Power/Performance Bits: Sept. 3

Flexible, organic solar cells
Work by a team of chemical engineers at Penn State and Rice University may lead to a new class of inexpensive organic solar cells.

If solar cells could be made as easily as posters or newspapers are printed, sheets of organic solar cells could be made, representing a fundamental shift in the way solar cells are made, the researchers said.

Today, most solar cells are inorganic and made of crystalline silicon. The problem with these is that inorganic solar cells tend to be expensive, rigid and relatively inefficient when it comes to converting sunlight into electricity. But organic solar cells offer an intriguing alternative that’s flexible and potentially less expensive.

However, not many organic solar cells currently exist because the bulk of them employ fullerene acceptors — a carbon-based molecule that’s extremely difficult to scale up for mass production. The Penn State-Rice University approach skips the fullerene acceptor altogether and seeks to combine molecules in a solution.

Work by a research team at Penn State and Rice University could lead to the development of flexible solar cells. The engineers’ technique centers on control of the nanostructure and morphology to create organic solar cells made of block polymers. (Source: Penn State)

Their organic solar cell made of block copolymers is 3% efficient, but the researchers believe they can do better than this.

Sensitive as a dog’s nose
Using carbon nanotubes, a research team including scientists from ETH Zurich and the Lawrence Livermore National Laboratory (LLNL) has developed a sensor that greatly amplifies the sensitivity of commonly used but typically weak vibrational spectroscopic methods, such as Raman spectroscopy. This type of sensor makes it possible to detect molecules present in the tiniest of concentrations.

Thanks to its unique surface properties at nanoscale, the method can be used to perform analyses that are more reliable, sensitive and cost-effective. In experiments with the new sensor, the researchers were able to detect a certain organic species (1,2bis(4-pyridyl)ethylene, or BPE) in a concentration of a few hundred femtomoles per litre. A 100 femtomolar solution contains around 30 trillionth of a gram of this organic species in one liter of solution.

Until now, the detection limit of common SERS systems was in the nanomolar range, i.e. some millionth of a gram of organic matters per liter.

Raman spectroscopy takes advantage of the fact that molecules illuminated by fixed-frequency light exhibit ‘inelastic’ scattering closely related to the vibrational and rotational modes excited in the molecules. Raman scattered light differs from common Rayleigh scattered light in that it has different frequencies than that of the irradiating light and produces a specific frequency pattern for each substance examined, making it possible to use this spectrum information as a fingerprint for detecting and identifying specific substances. To analyze individual molecules, the frequency signals must be amplified, which requires that the molecule in question either be present in a high concentration or located close to a metallic surface that amplifies the signal. Hence the name of the method: surface-enhanced Raman spectroscopy.

This technology has been around for decades but with today’s SERS sensors, the signal strength is adequate only in isolated cases and yields results with low reproducibility. Therefore, the researchers set themselves the goal of developing a sensor that massively amplifies the signals of the Raman-scattered light.

The substrate of choice turned out to be vertically arranged, caespitose, densely packed carbon nanotubes (CNT) that guarantee this high density of ‘hot spots.’ The team developed techniques to grow dense forests of CNTs in a uniform and controlled manner. The availability of this expertise was one of the principal motivations for using nanotubes as the basis for highly sensitive SERS sensors.

The basis of the high-sensitivity sensor are carbon nanotubes having curved tips. The numerous gaps let through the Raman scattered light. (Source: ETH Zurich)

The researchers envision installation of the technology in portable devices, for example, to facilitate on-site analysis of chemical impurities such as environmental pollutants or pharmaceutical residues in water. Other potential applications could include forensic investigations or military applications for early detection of chemical or biological weapons, biomedical application for real-time point-of-care monitoring of physiological levels, and fast screening of drugs and toxins in the area of law enforcement.

~Ann Steffora Mutschler