Next-Generation and Nano-Architectured Photovoltaics: A Field of Great Promises and Risks

Converting the widely available solar energy into electricity provides a necessary solution to the energy crisis we currently face. As we move towards a higher standard of living in the 21st century with an ever growing population, it will be difficult to live on a dwindling supply of fossil fuels. Although photovoltaic (PV) technology, more colloquially solar cells, has been widely used and established, the major obstacle that still hinders its development and large-scale application is the high cost of commercially available inorganic semiconductor-based solar cells. Recently developed photovoltaics (OPVs), however, offer an alternative route to solar cell technology by converting sunlight directly into current, creating electric power.[1] Though these devices show much promise, the most complicated issue is in electron transport through the layers of the devices. Despite this mechanistic challenge, the efficiency can be improved through systematic nano-engineering and the development of nano-architecture that is optimally matched to the properties of these photovoltaic materials.

Enter Titanium Nanorods!

A new approach towards higher efficiencies of solar cells incorporates the use of nanotechnology. For instance, at the National Taiwan University, efficient photo-induced charge transfer has been shown to occur at titanium nanorod/polymer interfaces, which enhances the separation of charges, resulting in current.[2] Approximately 35 nm in length and 4 nm in diameter, the TiO2 nanorods act as highways for charge transfer and extend the interfacial area for photogenerated charge transfer, due to their large surface-to-bulk ratio. Additionally, the introduction of TiO2 nanorods further reduces disorder in the morphology of the devices, which increasing efficiency in the photovoltaic devices. In terms of applications, nanorods will be more suitable for polymer solar cells with respect to the widely used CdSe nanorods. However, despite these promising results, there are still problems of carrier mobility, stability and the length and diameter of the TiO2 nanorods and their alignment within the polymer, which will affect the energy output.

Figure 1. TEM image of TiO2 nanorods with a size of 4–5 nm in diameter and 20–40 nm in length.

Figure 1. TEM image of TiO2 nanorods with a size of 4–5 nm in diameter and 20–40 nm in length.


Oooo….Quantum Dots!

Besides nanorods, quantum dots (QDs) are also another area of interest. QDs, or tiny semiconductor particles generally no larger than 10 nanometers, can be made to fluoresce in different colors depending on their size. What makes them special is that they last much longer than conventional dyes used to tag molecules, which usually stop emitting light within seconds. Also, only a single container, or “pot,” is needed to prepare these QDs within a few hours. In the area of energy applications, QDs can produce electrons when they absorb light, making possible extremely efficient solar-energy devices via multiple exciton generation (MEG). Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), collaborating with Innovalight, Inc., have shown that an important effect called Multiple Exciton Generation (MEG) occurs efficiently in silicon nanocrystals, or quantum dots. Essentially, MEG results in the formation of more than one electron per absorbed photon, and so far has only been reported to occur only in quantum dots of semiconductor materials. For comparative purposes, when today’s photovoltaic solar cells absorb a photon of sunlight, about 50% of the incident energy is lost as heat, but MEG is capable of converting this energy lost as heat into additional electricity.[3]

a 1 μm x 1 μm surface imaging of InAs quantum dots on GaAs/InP, (inset) a single InAs quantum dot.

AFM micrographs of: a 1 μm x 1 μm surface imaging of InAs quantum dots on GaAs/InP, (inset) a single InAs quantum dot.

Ambiguous Risks

It is clear that the field requires materials advances, fundamental advances in physical understanding, as well as technological improvements. The central challenges include improved physical understanding, materials, transport and surfaces, and more importantly, the health and environmental risks of this type of nano-engineering.

Whether it be quantum dots, nanorods, or carbon nanotubes, they are all microscopically sized and exhibit unique properties potentially useful in electronics, optics and various other materials. They are manufactured and synthesized in many different ways, and produce different results when trying to assess their safety. “To confound the situation further,” University of Oregon’s Jim Hutchinson writes, “the methods of production are still immature for most materials, often resulting in batch-to-batch variability in composition and purity.” [4] Impurities, are hard to detect, difficult to extract and may obscure the real effects of nanomaterials. He believes the way to go is to employ green chemistry before the field of nanomaterials and nanoparticles becomes fully launched.

Nanomaterials are complex, as are their interactions with biological organisms and the environment. However, based on known studies, the concentration and distribution of surface oxides exert a profound influence on the aquatic stability and sorption properties of multiwalled carbon nanotubes (MWCNTs).[5]

More aggressive oxidizing conditions lead to larger changes in the surface oxygen content without altering the physical characteristics of individual MWCNTs (e.g., length and structural integrity).

Oxides on carbon nanotubes

Oxides on carbon nanotubes

By identifying the interplay between the MWCNTs surface chemistry and other properties, we can more adequately prevent the dispersion of toxic chemicals in water and biological media like in people. It is important to understand the mechanistic aspects of CNT aggregation and deposition and to conduct transport studies to measure the mobility of oxidized MWCNTs through porous media with and without the presence of contaminants.

Influence of oxygen functional group distribution on the aquatic stability of oxidized MWCNTs. The solution chemistry of three vials, each containing an equal concentration of oxidized MWCNT particulates (overall oxygen concentration shown on the cap of each vial) at pH7, has been modified by the addition of 0.07MNaCl, vortexed and allowed to settle under the influence of perikinetic flocculation for 120 min.

Influence of oxygen functional group distribution on the aquatic stability of oxidized MWCNTs. The solution chemistry of three vials, each containing an equal concentration of oxidized MWCNT particulates (overall oxygen concentration shown on the cap of each vial) at pH7, has been modified by the addition of 0.07MNaCl, vortexed and allowed to settle under the influence of perikinetic flocculation for 120 min.

The Future of Nanomaterials

The rise of solar energy, in one form or another

The rise of solar energy, in one form or another

Though there are various benefits of nanomaterials, the quality of our environment, the future vitality of the American economy, and the health of workers and consumers are all at risk to some extent. Hence, it is especially ciritical to examine the risks and remedy them for the continuing progress of nanotechnology. Before nanotechnology can gain mainstream acceptance in solar cells, for example, there needs to be a more comprehensive research strategy, involving input from academia, industry, consumer and environmental groups.



[1] “Another Silicon Valley?” The Economist. http://www.economist.com/specialreports/displaystory.cfm?story_id=11565636. 30 January 2009.

[2] Lin, Yu-Ting Lin, et al. Nanotechnology 17 (2006) 5781.

[3] M.C. Hanna, et al. “Quantum Dot Solar Cells: High Efficiency through Multiple Exciton Generation”. NREL. 2007.

[4] “Potential Nanotech Hazards Are Hard to Determine, Researchers Urge Proactive Approach.” http://www.sciencedaily.com/releases/2008/03/080331130252.htm. 27 January 2009.

[5] Fairbrother, Howard, et al. Surface Oxides on Carbon Nanotubes (CNTs): Effects on CNT Stability and Sorption Properties in Aquatic Environments. Nanoscience and Nanotechnology. 2008.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: