Characterizing blends of Zn/Pc‐C60 for Organic Photovoltaic Cells using energy‐filtered Backscattered Electron ( BSE ) imaging in combination with Low Voltage Scanning Electron Microscopy ( LVSEM )

Abstract

Blends of conjugated polymer zinc‐phtalocyanine (ZnPc) and electron acceptor fullerene (C 60 ) are used as an active layer of organic photovoltaic (OPV) cells in bulk heterojunction architectures. In this study, the morphology of co‐evaporated small molecule blends of ZnPc (ZnC 32 H 18 N 8 ) and C 60 is investigated with Low Voltage Scanning Electron Microscopy (LVSEM). Compared to SEM studies carried out at standard conditions, these LVSEM studies present the advantages of a lower modification of the polymer structures during observation and of a better material contrast between the components which consist mainly on carbon. Energy‐filtered SEM imaging in combination with low primary beam energies (E p ) allows to detect the low loss backscattered electrons (LLBSE). These LLBSEs undergo a small number of inelastic scattering events, and therefore, they lose only a small amount of energy. The material contrast between the blend components is detectable and enhanced for low E p . These fine contrast differences obtained from the LLBSE are originated in the very shallow regions of the blend surface, and not from the volume where multiple inelastic scattered electrons, like the secondary electrons (SEs), are produced [1]. The blend morphology is imaged using a novel energy selective backscattered (EsB) electron detector whose grid voltage is set to high values in order to cut off the lower energy SEs, which are responsible for the topography information in the SEM images. This energy‐filtering technique allows to detect uniquely the LLBSEs, which mainly carry material contrast information. High atomic number (Z) elements have a higher backscatter electron coefficient for E p ≥ 1 kV [2]. Therefore, higher Z elements appear brighter due to a higher BSE emission. The material contrast is optimized and quantified as a function of the E p , the EsB grid voltage and the working distance (WD) at low landing energies. Figure 1 illustrates how topography information dominates the images for EsB grid voltages below 300 V, due to influence of SEs. The material contrast is obtained for EsB grid voltages ≥ 500 V. In Figure 2, the in‐lens SE image a) also shows the blend topography: long rods interlaced with each other and in between, cube‐shaped particles. For the identical sample region, the image detected with the EsB detector b) shows the material contrast between rods and particles. The bright rods are identified as the ZnPc phase, while the darker cube‐shaped component is assumed to correspond to C 60 . Depending on the SEM working conditions, the systematic study of the material contrast reveals that small differences in composition can be detected using the LLBSEs. Summarizing, it is possible to image the material contrast difference even for challenging materials like the ZnPc‐C 60 active layer in OPV cells by optimizing E p , EsB grid voltage and WD values.