Watching Electrons on the Move
Spectroscopic method could lead to improved polymers for solar cells.
Using ultrafast infrared spectroscopy, researchers at Pennsylvania State University in University Park have observed electrons moving in real time through a polymer blend, possibly pointing the way to more efficient organic solar cells. The group found that the mobility of the electrons in the polymer blend was less than a tenth of what it could be. The right polymer construction, therefore, might increase mobility, thereby leading to improved performance.
“Higher carrier mobilities would enable organic solar cells to operate as efficiently under full sun exposure as they do under reduced intensity. They also could be made thicker to absorb all of the incident light,” said John B. Asbury, research leader and assistant professor of chemistry, who cautioned that the task of improving organic solar cells is complicated because it involves balancing fundamental properties that interact with one another.
A pump pulse (blue beam) puts the molecules of an organic semiconductor into an excited state, which is then sampled with a probe pulse (rainbow beam) arriving a variable amount of time later. Spectra from the carbonyl (C = O) stretch contain information about electron movement as carriers hop from one molecular domain of the solar cell’s polymer to the next (schematic lower right). The inset on the lower right shows the electrons moving toward the centers of the polymer’s domains and the corresponding time evolution (indicated as drift time) of the carbonyl mode that results from the electrons’ motion. The arrows connect the position of the electrons with the corresponding drift time. Courtesy of John B. Asbury, Pennsylvania State University.
Organic solar cells promise to produce power at a significantly lower cost per watt than traditional silicon-based solar cells because of the cost differential between the core materials. However, the efficiency of organic devices has remained poor because of low charge carrier mobility. The effect also has led to the practice of using thin photovoltaic layers that do not absorb light well.
The root cause of low mobility is that the organic semiconductors at the heart of the solar cells are molecular in nature. Consequently, there is a need for high molecular order and high interfacial density at the same time — requirements that dampen mobility.
In their study, the investigators used a combination of two-dimensional IR, polarization IR pump and visible pump IR probe spectroscopic techniques. They did so with a Ti:sapphire laser from Quantronix Corp. of East Setauket, N.Y., pumping two optical parametric amplifiers from Light Conversion Ltd. of Vilnius, Lithuania. One amplifier generated a 5.8-μm pulse of 100-fs duration for the 2-D IR and as a probe for the visible-IR experiments. The other generated pump pulses at 550 nm with 100-fs duration for the visible IR.
The researchers mounted the sample on a translation stage, collecting data in either reflective or transmissive mode as required by the experiment. They used a 64-element HgCdTe from InfraRed Associates Inc. of Stuart, Fla., for signals from the sample, capturing 32 probe frequencies simultaneously through a spectrograph made by Jobin Yvon (now Horiba Jobin Yvon of Edison, N.J.).
Using a carbonyl stretch in the functionalized fullerene PCBM as a local vibrational reporter, they probed the dynamics in a blend of the fullerene and the conjugated polymer CN-MEH-PPV. They found that charge transfer occurs at the interfaces between the spherical domains of the fullerene molecules and the polymer. Because of differences in carbonyl vibration modes resulting from variations in location within the fullerene domains, the scientists could observe electron motion within individual domains.
The results showed that the electron velocity within the fullerene domains was 1 to 2 m/s — at least an order of magnitude higher than the velocity in the composite polymer blend. This discrepancy suggests that organic solar cell efficiency could be improved substantially if the interfacial boundaries that act as speed bumps along the charge migration path were eliminated.
The next phase of the research, according to Asbury, will seek to understand the influence of various morphology and structural changes on charge migration. When discussing equipment, he noted that the detection limit remains a problem, primarily because of the stability of the laser and the noise of the detector. Improvements in either could prove useful, he said.
“With lower detection limits, we could expand the studies to other vibrational modes and examine the materials with excitation intensities that more closely match the intensity of the sun.”
Journal of the American Chemical Society, Dec. 26, 2007, pp. 15884-15894.
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