These lucky scientists are the Wiesner Group at Cornell University. They are not lucky only because they are doing their Ph.D. in one of the best universities in US but also because of the great opportunities Wiesner Group members receive after graduation. Please click on this link to take a look at the group alumni. Wiesner Group graduates work in very prestigious labs / universities. We especially admire the graduates, who are now working in Grätzel Lab. This lab belongs to Michael Grätzel, one of the most significant names in solar energy field. He is the co-founder of dye-sensitized solar cells. What is more impressive is the fact that Prof. Wiesner and one of his students, Dr. Hooisweng Ow, created Cornell Dots (C-dots) and founded a company, Hybrid Silica Technologies, together in 2005.
Before discussing Wiesner Group’s solar energy related projects, let’s take a moment to look into Cornell Dots.
If you have read some of the team reviews in our website, you might have noticed our fascination with nano-technology. We consider nano-technology (materials a billionth smaller in size) as the second renaissance. One particular topic within nano-technology, Quantum Dots, is a very promising approach in solar cells. 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. By combining different size Quantum Dot particles, theoretically the whole spectrum of sun light can be absorbed, increasing the solar cell module efficiency significantly.
Wiesner Group’s discovery, C-Dots, is also a benefit of nano-technology. Originally developed as optical probes, the C-Dots are now an FDA approved drug that is used in cancer research. These particles glow tree times more than the dye used in imaging today. They are also, due to their nano size, easily removed from the body through kidneys. Scientists are aiming to use these particles as cancer cell tracers and more. The main benefit of using C-Dots over D-Dots is that the C-Dots are not toxic.
Being an interface between polymer science and solid state chemistry/physics, Wiesner Group works on multiple topics, including solar cells. Since some of Wiesner Group alumni work in Grätzel Lab, one can easily guess the type of solar cells the team is focused on: “dye-sensitized solar cells”.
One of the most promising solar cell technologies, dye-sensitized solar cells, is easy & cheap to produce. The drawback, of course, is the lower efficiencies observed compared to less cost effective technologies like Crystalline Silicon solar cells. The efficiency records for dye-sensitized solar cells have always been around 11% for the last decade. This graph is from National Renewable Energy Laboratory (NREL) and the most recent version can be seen by clicking on this link.
The most recent change in that chart is regarding a new type of solar cell Prof. Grätzel and a group of scientists created. As explained in their latest paper, “Sequential deposition as a route to high-performance perovskite-sensitized solar cells” (click here to download the paper), they used perovskite to create a cheaper & more efficient solar cell type. With close bonds to the Grätzel Lab, we would not be surprised to see the Wiesner Group taking the perovskite-sensitized solar cells concept a step further soon. In fact, reading through the team’s papers, we observed adaptation of new concepts in every project.
The Wiesner Group used self-assembly to create the materials in the projects we have reviewed. The team was actually the pioneers of this method as described in their paper “Organically Modified Aluminosilicate Mesostructures from Block Copolymer Phases” (click here to download the paper) published in 1997. One of the most significant projects, built on top of this concept, is explained in their paper “A Bicontinuous Double Gyroid Hybrid Solar Cell” (click here to download the paper). This is quite an interesting paper. In fact, it was featured in ACS Nano Letters cover (volume 9, issue 8) in 2009.
This was the first use of gyroid structure in an electronic device. Gyroids were discovered in 1970 by a NASA scientist Alan Schoen. A gyroid is the only known embedded triply periodic minimal surface with triple junctions. In other words, a gyroid is the infinite number of “minimal surfaces with no straight lines” connected together. The Wiesner Group indicates that these fully interconnected channels and struts make the gyroid structure ideal for hybrid solar cells.
The following picture is a gyroid created with a 3-D metal printer. The small holes are just an addition by the creator and they make it easier to see the whole structure.
In this project, the Wiesner Group used the self-assembly approach to create a gyroid semiconductor network in a solid-state dye-sensitized solar cell. To simply put, the team uses two materials to start with, poly-fluorostyrene and poly-lactide (PFS-b-PLA), and create two continuous gyroid networks interconnected to each other as shown in figure (a) below. Then PLA is removed, as shown in figure (b). The remaining inorganic material keeps the structure intact, thus, the gyroid shape is not lost. At this point, by using electrochemical replication, the team creates the shape of the empty space in the gyroid structure as a free standing inorganic semiconductor array. In other words, the team is removing the PFS but keeping the surface between PFS and the empty space to create a free standing structure in the form of a gyroid, as shown in figure (c). Now, they fill this free standing space with a “p-type” material, which provides the holes, and outside area with an “n-type” material, which provides the electrons, as shown in figure (d). This is basically how they create a bicontinuous bulk heterojunction solar cell using a gyroid structure.
With this approach, the team is able to reach 1.7% efficiency. Even though the number is lower than the efficiencies reported in solid-state dye-sensitized solar cells, this solar cell is much thinner. This means, with the same amount of material, this approach could compete with any solar cell out there.
Following their work in solid-state dye-sensitized solar cells, the Wiesner Group published another paper in 2010, “Monolithic route to efficient dye-sensitized solar cells employing diblock copolymers for mesoporous TiO2” (click here to download the paper), regarding their project in the liquid-state dye-sensitized solar cells. In this project, the team was able to create a 3.5 micro meter thick film with 6.44% efficiency in full sun conditions and 8.1% efficiency under low light levels. They also observed that the efficiency of the cell did not increase with thickness. At 9 micro meters, they were still able to get 6.4% efficiency but after that, the efficiency decreased. The team was able to create these cells for up to 30 micro meters thick with no cracks. At the 30 micro meters, the efficiency was recorded at 2%.
The figure below describes the team’s liquid-state dye-sensitized solar cell.
Going back to the solid-state dye-sensitized solar cells, the Wiesner Group published “Triblock-Terpolymer-Directed Self-Assembly of Mesoporous TiO2: High-Performance Photoanodes for Solid-State Dye-Sensitized Solar Cells” (click here to download the paper) in 2012. In this project, the team created a new self-assembly process for mesoporous TiO2 films that led to efficiencies over 5%. With these results, the team indicates that for the first time a self-assembled mesoporous TiO2 system performs better than nanoparticle-based electrodes produced and tested in same circumstances.
As we mentioned earlier, we observed new concepts adapted by the Wiesner Group in every project. In 2011, the team published a paper, “Plasmonic Dye-Sensitized Solar Cells Using Core − Shell Metal − Insulator Nanoparticles” (click here to download the paper), regarding their project where they integrated plasmonics with dye-sensitized solar cells.
Plasmonics is a relatively new concept. In 2000, Prof. Harry A. Atwater and his team (click on this link to see the Atwater Research Group review in our website) gave its name to “Plasmonics”. Simply put, when you have a metal & nonconductive material facing each other and you point light at the interface between these two materials, the light waves and the electrons at the surface of the metal can resonate. That is how you create surface plasmons. Prof. Atwater describes them as “density waves of electrons that propagate along the interface like the ripples that spread across the surface of a pond after you throw a stone into the water.”
Plasmonics already found its purpose in many fields, from imaging in medical field to plasmonic holograms (as seen in the video clip below).
Scientists have been working on utilizing plasmonics in solar cells for years. In this project, the Wiesner Group uses Au (gold) nano-particles covered with silica to improve the light absorption, photocurrent, and efficiency of their dye-sensitized solar cells with plasmonics. More importantly, the team calls out the fact that the previous implementations of plasmonics in solar cells were on unconventional devices and not compatible with the already available solar cell production techniques. The team indicates that the technique demonstrated in this work can easily be adapted to already available solar cell production techniques.
Wiesner Group ran several experiments before using Au-silica. The team used bare metals, Au and Al (silver), which severely diminished the solar cell performance due to corrosion and the metal acting as a charge recombination center. This was consistent with other studies in this area that recommend insulating the metal particles electronically & chemically. After confirming that the silica itself (without the metal) has no impact on the solar cell performance, the team used 3 nm silica shells to encapsulate the gold nano particles. The results were very encouraging especially when considering how thin these materials are. The team was able to observe 4% efficiency and the cell, even though no experiment was done regarding its stability, lasted for more than a month. Finally, it’s worth mentioning that conventional dye-sensitized solar cells perform better when the thickness of the device is higher than 8 micro meters.
Even though gold & silver are commonly used in plasmonics related projects, scientists are always looking for better alternatives. Very recently, a group in Purdue University published a paper, “Alternative Plasmonic Materials: Alternative Plasmonic Materials: Beyond Gold and Silver” (click here to download the paper), and suggested alternatives to gold & silver, i.e. “doped” semiconductors, transparent electrically conductive oxides, ultrathin layers of carbon called grapheme, and so on.
For more information about Prof. Wiesner and his team, please visit the team’s website by clicking on this link.
Prof. Ulrich B. Wiesner
Prof. Ulrich B. Wiesner received his Chemistry Diploma in 1988 from the University of Mainz, Germany. Later, in 1991, he earned his Ph.D. degree working on optical information storage in liquid crystalline polymers at the Max-Planck-Institute for Polymer Research, Germany.
After graduation, he started working as a postdoc in Ecole Superieure de Physique et de Chimie Industrielle de la ville de Paris (E.S.P.C.I.), France, where he met Prof. Grätzel. In 1993, he started working as a staff member at Max-Planck-Institute for Polymer Research. He joined Cornell as an Associate Professor of Materials Science & Engineering faculty in 1999. Later in 2005, he became a Full Professor and in 2008, he was elected Spencer T. Olin Professor of Engineering.
Prof. Wiesner’s academic career had already impressed us before we wrote this review. After digging out the details about Prof. Wiesner and his team, we are now infinitely more impressed. With over 188 publications and contributions to books, Prof. Wiesner is a big name in polymeric materials as well as dye-sensitized solar cell research, which directly appeals to us. His close relation with Prof. Grätzel and, more importantly, his discovery of C-Dots (FDA approved drug, as mentioned earlier), which may have strong impact on cancer research, are also hard to neglect. We certainly believe that the C-Dots will be a breakthrough in the medical field, possibly saving many lives. Prof. Wiesner also received many honors and awards throughout his academic career, including:
- Ph.D. Award of the Hoechst AG
- Carl Duisberg Memorial Award of the German Chemical Society
- IBM Faculty Partnership Award
- Mr. & Mrs. Richard F. Tucker’50 Excellence in Teaching Award
- Elected member of the Nanotechnology Technical Advisory Group (nTAG) of the President’s Council of Advisors on Science and Technology (PCAST)
- Elected Spencer T. Olin Professor of Engineering
For more information about Prof. Wiesner and his team, please visit the team’s website by clicking on this link.