The output of the new technique provides a precise reproduction of the device’s current density-voltage curve through the entire voltage range between the bias extremes allowing researchers to pinpoint where problems exist in the device and can serve as a blueprint for what to fix in the device.
The new method uses impedance spectroscopy to generate repeatable, non-destructive results and allows the device to be tested in real-word conditions.
"This measurement breakthrough should allow us to more rapidly optimize solar cells," Richter states. "We’re able to look at what happens electronically throughout the entire device. Importantly, how long does the charge exist once created and how long does it take to get the photogenerated charge through the semiconductor mixture to the electrodes? The larger the difference between the charge lifetime and device transit time greatly improves the likelihood that a photovoltaic device will be a more efficient source of electrical power."
Currently at the laboratory level, current-voltage testing of organic photovoltaic devices is typically done by analyzing device operation at either extreme of the device’s bias spectrum – that is, a short circuit or an open circuit – and trying to infer from those results what is happening electrically within the device. But, when the device does not perform as a ‘textbook’ or ‘ideal’ solar cell then the picture of what’s going on in the device between these bias extremes quickly becomes clouded.
"That approach only works if the recombination (where the charge carriers are eliminated rather than continuing to flow through the device) at one bias is nominally identical to the charge generation at the other," explained Gundlach. "In a good device, those should be about equal. In a non-ideal device, they could be vastly different. With our technique, we can actually map the full range of the characteristics from one extreme to the other and disentangle generation, transport, and different loss mechanisms throughout the entire bias range."
Combining the physical properties, lifetimes, and carrier concentrations with an accurate nanoscale picture of the semiconductor film’s microstructure really gives a complete picture of how the device operates and what limits these devices from reaching their theoretically predicted performance limits," said Gundlach. "We are now in a much better position to put all of the pieces of information together, and then we can develop more physically accurate device models, better informed materials design guidelines, and ultimately more closely connect materials properties with processing methods and solar cell performance."
And since the physical process governing organic photovoltaics is similar to other organic semiconductors (organic light-emitting diodes, for example, which are prevalent in electronic displays), future applications of this technique to other industries appears straight forward.
"A lot of the understanding being developed here can also be applied to make better organic light emitting diodes," said Richter. The organic photovoltaic samples used in this study were developed in house at NIST. The 100 nm thick device has a three-layer structure – a top semi-transparent electrode, the organic photovoltaic, and a bottom electrode – placed on a 1 inch piece of glass.
For the impedance spectroscopy measurements, the sample was installed beneath an LED broadband white light, calibrated to one Sun illumination (natural sunlight).
The measurement itself is conceptually simple: "We’re applying an oscillating voltage across the device and measuring the current that comes explained Richter. "We do this underneath the simulated sunlight. Mathematically, we’re looking at the phase shifting of the current out relative to the voltage in."
These results, combined with Basham’s analysis and methodology, provide a relatively inexpensive measurement that can improve understanding dominant loss mechanisms across the entire bias range of a device.
"Now, a small start-up company can go out and buy an impedance spectrometer and do this measurement with our paper in hand because it tells them how," suggested Gundlach.
1 J. I. Basham, T. N. Jackson, D. J. Gundlach, "Predicting the J-V Curve in Organic Photovoltaics Using Impedance Spectroscopy," Advanced Energy Materials, Vol. 4, No. 9, 7 p., (02-Jun-2014)
2 L. C. C. Elliott, J. I. Basham, K. P. Pernstich, P. R. Shrestha, L. J. Richter, D. M. DeLongchamp, D. J. Gundlach, "Probing Charge Recombination Dynamics in Organic Photovoltaic Devices under Open-Circuit Conditions," Advanced Energy Materials, 8 p. (12-Jun-2014)
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