Even though the Optoelectronic Component and Materials (OCM) Group has been calling University of Michigan “home” since 2006, the team was actually founded in 1985 by Prof. Stephen R. Forrest while he was a faculty member at University of Southern California. Prof. Forrest tried to keep the team intact over the years, thus, it is appropriate to say that today’s OCM Group is a continuation of the OCM Group founded in 1985.
Like other leaders of the phovoltaics field, Prof. Forrest strongly believes in diversity of backgrounds. He also makes sure the team has every level of expertise; from graduate students to post docs, visiting scientists, and professional research staff.
The OCM Group works on several projects grouped in 3 main categories, including Photovoltaic Cells. Among the projects under the Photovoltaic Cells category, we will be reviewing the following topics:
- Multiple growth of III-V thin film solar cells on single substrate using epitaxial lift-off
- Effect of Interface Structure on Organic Photovoltaic Efficiency
- Tandem Organic Solar Cells
- Cascade Organic Photovoltaic Devices
- Reliability of organic photovoltaic cells
- High Yield of Large-area Organic Solar cells
Multiple growth of III-V thin film solar cells on single substrate using epitaxial lift-off is a particularly interesting & promising project. Transferring the active region of multi-junction solar cells with epitaxial lift-off allows the re-use of the materials. The team observed about 50% reduction of the active region thickness to reach the same results III – V solar cells provide. Since the multi-junction cells are the highest efficient and the most expensive cells, with a 50% reduction in the substrate might make these cells more cost-effective. That is exactly what the team is aiming for here and they hope to achieve an efficiency/cost structure that is competitive to crystalline silicon solar cells.
The OCM Group published their first paper on this topic in 2010, “Multiple growths of epitaxial lift-off solar cells from a single InP substrate” (click here to download the paper). Following to that paper, in 2012, the team published “Reuse of GaAs substrates for epitaxial lift-off by employing protection layers” (click here to download the paper).
At this point, it is worth mentioning that the team lead in this project, Kyusang Lee, won this year’s IEEE Photovoltaic Specialist Conference student award in III-V & Concentrator Technologies with this work. To see the complete list of student awards & recipients, please click on this link.
The remaining OCM Group projects listed above are related to Organic PhotoVoltaics (OPV). The next 3 projects focus on efficiency topic whereas the last two focus on reliability & yield topics respectively.
Effect of Interface Structure on Organic Photovoltaic Efficiency project, led by Jeramy Zimmerman, uses solvent vapor annealing to create DPSQ / C60 thin films and observe the current/voltage nature of the devices in different scenarios. Even though the team aims to achieve OPV efficiencies over %35 OPV, which is much higher than the current commercial OPV efficiency average of %8-%10, the results OCM Group has seen are very promising.
The picture below describes the team’s approach. The material at the bottom is DPSQ and the material at the top is C60. The results show that, if the DPSQ is solvent vapor annealed before deploying C60 (B), the loss of current with increased voltage is worse than if the C60 is simply deployed over DPSQ (A). However, solvent vapor annealing the device after C60 is deployed over DPSQ results in a much higher current (C), hence much higher energy & efficiency. The picture also shows the surface structure in these 3 scenarios.
To read more about this project, please click on this link to download the team’s first related paper, titled “Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells”.
The next projects, Tandem Organic Solar Cells & Cascade Organic Photovoltaic Devices, provide different organic solar cell designs to achieve higher efficiencies.
As a part of the Tandem Organic Solar Cells project, the team integrates 2 separate cells (SubPc/C60 and DPSQ/C60) with complementary absorption, as seen in the picture below. This way, a larger spectrum of the light would be absorbed by the structure compared to the two cells used separately. Cascade Organic Photovoltaic Devices project, on the other hand, utilizes a cascade structure for multiple electron donor materials chosen for optimal interaction with the electron acceptor material, as demonstrated below.
With the Reliability of organic photovoltaic cells project, the OCM Group is tackling another significant problem with organic solar cells. Compared to the crystalline silicon solar cell average lifespan of 25 years, the organic solar cells last about 5 years. To increase the organic solar cell lifespan, the OCM Group is studying the degradation pathways. The following table, from the paper “Degradation mechanisms in organic photovoltaic devices” published by a group of scientists from Netherlands, provides a good list of pathways that lead to degradation for reference.
High Yield of Large-area Organic Solar cells is another very exciting & promising project the OCM Group has been working on. Led by Nana Wang, the OCM Group uses a new technique, Carbon Dioxide Snow Cleaning, to clean the substrates and the results show yield over %70 for large area deployments. The process is demonstrated in the picture below. To read more about this project, please click on this link to download the paper titled “Snow cleaning of substrates increases yield of large-area organic photovoltaics”.
For more information about Prof. Forrest and his team, please visit the team’s website by clicking on this link.
Prof. Stephen R. Forrest
University of Michigan is a respected leader in research in our country. In fact, the university is one of the main economic resources for the Ann Arbor area investing over a billion dollars in research every year. Prof. Forrest, being the Vice President for Research, is one of the main drivers of research at University of Michigan.
Prof. Forrest received his B.A. degree in Physics from University of California – Berkeley in 1972. Two years later, he received an M.S. degree in Physics from University of Michigan. Following his M.S. degree, Prof. Forrest earned a Ph.D. degree in Physics, again from University of Michigan, in 1979 and started working at Bell Labs. In 1985, Prof. Forrest went back to academia and joined the faculty at University of Southern California. That is where he founded the OCM Group. After carrying numerous prestigious titles, he joined University of Michigan in 2006 as Vice President for Research, and as a Professor in Electrical Engineering, Materials Science & Engineering, and Physics departments.
Aside from a huge career in academics, with over 500 published papers & 200 patents, Prof. Forrest has also had a strong impact on the industry. He has founded / co-founded multiple companies including Sensors Unlimited, Epitaxx, Global Photonic Energy Corporation, Universal Display Corporation and InsideChips, and is on the Board of Directors of Applied Materials (the Apple of Semiconductor field as we call it, click here to see our Applied Materials review) .
Several video clips that involve Prof. Forrest are available online:
Throughout his career, Prof. Forrest has received many honors & awards including:
- IEEE Daniel Nobel Award for innovations in OLEDs 2007
- Jan Rajchman Prize for invention of phosphorescent OLEDs 2006
- IEEE/LEOS William Streifer Scientific Achievement Award 2001
- MRS Medal for work on organic thin films 1999
- Thomas Alva Edison Award for innovations in organic LEDs 1998
- Co-recipient of the IPO National Distinguished Inventor Award 1998
- IEEE/LEOS Distinguished Lecturer Award 1996-97
To learn more about Prof. Stephen R. Forrest, please visit the OCM Group website by clicking on this link.