3D Nanoscale Characterization of Thin-Film Organic Photovoltaic Device Structures via Spectroscopic Contrast in the TEM 1

The three-dimensional characterization of third generation photovoltaic device structures at the nanometer scale is essential to the development of efficient, reliable, and inexpensive solar cell technologies. Electron tomography is a powerful method for three-dimensional characterization; however, the application of this method to the organic materials systems that comprise typical high-efficiency devices is complicated by the difficulty in generating contrast from the compositionally similar materials. Herein we report the application of low-loss energy-filtered transmission electron microscopy as a method of generating spectroscopic contrast from a common organic bulk-heterojunction thin film consisting of a polymer donor and a fullerene-derivative acceptor. Spectral imaging methods combined with principal component analysis are used to characterize the contrast generation mechanism and to determine the optimum data acquisition parameters for this particular combination of organic phases. A proof of method for using the low-loss spectral signal as a basis for electron tomography is presented, and the advantages and drawbacks of the technique as applied to multiphase organic systems relative to the more commonly employed bright-field imaging approach are outlined.