Characterization of Porous, TiO2 Nanoparticle Films Using On-Axis TKD in SEM -a New Nano-Analysis Tool for a Large-Scale Application

Abstract

Dye sensitized solar cells (DSSCs) attracted great attention as a low-cost alternative to fossil energy, but also to other photovoltaic energy conversion systems. The core of a DSSC mostly consists of a porous micrometer thick film such as titanium dioxide mostly in form of nanoparticles (NP) as electron carrier, easily screen-printed at industrial scale (Figure 1a and 1b). The performance of the cell is amongst others linked to the morphology and formation of the NPs within the layer [1]. Characterization of the NP network is a challenging task, since common techniques mainly work with isolated or dispersed particles rather than particle layers. The constituent NPs in our work are bipyramidal anatase crystals manufactured via a shape-controlled routine, of 50 nm mean length and 32 nm mean width measured by Transmission (Scanning) Electron Microscopy (T(S)EM) and, respectively, 67 nm by 44 nm as measured with atomic force microscopy (AFM) (dimensions are expressed as Feret max and Feret min, respectively [2]).The particulate layers were characterized using the new technique Transmission Kikuchi Diffraction (TKD) in a SEM [3]. In principle TKD works similar to the established Electron Backscatter Diffraction (EBSD) technique, which is a powerful method to identify grains/crystals via indexing Kikuchi pattern and determine their crystal orientation. However, on-axis TKD can reach a considerably better spatial resolution down to 2 nm and is, consequently, a technique well-suited for the investigation of nanocrystalline material including characterization of size and shape of NPs. This spatial improvement is gained by preparation of electron-transparent foils from the respective material leading to reduced background intensity, enhanced Kikuchi contrast and reduced beam broadening in comparison to conventional EBSD [4].Thus, TKD samples were prepared as thin as possible using conventional Focused Ion Beam technique (FIB) avoiding excessive amorphisation or Gallium ion implantation into the sample surface by thinning down with falling acceleration voltage beginning with 30 kV and finishing with 2 kV. To remove the very rest of amorphous skin and possible contamination from electron beam scanning, the FIB lamella was showered with low-kV Argon ions using the Fischione 1040 NanoMill® at Fischione instruments facility (Export, PA, USA). The on-axis TKD measurements were performed with the Bruker e-Flash FS retrofitted with the OPTIMUS® detector head (Bruker Nano GmbH, Berlin). Figure 1d shows a raw inverse pole figure map presenting the crystal orientation of a TiO2 NP layer obtained by on-axis TKD from which parameters like grain size and grain shape can be deduced. Some beam instabilities can be recognized in the acquired TKD orientation map caused by charging of the non-conductive, light and porous material. The drift areas were neglected (see black areas in Figure 1e) to avoid miscalculating the distributions of grain size in Figure 1f and aspect ratio in Figure 1g. For better statistics, incomplete grains at the fr...