Even though the U.S. government shows strong support for alternative energy related initiatives in our universities, including solar energy projects, it is still not even close to the support we observe in other countries like Germany, England, Canada, and so on. In fact, looking at the record breaking solar cell efficiencies in the last 15 years, as seen in the following graph, one can only notice a single American university (UCLA with Prof. Yang’s Tandem Organic Solar Cell in 2012) at a single spot. All the rest of the records belong to private research labs, government entities, and foreign universities.

Solar Energy Efficiency Chart

The graph above is from National Renewable Energy Laboratory (NREL) and the most recent version can be seen by clicking on this link.

For example, École Polytechnique Fédérale de Lausanne (EPFL) dominates the dye-sensitized solar cells field with Prof. Michael Grätzel’s work and University of Toronto keeps breaking its own records in quantum dot solar cells with Prof. Edward H. Sargent’s work. The latter is a very new & very promising emerging technology in photovoltaics. One of the strongest teams working on quantum dot solar cells in U.S. is the scientists in the picture above, the Asbury Group from Penn State University.

Before discussing the Asbury Group and their projects, we would like to thank Prof. Asbury for taking the time to provide us the documents & information we needed for this review. We are excited & honored to review one of the pioneers of quantum dot solar cells in United States.

Quantum Dots are nano particles that emit photons in different wave length (color) based on the particle size. This is a very distinct feature as other materials used in photovoltaics show the same features no matter what the particle size is. The following picture demonstrates what happens when different size Quantum Dot particles are exposed to light. The smaller the Quantum Dot, the closer the wavelength (color) is to blue and the larger the Quantum Dot, the closer the wavelength (color) is to red. Based on the size, the wavelength can also be set beyond what’s visible to human eyes, i.e. infrared.

Solar Energy

By combining different size Quantum Dot particles, theoretically the whole spectrum of light can be absorbed. With the materials currently used in solar energy field, like Crystalline Silicon, the energy band is fixed, thus, only a portion of light can be absorbed no matter what the particle size is.

One of the most significant & exciting projects the Asbury Group worked on, as explained in their 2011 paper “Colloidal-quantum-dot photovoltaics using atomic-ligand passivation” (click here to download the paper), resulted in quantum dot solar cell efficiencies up to 6%, an unprecedented number an American university has never seen.

The team used inorganic ligands (atom/ion that is bonded to a central atom/ion with covalent bonds); i.e. F- (Fluoro), Cl- (Chloro), Br- (Bromo), I- (Iodo); to passivate surface defects in quantum dot solar cells. This was the first reported use of inorganic ligands as photovoltaic devices all used organic ligands prior to this work. The reason behind the team’s move towards inorganic ligands was the bulkiness, vulnerability oxidation, and vulnerability to thermal degradation associated with organic ligands.

Solar Energy - Asbury Group - Penn State

Keep in mind that the highest certified quantum dot solar cell efficiency today is 7% (University of TorontoProf. Edward H. Sargent – 2012), as seen in the NREL graph earlier. In this effort, Prof. Asbury and his colleagues reach an NREL certified 5.1% (with a maximum of 6%) efficiency. That is extremely impressive and extremely promising. Moreover, these results were achieved in room temperature, in air, without any annealing. The following figure provides more details about the photovoltaic device they built and their results.

Solar Energy - Asbury Group - Penn State

It is worth mentioning that the team was able to create more than 30 solar cells in this fashion, all with efficiencies higher than 5.5%. National Go Solar Foundation is, of course, not the only organization impressed by Prof. Asbury’s work in quantum dot solar cells. Following the work with inorganic ligands, in 2012, Prof. Asbury received Department of Energy Early Career Research Award, which came with a minimum of $750K grant to be used over 5 years.

Another impressive work by Prof. Asbury and his colleagues was about mobility lifetime, as explained in their 2012 paper “Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics” (click here to download the paper).

The simplest way to explain mobility lifetime is to think about a simple solar cell with two separate layers for the electron donor & electron acceptor. When light reaches the donor with enough energy, an electron is released by the donor towards the acceptor. The thicker the donor is, the more photons the material absorbs. However, one cannot simply increase the size of the donor to increase the light absorption in order to improve the efficiency of a solar cell. The thicker these materials are, the more energy the electrons require to travel. The diffusion length, average distance an electron can travel before recombining with a hole, is proportional to the square root of mobility lifetime. By increasing mobility lifetime, we would increase the distance an electron can travel before recombination. In return, we would be collecting more electrons using thicker materials, which would increase efficiency of those materials.

In this paper, Prof. Asbury and his colleagues point out how crucial the selection of ligands is in quantum dot solar cells. As an example, the following figure shows the team’s results when 3-mercaptopropionic acid (MPA) and ethanedithiol (EDT) are compared. The Jsc (short circuit current density), which is proportional to the efficiency of a solar cell, in devices with MPA is twice as high as devices with EDT.

Solar Energy - Asbury Group - Penn State

Now, it is worth mentioning that the Asbury Group has a strong focus on organic solar cell research as well. In 2009, Prof. Asbury received an NSF CAREER Award, which came with a $624K grant to be used in organic solar cell research over 5 years. Since Prof. Asbury is a pioneer in quantum dot solar cell research in U.S., we wanted to focus on that aspect of his research. In fact, to the best of our knowledge, Prof. Asbury is the first scientist in an American university utilizing “Time-resolved vibrational spectroscopy” in solar cell research.

The Asbury Group published a paper earlier this year, “Vibrational Spectroscopy of Electronic Processes in Emerging Photovoltaic Materials” (click here to download the paper), explaining their use of Time-resolved vibrational spectroscopy in solar cell research. Simply put, the Asbury Group indicates that molecules have distinct vibrational features based on their location in a solar cell. In other words, a molecule in an electron donor material has different vibrational features than a molecule at the interface between the donor & acceptor materials. The Asbury Group also explains that the time-resolved vibrational spectroscopy is perfect for studying the dynamics of solar cells based on the molecule features as it combines ultrafast time resolution with measurement of the vibrational spectra of molecules.

Solar Energy - Asbury Group - Penn State

For more information about Prof. Asbury and his team, please visit the team’s website by clicking on this link.

Funding:
Asbury Group’s current projects are funded by the following entities:

Solar Energy - Asbury Group - Penn State

Joining the Team:
Interested candidates are encouraged to contact the Asbury Group via any of the methods provided in the “contact us” section of their website (click here to visit the respective section).

Prof. John B. Asbury

solar energy
Prof. Asbury started his academic career with a B.S. degree in Chemistry from University of Tennessee in 1996. He graduated at the top of his class and was named the Hoeschst-Celanese Best Junior Chemistry Major. In 2001, he earned his Ph.D. degree from Chemistry Department at Emory University, working with Prof. Tianquan “Tim” Lian. He spent the next four years working as a post-doc at Stanford University in Prof. Michael D. Fayer’s team. In 2005, Prof. Asbury joined Penn State University as an Assistant Professor of Chemistry.

Prof. Asbury is one of the pioneers of quantum dot solar cell research in U.S., in fact, as we discussed earlier, his work resulted in quite impressive solar cell efficiencies comparable to the highest numbers reported so far. His team collaborates with many other groups, including Prof. Edward H. Sargent’s from University of Toronto. His work in quantum dot solar cell research, as well as organic solar cell research, earned him very prestigious honors & awards including:

  • DOE Early Career Research Award (2012)
  • NSF CAREER Award (2009)
  • 3M Non-Tenured Faculty Grant (2008, 2009)
  • Eli Lilly Analytical Chemistry New Faculty Award (2007)
  • Camille and Henry Dreyfus New Faculty Award (2005)

For more information about Prof. Asbury and his team, please visit the team’s website by clicking on this link.