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|Language of final thesis:||English|
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|Title of the thesis:||Combining Surface and Vertical Profile-Studies of Organic Materials in Solar Cells |
|Summary:||In recent years, the technology of organic solar cells has become very attractive because of their advantages such as low cost processing, relative low energy polymer chemistry, flexibility, transparency and lightweight. However, intensive research is being and still need to be done in order to understand the degradation processes of used materials under standard working conditions and therefore to achieve a better stability of the devices for a large-scale application domains. The introduction of a new range of p-type polymers with a low bandgap allowed increasing the absorption of the red part of the solar spectrum and consequently the device efficiency which nowadays, is recorded above the value of 13 %. The photoactive layer is composed by a p-type polymer (electron donor molecule) blended together with an n-type (electron acceptor molecule) typically fullerene derivatives. The materials used in the photoactive layer are not stable under standard conditions and tend to degrade as it has been shown in the literature by different research groups. The typical stress conditions that the cell is submitted to are high temperature, high/low humidity changes, ambient atmosphere, and intense light with strong UV component etc. This leads to chemical/physical degradation e.g. triplet formation from charge transfer, singlet oxygen production, oxidation of polymer, oxidation of electrodes, light assisted doping by O2, morphological changes, inter-layer reaction, delamination, diffusion of O2 and H2O etc. The photoactive layer is the place where most of the listed degradation occurs and also oxygen constitutes the main reason of the device failure. In particular, the oxygen-light induced degradation known as photooxidation on the morphology parameters as well as electrical performance of the devices such as short-circuit current, fill factor, open-circuit voltage, efficiency, parallel and serial resistances, is a major contribution and require a well understanding of the processes on a nanometer scale. For these purposes, time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and laser beam induced current (LBIC) techniques have been applied. ToF-SIMS allows tracking the degradation processes by investigating the chemical changes on ion fragments, their intensity evolution with the depth and the inter-layer mixing that occurs during the degradation. The photoactive layer surface morphology and its evolution with the degradation were established using SEM methods, which provided information about the nanostructures on a broad range of scales from nanometer to millimeter. The nanostructure of the active layer and its roughness profile evolution with the degradation was investigated by the means of AFM techniques. The electrical device performance (photocurrent) losses that occur with the degradation was investigated using LBIC method. These investigations allowed to establish that the degradation of inverted geometry OSC structures at ambient atmosphere under photooxidation conditions affect mostly the PEDOT:PSS and P3HT:PCBM layer materials. Moreover, it has been shown that oxygen induces drastic changes of the device properties such as oxidation of materials, morphological changes and diffusion of interfacial layer toward semiconductors layer resulting in a deterioration of related interfaces and therefore results in a considerable loss of the device power conversion efficiency. Also, the obtained experimental results show, that water induces substantial degradation in organic solar cells with a pronounced deterioration of the device properties. It has been found that the degradation occurs by diffusion of water molecules through the solar cell device which interacts with the low work function electrodes and related interface. The water molecules diffusion leads to the observed changes in a form of induced inhomogeneities, in which the photocurrent density of the aged samples changes to a strong decrease of the power conversion efficiency. We have found that for long-term ageing, the higher oxidation of the photoactive layer as it has been observed in the Si-PCPDTBT:PC70BM blend layer structures, could be an explanation of the total absence of photovoltaic response in the cells as it has been described in the literature. All the above mentioned results give us a better understanding of the degradation processes of materials used in the photoactive layers at the nanoscale range. They allow us also to better understand the changes that occur in the PV cell during its operation and eventually figure out the device weak points. These results determine a remarkable achievements and therefore a considerable progress for the study of organic solar cells stability.|
|Key words:||Organic solar cells, Degradation study, TOF-SIMS, AFM, SEM, LBIC|
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