Testing Organic Solar Cells on the Nanoscale: Using Photoconducting Atomic Force Microscopy to Probe Structure-Property-Performance Relationships in Organic Photovoltaics
In order to address our current global energy and environmental challenges, low-cost, renewable forms of energy must be developed. The study of organic photovoltaics (OPVs) is a promising and intriguing area of research that may yield low-cost solar energy that can be produced by solution processing onto lightweight and flexible substrates. In OPVs, the photoactive materials are conjugated organic molecules or polymers that can by synthesized from inexpensive starting materials. OPVs comprise thin films made up of a network of two organic materials with dissimilar electronic properties such that one behaves as an electron donor and the other as an electron acceptor called the active layer, which is sandwiched between two electrodes. The active layer converts energy from incident sunlight into electrical energy, via processes that occur on the nanometer length scale. In order to understand these processes and thus be able to design more efficient OPV devices, we use photoconducting atomic force microscopy (pc-AFM) to characterize devices on the nanoscale. AFM exploits weak forces between a thin film sample and an atomically sharp probe in order to map the topography of a surface. By using a conductive probe and making electrical contact to our sample, we create a nanoscale device in which one electrode is the planar bottom contact and the other is the AFM probe. Pc-AFM can be used as a nanoscale analog to bulk solar cell testing, measuring the active layer morphology and photocurrent simultaneously, resolving nanoscale morphological features that either benefit or undermine the device performance.