Development of efficient, stable organic-inorganic hybrid solar cells
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Developing a fundamental understanding of photocurrent generation processes at organic-inorganic interfaces is critical for improving hybrid solar cell efficiency and stability. This dissertation explores processes at these interfaces by combining data from photovoltaic device performance tests with characterization experiments conducted directly on the device. The dissertation initially focuses on exploring how morphologically and chemically modifying the organic-inorganic interface, between poly(3-hexylthiophene) (P3HT) as the electron donating light absorbing polymer and titanium dioxide (TiO₂) as the electron acceptor, can result in stable and efficient hybrid solar cells. Given the heterogeneity which exists within bulk heterojunction devices, stable interfacial prototypes with well-defined interfaces between bilayers of TiO₂ and P3HT were developed, which demonstrate tunable efficiencies ranging from 0.01 to 1.6 %. Stability of these devices was improved by using Cu-based hole collecting electrodes. Efficiency values were tailored by changing TiO₂ morphology and by introducing sulfide layers like antimony trisulfide (Sb₂S₃) at the P3HT-TiO₂ interface. The simple bilayer device design developed in this dissertation provides an opportunity to study the precise role played by nanostructured TiO₂ surfaces and interfacial modifiers using a host of characterization techniques directly on a working device. Examples introduced in this dissertation include X-ray photoelectron spectroscopy (XPS) depth profiling analysis of metal-P3HT and P3HT-TiO₂ interfaces and Raman analysis of bonding between interface modifiers like Sb₂S₃ and P3HT. The incompatibility of TiO₂ with P3HT was significantly reduced by using P3HT derivatives with -COOH moieties at the extremity of a polymer chain. The role of functional groups like -COOH in interfacial charge separation phenomena was studied by comparing the photovoltaic behavior of these devices with those based on pristine P3HT. Finally, for hybrid solar cells discussed in this dissertation to become commercially viable, high temperature processing steps of the inorganic TiO₂ layer must be avoided. Accordingly, this dissertation demonstrates the novel use of electromagnetic radiation in the form of microwaves to catalyze growth of anatase TiO₂ thin films at temperatures as low as 150 °C, which is significantly lower than that used in conventional techniques. This low temperature process can be adapted to a variety of substrates and can produce patterned films. Accordingly, the ability to fabricate TiO₂ thin films by the microwave process at low temperatures is anticipated to have a significant impact in processing devices based on plastics.