Novel titanium dioxide nanocomposites and nanostructures for high performance dye-sensitized solar cells
Date of Issue2014
School of Chemical and Biomedical Engineering
Dye-sensitized solar cell (DSSC) has become one of the most promising alternatives to conventional Si-based photovoltaic devices, due to its low-cost, relatively high efficiency, easy fabrication process and light weight. Titanium dioxide (TiO2) is a widely used photoanode material with high stability, low toxicity and suitable electronic structure for DSSC. Most breakthroughs in power conversion efficiency (PCE) of the DSSCs are based on TiO2-photoanodes. To date, the PCE of nanocrystalline TiO2-based DSSC has reached 12.3% at AM 1.5G full sun (13.1% at 50.9% sun). However, further improvements in cell performance and stability are still necessary to deliver commercial viability of DSSC modules. Some unresolved issues in DSSC including the use of corrosive redox couple (i.e. Iˉ/I3ˉ) and limited film thickness of the photoanodes are attributed to the relatively slow electron transport in the mesoporous TiO2 anode films, which hinder the PCE improvement, stability and scale up of the DSSCs. Therefore, a major challenge in commercialization of the nanocrystalline TiO2-based DSSC is enhancing electron transport across the TiO2 matrix to further improve its efficiency and stability. The major contribution of this dissertation is the innovation of new TiO2-based nanocomposites and nanostructures which show significantly enhanced PCE of DSSC, and the interrogation and fundamental understanding of the mechanisms which contribute towards the enhancement. To improve electron transport property of the anode films, graphene was incorporated into the TiO2-based photoanodes. We developed a one-step solvothermal approach to synthesize uniform graphene-TiO2 composites with three different TiO2 nanostructures, including ultra-small 2 nm nanoparticles (labelled as USTG), 12 nm nanoparticles (STG) and nanorods (NRTG), controlled by simply adjusting the solvothermal reaction conditions. Using the three composites as photoanode materials, the effect of nanostructure of graphene-composited TiO2 on the performance of dye-sensitized solar cells was investigated. The photoanode based on ultra-small 2 nm TiO2-graphene composite exhibits the highest efficiency, which is attributed to the synergistic effects of the high specific surface area produced from the ultra-small TiO2-formed porous structure and efficient electron transport in the incorporated graphene with the compact TiO2 particles as electron leakage barrier. From the comparison between the USTG and STG based DSSCs of the above-mentioned work, a size effect of graphene-composited TiO2 on the DSSC performance was observed. The anode based on smaller-sized composite TiO2 gave a higher PCE, with a higher photocurrent density and lower electron transfer resistance. The mechanism of this phenomenon remained unclear from bulk device measurements. To understand the nanoscale electronic properties of the graphene-TiO2 films and reveal the size effect mechanism, conducting atomic force microscopy (c-AFM) was applied for nanoscale characterization of DSSC photoanodes. This is the first demonstration of applying c-AFM for DSSC studies. Three graphene-TiO2 composites with different TiO2 particle size were prepared using the one-step solvothermal approach and utilized to prepare the electrode films, of which the nanoscale electronic properties were measured and ultimately correlated to the bulk device performance, revealing a size-dependent electron transport property of the graphene-TiO2 composite photoanodes. The short-circuit current density and fill factor of the devices are improved as TiO2 particle size decreases, which is attributed to a more continuous electron transfer network and higher electron mobility of the smaller-sized TiO2 based electrode. The continuous conduction network, from the more flexible smaller-sized TiO2-graphene nanosheets, reduces the internal resistance at TiO2/TiO2 and TiO2/FTO interfaces. The high electron mobility, due to the continuous conduction network renders electron transfer fast and efficient. To integrate the features of large specific surface area, strong scattering and efficient electron transport into a single photoanode, hierarchical microspheres constructed with single crystal rutile TiO2 nanorods as multi-functional photoanode materials were designed and synthesized by combining an acid thermal crystallization and a self-assembly process of the nanorods via a solvothermal amphiphile-water microreactor strategy. The morphological evolution and crystallinity of the hierarchical microspheres are reaction temperature dependent. A great improvement (20%) in PCE for microsphere-based device was achieved compared to P25 reference cell, which is attributed to the synergistic effects of the large specific surface area, enhanced scattering effect, and efficient electron transport property of the microspheres. Using conventional P25 nanoparticles as a void filler, the microsphere-based photoanode exhibits a maximum PCE of 7.95% at an anode film thickness of 27.2 µm. In summary, this thesis dissertation, which reports interdisciplinary research works involving the design and synthesis of novel TiO2 nanocomposites and nanostructures as high performance photoanode materials and their fundamental studies, has contributed to further PCE enhancement of DSSC, and revealed the mechanisms of the novel anode materials in DSSC performance improvement.