Going Over to the Dark Side

The University of Texas at Austin’s press release spells out the quantum-like behavior of photons striking solar cells, and provides some insight into why obtaining higher efficiencies so far has perplexed researchers.

“AUSTIN, Texas — The efficiency of conventional solar cells could be significantly increased, according to new research on the mechanisms of solar energy conversion led by chemist Xiaoyang Zhu at The University of Texas at Austin.

“Zhu and his team have discovered that it’s possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.

“’Plastic semiconductor solar cell production has great advantages, one of which is low cost,’ said Zhu, a professor of chemistry. ‘Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies.’”

Pentacene stick and ball model

Zhu and his team published their discovery December 16 in the journal Science, under the title “Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer,” although the abstract may bring more darkness than light to this modest attempt to enlighten.

“Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation.  We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.”

The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun’s energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of “hot electrons,” is instead lost as heat. Capturing hot electrons “could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.”

The press release provides a four-point explanation of the quirky behavior that now limits solar cell efficiency, but also shows how the current causes of the waste heat could be exploited for greater gain.

  • Absorption of a photon in a pentacene semiconductor creates  an excited electron-hole pair called an exciton.
  • The exciton is coupled quantum mechanically to a dark  “shadow state” called a multiexciton.
  • This dark shadow state can be the most efficient source of  two electrons via transfer to an electron acceptor material, such as fullerene,  which was used in the study.
  • Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

Trying to capture that waste energy as reported in Science in 2010, Zhu and his team used semiconductor nanocrystals, but noted the challenges involved in that technology.

“For one thing,” said Zhu, “that 66 percent efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel.  This creates problems when considering engineering a new material or device.”

As an alternative, the team discovered that a photon produces a dark quantum “shadow state” from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene, an organic semiconductor consisting of five linearly-fused benzene rings.

This plastic semiconductor material can  apparently be easily and inexpensively manufactured, adding to its attractiveness for further research and development.

The UT press release continues, “Zhu said that exploiting that mechanism could increase solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more  widespread use of solar technology.

“The research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in Zhu’s group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was supported by the National Science Foundation and the Department of Energy.”

The Los Angeles Times, noting the new technology is “way  cheap,” reports, “All of this goes to reinforce a recent study by Joshua Pearce at Queen’s University in Kingston, Ontario, who found that cost estimates for solar technology used by energy analysts are greatly inflated. The technology is changing so fast that many studies don’t reflect the newest realities. For instance, the cost of solar panels has dropped 70% since 2009, and their productivity only declines 0.1% to 0.2% per year, rather than the 1% per year that was the norm.

“The bottom line? Commonly used studies have listed dollars-per-watt of electricity as high as $7.61. According to Dr. Pearce, the real cost in 2011 is under $1 per watt.”

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