| NSF Org: |
DMR Division Of Materials Research |
| Recipient: |
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| Initial Amendment Date: | May 6, 2015 |
| Latest Amendment Date: | May 6, 2015 |
| Award Number: | 1507988 |
| Award Instrument: | Standard Grant |
| Program Manager: |
James H. Edgar
DMR Division Of Materials Research MPS Directorate for Mathematical and Physical Sciences |
| Start Date: | July 1, 2015 |
| End Date: | June 30, 2019 (Estimated) |
| Total Intended Award Amount: | $298,262.00 |
| Total Awarded Amount to Date: | $298,262.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
3141 CHESTNUT ST PHILADELPHIA PA US 19104-2875 (215)895-6342 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
3141 Chestnut St Philadelphia PA US 19104-2816 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): |
DMR SHORT TERM SUPPORT, ELECTRONIC/PHOTONIC MATERIALS |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.049 |
ABSTRACT
Non-technical Description: Copper zinc tin sulfide selenide (CZTSSe) is a promising candidate material for use in solar cells because it strongly absorbs visible light and is composed primarily of earth-abundant, non-toxic elements. However, fundamental scientific understanding of CZTSSe has been limited by difficulties in fabricating thin films of high quality. In this project, researchers at Drexel University and the University of Delaware grow bulk crystals of CZTSSe and characterize their response to light. This approach enables identification of relationships between elemental composition and photovoltaic response, which can lead to both near-term increases in efficiencies and improved estimates of the practical performance limits of solar cells made from this emerging material. Multiple graduate and undergraduate student researchers participate in this project. Additionally, researchers use a mobile solar module to bring concepts in solar energy conversion to K-12 students in the Philadelphia and Newark communities, especially from under-represented groups, through events such as Philly Materials Day.
Technical Description: CZTSSe thin films have shown promising photovoltaic efficiencies up to 12.6%, but they are still far below the theoretical limit of over 30%. Photocurrent and photovoltage in CZTSSe solar cells are limited by short (nanoseconds) photoexcited carrier lifetimes. Further improvements in efficiency will require full understanding of how materials composition, intrinsic point defects, and interfaces affect ultrafast photoexcited charge carrier dynamics. However, complex defect chemistry and highly non-equilibrium conditions of thin film growth result in high densities of grain boundaries and secondary phases, posing a significant impediment to fundamental understanding. In this project, the collaborative research team grows high-quality, quasi-equilibrium CZTSSe single crystals and interrogates them using ultrafast spectroscopic probes to understand how carrier dynamics depend on composition, defects, and interfaces in CZTSSe single crystals. This work is expected to lead to new understanding of the relationships between ultrafast carrier dynamics, processing, material and interface properties, and photovoltaic performance. Specifically, the project relies on terahertz spectroscopy and transient reflectance spectroscopy coupled with finite element transport-recombination models to determine photoexcited carrier lifetimes, mobilities, and dominant recombination mechanisms. Lifetimes and mobilities are measured as a function of Cu:Zn:Sn and S:Se ratios and are correlated to device performance. Additionally, studies of surface/interface recombination in CZTSSe-CdS heterojunctions and the effects of grain boundaries in quasi-equilibrium polycrystals enable extrapolation of new fundamental scientific understanding to thin film photovoltaic devices.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Principle Investigators Jason Baxter at Drexel University and Brian McCandless and Robert Birkmire at the University of Delaware Institute of Energy Conversion recently completed research for their collaborative award DMR-1507988/1508042, studying relationships between ultrafast carrier dynamics and solar cell performance in copper zinc tin selenide (Cu2ZnSnSe4, CZTSe) single crystals. CZTSe is a promising candidate material for use in solar cells because it strongly absorbs visible light and is composed primarily of earth-abundant, non-toxic elements. However, fundamental scientific understanding of CZTSe has been limited by difficulties in fabricating thin films of high quality. In this project, researchers at Drexel University and the University of Delaware grew bulk crystals of CZTSe and characterized their response to light. The use of bulk monocrystals grown under quasi-equilibrium conditions eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin-film growth.
The team investigated new combinations of ultrafast spectroscopic methods to track the behavior of photoexcited electrons on picosecond to nanosecond time scales (trillionths to billionths of a second). In combination with appropriate models, these results enabled calculation of bulk carrier lifetime and surface recombination velocity that can be used to understand limitations in solar cell performance. The team applied this approach to a series of monocrystals with a range of Cu:Zn:Sn ratios, finding that Cu-poor, Zn-rich compositions yielded the longest carrier lifetimes and, correspondingly, the highest power conversion efficiencies. In contrast, more stoichiometric compositions resulted in poor performance. Using parameters obtained from these experiments, the team simulated CZTSe solar cells under a wide range of conditions and identified pathways for increased open circuit voltage and efficiency. The approaches developed as part of this work on CZTSe can also be applied to other materials for thin film solar cells, as well as for other optoelectronic devices. In related work, the team also elucidated relationships between carrier dynamics and material properties and/or interface treatments in other earth-abundant chalcogenides and oxide thin films. Collectively, this project has provided new insights relating carrier dynamics to material properties, processing, and solar cell performance.
Detailed research results have been disseminated to the research community and are also accessible to the general public. This grant resulted in 7 publications in peer-reviewed journals and 2 conference proceeedings. Research was also presented at 6 national and international conferences.
This grant provided partial support for education and training of 4 PhD students. One of these went on to a postdoctoral fellowship at the National Renewable Energy Laboratory, one works at Intel developing next-generation processes for the microelectronics industry, and the other two are completing their thesis work. In related outreach, Drexel University has hosted annual Philadelphia Junior Solar Sprint competitions, in which over 300 middle school students built and raced shoebox-sized solar powered cars over the last two years.
Last Modified: 09/28/2019
Modified by: Jason B Baxter
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