Conventional Electron Microscopy and Electron Holography of Organic Solar Cells

Abstract

Organic solar cells are a promising route towards large‐area and low‐price photovoltaic systems. The devices are composed of at least two layers: the hole‐transport layer and the electron‐transport layer. The light absorption can occur in one or both layers. At the interface of the layers the excitons are separated into charge carriers, and every layer deals with one type of carrier. Higher efficiencies of the separation process can be obtained by using a mixed layer containing both materials to obtain a very high interface area. Although the structure of the mixed layers used plays a crucial role for the device performance, until now the morphologies have not been elucidated. In order to correlate physical and optical findings with structure and morphology for the materials in question, electron microscopy experiments were performed on the single components as well as on the layer systems. The conventional electron microscope is a poor phase microscope. As consequence, weak‐phase objects like organic molecules have to be stained or imaged under strong defocus to produce an observable contrast. Artifacts caused by chemical staining and the appearance of Fresnel diffraction using the defocus technique represent the main problems of conventional microscopy. These artifacts can be avoided using electron holography. Holograms of ultrathin sections of thin layers composed of organic dye molecules were recorded. Subsequently, the phase images were reconstructed. In this manner, we succeeded in obtaining high‐contrast electron micrographs without applying staining or defocus. In addition, holograms of crystalline C60 and zinc phthalocyanine were successfully recorded. Holography has been shown to be a useful tool to image beam‐sensitive and weak‐phase objects without artifacts.

Donor–acceptor nanocomposite structures for organic photovoltaic applications

“…Figure 4b shows significant increase in number and crystallite sizes of 15–30 nm in the DAN film prepared at 60 °C, comparable to earlier results 5. No crystalline ZnPc regions can be spot indicating that the amorphous phase of ZnPc is stable at these temperatures, which is in good agreement with reference 9 where stability up to 50 °C is reported.…”

Fluorinated Zinc Phthalocyanine as Donor for Efficient Vacuum‐Deposited Organic Solar Cells

“…For 70 nm F 4 ZnPc:C 60 blends (1:1 ratio), there are no peaks visible in the XRD diffractograms (see Figure 10b) that could be associated with organic materials. This indicates that these films are at least X‐ray amorphous, which was also observed by Maennig et al46, 47 using electron diffraction for ZnPc:C 60 blends that were deposited at room temperature. X‐ray studies of similar F 4 ZnPc:C 60 samples where the substrate was kept at 110 °C during deposition revealed that, even then, the blend layers are X‐ray amorphous and no crystal structure could be obtained (data not shown).…”

Efficient Solar Cells Using All‐Organic Nanocrystalline Networks

“…[17] Both electron and X-ray diffraction confirm the existence of crystalline domains of CuPc and C 60 . Crystalline domain sizes range from 5 nm to 10 nm, similar to those found in homogeneous films of C 60 but smaller than ZnPc crystals obtained by thermal evaporation [18,19] . We obtained similar TEM images of [CuPc(3.1 nm)/C 60 (3.1 nm)] 17 , where the crystallites are similar in shape but have slightly smaller sizes, as expected.…”

“…2b), where the crystalline phase separation is limited. [4,5,18,19,22] As shown in Figure 2c, the surface morphology of the same [CuPc(6.1 nm)/C 60 (6.1 nm)] 10 film observed by AFM shows the crystalline texture with an rms roughness of 12.7 nm, reflecting the roughing effect (c.f. Fig.…”