Design and fabrication of excitonic solar cells

• Main Author: Aung Ko Ko Kyaw
• Other Authors: Sun Xiaowei
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Nanotechnology; DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: https://hdl.handle.net/10356/48662
• Institution: Nanyang Technological University

The excitonic solar cells (XSCs), including organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs), have attracted a great interest due to their huge potential of low cost technology compared to conventional silicon solar cells. Although the technologies of XSCs have advanced significantly, XSCs still need to be improved in various aspects to become tangible in energy market. The important criteria of a solar cell are efficiency, cost and life time. Hence, the research in this dissertation focuses on the design of XSCs with better choice of materials and device architecture for either enhancement in stability and efficiency or reduction of cost. In spite of over 7% power conversion efficiency, the OSC based on bulk-heterojunction concept has limitation in device stability due to diffusion of oxygen into the organic layer through pinholes and grain boundaries in Al cathode and the degradation of transparent conductive oxide (TCO) electrode, which is etched by poly (3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) buffer layer. To overcome this problem, an inverted structure was implemented. The reverse polarity of charge collection in an inverted structure allows the usage of air-stable high-work-function metal as top electrode and gets rid of TCO/PEDOT:PSS interface. In our design, TCO is modified with sol-gel derived zinc oxide (ZnO) to exclusively collect electrons from active layer and block holes. A thermal-evaporated molybdenum oxide (MoO3), which is inserted between active layer and top electrode, increases the fill factor of the device due to exciton/electron blocking property. It was observed that the efficiency of an inverted structure OSC can be further improved by manipulating the resistivity, energy level and optical property of ZnO layer with appropriate amount of indium doping. We also verified that the stability of device in air is significantly improved by inverted structure. DSSC, another type of XSC, is also a promising alternative to silicon photovoltaic technology. However, it is estimated that conducting glass is the most expensive part of DSSC and it incurs 60% of total cost. Therefore, we designed top-illuminated structure which can be fabricated on inexpensive opaque substrates such as metals or plastic foils with metal coating. Although the efficiency of the top-illuminated cell is about 20% lower than the traditional bottom-illuminated cell, it reduces the cost of DSSC tremendously by eliminating the usage of expensive TCO. Ti is more suitable to be used as electrode in top-illuminated DSSC than other metals because of minimum catalytic activity on redox reaction and high resistance to corrosion. Another approach to eliminate TCO is replacing with transparent carbon nanotube (CNT) electrode. However, the catalytic activity to redox reaction limits its application as working electrode in DSSC. Therefore, the implementation of DSSC with CNT electrode was realized by modifying CNT with titanium-sub-oxide (TiOx) which inhibits the charge-transfer kinetic at CNT/redox solution interface and facilitates the unidirectional flow of electrons in the cell. To our best knowledge, this is the first demonstration of CNT as working electrode for liquid-type DSSC. Based on this finding, we also realized DSSC with all carbon electrodes.