This large group of scientists is the Yang Yang Laboratory team at University of California, Los Angeles (UCLA). There is no better place for a young scientist to work on organic solar cells in an American university than this laboratory. Prof. Yang is one of the most well known & respected organic solar cell researchers in the world. This is not an opinion but a fact supported by Prof. Yang’s research papers that were cited countless times and the record breaking solar cell structures that were certified by National Renewable Energy Laboratory (NREL).

There is an apparent domination of some teams in certain solar cell types. For example, Prof. Michael Grätzel at École Polytechnique Fédérale de Lausanne (EPFL) leads in dye-sensitized solar cells, Prof. Edward H. Sargent at University of Toronto leads in quantum dot solar cells, and we are proud to say that Prof. Yang Yang at University of California, Los Angeles (UCLA) leads the organic tandem solar cells field. All three are emerging technologies that are very promising. Even though the solar panel market is dominated by Crystalline Silicone, any one of these technologies is a better alternative in large scale deployments due to their ease of production, light weight, and ease of deployment. In fact, there are special setups with printers used to print organic solar cells, as demonstrated in the video clip below.

Aside from solar cells, Prof. Yang also works on digital memory units, light emitting diodes and thin film transistors. The team’s current research is categorized into four groups:

  • Organic Solar Cells
  • CIGS Solar Cells
  • Graphene
  • Memory

Although hundreds of published research papers combined over the years explain the story behind Prof. Yang’s success, his research will be discussed here by starting with one of his most cited papers, “Polymer solar cells with enhanced open-circuit voltage and efficiency” (click here to download the paper).

The work that led to this paper was regarding single organic solar cells, not tandem organic solar cells. In 2009, Prof. Yang’s team used a low band polymer, PBDTTT, as a base to test different functional groups to reach higher open circuit voltage (Voc), which is directly proportional with the efficiency of a solar cell. Their tests resulted in PBDTTT–CF achieving a world record efficiency of 7.73%. Even the average efficiency of 75+ devices they built with PBDTTT–CF was a record, 7.38%. The team sent their solar cells to NREL for certification and even though the result at NREL labs was lower, it was still a world record, 6.77%. The record can be seen in the following graph under the name Solarmer, a company that spun off from Yang Yang Laboratory.

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.

It is widely believed that by optimizing the materials with carrier mobility, energy levels, and appropriate band gap the highest efficiency that can be achieved with a single organic solar cell is between 10 & 12 percent. One of the highest predictions was made by scientists at Konarka Austria, which is explained in their paper “Design Rules for Donors in Bulk-Heterojunction Tandem Solar Cells Towards 15% Energy-Conversion Efficiency” (click here to download the paper). The same limitation does not apply to carefully selected tandem cells. In fact, even in inorganic solar cell research, tandem cells achieve amazing efficiency levels. For example, as of today, the highest efficiency solar cell certified by NREL is a multi-junction solar cell created by SHARP, with a 44.4% efficiency.

Organic tandem cells can be created by connecting two organic cells in a parallel or series fashion. The series connection approach has been the most commonly used. In the earlier days of organic tandem cells, the materials used for front cell and the rear cell were the same. The problem is that by using the same material, the cells would capture the same part of the light spectrum. In other words, when light hits these cells, a certain bandgap will result in current. The photons with higher energy than this bandgap will still result in current but the excess energy will be lost in form of heat. The photons with lower energy than the bandgap, however, will simply go through the cell. If the tandem cells are made from the same material, the same photons would result in current in these cells. If the front cell already captured these photons, there is not much left for the rear cell to produce.

Finally, in 2006, a group of scientists from universities in Netherlands used different materials to create tandem organic cells, as explained in their paper “Solution-Processed Organic Tandem Solar Cells” (click here to download the paper). Even though the efficiency of their organic tandem cell, 0.57%, may not sound like a breakthrough but it was. The organic tandem cell’s efficiency was significantly better than the subcells that that were connected to create it.

Following their ground breaking work in single organic solar cells, Prof. Yang’s team turned to organic tandem solar cells.

As explained in one of Prof. Yang’s popular papers, “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer” (click here to download the paper), the team focused on the biggest problem with organic tandem solar cells, finding a low bandgap polymer to maximize efficiency. Basically, when light hits an organic tandem cell, the high energy photons will be absorbed by the front cell, leaving the challenge of collecting lower energy photons to the rear cell. The phrase “a chain is only as strong as the weakest link” applies here in the form of current. Since these cells are connected in series, the current of the tandem cell is going to be dictated by the cell with the lower current. If the front cell captures the high energy photons and the rear cell is left with lower energy photons, the current in the rear cell will be lower than the front cell, in other words, the tandem organic cell’s current will be lower than a single organic cell. That is why finding a low bandgap polymer to maximize efficiency is the biggest problem with organic tandem cells. By using PBDTT-DPP, a low bandgap polymer, the team created the following organic tandem cell and broke another world record in July 2011, 8.62%.

Solar Energy

The following table shows the open voltage, short circuit current density, and the efficiency values associated with individual cells as well as the tandem cell that they form.

Solar Energy

Less than a year later, Prof. Yang’s team collaborated with Sumitomo Chemical to break another world record. The key, as Prof. Yang’s research clearly shows, is the low bandgap polymers. By utilizing an infrared absorbing polymer material by Sumitomo Chemical, the team was able create an organic tandem solar cell with an amazing 10.6% efficiency. As of today, the highest NREL certified organic tandem cell efficiency is still 10.6%. As explained in their paper, “A polymer tandem solar cell with 10.6% power conversion efficiency” (click here to download the paper), the team used the following cell structure.

Solar Energy

The following table shows the open voltage, short circuit current density, and the efficiency values associated with individual cells as well as the tandem cell that they form.

Solar Energy

Among the 30 research papers the team published this year, 2013, perhaps the following two provide important clues about Prof. Yang’s goals for 2014.

Recent trends in polymer tandem solar cells research (click here to download the paper)

Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency (click here to download the paper)

The first paper is a phenomenal reading material that covers the latest improvements in organic tandem cells. One of the most important points is the reference made to Mitsubishi Chemical’s 80% external quantum efficiency (EQE), twice. The same reference was made in “A polymer tandem solar cell with 10.6% power conversion efficiency” as well with a statement about possibly reaching 14% efficiency level soon. Keeping in mind that even the commercially unavailable state of art organic tandem cells have an average EQE of 50%, we strongly believe that the team will utilize Mitsubishi Chemical’s breakthrough in order to improve their record breaking organic tandem cells.

The second paper shows the team’s interest in the small molecule organic tandem solar cells. With so many references, in several of their papers, to Heliatek, a German company that keeps improving the small molecule organic tandem cell efficiency every year, we believe the team might find an opportunity to integrate Heliatek’s expertise in their research.

No matter which direction they take, we will closely follow Prof. Yang’s team with great interest in 2014. For more information about Prof. Yang’s research, please visit the Yang Yang Laboratory website by clicking on this link.

Funding:
Prof. Yang’s current projects are partly funded by the following entities.

National Science Foundation
U.S Air Force Office of Scientific Research
U.S. Office of Naval Research
U.S. Department of Energy
National Renewable Energy Laboratory
The Kavli Foundation
Henry Samueli School of Engineering and Applied Science
Joining the Team:
Even though there are no details in the team’s website about open positions, we encourage the candidates to contact Prof. Yang directly with their updated resume.

Prof. Yang Yang

solar energy
Prof. Yang’s academic career started with a B.S. degree in Physics from National Cheng-Kung University, Taiwan, in 1982. Prof. Yang proceeded with earning an M.S. degree in physics and Ph.D. in applied physics in 1988 and 1992 respectively, both from University of Massachusetts, Lowell.

Following his Ph.D. degree, Prof. Yang worked as a post-doc at University of California Riverside and as a research staff member at UNIAX (DuPont Displays). In 1997, he joined UCLA. As of 2011, Prof. Yang is the Carol and Lawrence E. Tannas Jr. Endowed Chair Professor of Materials Science and Engineering at UCLA. He is also a director at Nano Renewable Energy Center in UCLA and at Center for Organic Opto-electronics Technologies in Zhejiang University.

Prof. Yang has created solar cells that broke world records, which earned him a permanent spot in the history of solar energy field. He has published about 250 papers, filed 40 patents and given 120 invited talks.

A world renowned name in organic solar cell research, Prof. Yang has received many honors & awards including:

  • The Carol and Lawrence E. Tannas Jr. Endowed Chair in Engineering, 2011
  • Top 11 Hot-Researchers in 2010, Science Watch
  • Highest cited Paper in 2010, Advanced Functional Materials
  • Highest cited Paper in 2008-2010, Journal of American Chemical Society
  • IEEE Photovoltaic Field Expert, 2009
  • Microelectronics Advanced Research Corporation Inventor Recognition Award, 2007
  • NSF Career Award, 1998
  • 3M Young Investigator Award, 1998

Several video clips about Prof. Yang are also available online:

  • Video-1
  • Video-2
  • Video-3
  • Video-4

For more information about Prof. Yang, please visit the Yang Yang Laboratory website by clicking on this link.