Characterization of Thin Polymer Blend Films using ESEM - No Charging, No Staining
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DD11.8.1/AA10.8.1
Characterization of Thin Polymer Blend Films using ESEM – No Charging, No Staining.
Ian C. Bache, Catherine M. Ramsdale, D. Steve Thomas, Ana-Claudia Arias, J. Devin MacKenzie, Richard H. Friend, Neil C. Greenham and Athene M. Donald Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK ABSTRACT
Characterising the morphology of thin films for use in device applications requires the ability to study both the structure within the plane of the film, and also through its thickness. Environmental scanning electron microscopy has proved to be a fruitful technique for the study of such films both because contrast can be seen within the film without the need for staining (as is conventionally done for electron microscopy), and because cross-sectional images can be obtained without charging artefacts. The application of ESEM to a particular blend of relevance to photovoltaics is described.
INTRODUCTION
It has recently become apparent that quantum efficiencies of photovoltaic devices can be significantly enhanced in conjugated polymer blends in which phase separation occurs on a lengthscale comparable to the exciton diffusion distance[1]. In such blends, exciton dissociation is promoted at the many polymer-polymer interfaces. This leads to enhanced generation of charge carriers which can then migrate towards the appropriate electrodes, as long as there is a suitable continuous path for this to occur. One can therefore imagine an idealised structure for a photovoltaic device, consisting of an interdigitated bicontinuous polymer network, in which there is a concentration of the electron acceptor in the vicinity of the cathode and the hole acceptor polymer at the anode. It is therefore necessary, when polymer blends are being prepared as thin films for photovoltaic device application, to examine the morphology to test how far such an idealised structure is being realised. Most commonly atomic force microscopy (AFM) is used to explore the morphology, and for many purposes this is excellent, especially where there is significant topography present[2]. However, it does have limitations, particularly if the polymers are very similar and there is little topographical detail. Also AFM provides information about the surface and, except in certain circumstances, information about the phase- and compositiondistribution through the cross section of the film can only be inferred from this technique with additional data from other techniques. However, it is precisely this structure which is directly relevant to device performance in photovoltaics and LED’s. This paper describes the application of environmental scanning electron microscopy (ESEM) to the study of polymer blends relevant to photovoltaic devices. It will be shown that the mode of operation of the ESEM has particular advantages for the study of phase-separated polymers, enabling composition contrast and crosssectional information in conjugated polymer films to be obtained.
DD11.8.2/AA10.8.2
BACKGROUND TO ESEM
The ESEM is a commercial instrument,
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