Conventional electron microscopy and electron holography of organic solar cells

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-tran sport 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 C-60 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.