Integration of polymer and dye-sensitized solar cells for high voltage source

Abstract

Organic solar cells (OSCs) such as polymer solar cells (PSCs) and dye-sensitized solar cells (DSCs) show great potential as a cost effective route to wide use of solar energy for electrical power generation. In the past decade, much effort has been focused on enhancing the power conversion efficiency (PCE) of OSCs. However, for practical electric power generation, the integration of OSCs is required as well as the efficiency enhancement of single OSC. In order to integrate OSCs, few unconventional patterning approaches have been proposed based on softlithographic or photolithographic methods. In spite of their many desirable properties including high pattern resolution, those methods have a limited applicability in the development of high-resolution OSCs with advanced performance due to the pattern reliability and the restriction of patternable materials, and thus the development of a novel patterning method for organic/inorganic layers of OSCs is inevitably required In this thesis, high-fidelity patterning techniques based on the use of a chemically compatible sacrificial layer, which is complementary transfer assisted lift-off (CTAL) technique, is studied in the viewpoint of the scientific research and practical applications to the integration of OSCs. The use of chemically compatible sacrificial layer of fluorous-polymer in combination with the softlithographic transfer-patterning provides the high-fidelity in patterning organic/inorganic layer without any detrimental effects on their electrical properties as well as the capability of high-resolution patterning down to a few micrometers. First, I demonstrate integrated solid-state DSCs using a CTAL technique that can be used for patterning the nanocomposite TiO2 layer in a small area. By patterning electron transport layer of nanocomposite TiO2, the individual cell can be integrated about a hundred micrometer scale. In addition, to resolve leakage and evaporation problem of liquid electrolyte, solid-state hole transport material of spiro-OMeTAD (2,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)9,90-spirobifluorene) is used. This concept show promise applicability of integrated DSCs to be a compact electric system. Next, the PCE enhancement of PSCs in a bilayer configuration is demonstrated. Specifically, I describe the solvent-dependent morphology of a conjugated polymer, poly(3-hexylthiophene) (P3HT) interfaced with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and the evolution of the resulting inter-connected fibrillar structure (ICFS) in a diffusive bilayer heterojunction (DBHJ). It is found that the fibrillar structure of the P3HT is preferentially developed in a poor solvent and less soluble than an amorphous structure and the ICFS improves the PCE significantly. Among four different solvents of chlorobenzene, dichlorobenzene, chloroform, and p-xylene studied, p-xylene results in PCE ≈ 2.7% whereas chlorobenzene yields only 0.5%. This indicates clearly that the formation of the ICFS plays a critical role to induce high exciton generation/dissociation, to produce spatially uninterrupted pathways for the charge transport, and to enhance PCE of a single PSC device in the DBHJ configuration. Finally, I develop a powerful platform of constructing polymer solar cells (PSCs) in a serial architecture within the framework of CTAL technique. The CTAL approach allows the feature resolution down to a few micrometers and provides high integration capability of different classes of functional materials including organic semiconductors, charge transporting materials, and metals. The sacrificial layer of hydrophobic fluorinate-polymer was patterned complementary to active regions, and lift-off of the sacrificial layer using a chemically inert solvent to leave only active patterns on a substrate. The reduced dark current by patterned hole transport layer of molybdenum oxide (MoOx) results in high open circuit voltage of integrated PSC array. A high voltage power source for electronic device was realized by interconnecting a PSC array to the liquid crystal display device. A monolithic integration of electronic device with self-power source will be the next step towards integrated organic devices. In conclusion, through this thesis, a novel high-fidelity patterning technique, which is inevitably required for advanced solar cell systems, was shown with integrated PSCs and DSCs. Moreover, solvent-dependent fibrillar morphologies of the P3HT layer and the resultant PCBM-on-P3HT film play an important role in the PV characteristics were successfully demonstrated with sufficient experimental results. All this concept of highly integrated OSCs by CTAL patterning technique provides a versatile platform to devise a variety of compact electric and optoelectronic systems.