in the science behind solar power itself – solar panel efficiency. Businesses, homeowners and developers are not likely to invest heavily in solar energy until solar panel efficiency reaches a tipping point – delivering performance that spurs large scale purchasing and lower manufacturing costs. With this goal in mind, several companies are actively seeking solutions on ways to improve solar panel efficiency – accelerating innovation and bringing the concept of ubiquitous solar energy closer to a reality.

In 2016, Kaneka Corp. achieved a significant milestone in solar panel efficiency with a conversion efficiency of 26.33 % in a practical size monocrystalline silicon solar cell – surpassing the previous record by 0.7 %. This past November, Natcore Technology demonstrated an efficiency of 19.4 % – showcasing their efforts at improving the all-black-contact Natcore Foil Cell. Natcore’s efforts at increasing the efficiency of this laser-formed base contact has been their primary focus – and has been limited by the higher resistance of the contact itself, in addition to damage from the laser process. A new laser-based contacting process has generated quick results for Natcore, increasing efficiency by 2 % in less than six months.

MetaShield LLC has approached this challenge with the development of MetaShieldPV, a nanoparticle-based light trapping coating for III-V triple junction solar cells that, when applied on top of current anti-reflective coatings, increases cell efficiency by up to 1.2 % (absolute).  The company is currently working with PV manufacturers on reducing the cost of silicon solar panels by reducing the thickness of the wafers, using gains from its MetaShieldPV coating to compensate for the resulting lost efficiency. 

A particular high water mark was reached in 2016 by U.S. wafer producer 1366 Technologies and Hanwha Q CELLS, who combined their efforts to set a new record of 19.6 % efficiency this past December using the “direct wafer” process. This was achieved using 1366’s kerfless, drop-in 156-mm multi-crystalline wafers and Hanwa’s Q-ANTUM PERC cell process – creating crystalline wafers directly from molten silicone in place of additional steps that would require more time and energy to produce. 2016 has been a tremendous year for innovation in solar technology – and we can expect more of the same in 2017. Not only are manufacturers eager to build on previous advances to achieve even higher levels of efficiency, but many are also motivated to differentiate themselves from the competition as the market moves closer to broader adoption.

At some point in the future, researchers will be limited by the principles of physics in terms of increasing solar cell efficiency. As the industry moves toward that point, however, individual methods of testing the limits of solar technology are closely guarded by each manufacturer – increasing the diversity of research itself and accelerating progress. That said, most players in this space focus on a few key approaches to improving solar cells.  Overall, most researchers are concentrating on the following areas:

  • Improved defect passivation at the solar cell’s material surface and bulk to reduce charge carrier recombination;
  • Reduced series resistance at contacts;
  • Developing all back-contact solar cells to avoid light-shading;
  • Developing more ways of trapping light, including better texturing and incorporating nanoparticles; and
  • Developing materials for broadband-spectral shift to make use of wavelengths that solar cells currently cannot absorb.

While monocrystalline solar cells are approaching their theoretical limit of 30 %, some have predicted that spectral downshift and upshift can raise this limit to 40 and 50 %, respectively. This advance may still be a long way off, however, as spectral shift technology is still in the early stages of development with necessary materials absorbing light in a narrow spectral range and shifting wavelengths with very low efficiency.

Given this pace of innovation, the limits of solar cell technology and the competitive market, how will the industry move ahead as a whole? First, steps need to be taken to increase profitability by reducing manufacturing costs. Some companies are responding through an increased focus on research and developing, including Canadian Solar’s move to retain 350 R&D professionals in the midst of a cost reduction. MetaShield is leading this effort by researching the most efficient methods to absorb the maximum number of photons available in the solar spectrum, including taking a close look at plasmonics and dielectric nanoparticles because of their light trapping properties. MetaShield is also studying the spectral shift of solar spectrum to absorb the spectrum portion lost due to transmission or heat. Looking ahead, it is expected that plasmonics-based technologies will accelerate the use of thin-film solar panels, which is turn will reduce overall manufacturing costs. MetaShield’s MetaShieldPV coating incorporates the latest advances to generate more power with less balance-of-system – making marginal PV projects more cost-effective.

While solar energy has always been viewed as an environmentally-responsible choice, economics remains the most influential motivator to adoption. In fact, the installed price of solar energy has declined significantly in recent years, resulting in a rising trend in installations that is not expected to slow down in the near future. The good news is that plasmonics-based technology promises low-cost solar panels with a much thinner absorption material, using nanoparticles for optical enhancement. Once this technology becomes widely available, the market moves increasingly closer to producing utility-scale solar energy – and achieving the tipping point necessary to make solar energy commonplace.

Puruswottam Aryal, PhD., is an R&D project manager in MetaShield, LLC’s solar division. He specializes in physics and materials science with a special focus on thin film photovoltaics. Aryal has authored a number of peer-reviewed journals and conference proceedings in his field. He can be reached at paryal@metashield.com or 212-938-3699.