The performance of organic solar cells is highly dependent on film morphology. However, directly correlating local film structures with device performance remains challenging. Photoconductive atomic force microscopy (pcAFM) can be used to map local photocurrents with 20 nm resolution in donor/acceptor blend solar cells of the conjugated polymer poly[2-methoxy-5-(3‘,7‘-dimethyloctyl-oxy)-1,4-phenylene vinylene] (MDMO-PPV) with the fullerene (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) spin-coated from various solvents. Photocurrent maps under short-circuit conditions (zero applied bias) are presented here as well as under various applied voltages. Significant variation is found in the short-circuit current between regions that appear identical in AFM topography. These variations occur from one domain to another as well as on larger length scales incorporating multiple domains. These results suggest that the performance of polymer−fullerene blends can still be improved through better control of morphology.
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David S. Ginger et al, University of Washington, Department of Chemistry, Box 351700, Seattle, Washington 98195-1700, Nano Lett., 2007, 7 (3), pp 738–744
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 pcAFM, we typically use focused laser illumination to photoexcite the sample. The small collection area leads to a small photocurrent, and even 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, 5mW, 532nm, 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 ~ 0.2N/m), are used for measurement. A small setpoint value is used to minimize destruction of the polymer layer whilst also to keep 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.
Figure 1. (A) Schematic of a typical bulk heterojunction organic photovoltaic device. Schematic diagrams of the (B) trEFM and (C) pcAFM experimental setups based on Asylum Research’s MFP-3D-BIO™ AFM System.