Revealing the Morphology of Organic Bulk Heterojunction Solar Cells Using EFTEM and Low‐Energy STEM

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The morphology of organic bulk heterojunction (BHJ) solar cells decisively influences the device performance and efficiency and therefore is an important factor that needs to be investigated to gain a better understanding and improvement of the devices. Especially the nanoscale morphology of the active layer plays an important role as it determines the charge separation at the interfaces and the percolation pathways to the electrodes. This nanoscale morphology depends not only on the involved materials but also on their molecular weight and their treatment like thermal annealing and solvent vapor annealing. Transmission electron microscopy (TEM) is an approved technique to study the morphology of organic solar cells. Due to the similarity of the organic materials involved in the BHJ active layer regarding the chemical composition and the formation of homogeneously thin films, the contrast in TEM images is often uniform and no significant structures can be seen. Thus conventional imaging techniques are often not sufficient to identify and distinguish the polymers and the fullerene derivatives. Here we will demonstrate that energy‐filtered TEM (EFTEM) is a powerful technique to visualize the material distribution in organic BHJ active layers. We present three concepts using different information gained in EFTEM investigations: i) the elemental information, ii) the plasmonic information, and iii) pre‐carbon imaging. We demonstrate that the results of these three concepts are in good agreement and that the morphology can be reliably and consistently determined using EFTEM. To corroborate the reliability of these three concepts we present different material systems. Figure 1 shows the results of the EFTEM investigation for a P3HT:PCBM BHJ film. Due to the different plasmon energies of the materials (P P3HT = 21.9 eV, P PCBM = 24.5 eV) the respective plasmonic energy regions represent P3HT and PCBM in the blend. The elemental maps of sulfur and carbon are used to represent P3HT and PCBM, respectively, due to the different elemental compositions (S P3HT = 4.0 at%, S PCBM = 0.0 at%, C P3HT = 40.0 at%, C PCBM = 81.8 at%). Additionally, the pre‐carbon image represents P3HT as the carbon signal is suppressed and the sulfur signal enhanced. These EFTEM investigations clearly elucidate the morphology of this blend exhibiting P3HT fibers with diameters of 10 nm. Comparing the results of the three concepts clearly shows the good agreement of the determined morphology. Furthermore, we demonstrate the capabilities of low‐energy scanning transmission electron microscopy (STEM). STEM at low electron energies exhibits enhanced material contrast and can therefore be used to visualize the material distribution of polymers and fullerene derivatives in a BHJ film. Figure 2 shows a STEM BF image of the same P3HT:PCBM BHJ film at a high tension of 15 kV. The P3HT fibers are clearly visible and consistent with the EFTEM investigation. The origin of material contrast will be discussed. Low‐energy STEM is a highly promising alternative for determination of the morphology of organic BHJ solar cells as it features a high throughput SEM based technique. Using EFTEM and low‐energy STEM the morphology of various organic BHJ solar cells can be elucidated, leading to a better understanding and improvement of the device performance.