High-resolution morphological and electrical characterisation of organic bulk heterojunction solar cells by scanning probe microscopy

Abstract

State-of-the-art organic bulk heterojunction (BH) solar cells, also called excitonic solar cells, are based on intimate mixtures of donor and acceptor organic materials of which the nanoscale morphology strongly influences both the photovoltaic performances and the stability of the device. In particular, the form and the size of the three-dimensional (3D) interpenetrating network of donor/acceptor material is shown to be crucial for the electrical transport properties andthe resulting photovoltaic properties. Powerful high-resolution characterisation tools to locally map the morphology of these material systems are Scanning and Transmission Electron Microscopy (SEM/TEM), Atomic Force Microscopy (AFM) and Nuclear Magnetic Resonance (NMR). Yet, to correlate morphology with local electrical properties, significant progress has been made by the recent introduction of advanced Scanning Probe Microscopy (SPM) methods based on electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM). EFM related methods measure the electrostatic interaction between the probe and the surface of the organic thin films, hence to derive the variations in the sample surface potential C-AFM based methods perform two-dimensional (2D) current mapping of sample conductivity and local spectroscopy to analyse transversal charge transport mechanisms in the blends. In case the space charge limiting current (SCLC) regime is dominating the charge transport mechanisms, carrier mobility can also be determined. Finally, the sensitivity of C-AFM to photovoltaic properties is reported. In this paper a review dealing with the different SPM methods currently used and the respective achievements performed on organic blends for BH solar cells is proposed. Copyright © 2007 John Wiley & Sons, Ltd.