Group's Measuring Tool Probes Solar-Cell Materials

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Next-generation solar cells made from organic compounds hold great promise in meeting future energy needs, but researchers are still striving to gain a deep understanding of the materials involved – including the efficiency with which they convert light into mobile charge, known as photocapacitance.

A Cornell research group led by John Marohn, professor in the Department of Chemistry and Chemical Biology, has proposed a unique method for recording and measuring light-induced mobile charge – at nanoscale lengths and nanosecond time scales – at different areas in a heterogeneous solar-cell material.

Their approach involves a charged microcantilever, which experiences a slight shift in oscillation phase as a result of interaction with a nearby electrically charged material. Marohn likens the technique to how a clock might get affected by an electrical charge, where the difference cannot be seen in real time but the charge’s effect is evident when you compare that clock to an unaffected one.

“The clocks both go around once an hour,” Marohn said, “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.”

Their paper, “Microsecond photocapacitance transients observed using a charged microcantilever as a gated mechanical integrator,” was published June 9 in Science Advances. Marohn’s collaborators were doctoral students Ryan Dwyer and Sarah Nathan, who share lead-author credit.

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

One of the inefficiencies of organic solar-cell materials that Marohn and his group are addressing is recombination. When sunlight hits the material, it creates free charges (negatively charged electrons and positively charged holes) that get turned into electric current. But not all of those free charges escape the cell and turn into current; those that do not turn into current recombine, with the byproduct being heat.

The ability to “see” – or, more accurately, measure – charge generation and recombination following a burst of light was the group’s thrust behind developing pk-EFM. A conductive cantilever is placed near an organic semiconductor film; a voltage pulse is applied to the cantilever, while a carefully timed light pulse is applied to the sample.

The cantilever’s oscillation frequency is shifted slightly by the electrostatic interactions with the mobile charges in the sample. Those interactions result in a phase shift, or “phase kick” as the group calls it. This phase shift persists for a long time (nearly a second) and is therefore relatively easy to measure accurately.

The researchers study this phase shift as a function of the nanosecond time delay between the light pulses and voltages pulses. In this way, the researchers are able to indirectly infer what happened to charges on the nanosecond time scale without having to observe the charge directly, 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,” Marohn said. “We’re trying to watch things that are both very fast and very small.”

The group’s work is trying to probe more deeply the photocapacitance of organic bulk materials that have previously been examined using time-resolved electric force microscopy. Future work will focus on getting even better spatial and temporal resolution in hopes of ultimately determining which combination of materials is optimal for efficient solar power.

“Solar cells work OK, and we don’t really understand how they work,” Marohn said. “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.”

Funding for this work came from the National Science Foundation.