This dynamic group of scientists is the Marohn Group at Cornell University. Prof. John A. Marohn founded the group in 1999, when he joined Cornell University as an Assistant Professor of chemistry and chemical biology. Later, in 2005, he was promoted to his current title, Associate Professor.
There is one very significant reason why we are interested in this group. Prof. Marohn’s solar energy related research goals & outreach activities are almost in complete alignment with NGSF’s interests & goals. The outreach activities will be mentioned later in another section. As for the research goals, let us start by indicating that the Marohn Group is very strong in nano-technology. If you have read other academic reviews in our site, you might already know that NGSF considers nano-technology as the second renaissance. Working on materials a billionth smaller in size than materials from pre-nano-technology era is worth being called second renaissance in our opinion.
The Marohn Group lists their research projects in 4 categories, including Electric Force Microscopy of Organic Semiconductors. This category is also divided into 3 sub-categories; Charge Trapping, Charge Injection, and Organic Photovoltaics. Even though the Marohn Group combines all 3 sub-categories in some of their recent work, i.e. “Spectroscopic Imaging of Photopotentials and Photoinduced Potential Fluctuations in a Bulk Heterojunction Solar Cell Film” (click here to download the paper), we will be focusing on Organic Photovoltaics in this review.
The most efficient, but also the most expensive, solar cells are made with Multi-junction Cell technique, as seen in the graph below. This graph is from National Renewable Energy Laboratory (NREL) and the most recent version can be seen by clicking on this link.
Even though the organic cells provide efficiencies around 11% at most and these numbers are not even close to the current efficiency record, 44.4%, of Sharp’s triple-junction solar cell; the organic solar cells are one of the most cost effective & promising technologies. The fact that we can now use special printers to print organic solar cells makes this technology very desirable for large-scale deployments. Due to space restrictions, we do not expect household solar energy market to switch from Crystalline Silicon to Organic Solar Cells any time soon.
The Marohn Group, under Organic Photovoltaics topic, focusses on polyfluorene blend (PFB:F8BT) bulk heterojunction devices, hexabenzocoronene/C60 bilayer devices, and nanocrystal quantum dots. Quantum Dots are one of the most exciting results of nano-technology. These 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.
Using Quantum Dots in photovoltaics is a relatively new and extremely promising approach. Simply put, by combining different size Quantum Dot particles, theoretically the whole spectrum of sun light can be observed, increasing the solar cell module efficiency significantly. With the materials currently used in solar energy field, like Crystalline Silicon, the energy band is fixed, thus, only a portion of the sun light can be absorbed no matter what the particle size is. This is where the Marohn Group comes into play. The team created a tandem organic solar cell by applying this theory, as demonstrated in their paper “Solution-Processed Nanocrystal Quantum Dot Tandem Solar Cells” (click here to download the paper).
In this effort, the Marohn Group creates the first tandem solar cells produced from solution processed nano-crystal quantum dots (NQD). It is worth noting that the team put special emphasis on optical absorption profile of the whole device as well as the charge transport at the interlayer.
The following picture explains the device layers in detail. The cell at the top uses PbS NQD with a particle diameter of 4.8nm whereas the cell at the bottom uses PbS NQD with a particle diameter of 3nm. Both cells have ZnO on one side to receive the electrons and PEDOT:PSS on the other side to receive the holes.
As we mentioned earlier, the team put special emphasis on optical absorption profile of the whole device as well as the charge transport at the interlayer. That is why they clearly call out the fact that the PEDOT:PSS is pH-neutralized (pH 0 < pH 7 Acid / pH 7 Neutral / pH 7 < pH 14 Base) to avoid performance reduction observed in other studies. Moreover, the team adds a thin layer of gold, (Au / 1 nm) to compensate the energy off-set between ZnO and PEDOT at the interlayer. They also call out the fact that using Aluminum (Al) or Silver (Al) instead of Gold (Au) causes strange S-type current – voltage graphs.
Interestingly, the gold layer creates a few nano-meter size islands on top of ZnO rather than a continuous film. The picture below shows a ZnO layer (figure a) followed by a ZnO layer with Au deposit.
With this setup, they were able to also confirm that the open circuit voltage for the whole device was about the total open circuit voltage of the two cells that create the device.
This is a very exciting application of tandem organic cell theory into practice. We will be following the team’s progress in this area very closely.
For more information about Prof. Marohn and his team, please visit the team’s website by clicking on this link.
Prof. John A. Marohn
A New York native, Prof. John A. Marohn, attended Kenmore West Senior High School in Buffalo NY before starting his undergraduate studies in University of Rochester. Prof. Marohn received a B.S. degree in chemistry as well as a B.A. degree in physics from University of Rochester in 1989. Continuing his academic career further, Prof. Marohn earned a Ph.D. degree in chemical physics from California Institute of Technology in 1996. His supervisor was Prof. Daniel P. Weitekamp and his Ph.D. thesis title was “I. Multiple-pulse radio-frequency gradient nuclear magnetic resonance imaging of solids ; II. Optical nuclear magnetic resonance analysis of epitaxial gallium arsenide structures” (click here to download the thesis).
After earning his Ph.D. degree, Prof. Marohn worked as a National Research Council Postdoctoral Associate for 3 years in U.S. Army Research Laboratory in Maryland. In 1999, Prof. Marohn joined Cornell University as an Assistant Professor of chemistry and chemical biology. Later, in 2005, he was promoted to his current title, Associate Professor. Prof. Marohn also holds the following titles:
- Member of the Cornell Center for Nanoscale Systems
- Member of the Cornell Center for Materials Research
- Member of the Kavli Institute at Cornell for Nanoscale Science
- Member of the Executive Committee of the Cornell Center for Materials Research
- Member of the Executive Committee of the Cornell NanoScale Science and Technology Facility
Prof. Marohn is a very research oriented faculty member with over 70 published papers and numerous awards & honors, including:
- NSF Career Award 2002
- National Research Council / U. S. Army Research Laboratory Postdoctoral Fellow 1996
- W. R. Grace and Company Graduate Fellowship
The NSF Career Award is particularly impressive as it came with a $500K grant to be used over 5 years. Aside from his research in solar energy field, we are also very impressed with Prof. Marohn’s outreach activities. As we mentioned earlier, Prof. Marohn’s solar energy related research goals & outreach activities are almost in complete alignment with NGSF’s interests & goals.
Prof. Marohn, as a member of the Cornell Center for Materials Research (CCMR), participates in activities that appeal to teachers & students as explained in CCMR website’s education section (click here to visit the webpage). A blogger even described in great detail a session he attended in Marohn Group’s lab. In that session, Prof. Marohn first explained teachers how the solar cells work and then helped them create their own dye-sensitized organic solar cells together. How exciting! (click here to see the blog about this session)
It is also worth mentioning at this point that Prof. Marohn is also a professional guitar & banjo player. He is a member of the Fall Creek Bluegrass Partners band. If you attend one of his classes at Cornell, you might even be lucky enough to catch him play & sing in class:
Several other video clips that involve Prof. Marohn are available online: