After TiO₂ nanocrystals were put to use in dye sensitized solar cell, the power conversion efficiency (PCE) has been delivered to above 10% at the current stage after persistent optimization. Nanomaterial synthesis, nanostructure assembly and interfacial engineering have played and will continue to play an imperative role in improving the performance of sensitized solar cell. For a significant improvement of PCE, the materials and nanostructure should be multifunctional and highly compatible in the device. In my thesis project, I focused on the understanding and the building up of photoanode materials to improve light harvesting and charge collection, and ultimately improve the overall solar cell performance.

In chapter 3, I report the development of a novel double layered photo...[ Read more ]

After TiO₂ nanocrystals were put to use in dye sensitized solar cell, the power conversion efficiency (PCE) has been delivered to above 10% at the current stage after persistent optimization. Nanomaterial synthesis, nanostructure assembly and interfacial engineering have played and will continue to play an imperative role in improving the performance of sensitized solar cell. For a significant improvement of PCE, the materials and nanostructure should be multifunctional and highly compatible in the device. In my thesis project, I focused on the understanding and the building up of photoanode materials to improve light harvesting and charge collection, and ultimately improve the overall solar cell performance.

In chapter 3, I report the development of a novel double layered photoanode for dye sensitized solar cell made of highly crystalline TiO₂ octahedral nanocrystals and agglutinate mesoporous TiO₂ microspheres. The underlayer of nanooctahedra serves as a transparent photoanode for copious and strong dye adsorption on the smooth (101) surfaces and for facilitated electron transport. Although the nanooctahedra are extremely small, our synthetic route has ensured a well-faceted crystalline shape with sharp edges and smooth surfaces, resulting in a 7.61% power conversion efficiency, much higher than that of P25 (5.76%). Separately, the overlayer of hierarchical TiO₂ mesoporous microspheres plays multiple roles of efficient light scattering, dye absorption and electrolyte permeation. Especially noteworthy is the agglutination of the microspheres through our 3D necking process, which has yielded an electron diffusion coefficient five times that of the P25 network and four times that of the nanooctahedra network. This is a significant breakthrough in DSSCs, which ensures that the photogenerated electrons in the overlayer can be effectively transported through such highway-like paths and ultimately collected at the FTO electrode. Therefore, this double layered photoanode has taken into consideration a number of disparate factors aiming at enhancing the overall DSSC performance. Drawing on the judicious combination of materials synthesis and engineering of nano-architectures and interfaces, solar cells based on this double layered structure have achieved 8.72% power conversion efficiency even with simple device fabrication procedures, showing promise as a new photoanode design for high efficiency dye sensitized solar cells.

In chapter 4, I have significantly improved open circuit voltage and fill factor with Pt counter electrode of quasi-solid state quantum dot sensitized solar cells (QDSSCs) by achieving compact coverage of QDs on TiO₂ matrix through a linker seeding chemical bath deposition process, leading to 4.23% power conversion efficiency, nearly two times that with conventionally deposited control photoanode. The distinguishing characteristic of our linker seeding synthesis is that it doesn’t rely on surface adsorption of precursor ions directly on TiO₂ (TiO₂~Cd_x) but rather nucleates special ionic seeds on a compact linker layer (TiO₂-COORS-Cd_x), thereby resulting in a full and even coverage of QDs on the TiO₂ surface in large area. We have shown that the compact coverage not only helps to suppress recombination from electrolyte but also gives rise to better charge transport through the QD layer. This LS-CBD method is general and expected to reinforce the hope of quasi-solid state QDSSCs as a strong competitor of dye sensitized solar cells after further optimization and development.

Chapter 5 demonstrates the first use of a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20 % PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent and fill factor. Time resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core-shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed.

The research in chapter 6 builds on the work of the QW structure in Chapter 5. Because the previous aqueous synthesis of the QW structure failed to generate photoluminescence (PL) from CdSe, presumably stemming from the low crystallinity and numerous defects, I opted to develop an organic solution process coupled with a layer-by-layer approach at much higher temperature to synthesize the QW structure. Well to my expectation, strong PL was observed even with the naked eye. Through optimization of the QW structure, the ZnSe/CdSe/ZnSe sandwiched QW supported on the ZnO tetrapod (ZnO/QW) showed 17 times stronger PL than the ZnSe/CdSe heterojunction (HJ) supported on the ZnO tetrapod (ZnO/HJ) at single particle level. Ensemble measurements also showed 10 times stronger PL of the former than the latter.