ORGANICPV
we typically use a 405 nm LED. trEFM is sensitive enough that low-intensity LED illumination, equivalent to 1 Sun or less, is sufficient to produce a useful signal. Attenuation of the light intensity using various ND filters can be used to adjust the total charging time so that it is long enough to be resolved cleanly, but fast enough to make repeated measurements tractable.
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Figure 3. (A) Line traces of the height, charging rate, frequency shift magnitude, and charging rate fit error at the polymer-ITO interface. Data were taken along the area indicated in the corresponding charging rate image (right). The charging rate decreases to 80% the typical polymer charging rate value approximately 90 nm from the polymer-ITO interface, as indicated by the yellow circle. (B) Time-resolved voltage pulses on an ITO sample, separated by 100 µs (left) and 50 µs (right). The 100 µs data indicates the time resolution of trEFM; the 50 µs-separated pulses cannot be clearly-resolved by the current instrumentation
In order to generate an image, we pulse the LED at each position on the sample and then record the resulting frequency shift as a function of time, resulting in plots such as that in Figure 2b. An exponential decay function is then fit to the decay of the frequency shift, and this decay constant relates to the photocurrent we wish to measure. We repeat this process for every pixel to generate simultaneous charging rate and topography images. Because the charging process is relatively fast, a 256 x 256 resolution trEFM image takes about twenty minutes to acquire, which is comparable to standard SKPM techniques. The ability to record the cantilever motion with faster time resolution, which should be possible with the new generation of AFM hardware, would allow the use of higher intensity light pulses and make imaging even faster.
As one example of the capabilities of this technique, we have used trEFM to explore the photoinduced charging behavior in all-polymer OPV blends,24
photogenerated charge carriers migrate to opposite sides of the active layer. The resulting accumulation of charge changes the capacitance and electrostatic force gradient, in turn causing a resonance frequency shift according to Equation 1. By continuously measuring ∆f with ~100 µs time resolution, we are able to record a charging curve and determine the local charging rate in the material (Figure 2b). While the ability to study dynamic processes is an obvious advantage of trEFM, we have found that another significant advantage of trEFM is that it appears to be less sensitive to tip contamination than conventional EFM or SKPM. We attribute this added robustness to the fact that trEFM measures a rate of change rather than an absolute value and so is less sensitive to contamination and, consequently, is a more repeatable and robust technique.
In our experiments we often use commercial Pt- coated cantilevers (such as Budget Sensors ElectricTap-300G) with spring constant k ~ 40 N/m and resonance frequency f ~ 300 kHz. We photoexcite the sample with illumination pulses from LEDs operating at different wavelengths depending on the absorption properties of the sample. For our studies of polyfluorene polymers,
in this case poly(9,9’-dioctylfluorene-
co-benzothiadiazole) (F8BT) and poly(9,9’- dioctylfluorene-co- - bis-N,N’-phenyl-1,3- phenylenediamine) (PFB). We chose PFB:F8BT blends as a model system because of the wide literature discussing the effects of processing and blend morphology on their performance. By comparing the topography (Figure 2c) with the charging rate image (Figure 2d), we can analyze the relationship between charging behavior and the local PFB:F8BT film composition. We have confirmed the utility of trEFM as an analytical technique by showing that the spatially-averaged local charging rate and the measured external quantum efficiency (EQE) are correlated for a wide range of blend ratios (Figure 2e). This is an exciting result– with only a single calibration factor, a trEFM image of a polymer blend can be used to accurately predict the efficiency of the polymer solar cell that will be fabricated from a particular film. One can imagine using such a method both to screen new materials in the lab, or as a rapid quality control diagnostic in a production facility. Additionally, we note that it is possible to use trEFM to monitor other quantities of interest, such as spatially-correlated charge trapping and detrapping.
www.solar-pv-management.com Issue II 2010
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