New Method for Recording and Measuring Light-Induced Mobile Charge at Nanosecond Time Scales

Next-generation solar cells produced from organic compounds have proved themselves to be promising in fulfilling future energy requirements. However, Researchers are still working towards gaining an in depth understanding of the materials involved in this process – including the effectiveness with which they transform light into mobile charge, called photocapacitance.

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A Cornell research group headed by John Marohn, Professor in the Department of Chemistry and Chemical Biology, has introduced a unique technique for both recording and measuring light-induced mobile charge, at nanosecond time scales and nanoscale lengths, at diverse areas in a heterogeneous solar-cell material.

This new technique involves a charged microcantilever, which goes through a small shift in oscillation phase due to the interaction with a close by electrically charged material. Marohn likens the technique to how a clock could get affected by an electrical charge, where it is not possible to see the difference in real time but the effect of the charge is evident when that clock is compared to an unaffected one.

The clocks both go around once an hour. But one will advance slightly as a result of the interaction with the charge. And by comparing the two clocks, you can see that the one picked up a little extra angle.

John Marohn, Professor in the Department of Chemistry and Chemical Biology, Cornell University

Their paper, “Microsecond photocapacitance transients observed using a charged microcantilever as a gated mechanical integrator,” was published in Science Advances on June 9th. Doctoral Students Ryan Dwyer and Sarah Nathan, who share Lead-Author credit, were Marohn’s collaborators.

The group has filed for patent protection for the technique it produced for this research– phase-kick electric force microscopy (pk-EFM) – with Cornell’s Center for Technology Licensing.

Marohn and his group are addressing the concept of recombination, which is considered to be one of the inefficiencies of organic solar-cell materials. When sunlight falls on the material, it produces free charges (positively charged holes and negatively charged electrons) that get converted into electric current. However, only few of these free charges escape the cell and transform into current; and those that do not get converted into current recombine, with the byproduct being heat.

The group’s thrust behind developing pk-EFM refers to the potential to “see” – or, more precisely, measure – charge generation and recombination following a burst of light. A conductive cantilever is positioned close to an organic semiconductor film; voltage pulse is used on the cantilever, while a cautiously timed light pulse is applied to the sample.

The cantilever’s oscillation frequency is moved to some extent by the electrostatic interactions with the mobile charges in the sample. Those interactions lead to a phase shift, or “phase kick” as the Researchers call it. This phase shift exists for a prolonged time period (almost a second) and is thus comparatively easy to measure in a precise manner.

This phase shift is studied by the Researchers as a function of the nanosecond time delay between the voltages pulses and the light pulses. In this manner, the team was able to indirectly assume what happened to charges on the nanosecond time scale without the need to directly observe the charge in real time.

What we wanted was a way to see, in these tiny regions where different molecules are concentrated, how the charges recombine in the various regions of the sample. We’re trying to watch things that are both very fast and very small.

John Marohn, Professor in the Department of Chemistry and Chemical Biology, Cornell University

In their work, the Researchers are trying to investigate in a more detailed manner the photocapacitance of organic bulk materials that have earlier been analyzed using time-resolved electric force microscopy. Work to be conducted in the future will concentrate on acquiring even better temporal and spatial resolution in order to eventually determine which combination of materials is most favorable for efficient solar power.

Solar cells work OK, and we don’t really understand how they work. It seems like, if you really understood how they worked, you could make them a lot better. And this is one way to try to figure that out.

John Marohn, Professor in the Department of Chemistry and Chemical Biology, Cornell University

The National Science Foundation funded this research.

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