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A unique method for processing ZnO nanowires and polythiophenes results in a hybrid structure with nanometer scale ordering and a high crystallinity, showing great promis for the future of hybrid photovoltaics. Credit: Advanced Materials; Wiley.

Inorganic/organic hybrid materials show great promise for use in photovoltaic devices, where the advantages of both types of materials could be combined to better harvest the sun’s energy. For the potential of hybrid materials to be fully realized, it isn’t sufficient simply to select the right materials, but the precise arrangement of these materials at small length scales can also be crucial to determining their ultimate properties and usefulness. Professor Chinedum Osuji and coworkers have developed a unique method for processing ZnO nanowires and polythiophenes that results in a hybrid structure with nanometer-scale ordering and a high crystallinity, showing great promise for the future of hybrid photovoltaics.

In order to understand why ordering is so crucial to the performance of hybrid photovoltaics, it is instructive to look closer at how such devices work. In its most basic form, the function of a photovoltaic device is to convert light energy from the sun into electrical energy (electrons and holes with negative and positive charges respectively) with a high efficiency. The light absorption usually occurs in the organic material, thereby creating an electron-hole pair. In order to extract useful work from the device, the electron must be transferred to the inorganic material, and then the electron and hole can be pulled to opposite sides of the device by the electric field in order to complete the electronic circuit. By ordering the materials at the nanoscale, it can be assured that the charge carriers have only a short distance to travel in order to complete this process. The molecular ordering within the polymer layer is very important where charge transport is much more favorable in a crystalline polymer than an amorphous one. Furthermore, the orientation is also key, where charge transport along polymer chains is more favorable than lateral hopping between chains. By controlling the arrangement of these materials as precisely as possible (and at multiple length scales), the losses from the device can be minimized to achieve a high power conversion efficiency.

Achieving this type of complex ordering at small length scales is not easy. To arrange each of these domains by hand, we would require nano-sized tools, and a macro amount of patience. However, by taking advantage of intermolecular and other long-range forces we can coerce the materials to order themselves. This class of methods is fittingly called “self-assembly.” Professor Osuji and coworkers were able to take advantage of electrostatic forces to graft a thiophene-based polymer onto the surfaces of ZnO nanowires, creating a new crystalline phase that does not exist when the polymer is processed alone. Then, in order to arrange the coated nanowires, a shear force was applied to the hybrid material to achieve the ordering shown in the image above. As an alternative method, the authors also achieved very similar ordering by applying a magnetic field after a small amount of cobalt was incorporated into the nanowires. In both cases, the result was an ordered hybrid material boasting a near-ideal geometric arrangement for charge collection in photovoltaic devices. With creative methods such as these, the future of hybrid materials appears bright, and we may reap the rewards in the next generation of solar cells.

The paper is: 10.1002/adma.201103708 ; DOI: Shanju Zhang et. al., Advanced Materials

John Ulrich is a writer for MaterialsViews.com. This story first appeared in the 1/5/2012 edition.

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Pyreheliometers are pointed toward the sun for their yearly calibrationat NREL. Accurate measurement of solar energy is needed for due diligence of photovoltaic installations. Credit: NREL.

If you have one, you probably were at the 16th annual convening of the NREL Pyrheliometer Comparisons in October in Golden, Colo.

If you don’t have one, it helps to know that a pyrheliometer is a type of radiometer for measuring the sun’s solar energy, and like all instruments, it needs to be calibrated regularly in order for its measurements to be valuable.

And, valuable they are. Solar energy measurements are used by almost anyone in the sunshine business: researchers, universities, the solar industry, and the bankers, venture capitalists and financiers who invest in them.

Calibrations are done against standards, and the international standard for solar energy is kept in Davos, Switzerland, at the World Radiation Center. Every five years instruments are calibrated against it in Davos. NREL is the only facility in the world that holds an annual event for intermediate recalibrations. According to the website, the goal of the gathering is to “transfer the World Radiometric Reference to international, national, and regional researchers.”

In the news release, NREL group manager Tom Stoffel says, “It’s all about traceability.” He continues, “This is all part of due diligence. … [if] you’re proposing a $30 million concentrating-solar-power plant, exactly how much direct sun are you going to get?”

The due diligence requirements have become very demanding since the 1980s. Twenty-five years ago, daily updates on the sun’s intensity in a few zones across the country were typical. Fifteen years ago, hourly reports became available. Now, according to the news release, “The gold standard is once-a-second updates at the exact spot where the solar installation is envisioned.”

NREL keeps a solar calendar that dates back to 1981 with data on the sun’s radiation starting at sunrise. DOE, through its Climate Research Facility in Lamont, Okla., maintains radiometers at 25 sites, three of which are internationally based and two that are mobile (and currently deployed overseas). The CRF, through its Atmosphereic Radiation Measurement program, has 20 years of solar and atmospheric radiation data. These days, data is taken every 60 seconds.

CRF electronic technician, Craig Webb was one of the participants at this year’s calibration event. In the news story, he sums up, “They want everybody to be tied to the same base so we can measure accurately the watts from the sun. That’s why we’re here every year.”

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At top, domains with opposite electrical polarization, averaging about 140 nanometers wide and separated by walls 2 nanometers thick, form a well-aligned array in a thin film of bismuth ferrite. When illuminated, electrons collect on one side of the walls and holes on the other, driving the current at right angles to the walls. Voltage increases as excess electrons accumulate stepwise from domain to domain. Credit: LBL

A news release out of Lawrence Berkeley National Lab reports that “the average installed cost of residential and commercial PV systems completed in 2010 fell by roughly 17 percent from the year before, and by an additional 11 percent within the first six months of 2011.” The information is from a recent LBL report (pdf), “Tracking the Sun IV: An Historical Summary of the Installed Cost of Photovoltaics in the United States from 1998 to 2010.”

The cost reductions are attributed to reductions in the cost of PV modules and to nonmodule costs, like labor, marketing, business overhead and nonmodule system components. The report authors note that the drop in nonmodule costs is significant because they can be influenced easily by policies intended to reduce market barriers and expedite deployment. Module cost reduction, however, is R&D dependent and more difficult to influence timewise.

One aspect of module cost reduction is to develop more efficient photovoltaic materials. High efficiency in solar cells is a function of voltage and current, and more (of both) is better.

Ferroelectric materials have very high photovoltaic responses to illumination, but the mechanism has been unknown. Some new research, also out of LBL, describes a model for the high voltages seen in thin-film bismuth ferrites (BFOs), which may provide some insight to developing more efficient PV materials. BFOs themselves are not candidate PV materials because they respond only to a very small slice of the solar spectrum (blue and near ultraviolet).

BFO thin films have a highly periodic domain structure (regions where the electrical polarization orients in different directions). Joel Ager, the lead researcher said in a press release, “When we illuminated the BFO thin films, we got very large voltages, many times the band gap of the material itself.”

The question is, why? The voltage measured across the film increased as the number of domains between electrodes increased, showing researchers that somehow the domain walls were involved.

The press release describes the charge-transport model that was developed:

The model presented a surprising, and surprisingly simple, picture of how each of the oppositely oriented domains creates excess charge and then passes it along to its neighbor. The opposite charges on each side of the domain wall create an electric field that drives the charge carriers apart. On one side of the wall, electrons accumulate and holes are repelled. On the other side of the wall, holes accumulate and electrons are repelled.

While a solar cell loses efficiency if electrons and holes immediately recombine, that can’t happen here because of the strong fields at the domain walls created by the oppositely polarized charges of the domains.

“Still, electrons and holes need each other,” says Ager, “so they go in search of one another.” Holes and electrons move away from the domain walls in opposite directions, toward the center of the domain where the field is weaker. Because there’s an excess of electrons over holes, the extra electrons are pumped from one domain to the next - all in the same direction, as determined by the overall current.

“It’s like a bucket brigade, with each bucket of electrons passed from domain to domain,” Ager says, who describes the stepwise voltage increases as “a sawtooth potential. As the charge contributions from each domain add up, the voltage increases dramatically.”

Ager expects that the mechanism will apply to all materials with “sawtooth” potentials, which opens the possibility of developing new PV materials with high voltage and high current.

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Solar power in Germany jumped 76 percent in first six months of 2011. Credit: NREL.

According to the German Association of Energy and Water Industries (BDEW), renewable energy grew—in the first half of 2011, alone—from 18.3 percent of total demand in 2010 to 20.8 percent. That’s a significant jump and achievement.

What makes it even more remarkable is that, for sheer timing reasons, it cannot be connected to Germany’s post-Fukushima policy decision to shut down its nuclear power facilities. Indeed, as Spiegel Online International reports, “[the increase] does give a boost to Germany’s long-term effort to phase out nuclear power completely by 2022.” Spiegel speculates that trend will give Chancellor Angela Merkel new political capital to offset dissension about the elimination of in-country nuclear power sources.

I don’t have access to an English version of the BDEW report, but Spiegel says the group attributes most of the change to photovoltaic installations. PV power now ranks third in the list of renewable sources (in terms of kilowatt-hours consumed), knocking wind power to fourth place.

Why the surge in the recent months? BDEW, according to news reports, cites two major factors: 1) a big drop in PV equipment prices (50 percent since 2006) and 2) continuation of federal subsidies for private PV generation (which had been targeted for elimination).

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