Utilizing Scanning Probe Microscopy for Organic Photovoltaic Research

            Organic molecules present a unique and potentially affordable solution to renewable photovoltaic energy.  The ability to tune the absorption of organic molecules to solar conditions, and the potential ease of device fabrication have raised much interest in the field of organic photovoltaics.  Currently, however, organic solar cells suffer from low efficiencies due in part to low charge extraction rates caused by recombination in the photovoltaic devices.

            Scanning probe microscopy is increasingly being utilized to study the properties of organic photovoltaic materials at both the nanoscale and the single molecule level.  The Atomic Force Microscope (AFM) has the ability to study both the morphological properties of organic film devices, as well as electronic properties such as conductance and surface potential.  The scanning tunneling microscope (STM), similarly, can study the electronic properties of single photovoltaic molecules.

            We have studied the conductance properties of photovoltaic substrates and organic thin films of light absorbing molecules using current sensing atomic force microscopy (CS-AFM).  We have also directly observed the conductance properties of single porphyrin-fullerene dyad molecules on Indium Tin Oxide electrodes using STM by observing tunnel junction conductance switching caused by single molecule binding events in the time domain.  We find that the scanning probe microscope is a useful tool in the electrical characterization of organic photovoltaics and has the potential to help influence organic photovoltaic design and improvement.