Scanning Probe Microscope Works Out Solar Cell Profiles

Scanning probe microscope techniques can be used to study nanostructured organic photovoltaic cells, giving insights into film behavior and processing needs. Knowing about the chemistry of different film materials being considered for solar cells is not enough. Data about their nanostructures is vital to obtain the maximum efficiency when they are brought together in the finished cell.

Alexander E. Braun, Senior Editor -- Semiconductor International, 7/31/2008 9:00:00 AM

There is an ongoing effort by the photovoltaics (PV) industry to use polymers to create solar cells because this would not only enable processing to be done at room temperature and ambient conditions, but would also allow the application of processing techniques, such as various forms of printing, that are relatively inexpensive when compared with processing silicon. However, for the greatest efficiency possible, it is not enough to have the chemistry and chemical composition of these different materials well in hand, but to also know as much as possible about their nanostructures before they are all brought together in the finished cell.

Professor David Ginger and his students at the University of Washington’s Department of Chemistry (Seattle) have been researching this problem, using scanning probe microscopy (SPM) to solve it. “The idea is that if we can measure the PV and some of its more fundamental electronic properties with this level of resolution, we might be able to find out what limits performance and how to improve it,” Ginger said.

For quite some time, Ginger and his group have used different kinds of (SPM) — from electrostatic force microscopy (EFM) to conductive atomic force microscopy (c-AFM) — to develop maps of the photocurrent distribution in these solar cells to be able to determine the size and kinds of domains that the photocurrent originates from. “That’s really just step one — getting a more microscopic picture,” Ginger said. “You don’t just want to know where the photocurrent comes from — you also want to find out where these charges are being created, where they are being lost to recombination, and how charge transport varies region to region in these extremely heterogeneous thin films. We’re trying to look at transport differences.”

The hurdle is that there is a quantitative issue with how to interpret that data. Several researchers who have previously attempted it report that if SPM techniques are used to measure the charge carrier mobility, it is soon realized that the values obtained can be up to two orders of magnitude higher than they should have been. Ginger’s group has been trying to solve that problem and determine whether it is possible to use the method to get quantitative, rather than just qualitative, measurements. “The fact that there’s a two-orders-of-magnitude discrepancy between what the known mobility values in these materials are and what’s coming out of the scanning probe measurements made us hesitate a bit before using this technique,” Ginger said, “because even if it gives a relative value, you must wonder whether it’s that accurate since it is so much larger than what you thought it would be.”

The researchers hypothesized that the differences lay primarily in the experiment’s electrode geometry. After doing the numerical work to check that possibility and experimenting to do comparisons with these numerical simulations, it was discovered that, indeed, most of the variation could be attributed to the very sharp needle plane geometry present in the conducting AFM experiment vs. planar geometry, where bulk photodiode measurements are performed. “When you compare these two, you realize that you must scale your experimental conductive AFM results based on the film thickness and the tip diameter,” Ginger said. “When you take those two factors into account, you collapse what looks like a whole scatter plot of conductive AFM data for mobility values on different films taken with different tips onto a nice single line that agrees with a fundamental underlying film property.”

Several scanning probe microscopy techniques are useful to study nanostructured organic solar cells. Illustrated (center) are atomic force microscopy’s (AFM) two primary modes. In non-contact mode (A), a vibrating cantilever measures electrostatic forces between the tip and surface. The photo-induced charging rate in a solar material is mapped using a pulsed light source (top left). In contact mode (B), a static cantilever measures tip sample currents to map photocurrent generation in solar materials (bottom right). Contact mode can be used to extract quantitative values of local charge carrier mobility through analysis of current voltage curves taken by the AFM tip. The figure at the bottom left shows numerical simulation results for current density (J) and potential (V) distributions generated in a solar material under a conductive AFM tip. (Source: University of Washington)

This breakthrough enabled the group to reconcile why a certain tip gave a different measurement on a certain film, and that film gave a different value than another. Now it is possible to obtain reasonable carrier mobility, enabling the use of this technique to locally map quantitatively changes in the mobility from region to region in heterogeneous thin films. The next step is to use the technique and its results to characterize local variations, because it is now known that, quantitatively, it provides the right values.

By understanding these local variations in transport, it will be possible to correlate them with inter-device performance changes with processing and possibly come up with a more rational optimization of film processing, which is currently being done in a somewhat hit-or-miss fashion. “You try different solvents, different heating conditions, different anneals for different times and different temperatures, and you know that affects the texture and nanostructure morphology of the film,” Ginger said. “However, you’d really want to correlate that with a change in a local hole mobility in the domain, for instance.”

In the future, the researchers would like to couple these transport measurements with local measurements of carrier generation and carrier trapping and recombination. Once all of these are obtainable, it becomes possible to build up a truly microscopic picture of how these nanostructured organic solar cells work. This would transform what at present is an art into a technology. The overall goal is, of course, to actually develop new polymers that harvest a wider range of the solar spectrum and have greater stability; materials that have better energy level alignments so that not as much of the voltage obtained from the photons that are absorbed is not wasted. However, there still remain many outstanding fundamental research as well as technological — processing and characterization — challenges in the organic electronics and polymer PV field.

The UW researchers know what needs to be done. But as Ginger put it with a grin, “The doing is easier said than done.”