A Study on Multi-functional Composite Scattering Structures for Highly Efficient Dye-Sensitized Solar Cell

Abstract

Today, fossil fuels are the main source of energy throughout the globe, and the use of these fuels is a major source of carbon dioxide gas emission and greenhouse effect which is leading towards the global warming. To overcome this problem, alternative green energy sources are necessary for a safer future of our environment. There are various renewable energy sources including wind power, wave power, hydropower, and solar power, available throughout the world according to the geographical locations and possibilities. Specifically, solar power has a great potential to overcome the energy crises and to fulfil the major portion of the renewable and green energy demand in the future. Community of solar devices and material’s scientists have always been motivated to develop the inexpensive and highly effective photovoltaic (PV) technologies. During the last three decades, major improvements in three different aspects of PV have to be made: cheaper production, sustainable materials and applicability. Recently, majority of researchers has been trying to find the alternatives of silicon based expensive photovoltaic technology, and after the breakthrough of nanotechnology, nanotechnology based photovoltaic devices (3rd generation PV) has gained significant potential as a cheaper and highly efficient alternatives. One of the most applicable technology is Dye Sensitized Solar Cells (DSSCs), which is developed by O’Regan and Grätzel in 1991. This new technology has already attracted a lot of interest by the academia and industry due to its lower cost (based on the use of inexpensive components), its affordable and simpler production technology and also its wider applicability. Now DSSC is considered as a promising future technology. The substantial efforts of academia and industry over the past 20 years have not only improved the efficiencies but also have introduced numerous new ways to develop durable and robust DSSCs which are fairly low-priced with good efficiencies. 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 optimized multifunctional efficient scattering structures 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. Out of many ways, addition of multifunctional scattering structures into the photoanode is a promising and economical way to enhance the photovoltaic performance of DSSC. The objective and goal of the present research work was to synthesize cheaper and highly productive scattering materials for DSSC, ways for easy fabrication of cells, their characterization, and improving the efficiency of DSSCs based on multifunctional scattering structures. This work commences by a broader introduction and background of photovoltaics, different energy harvesting approaches, energy conservation devices which include solar cells, both traditional Silicon Vs third generation solar cells, and discussion in details about the advantages and disadvantages of both types of solar cells. Later in the chapter we point out the novelty, design and strengths of Dye Sensitized Solar Cells (DSSCs) which includes use of nanomaterials engineering, optimizing device architecture etc. Current status of research which includes literature survey, followed by DSSC applications and later the technology position of DSSC, are discussed in details. The maximum theoretically predicted efficiency of DSSCs, the areas of scientific goals necessary to enhance DSSC performance especially scattering phenomena and history of different scattering structures used in DSSC are examined in detail. One-dimensional (1D) nanostructured photoanodes (e.g. nanotubes, nanowires, and nanorod) have been proved as a favorable material to use in photoanode due to their slow recombination, fast electron transport, and exceptional light scattering ability. Hydrothermally synthesized TiO2 nanotubes (TNTs), which have the advantages of high aspect ratio, simple preparation procedure, and controllable morphology, has become the recent research focus within the 1-D nanostructure category. Composite functional nanostructured scattering layers is a promising strategy for more improving the performance of DSSC. Optimized nanotubes/nanoparticles composite was used as over/functional layer in DSSC, which offered high surface area leading to high dye loading and high charge generation, better light scattering, better photon-electron generation suppressing the effect of recombination, long electron lifetime, better conductivity, and better electron collection efficiency. Composite of 75 % TNT and 25 % P25 was found to be excellent for better performance of DSSC due to TNT/P25 homogeneous distribution, more conductive surface, and longer electron lifetime. Although performance of DSSC have been improved by TNTs prepared by P25 due to better scattering and electron kinetics but the dye adsorption of these TNTs was lower than TiO2 particles due to smaller internal diameter of tubes which hinders the entrance of dye molecules into the tubes. To overcome this shortcoming, highly crystalline and pure bigger sized TNTs were prepared through hydrothermal process with commercial precursor (SG-T0200). Composite of bigger TNTs and P25 was also prepared. Smaller and bigger TNTs and their composites were used as scattering layer in DSSC. Bigger TNTs showed slightly less BET surface area than smaller TNTs but much higher dye loading ability due to higher functional surface area. B-TNT dyed films also showed much higher light absorbance, better light scattering, and low transparency than S-TNT dyed films due to their bigger size. The DSSC having B-TNT multi-functional over layer provided effective light absorption, better light scattering, longer electron life time, and rapid electron transfer and consequently JSC and light harvesting efficiency have been improved. B-TNTs show remarkable dye adsorption and light scattering ability but the S-TNTs show better dye loading and transparency. Taking advantage of both types of TNTs, a sequential scattering structure was fabricated with S-TNT as first scattering layer and B-TNT as second scattering layer. This new TNT sequential structure showed much higher dye loading, light absorbance, lowest transparency, and marvelous light scattering properties. The DSSC with multi-functional sequential scattering structure provided effective light absorption, better light scattering, longer electron life time, and rapid electron transfer and consequently JSC and broader spectrum light harvesting efficiency have been improved. This new idea of TNT based sequential scattering structure enhanced photovoltaic and electrochemical properties and showed about 14 % improvement in photo conversion efficiencies as compared to conventional structures. Size dependent scattering is a well-known phenomenon in DSSC. Smaller size particles are prone to transmit most of light but the larger size particles are more efficient in scattering the light. Taking advantage of smaller and bigger particles, three layered sequential scattering structure was fabricated with 100 nm/20 nm composite as first scattering layer and 200 nm/20 nm composite as second scattering layer. Sequential structured dyed films showed much higher dye loading, outstanding light absorbance, and better light scattering properties. The DSSC having multi-functional sequential scattering structure provided efficient charge generation, longer electron life time, higher recombination resistance, and rapid electron transfer and consequently JSC and broader spectrum light harvesting efficiency have been improved leading to achieve a new record efficiency of 10.9 % with commercial D719 dye. Combination of internal and external scattering is a novel way to get maximum scattering and enhanced performance in DSSC. To make this idea as practical, three layered sequential scattering structure was fabricated with 100 nm/20 nm composite as first scattering layer, 200 nm/20 nm composite as second scattering layer and 500 nm/20 nm composite as third scattering layer. Three layered sequential scattering dyed films showed much higher dye loading, outperforming light absorbance, least transmittance, and remarkable light scattering properties. The DSSC having multi-functional three layered sequential scattering structure provided efficient charge generation, longer electron life time, higher recombination resistance, better conductivity, lower interfacial resistance leading to outstanding JSC and broader spectrum light harvesting efficiency (EQE) were achieved with new record photovoltaic efficiency of 11.2 %. These all composite multi-functional scattering structures are promising strategies for further improving the efficiencies of DSSC and these will be concrete fundamental background toward the development of the applications of the next generation dye-sensitized solar cells. Optimized compositions, better dispersion of all composites, TNTs composite with bigger functional particles, optimized thickness of composite scattering-layers, and combination of panchromatic and organic dyes with these composites will enlighten the future course leading towards the extremely efficient DSSCs. The thesis is concluded with general conclusion and directions for future work.