Abstract: Printable materials and technologies to realize low-cost dye-sensitized solar cell fabricated on thin plastic substrates are reviewed. Mesoscopic conductive materials and pastes that enable low-temperature coating of electrochemically active films for photoanode and conterelectrode are described in aspects of material preparation, electrochemical and photovoltaic behavior, and stability of the plastic electrode. Performance of plastic solar cells and modules are discussed with respects to the structure and thickness of the non-sintered mesoporous films, light-harvesting functions of dyes, and optimization of electrolyte compositions. Commercial advantages of the lightweight, flexible cell in power generation are also introduced based on proof-of-concept tests with large-area modules.
Abstract: Dye-sensitized solar cell(DSC) is a new type of photovoltaic device. This paper mainly describes the research results of the development of a novel nitrogen-doped photoanode for DSC in our group. Highly efficient dye-sensitized solar cells (DSCs) of 7.6-10.1% were fabricated using nitrogen-doped titania electrodes. The photoelectrochemical properties of the nitrogen-doped titania powder, film, and solar cell were systemically investigated. We confirmed the substitution of oxygen sites and oxygen deficiency with nitrogen atoms in the titania structure by X-ray photoemission spectroscopy (XPS). The UV-Vis spectra of the nitrogen-doped powder and film showed visible light absorption in the wavelength range between 400 nm and 535 nm. The results of the stability test indicated that the DSCs fabricated by the nitrogen-doped titania exhibited great stability.
Abstract: Recently, dye-sensitized solar cells have attracted much attention relevant to global environmental issues. So far ruthenium(II) bipyridyl complexes have proven to be the most efficient TiO2 sensitizers in dye-sensitized solar cells. However, the highest power conversion efficiency has been stagnated in recent years. More importantly, considering that ruthenium is rare and expensive, novel dyes without metal or using inexpensive metal are desirable for highly efficient dye-sensitized solar cells. To fulfill the requirement, it is crucial to develop inexpensive novel dyes that exhibit high efficiencies in terms of light-harvesting, charge separation, and charge collection. Porphyrins are important classes of potential sensitizers for highly efficient dye-sensitized solar cells owing to their photostability and potentially high light-harvesting capabilities that would allow applications in thinner, low-cost dye-sensitized solar cells. However, typical porphyrins possess an intense Soret band at 400 nm and moderate Q bands at 600 nm, which does not match solar energy distribution on the earth. Therefore, the unmatched light-harvesting property relative to the ruthenium complexes has limited the cell performance of porphyrin-sensitized TiO2 cells. Elongation of the -conjugation and loss of symmetry in porphyrins cause broadening and red-shift of the absorption bands together with an increasing intensity of the Q bands relative to that of the Soret band. On the basis of the strategy, the cell performance of porphyrin-sensitized solar cells has been improved remarkably by the enhanced light absorption. The efficiency of porphyrin-sensitized solar cells could be improved significantly if the dyes with larger red and near-infrared absorption could be developed.
Abstract: Ionic liquid oligomer, 1-oligo(ethyleneoxide)-3-methylimidazolium salt (PEO(X)MIm) and Ionic liquid polymer, poly(1-oligo (ethylene glycol) methacrylate-3-methylimidazolium) salt (P(MOEMIm)) prepared by incorporating imidazolium ionic liquid with PEO oligomer and polymer were investigated as electrolytes for dye-sensitized solar cells (DSCs). Ionic liquid electrolytes were composed of LiI, I2, and PEO(X)MImCl or the mixture of 1-hexyl-3-methylidazolium iodide (HMImI), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and PEO(X)MImCl. Quasi-solid-state electrolytes were prepared by employing the imidazole polymers P(MOEMImCl) to solidify the liquid electrolyte containing lithium iodide, iodine and ethylene carbonate (EC)/propylene carbonate (PC) mixed solvent. Ionic liquid based quasi-solid state electrolytes were prepared by solidifying the ionic liquid electrolytes containing HMImI or a binary mixture of HMImI and EMImBF4 with an ionic liquid polymer P(MOEMImCl), respectively. The influences of PEO molecular weight, polymer content, addition of alkyl ionic liquid and various anions of the ionic liquid oligomers and polymer on the ionic conductivity, apparent diffusion coefficient of the redox species in the electrolytes and the performance of solar cells were examined. The influences on the kinetic behaviors of dye regeneration and triiodide reduction reactions taken place at nanocrystalline TiO2 electrode and Pt counter-electrode, respectively, were also studied by cyclic-voltammetry and electrochemical impedance spectroscopy measurements. By using ternary ionic liquid electrolyte containing 1M lithium iodide and 0.5M iodine in the ionic liquid of the ionic liquid mixture of PEO(X)MImCl), HMImI and EMImBF4, quasi-solid-state electrolytes and ionic liquid based quasi-solid state electrolytes the photoelectron conversion efficiency of DSCs is 7.89%, 7.6% and 6.1%, respectively(AM 1.5, 100mWcm−2). These results show the potential application of PEO based ionic liquid in SCs.
Abstract: This review covers recent research on counter electrodes using platinum (Pt), carbon and conducting polymers as catalysts for the reduction of triiodide in the dye-sensitized solar cell. Different types of counter electrode preparation methods and their advantages and disadvantages are compared. The inadequacy of the counter electrode with the precious metals and the advantages of the non-metallic-type counter electrode are pointed out. Especially, recent research on the Pt counter electrode in our group is discussed concerning with the reaction mechanism of the triiodide (I3-)/ iodide (I-) redox couple on the Pt counter electrode.
Abstract: The efficiency of electron injection (inj) in dye-sensitized nanocrystalline films has been studied by means of transient absorption spectroscopy. We observed inj of nearly unity for N3 dye adsorbed on nanocrystalline TiO2 films (N3/TiO2). We examined the effects of various experimental conditions, such as light intensity, excitation wavelength, and presence of additives (4-tert- butylpyridine, tBP and Li ions), on inj. We also used various semiconductors and sensitizer dyes to study the effect of free energy change (G) on inj. These results give us new insights for developing high-performance solar cell devices.
Abstract: General characteristics of dye-sensitized nanoporous semiconductor electrode systems are summarized, with a particular emphasis on dye-sensitized solar cells. Properties of these electrode systems which distinguish them from conventional bulk semiconductor electrodes are highlighted. Current understanding of electron transport in dye-sensitized solar cells, in terms of the diffusion and multiple trapping models, is reviewed. Alternative transport and recombination theories are also briefly reviewed. Electron transfer at the semiconductor/electrolyte interface in dye-sensitized solar cells is reviewed and recent experimental results obtained by the authors are highlighted. As applicable, common techniques for characterization of electron transport and transfer in dye-sensitized solar cells are described, with reference to case studies where the electron diffusion length in dye-sensitized solar cells has been estimated. The steady-state aspects of the dye-regeneration process are also reviewed, together with the cross-surface percolation of holes in the dye monolayer and the finite-length diffusion of redox species in the electrolyte.
Abstract: Research on the flexible dye-sensitized solar cells (DSCs) has been more and more extensively conducted during the recent years both academically and comercially for the sake of its further reduced expense and even broader application. However, significant promotion of electron transport properties and consequently the photovoltaic performances of such devices are perpetually hindered by the key problem that the poor heat tolerance of the plastic substrates employed in flexible DSCs makes high-temperature sintering of the photoanode films impossible. Based on a brief overview of the current state of research on flexible DSCs, including new materials and delicate processing techniques, and the research results from the author’s own group, this chapter specially treats the profound mechanistic issue of electron transport and recombination in flexible DSCs, which is rarely discussed and relatively less well understood up to now. It is pointed out that the electron transport and recombination dominate photovoltaic performance of the flexible DSCs and suppressing the recombination of injected electrons with electrolyte redox species is of crucial sense for performance promotion. Besides, the methods for restraining electron recombination are proposed and the developing trend and prospects of flexible DSCs are also presented.