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In Hot Pursuit of Cost-Effective Solar Power

RCSA President Robert Shelton told the 70 or so participants in the 5th-annual Scialog Conference on Improving Efficiencies in Solar Conversion, held near Tucson, Arizona, Oct. 14-17, that the path to widespread use of solar energy remains complex, and the lay public still finds it difficult to perceive how our nation can realize the immense potential of converting sunlight into usable power.

“But that, of course, is where you come in,” Shelton told the gathered chemists, physicists, materials scientists and mathematicians. “Collectively you hold the keys to success. You have the knowledge, both theoretical and applied, to make this happen. You all understand that we are at a critical juncture in our history. The methods and approaches of yesterday are failing us, and we need to turn in new directions that are inherently sustainable, economical and lead to economic growth without the kind of damage to the environment that we’ve been so willing to overlook in the past.”

The unofficial theme of this year’s conference clearly was that steady progress is being made in developing inexpensive, earth-abundant materials to produce electricity and liquid fuels directly from sunlight.

While photovoltaic solar cell efficiency has risen, very dramatically in some cases, with advances in materials to optimize electron/hole transport and band gap properties, many of these devices, traditionally based on silicon wafers, continue to rely on expensive rare earth elements and complex manufacturing processes.

Next Generation PV Technologies

Meanwhile, “The next generation of PV technologies based on thin films of compound semiconductors or even organic materials are showing excellent potential for low-cost electricity generation,” said Gian-Luca Bona, CEO of Empa, the Swiss Federal Laboratory for Materials Science and Technology, and a Scialog keynote speaker.

Many Scialog Fellows are engaged, directly or indirectly, in fundamental research that could one day improve thin film efficiencies. Eventually their work may lead to spray-on roof coatings and sun-absorbing window tinting that produce electricity for homes and offices.

Another keynoter, Prashant Kamat, of Notre Dame, mentioned such possibilities as he discussed recent progress in organic metal halide perovskite photovoltaics.

Perovskite is a calcium titanium oxide (CaTiO2) abundant in the earth’s mantle. Although perovskite is relatively new to the photovoltaic process, Kamat noted that efficiencies in the range of 17.9 % to 19.3 % have already been reported.

By contrast silicon, when first put to work in 1941 converting photons to electrons, was less than 1% efficient; in 2014, three companies broke the record of 25.6% for a silicon solar cell  [http://onlinelibrary.wiley.com/doi/10.1002/pip.892/pdf] . These efficiencies are achieved by doping the crystal-lattice structure of the silicon molecules, as well as by coating the surface of the cells with exotic molecules and other photon-absorbing/electron-producing substances that are sensitive to increasingly wider areas of the visible -- and infrared -- spectrum.

Kamat noted that the interplay between the charge transfer complex state and charge separated state in the hybrid organic–inorganic perovskite variant CH3NH3PbI3 leads to efficient charge separation. “Insights into the charge recombination kinetics and band filling aspects of [this material] provide new ways to tune the efficiency of perovskite solar cells,” he said.

Currently, the major stumbling block to producing cost-effective pervoskite-based thin films seems to be the material’s susceptibility to humidity, Kamat said.

The OPV Holy Grail

Another keynoter, Mark E. Thompson, University of Southern California, discussed his attempts to get around the current/voltage trade-off in organic (carbon/hydrogen-based) photovoltaics (OPVs). Carbon and hydrogen are among nature’s most abundant materials, making high-efficiency electrical generation from OPVs a sort of Holy Grail among those seeking the ultimate in cheap solar power.

Thompson noted that the exciton is a critical part of each of the processes leading to photocurrent generation in OPVs, and that being able to control the location, lifetime and energy of the exciton is essential to achieving high efficiency.

(An exciton is a mobile concentration of energy in a crystal formed by an excited electron and an associated hole. In the photovoltaic process the electron is excited by its interaction with a photon, the smallest particle of light. A “hole” in this context is roughly defined as a space that an electron can move through within the molecular lattice structure of a crystalline semiconductor.)

“We have investigated methods for tuning exciton energies and controlling their migration paths, both intramoleculary and within a thin film,” Thompson said, adding that his most recent OPV work has focused on organic dyes – squarines and dipyrrins – as well as porphyrric materials.

“This involves a careful materials design study that leads to both low energy absorption into the near infrared and the efficient use of multiple absorbers to efficiently harvest photons through the entire visible spectrum,” he said.

Focus on Nanoscience

Much of today’s innovative research into improving photovoltaic conversion efficiency has been centered in nanoscience, the exploration of molecular-sized, or near-molecular-sized, bits of material.  Matter and energy interact in sometimes surprising ways at extremely small scales.

Susan Kauzlarich, UC Davis, delivered a keynote address that included a discussion of her work with germanium (Ge) and silicon (Si) nanoparticles for photovoltaics.

“We have developed a simple approach for the preparation of crystalline Ge nanoparticles with control on size by microwave assisted heating,” she said, adding that her research has also included examinations of the function of Ge nanocrystals based on their size.

In addition she discussed her work with Zintl compounds – named for German chemist Eduard Zintl. They are brittle, high melting point intermetallic compounds which Kauzlarich and her colleagues have been probing for their ability to produce electricity from heat, including perhaps one day, heat from the sun.

Of course the sun is also the energy source driving photosynthesis, the 3.4-billion-year-old process by which plants convert sunlight into chemical energy. In his keynote address Caltech’s Jonas C. Peters discussed the possibilities for productive synergy between nitrogen fixation catalysis and artificial photosynthesis, an achievement that might eventually enable the production of nitrogen fertilizer without the massive energy requirements and resulting CO2 pollution of the Haber-Bosch process.

Although admitting this line of research is a long shot, “ultimately, coupling sunlight with water oxidation, fuel generation, and nitrogen fixation can provide a path to a more sustainable future,” Peters predicted.

Meanwhile, at the federally funded Joint Center for Artificial Photosynthesis (JCAP), rapid progress has already been made toward increasing the efficiency of artificial photosynthesis for the production of liquid fuels, reported JCAP Scientific Director Nate Lewis, of Caltech.

Lewis, designated a Distinguished Speaker at the conference,  is chair of the Scialog Review Panel, the nine-member group that vets researchers’ proposals to ensure RCSA funding supports work that is high-risk and potentially high-reward when it comes to overcoming roadblocks to high-efficiency solar conversion.

Molecular-Level Energy Conversion Machines

JCAP, funded at $122 million over five years, has as its ultimate goal nothing less than the creation of highly efficient, non?biological, molecular?level energy conversion “machines” that generate fuels directly from sunlight, water, and carbon dioxide. 

In the two years since the creation of JCAP, Lewis said, researchers have met a number of initial goals associated with “synergistic innovation,” which he defined as the principle “that everything would be advantageous in the presence of everything else.”

Basically, Lewis and his colleagues in JCAP’s Molecular and Nanoscale Interfaces project are, as their website states, seeking “to couple light absorbers, catalysts, and half-reactions for optimal control of the rate, yield, and energetics of electron and proton flow at the nanoscale.”

That means, for example, “designing a structure that includes a long axis to absorb the light but integrated with a short axis to move the excited state sideways over a short distance instead of a long distance; and we could therefore use impure material in a way that we couldn’t do in a photovoltaic separated cell. That was concept one, it was proven by modeling and it turned out to be true,” Lewis said.

He also reported that researchers have found materials to reduce proton resistance in the transfer of charged particles, among other recent accomplishments.

Another distinguished speaker, Krishnan “Raj” Rajeshwar, University of Texas at Arlington, also a member of the Scialog Review Panel, delivered what he termed a “tutorial” on underlying concepts in inorganic semiconductor-based photocatalysis.

Forging a Common Language

Rajeshwar’s said his talk was prompted by the explosive growth of irradiated semiconductor coated surfaces for use in solar photoelectrolysis of water as well as photovoltaic solar cells, chemical/biosensors, self-cleaning surfaces, air purification, and anti-fogging devices. All of which, he observed, “brings with it unique challenges in forging a common ‘language’ and conceptual framework for communicating results and ideas” among various fields of research, much less explaining it to the public.

“This is an area where much confusion reigns in terminology, definitions, and operating terms,” he noted. Rajeshwar introduced a vector diagram to help people visualize electrochemical cell process and to explain simply and clearly cell thermodynamics, voltages, half-cell potentials, and spontaneous vs. non-spontaneous electron flow directions.

In addition to presentation by distinguished scientists, the emphasis at the 5th-annual conference, as at previous events, was also on dialog and team building, with the goal of creating novel strategies for overcoming research bottlenecks. A record 15 impromptu teams developed during the conference and presented proposals that will be evaluated for innovative thinking.

RCSA Program Director Richard Wiener said the Foundation will fund the winners of the competition, although no set number of grants had been determined.

“Innovation and discovery have no boundaries,” Wiener said. “Our goal in evaluating these proposals is to consider the possibility of unorthodox or unusual ideas without immediately dismissing them.”

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