Imaging charge recombination dynamics in organic semiconductor films

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.