SILICON PHOTOSENSITIZATION AND LIGHT HARVESTING USING LANGMUIR BLODGETT MONOLAYERS

Banks, Henry and Danos, Lefteris (2023) SILICON PHOTOSENSITIZATION AND LIGHT HARVESTING USING LANGMUIR BLODGETT MONOLAYERS. PhD thesis, Lancaster University.

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Abstract

This thesis looks at potential modifications to silicon wafers to reduce the amount of material required to absorb light energy and increase electron hole pair generation.1,2 To achieve this, the separation of the light absorption and electron hole pair generation is suggested so that the light absorbing component can be optimised independently. This is known as silicon photosensitization. To study this phenomenon light absorbing Langmuir Blodgett films were deposited onto silicon wafers and characterised using time resolved fluorescence techniques to estimate the light harvesting efficiency. Organic absorbing structures which can perform light harvesting were investigated using mixed monolayers of 3,3'-Dioctadecyloxacarbocyanine Perchlorate (DiO) and 1,1'-Dioctadecyl-3,3,3',3'-Tetramethylindocarbocyanine Perchlorate (DiI) deposited on quartz glass slides in ratios of high donor (DiO) to low acceptor (DiI). Energy transfer was shown from DiO to DiI. An efficiency of 95% was calculated for a donor acceptor ratio of 1-1 and energy transfer was shown to occur up to the 100-1 ratio. Triplet state energy transfer directly to silicon from a Langmuir Blodgett monolayer was then investigated. Langmuir Blodgett monolayers of a Rhenium (ReC18) and Ruthenium (RuC18) complex were both found to transfer energy to unmodified silicon wafer. Using inert stearic acid spacer between the emitting layers and the surface of silicon to alter the distance, the change in phosphorescence emission as a function of distance from the surface on silicon was observed. Following on from this the photoluminescence of silicon wafers was studied. Tetraphenylporphyrin (TPP) was mixed in a 1-1 ratio with stearic acid and deposited on silicon passivated with aluminium oxide, the luminescence of silicon and TPP film was observed. Enhancement of the silicon luminescence was shown but was not repeatable. The TPP emission also showed reduction in intensity but enhancement of the lifetime. This thesis provides examples of silicon photosensitization which can be applied directly on the top of silicon wafers and shows the potential for an ultrathin device using sensitization.