Investigating Charge Transport Mechanisms and Spatially Localized Photocurrent Variation in Organic Photovoltaic Devices.

摘要

Although the performance of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices is known to be closely related to the interpenetrating phase-separated network of the photoactive layer nanostructure, much initial work focused on improving relatively simplistic metrics such as efficiency and spectral response. Electron microscopy and tomography have yielded important insights into the nature of device morphology, but these methods are often expensive, time-consuming, and most significantly, do not allow for in situ analysis of operating devices. This dissertation focuses on better understanding the role of interfaces and active layer morphology through approaches that enable the analysis of operating devices.;The basic device architecture analyzed here is a glass substrate coated with an indium tin oxide (ITO) anode, a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) interfacial layer, a poly(3-hexylthiophene):[6,6]-phenyl-C 61-butyric acid methyl ester (P3HT:PCBM) photoactive layer, and an aluminum cathode. The primary analysis techniques include atomic force photovoltaic microscopy (AFPM), developed as part of this work, and impedance spectroscopy with equivalent circuit modeling. Conductive atomic force microscopy and photoelectron spectroscopy techniques are also extensively employed, particularly in the investigation of anode surface properties.;AFPM analysis demonstrates spatially localized photocurrent variations in operating micron-scale devices, which are too large to be related to P3HT and PCBM segregation alone. By varying anode surface treatments we show a correlation between the conductive uniformity of the anode surface and the variability observed in the photocurrent, suggesting that electrical inhomogeneities in the anode surface are passed through the active layer film. In situ impedance analysis of P3HT:PCBM devices provides an indirect measure of active layer morphology. We acquire and analyze the impedance response of these devices, which we relate to illumination and processing conditions, average charge carrier lifetime, and efficiency among other factors. We present a simple equivalent circuit model and show how the time constant extracted from this model can be related to device performance. Taken together, these results suggest that tailoring the electrical properties of the anode could be important for improving device efficiency and that impedance spectroscopy could be a relatively simple tool for optimizing new BHJ materials systems.