Layer-by-layer Self-assembly Of (Polyaniline/Carbon)_n Complex Counter Electrodes And Study On Their Photovoltaic Performances In Dye-sensitized Solar Cells

Dye-sensitized solar cell (DSSC) is one of the promising solutions to solve global energy and environmental problems because of its merits on clean and easy fabrication. The counter electrodes could effect the power conversion efficiency of DSSCs directly because of its task of collecting electrons from external circuit to the electrolyte/coumter electrode interface and catalysting the reduction reaction of I3-+ 2eâ†'3I-. However, high cost of commonly used Pt counter electrode has been one of the main restrictions for the commercialization of DSSCs. The combination of conducting polymers and carbonaceous materials can gather electron-conduction of carbonaceous materials and electrocatalytic activity of conducting polymers on their composites. In order to enhance the charge-transfer ability and to reduce the interface resistance, conducting polyaniline-carbonaceous materials complexes are synthesized by a reflux method instead of the traditional methods of blending and electrochemical deposition, which is restricted the enhancement of the power conversion efficiency. The covalent bonds between conducting polyaniline (-NH-) and carbonaceous materials (-C=) are validated to significantly accelerate the charge transfer. Positively charged complexes are assembled into multilayers with negatively charged materials by layer-by-layer (LbL) self-assembly technique to increase the reaction area of I3-and enhance the power conversion efficiency of DSSCs.With an aim of revealing the growth mechanism, electrical and photoelectric behaviors of conducting multilayer films, in the second chapter, the [polyaniline/poly(styrene sulfonate)]n [(PANi/PSS)n] multilayers are fabricated via LbL self-assembly technique. The experiments show linearly growth with the bilayer number. This indicates that the same amounts of PANi and PSS are deposited in each dipping cycle, and the uniform deposition process obeys higher order kinetics. Redox reaction in the sulfuric acid aqueous solution is controlled by the diffusion of charges in the (PANi/PSS)n multilayers. The electron tunneling mode is a potential mechanism for disclosing linearly the increased charge quantity from the bottom to the top of the PANi layer across the insulating PSS layer. As a result, the film resistance of (PANi/PSS)n multilayers is an approximate constant, by constrast, the electrical conductivity has an increasing linear correlation with the bilayer number. There is also a linear increase in the photocurrent density with the number of bilayers.In order to enhance the electrocatalytic activity and the charge-transfer ability of counter electrodes, positively charged PANi-graphene (or single-walled carbon nanotube) complexes are synthesized by a reflux method and assembled into multilayers with negatively charged graphene oxide (GO) in the third chapter. The covalent bonds between PANi (-NH-) and carbonaceous materials (-C=) can significantly accelerate the charge-tansfer and improve the electric catalytic activity of the multilayer counter electrodes. The counter electrodes from [PANi-graphene (or single-walled carbon nanotube)/GO]n [PANi-G (or SWCNT)/GO]n multilayers show superior electrocatalytic activity and electrical conductivity because of their porous structure. The experimental results show that the electrocatalytic activity of the multilayer counter electrodes increases with bilayer number and G (or SWCNT) dosages. G (or SWCNT) at high dosages can improve the power conversion efficiency of DSSCs as the chemical bonding between PANi and G (or SWCNT) can significantly accelerate the charge-transfer. The DSSC fabricated by (PANi-4 wt%oSWCNT/GO)5 multilayer counter electrode yields a power conversion efficiency of 6.88%, while the DSSC from (PANi-10 wt%oG/GO)io multilayer counter electrode yields a power conversion efficiency of 7.88%.With an aim of accelerating charge-transfer ability, positively charged PANi-G complexes, chemically polymerized from aniline-G complexes, are combined with negatively charged platinum (Pt) sol by LbL self-assembly technique in the fourth chapter. The electrocatalytic activity of the (PANi-G/Pt)n multilayer counter electrodes increases with bilayer number and G (SWCNT) dosages. The DSSC fabricated by (PANi-10 wt%oG/Pt)9 multilayer counter electrode yields a power conversion efficiency of 7.45%.