ORGANICPV
In order to gauge the utility of trEFM in analyzing such materials, it is necessary to determine the spatial and time resolution limits. We estimate the spatial resolution using the data in Figure 3a, where a PFB:F8BT blend is partially removed to expose the underlying substrate (indium tin oxide, ITO) and a charging rate image acquired. Since photoinduced charging does not take place on bare ITO, we can estimate the lateral resolution by determining the point where charging decreases across the polymer-ITO interface. The charging rate at the interface is half that over the polymer. Approximately 90 nm away from the interface, the charging rate is 80% of the normal value; this implies a lateral resolution on the order of ~100 nm is attainable using trEFM. The corresponding charging rate image and the linear section depicted in the plot are also shown. By applying voltage pulses to a metallic substrate, we can also determine the time resolution of our current apparatus. In Figure 3b, we apply voltage pulses separated by 100 µs and 50 µs. At 100 µs we can clearly observe distinct pulses, but at 50 µs the time resolution limit of the setup results in an overlap of signals, this is consistent with our experience resolving charging rates on actual polymer films. Based on such data, we claim that trEFM has a spatial/time resolution of ~100 nm/~100 µs, respectively. Improvements in time- resolution beyond the current 100 µs limit would not only allow us to image faster and better study local carrier dynamics (trapping, transport, detrapping) on shorter time scales, but it would also enable the study of more highly performing polymer:fullerene blends without significant attenuation of the LED pump pulse (which can cause additional experimental complications).
Photoconductive Atomic Force Microscopy (pcAFM)
Macroscopic characterization of device parameters such as open circuit voltage, short circuit current and fill factor provide information about overall device performance; however, on the microscopic level, it can be difficult to explain how these parameters are affected by various processing conditions and blend morphologies without direct measurements that can correlate the local electronic properties of the film with local structural features.
Thus, in addition to trEFM, we have used photoconductive AFM (pcAFM) as a complementary tool for the microscopic characterization of heterogeneous OPV films. A relative of conductive AFM (cAFM), pcAFM records local photocurrents directly in contact mode, essentially by using a metalized AFM probe as the top contact to form a nanoscale solar cell. In
Figure 4. Microscopic heterogeneity in (A) topography and (B) photocurrent on P3HT/PCBM blends. (C) Correlation between spatially-averaged photocurrent measured via pcAFM and EQE measurements for P3HT/PCBM blends annealed for different lengths of time again indicate that pcAFM data are qualitatively consistent with expected device performance
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pcAFM, we typically use focused laser illumination to photoexcite the sample. The small collection area leads to a small photocurrent and, even for high-quality devices with external quantum efficiencies over 50%, we find it beneficial to use high-intensity illumination to improve signal to noise. For instance, a green laser (Crystal Laser GCL-005L, 5 mW, 532 nm, see Figure 1c) is focused to a diffraction-limited spot on the sample and aligned with the tip; after attenuation, the laser intensity, and therefore the expected sample damage, is often comparable to that in confocal microscopy experiments on biological samples. We also use blue and red lasers as required to match the absorption spectrum of the material being studied. Contact AFM tips with metal overall coating, usually Au (Budget Sensors, ContE-GB, k ~ 0.2 N/m), are used for measurement. A small setpoint value is used to minimize destruction of the polymer layer whilst also keeping the conductive coating free from surface contamination. Perhaps one of the most significant practical challenges to using pcAFM is obtaining a good electrical image without causing significant damage to the sample. Patience, and a willingness to sacrifice many AFM cantilevers in the name of science, are often necessary.
www.solar-pv-management.com Issue II 2010
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