| AVS 58th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Wednesday Sessions |
| Session EN+NS-WeM |
| Session: | Organic Photovoltaics |
| Presenter: | Christopher Bailey, Air Force Research Laboratory |
| Authors: | B.J. Leever, Air Force Research Laboratory C.A. Bailey, Air Force Research Laboratory T.J. Marks, Northwestern University M.C. Hersam, Northwestern University M.F. Durstock, Air Force Research Laboratory |
| Correspondent: | Click to Email |
Numerous reports have established that improving the performance of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices requires not only the development of materials systems with improved spectral response and higher charge carrier mobility but also the ability to understand and tailor the morphology of these systems. Approaches for morphological characterization have included microscopic techniques such as scanning electron microscopy, transmission electron microscopy, and various atomic force microscopy techniques. Other methods have provided indirect information about active layer morphology by enabling the determination of charge carrier lifetimes. These approaches have included transient absorption spectroscopy, transient photovoltage, and time-of-flight techniques among others. While these methods have proven quite informative, the relationships between nanoscale morphology, device performance, and the underlying electrical characteristics of functioning devices are not yet fully understood.
Recently, impedance analysis has begun to be applied to BHJ OPV devices. These reports demonstrate that the impedance analysis framework established for dye-sensitized solar cells can, to some extent, be extended to bulk heterojunction devices in order to calculate average charge carrier lifetime, electron densities-of-states, and charge carrier concentrations. However, a detailed analysis of the impedance response of active devices, and its dependence on device processing history, morphology, and operating conditions is still needed. In this work we acquire and analyze the impedance behavior of operating P3HT:PCBM bulk heterojunction devices and its dependence on illumination and bias conditions, active layer composition, and annealing history. We also report a simplified equivalent circuit model that successfully describes bulk heterojunction devices over a range of illumination conditions and applied biases. We use this model to extract relevant device performance characteristics such as average electron lifetime and find, in agreement with other reports, that bimolecular recombination losses play a significant role in these devices. To this end, we demonstrate a correlation between device efficiency and lifetime, and describe how values extracted from the present equivalent circuit model can be used to optimize device performance with new materials systems.