Ryan Dwyer and Sarah Nathan have published a paper in Science Advances: Dwyer, R. P.; Nathan, S. R. & Marohn, J. A.“Microsecond photocapacitance transients observed using a charged microcantilever as a gated mechanical integrator”, Sci. Adv., 2017, 3, e1602951 [doi:10.1126/sciadv.1602951].
In the paper we introduce and apply a new scanning probe microscope method for studying charge generation and recombination in solar cell films. The method builds upon the widely used “electric force microscope” (EFM) experiment in which an electrically charged microcantilever interacts with charges in a thin-film sample. The time evolution of sample charges is followed by observing changes in the mechanical oscillation frequency of the cantilever. The problem with this EFM experiment is that it cannot observe changes in sample charge on a timescale faster than a cantilever oscillation period, a few microseconds.
In the paper we introduce a new method, phase-kick electric force microscopy (pk-EFM), in which we measure cantilever phase as a function of the time delay between a light pulse, exciting charges in the sample, and a voltage pulse, controlling the charge on the cantilever. By studying the cantilever phase as function of pulse time we learn about charge generation and recombination dynamics in the sample. We show that the time resolution of pk-EFM is not limited by the cantilever period, as in EFM, but is limited instead by the rise time of the optical and voltage pulses. In the paper we demonstrate 35 ns time resolution. We believe this resolution could be extended to picoseconds or even shorter.
The pk-EFM breakthrough was described in press releases at the Cornell Chronicle [link] and at phys.org [link]. We applied for patent protection for the pk-EFM invention with Cornell’s Center for Technology Licensing.
This work was funded by the U.S. National Science Foundation.