Characterization Of PCDTBT:PCBM Organic Solar Cells Using Atomic Force Microscopy And Related Imaging Artifacts
Open AccessFor over a decade, Organic Photovoltaics (OPVs) have attracted the scientific community around the globe as an alternative renewable energy technology owing to its low cost, solution processability, ease of manufacturing, environment friendliness, light weight and mechanical flexibility. The nanoscale film morphology of the donor/acceptor blends in most OPVs plays a critical role in photovoltaic performance and in its optimization to scale up from laboratory to commercial production. Most of the currently employed scientific tools for OPV characterization do not provide direct correlation between local morphology and local electrical properties. As a result, important conclusions are often drawn from macroscale measurements, which may be incorrect, making Atomic Force Microscope (AFM) an important tool for investigating local morphology-property relationship and optimization of OPVs. In this thesis work, both macroscale and AFM based microscale electrical transport experiments were conducted on an organic bulk heterojunction solar cell consisting of a semiconducting conjugated polymer (PCDTBT) and a fullerene derivative (PCBM). The microscopic and macroscopic experimental outcomes were analyzed and compared with the electrical transport calculation of the model system based on computational quantum mechanical methods. Simple models of few-molecule conductance, based on the quantum mechanics results, were applied to predict the bulk and local behaviors of the OPVs. Particular attention was paid to effects such as spatial experimental length scale, spatial molecular arrangement, localized electronic states, inelastic charge transport and broadening of conductance energy bands due to factors such as electron-phonon coupling and inelastic transport, whose influence was modeled using statistical methods applied to the quantum mechanics results.Finally, the invasive effects of contact mode AFM techniques, frequently used in OPV characterization, were investigated. It was shown that contact mode AFM scan can induce invasive morphological changes on this class of material, which can go unnoticed in the conventional AFM imaging modalities but can have non-negligible effects in material characterization. The detection, effects and nature of these induced surface alterations were investigated using a series of AFM techniques such as- Multifrequency AFM, Conductive AFM, Kelvin Probe Force Microscopy, Lateral Force Microscopy, Raman spectroscopy and Scanning Electron Microscopy.
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