A commonly employed design for producing high-efficiency photovoltaic cells using organic semi-conductors is the bulk-heterojunction (BHJ) architecture. Typically, this consists of an interpenetrating, three-dimensional (3-D) network of donor and acceptor materials produced via spin- or blade-coating from a co-solution of the two precursors. A number of morphological features are expected to produce a high-performance device, including a degree of phase separation that is on the order of the mean-free-path for excitonic recombination and a network configuration which allows for an uninterrupted vertical pathway to facilitate charge transport to the relevant electrode. These and other features are strongly influenced by the processing parameters used in fabrication such as the film drying rate after deposition, the extent of polymer crystallization, and any subsequent thermal or chemical treatments. In order to more fully understand the complex interplay between fabrication conditions, morphology, and device performance, robust characterization techniques are required that, ideally, can reveal the 3-D distribution of the two phases within the film at nanometer length scales.<br/><br/>While the transmission electron microscope (TEM) can readily achieve the required spatial resolution, traditional bright-field imaging methods based on differences in crystal structure or orientation are often inadequate. This is because the underlying materials tend to be poorly crystallized and, even when fully ordered, can quickly be rendered amorphous by the high energy electron beam. However, contrast between the donor and acceptor materials can often be generated by utilizing electron energy-loss spectroscopy in which images are formed using only electrons that have lost a characteristic amount of energy due either to inner-shell ionization of atoms or to the excitation of valence electron oscillations in the material. In this way, the compositional differences and spatial extent of the two phases can be quantitatively assessed. Furthermore, this type of imaging can also be used as the basis for 3-D characterization when coupled with electron tomography techniques and advanced specimen preparation routes. In this talk, the use of spectroscopic imaging in the TEM for morphological characterization of several BHJ systems will be detailed.

Nanoscale morphological characterization of the active layer in bulk-heterojunction organic photovoltaic systems using electron microscopy methods

256th National Meeting (2018)

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