A novel electrical model for organic photovoltaic cells

Research Doctorate - Doctor of Philosophy (PhD)Organic photovoltaic cells (OPVs) are an essential part of the future global energy production. However, OPVs are very complex electric systems and one approach to aid the understanding of their electrical properties is to model the devices using electrical components, observe how the model-fit parameters change with any treatment that is applied to the OPV, and to give a realistic and scientific physical explanation for this OPV behaviour. The most common equivalent electrical circuit used to model OPVs are the one diode model (ODM) or two diode model (TDM). While the ODM or TDM are useful in modelling the I-V characteristics of “ideal” OPVs, they are unable to account for “s-shaped” I-V curves. A new equivalent circuit model, i.e. back to back diode model (B2BDM) is proposed to solve this problem. The successful modelling of “s-shaped” I-V curves is a major advance in the formation of a meaningful equivalent circuit model for OPV devices. The application of the model has demonstrated an excellent correlation between the B2BDM electrical model and experimental results. Most importantly, the model allows the extraction of the series and shunt resistances that explain the mechanism of OPV cell function during the production of an “s-shaped” I-V curve and the role of cathode interfacial material in reducing this effect and thereby enhancing the performance of the OPV cells. The equivalent circuit models have also shown a good correlation between the electrical model-fit parameters and experimental data and this correlation can support in explaining the experimental results for thermally-annealed devices with a thin aluminium cathode layer which have a high series resistance. However, reordering of the active layer due to the aluminium oxide which occurs in this case compensates for the higher series resistance and depositing a thicker aluminium layer on top can then provide a better electrode to rectify this potential issue, as well as reduce the series resistance of OPV cells. Finally, degradation results are shown and the equivalent electrical circuit model effectively extracts the model-fit electrical PV parameters, especially the series (R_s) and shunt (R_sh) resistances. An excellent correlation is achieved between the resistances of the devices and performance of the device over the degradation process in this study where the devices are deliberately exposed to ambient conditions under constant illumination. The degradation of the device is found to primarily be caused by the degradation of the aluminium electrode by water and oxygen which correlates to the series resistance (R_s). However, it is possible degradation of the bulk active layer can also happen due to the constant illumination on the device which will also cause a reduction of photocurrent