This summer has been very exciting for solar energy advocates. First, the PlanetSolar’s yacht, which has the Guinness record for the first solar boat to sail around the world, visited our coasts (click here to read more about the solar yacht). It’s hard to top a yacht that is capable of a world tour with solely depending on solar energy. However, the only thing that is more exciting, a solar plane, has also visited us this summer.
The Solar Impulse plane flew from San Francisco to New York City, with a few stops along the way, and only relied on solar energy (click here to read more about the solar plane). To celebrate this achievement, Solar Impulse Innovation Conference was organized by Solvay in New York. The scientists in the picture above are the Loo Group from Princeton University, who attended this conference and, as the picture clearly demonstrates, enjoyed it very much. The group leader, Prof. Yueh-Lin (Lynn) Loo, was in fact invited as a technical speaker to this conference.
Before discussing Prof. Loo and her team, we would like to thank Prof. Loo for taking the time to provide us the documents & information we needed for this review. This review was particularly important to us. Our founder & president, who is a woman entrepreneur, and the rest of our team strongly believe in empowering women. Therefore, it is a great honor for us to review such a bright & successful scientist, Prof. Loo.
Plastic Electronics, a relatively new field, offers a wide variety of opportunities to improve our lives. Quoting from an interview with Prof. Loo, who spoke at World Economic Forum’s Annual Meeting of the New Champions (Summer Davos) in 2010, “Imagine electronic wallpaper that changes patterns from green stripes to pink polka dots at a click of a switch. Imagine tinted windows that can also generate power during the day. Imagine disposable sensors that would change color if the water source is contaminated, or yet, think of smart plastic patches that can monitor your health and deliver medication when you’re sick. The possibilities are endless.”
Prof. Loo is one of the pioneers of Plastic Electronics. In fact, she received numerous honors & awards for her work in this field, including National Science Foundation (NSF) Early Career Development (CAREER) Award that came with a $440,000 grant & Young Investigator Award from the Arnold and Mabel Beckman Foundation that came with a $264,000 grant.
The Loo Group categorizes their current research into 4 groups:
- Size and shape tunable periodic structures derived from functional block copolymers
- Solution-processable organic conductors and semiconductors for thin-film electronics
- Soft lithography and soft-contact lamination for plastic electronics
- Self-assembled monolayers facilitate interfacial engineering in organic solar cells
One of the most impressive things about the Loo Group is their creativity. We appreciate every single research project that aims to increase the efficiency or lower the cost of solar cells. However, a significant portion of these projects focus solely on different materials to achieve better results. Time to time, we encounter extremely creative projects that touch different aspects of solar cells for better results. Strong creativity is indeed what distinguishes the Loo Group from others.
Loo Group’s work regarding optical properties wrinkles & folds and their effects on organic solar cells, as explained in their paper titled “Wrinkles and deep folds as photonic structures in photovoltaics” (click here to download the paper), focuses on one of the aspects of solar cells that most of the scientists overlook. Inspired by the shape of leaves, the Loo Group studied the effects of wrinkles and folds in solar cells. The following figure explains the difference between winkles and folds at a microscopic level.
The team compared solar cells structured on a flat surface with solar cells structured on wrinkled as well as composite (wrinkles & folds) surfaces. The results were quite impressive even for the team as Dr. Jongbok Kim from Kumoh National Institute of Technology in Korea, a post-doc in Prof. Loo’s team at the time, states. We can summarize the results under three topics; efficiency, light absorption, and durability.
Efficiency of a solar cell is simply defined as the maximum power output divided by power input. Therefore, the higher the maximum power output, under same power input, the higher the efficiency of a solar cell. The maximum power output is directly proportional to Voc (open circuit voltage) and Isc (short circuit current). As the following figure demonstrates, while the Voc is the same for all three solar cell structures, the Isc differs. The composite surface results in the highest Isc followed by the surface with wrinkles and the flat surface. The team indicates that this increase in photocurrent is mostly due to the light trapping effect of the wrinkles & folds on the surface.
With this observation in composite solar cells, the team was able to reach up to 47% more efficiency compared to solar cells with flat surfaces. This improvement was also a result of the following topic, light absorption.
Another exciting observation the Loo Group made was the dramatic increase in light absorption at near-infrared light in spectrum. Simply put, the composite surface solar cells increased the useful range of light by 200nm. In other words, given the same amount of light, the composite surfaces absorbed more photons. This directly impacts the external quantum efficiency (EQE), which is defined as the number of photons collected by the solar cell divided by the number of photons that entered the solar cell. At the near-infrared region, after 650nm, the light absorption of conventional solar cells built on flat surfaces diminish significantly whereas the solar cells with composite surfaces show a 600% increase in EQE, as demonstrated in the following graph.
It may come as a surprise to a lot of people but, the solar cells with composite surfaces also showed better durability than the solar cells with flat surfaces during the team’s tests. The figure below explains the results in detail. Basically, the team observed up to 70% decrease in photocurrent in solar cells on flat surfaces with increased mechanical stress. The solar cells with composite surfaces, on the other hand, showed no change in photocurrent under same mechanical stress levels.
If you believe in evolution theory (survival of the fittest), perhaps it’s no surprise to see the imitation of leaves in solar cells resulting in better results than strictly human made solar cell designs. This is consistent with the fact that there is an optimum level of wrinkles, 11% per the Loo Group, after which the efficiency of a solar cell starts to decrease.
Another extremely creative project that caught our attention was explained in the team’s paper titled “Modular Construction and Deconstruction of Organic Solar Cells” (click here to download the paper).
Organic solar cell technology is relatively new and extremely promising. Even though the solar panel market is dominated by Crystalline Silicone, due to its cost effectiveness & ease to deploy, the organic solar cells provide a better alternative for large scale deployments. In fact, there are special printers used to print organic solar cells, as demonstrated in the video clip below.
The efficiency records for organic solar cells have been around 10-11% for the last few years. The following graph is from National Renewable Energy Laboratory (NREL) and the most recent version can be seen by clicking on this link.
Even though it has not been reflected in NREL’s latest solar cell efficiency chart, it is worth mentioning that Heliatek, a Germany based company, raised the bar by achieving 12% efficiency with their tandem organic solar cells earlier this year (click here to read the respective Healitek press release).
Organic solar cells are typically created with P3HT (electron donor) and PCBM (electron acceptor), as illustrated in the figure below.
The simplest approach to build an organic solar cell is to use two separate layers for the donor & acceptor. In this case, planar-heterojunction, when light reaches the donor with enough energy, an exciton (electron & proton pair) is released by the donor towards the acceptor. Once the exciton reaches the interface between the donor and the acceptor, if it has enough energy, the pair is split into an electron and a proton. The electron is passed over to the acceptor whereas the proton heads back to donor to eventually meet the electron after it travels the circuit, resulting in a current. The problem with this approach is that, one cannot simply increase the size of the donor & acceptor to increase the light absorption in order to improve the efficiency of a solar cell. The thicker these materials are, the more energy the excitons require to travel.
In bulk-heterojunction organic solar cells, the donor and the acceptor are mixed. As a result, an exciton does not have to travel all the way through the whole donor material to reach the interface with the acceptor. Since the materials are mixed, the surface allows a much easier travel of the excitons.
The inverted bulk-heterojunction organic solar cells are simply bulk-heterojunction organic solar cells with anode & cathode components swapped. The reason behind this approach is to avoid a common problem, oxidization of the anode.
No matter which approach is used, there is a common significant problem with organic solar cells. The processes to build an organic solar cell are irreversible. This creates numerous challenges. Most significantly, the layers cannot be fine-tuned or replaced once a solar cell is built. Prof. Loo and her team made a breakthrough when they applied soft-contact lamination (ScL) to create a modular organic solar cell. ScL is a technique Prof. Loo initially used in her 2002 paper “Soft, conformable electrical contacts for organic semiconductors: High-resolution plastic circuits by lamination” (click here to view the paper). Since then, she has successfully applied this technique in many projects. In their modular organic solar cells, the Loo Group uses polydimethylsiloxane (PDMS) as an interface for electrodes, single layer, or multilayers. Thanks to the Van Der Waals forces, this material sticks well and can easily be removed, just like a gecko in the following picture. Once again the evolution theory (survival of the fittest) is in play by imitating a feature of geckos that allowed them to survive for millions of years.
As clearly seen in the following graph, the Loo Group confirmed an important fact that the re-use of the PDMS has no negative impact on the performance. As mentioned earlier, by using ScL, the Loo Group was now able to replace/study any layer of an organic solar cell. Their studies showed multiple interesting results that were not feasible to observe prior to modular structure. The single most exciting outcome of this effort, in our opinion, is the opportunities ScL offers in tandem organic solar cells. Tandem organic solar cells have always been very difficult to setup. With the introduction of ScL, each layer of each cell can easily be fine-tuned prior to construction of the cell and can be replaced with ease afterwards. This is extremely exciting and we will closely follow Prof. Loo’s efforts in this area going forward.
Prof. Yueh-Lin (Lynn) Loo
Prof. Loo started her academic career with B.S. degrees in 1996 from Materials Science and Engineering department & Chemical Engineering department at University of Pennsylvania. Later in 1998, she earned her M.A. degree from Chemical Engineering department at Princeton University. Continuing her academic career at Princeton, she earned her Ph.D. degree working with Prof. Richard A. Register in 2001 from Chemical Engineering department.
After working in the industry as well as at University of Texas at Austin as an Assistant Professor for a few years, following her graduation, she went back to Princeton University to start as an Associate Professor of Chemical and Biological Engineering in 2007. In 2011, she was promoted to Professor of Chemical and Biological Engineering and also became Associate Director at Andlinger Center for Energy and the Environment at Princeton University. Earlier this year, 2013, she was named the Theodora D. ’78 & William H. Walton III ’74 Professor in Engineering.
Prof. Loo is one of the brightest scientists of our era. She is one of the pioneers of Plastic Electronics and her work in this area resulted in over 100 publications/book chapters and an impressive list of 8 patents. Perhaps none of this is surprising as Prof. Loo, as a kid, was always interested in chemistry. Growing up, her favorite toys were test tubes her father brought from work and she loved stirring things in them. In a sense, Prof. Loo was destined to be a scientist. With such interest and dedication from early ages, Prof. Loo has had a very impressive academic career full of numerous honors & awards including:
- 03/13 Fellow, American Physical Society
- 11/12 Owens Corning Award, AIChE
- 06/12 Young Global Leader, World Economic Forum
- 10/10 US Young Scientist Representative, Summer Davos Meeting
- 03/10 John H. Dillon Medal Recipient
- 02/08 Alfred P. Sloan Research Fellow in Chemistry
- 11/07 Ernest Thiele Lectureship, Chemical and Biomolecular Engineering Department, University of Notre Dame
- 11/06 Allan P. Colburn Award for Excellence in Publications, AIChE
- 01/06 Peter and Edith O’Donnell Award in Engineering, The Texas Academy of Medicine, Science and Engineering
- 01/06 Diverse magazine: “Top 10 Emerging Scholar”
- 08/05 General Dynamics Endowed Faculty Fellowship in Engineering, University of Texas at Austin
- 03/05 Beckman Young Investigator Award
- 09/04 MIT’s Technology Review: “Top 100 Young Innovator”
- 06/04 Selected as one of top 100 young engineers for the Frontiers of Engineering Symposium, sponsored by the National Academy of Engineering
- 05/04 PROGRESS/Dreyfus Lectureship, Rising Stars Program, American Chemical Society
- 01/04 National Science Foundation, Faculty Early CAREER Development Award, Polymers Program
- 10/03 DuPont Young Professor
- 07/02 Camille and Henry Dreyfus New Faculty Award
As a non-profit, our organization is also extremely impressed with the outreach activities Prof. Loo leads. Some of these activities are shown in the team’s website (click here to see the respective page). Prof. Loo is also the co-founder of Women in Nanoscience (WIN) (click here to visit the organization’s website), with a goal to celebrate the accomplishments of women in nanoscience. 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. More importantly, WIN appeals to one of our most important values. Our founder & president, who is a woman entrepreneur, and the rest of our team strongly believe in empowering women. Therefore, it is a great honor for us to review such a bright & successful scientist, Prof. Loo.
Several video clips about Prof. Loo are also available online: