Award Abstract # 2113994
Imaging charge recombination dynamics in organic semiconductor films

NSF Org: DMR
Division Of Materials Research
Recipient: CORNELL UNIVERSITY
Initial Amendment Date: March 9, 2021
Latest Amendment Date: June 20, 2023
Award Number: 2113994
Award Instrument: Continuing Grant
Program Manager: Birgit Schwenzer
bschwenz@nsf.gov
 (703)292-4771
DMR
 Division Of Materials Research
MPS
 Directorate for Mathematical and Physical Sciences
Start Date: June 1, 2021
End Date: August 31, 2024 (Estimated)
Total Intended Award Amount: $592,636.00
Total Awarded Amount to Date: $592,636.00
Funds Obligated to Date: FY 2021 = $217,865.00
FY 2022 = $184,734.00

FY 2023 = $190,037.00
History of Investigator:
  • John Marohn (Principal Investigator)
    jam99@cornell.edu
Recipient Sponsored Research Office: Cornell University
341 PINE TREE RD
ITHACA
NY  US  14850-2820
(607)255-5014
Sponsor Congressional District: 19
Primary Place of Performance: Cornell University
150 Baker Laboratory
Ithaca
NY  US  14853-1501
Primary Place of Performance
Congressional District:
19
Unique Entity Identifier (UEI): G56PUALJ3KT5
Parent UEI:
NSF Program(s): SOLID STATE & MATERIALS CHEMIS
Primary Program Source: 01002122DB NSF RESEARCH & RELATED ACTIVIT
01002223DB NSF RESEARCH & RELATED ACTIVIT

01002324DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s): 7237, 7697, 8396, 8607
Program Element Code(s): 176200
Award Agency Code: 4900
Fund Agency Code: 4900
Assistance Listing Number(s): 47.049

ABSTRACT

NON-TECHNICAL SUMMARY:

Our economy requires energy to run. Sunlight is a free and essentially limitless source of energy. To exploit this energy source, economical solar cells that can convert sunlight into electrical current and voltage are needed. Existing silicon solar cells are too expensive to create, pattern, and install on a massive scale. Solar cells made from plastics and small molecules, on the other hand, can potentially be as inexpensive as paint to create and as easy as newsprint to pattern at high speed. Plastic/molecular solar cells are being intensely studied worldwide, but how these complex materials convert light to electricity remains a puzzle. With this project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, Professor John Marohn and his research group at Cornell University will develop methods for watching how electrical charges in a film activated by sunlight move and relax. The research team will utlizie a specialized microscope which enables the observation of charges moving distances of nanometers (one billionth of a meter) on the timescale of nanoseconds (one billionth of a second). By allowing the observation of charge motion at the molecular level, it is expected that these measurements will significantly advance our understanding of how plastic/molecular solar cell materials convert light into electricity. This research will open new ways to study semiconductor chips and batteries, two growth technologies central to our economy. Researchers funded by this project will develop virtual high-school science experiments on the physics of waves suitable for both in-person and remote learning.

TECHNICAL SUMMARY:

In the best organic photovoltaic materials, the photocarrier recombination time is anomalously long. If this anomalous behavior could be understood then it could be exploited to improve the open-circuit voltage and current of organic solar cells. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, will study charge generation and recombination in organic donor/acceptor (D/A) blends at nanoscale spatial resolution and nanosecond temporal resolution. Proposed experiments include scanning Kelvin probe force microscopy, measuring local electrostatic potential and electric field; broadband local dielectric spectroscopy, measuring local steady-state conductivity and energetic disorder; and "phase-kick" electric force microscopy (pk-EFM), measuring transient conductivity. Charge mobility will be studied in single-component films by simultaneously measuring device current and local electric field. Charge recombination transients in D/A blends prepared on both insulating and metallic substrates will be recorded using pk-EFM and compared to bulk time-resolved microwave conductivity measurements. The drift and diffusion of photogenerated charges in inhomogeneous D/A blends will be observed stroboscopically via pk-EFM. Films comprised of polymer donors with both fullerene and non-fullerene acceptors will be examined. It is expected that the microscopic material parameters gleaned from these measurements will enable the rigorous testing of charge-recombination hypotheses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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.

This grant led to two major scientific outcomes. The first was a new way to measure a solar cell’s transient electrical conductivity. When light enters a solar cell material, energy is absorbed, creating energetic positive and negative charges that increase the material’s electrical conductivity. In a good solar cell, most of these charges leave the solar cell and do useful electrical work. In a bad solar cell, these charges promptly recombine.  Mobile charges recombine in many next-generation solar-cell materials on the nanosecond timescale (a nanosecond is one billionth of a second). Our new method for measuring transient changes in conductivity works on the nanosecond time scale and the tens of nanometers spatial scale (a nanometer is one billionth of a meter). Tens of nanometers is the length scale on which molecules are organized in next-generation solar-cell materials.

Implementing this measurement required building a new kind of scanning-probe microscope. In the measurement, a microscopic needle, smaller than a human hair, was coated with metal and brought within ten to one hundred nanometers of the solar-cell material. A pulsed force was applied to oscillate the needle, a pulsed voltage was applied to charge the needle, and a laser pulse was applied to excite charges in the sample. The presence of sample charges was observed as a change in the needle’s phase of oscillation. Changes of the sample conductivity with time were inferred by varying the timing of the voltage pulse and laser pulse. A U.S. patent was awarded for this invention. Reducing this invention to practice required machining new microscope parts; implementing circuits to synchronize pulses of force, voltage, and light; and developing a mathematical theory for the phase-shift signal.

The new microscope was used to study two potential next-generation solar-cell materials, ones made from blends of small molecules and plastics and ones made from lead-halide perovskites. We compared our transient conductivity measurements to time-resolved photoluminescence measurements, observing the light emitted from a solar cell when charges recombine. Our method observed long-lived charges that are difficult or impossible to observe in photoluminescence measurements.

The second outcome was a scanning-probe microscope method for simultaneously quantifying the density of charge and the electrical conductivity of a solar cell. In this method, the voltage applied to the needle was oscillated. The average oscillating frequency of the needle was plotted versus the voltage-oscillation frequency to yield a spectrum. We found that the spectrum’s roll-off frequency was proportional to sample conductivity, while the size of the spectrum was proportional to charge density. Here we developed new measurement protocols, invented a mathematical theory of the measurement, and wrote thousands of lines of computer code to reduce the mathematical description to numbers that could be compared to experimental data. The mathematical theory involved the application of response functions, a key idea in chemical physics that was challenging to apply to a scanning-probe microscope measurement. The measurement was applied to both plastic solar cells and lead-halide perovskites, where it was used to evaluate materials after they were processed into thin films and draw conclusions about the reproducibility of the processing methods.

This grant led to several broader impacts. This grant was the sole source of funding for the PhD research of one graduate chemistry student and was a significant source of research support for a second chemistry PhD student and a materials science master’s degree student. The grant supported the research of two chemistry undergraduates. The many thousands of lines of computer code developed in support of this funded research were released to the public under free and open-source licenses as two large Python packages available on Github. One of the funded graduate students laid the groundwork for commercializing the patented invention by participating in the U.S. National Science Foundation’s i-Corps program. The graduate students supported by this program held leadership positions in a city scientific-demonstrations program and in Cornell’s Expanding Your Horizon’s seminar-day organization, encouraging hundreds of middle-school students (and their parents) in central New York and Pennsylvania to become interested in science as a career. 

We anticipate future broader impacts of this work. The manufacturing of next-generation solar cells represents a major economic opportunity for the United States, yet achieving the reproducible processing of next-generation solar-cell materials has created a major bottleneck to commercialization. The measurements and theory developed during this grant can contribute to solar-cell commercialization by providing a potentially better way to evaluate reproducibility and stability by probing the materials on the nanosecond time scale and nanometer spatial scale.  They can contribute to the commercialization of other new materials like oxide semiconductors for high-power electronics.  Quantum computing is another economic opportunity that our work could positively impact.  For example, our new mathematical description of electrical scanning-probe microscopy could stimulate new methods for measuring quantum defects in materials like two-dimensional semiconductors.

 


Last Modified: 03/24/2025
Modified by: John A Marohn

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