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Solutions for R&D on dye-sensitized solar cells

The solar cell market grew to more than USD 35 billion worldwide in 2015 and is forecast to see a CAGR of over 12.0% from 2016 to 2024, according to research from Global Market Insights.  Solar’s growth is driven by a combination of environmental concern, technological innovation and bottom-line economics. Demand for dye-sensitized solar cells is showing strong growth as they can be developed as lightweight products with a design quality comparable to other silicon cells. However, the photoelectric conversion efficiency of dye sensitized cells poses serious challenges to R&D and quality control. Waters is taking a different path to solving these issues with a range of cutting-edge analytical solutions and efficient methods.

  • Types of solar cells
  • Mechanisms of dye-sensitized solar cells
  • Types of dye used in dye-sensitized solar cells and in ruthenium complexes
  • Examples of the analysis of ruthenium complexes using Xevo G2-XS QTof
  • Systemic solution for the analysis of dye for dye-sensitized solar cells

Types of solar cells

A solar cell is a physical battery that directly converts light to electrical energy using a semiconductor. Depending on the materials used in manufacture, solar cells can be broadly classified into four categories: silicon-based, polycrystalline silicon solar cells, compound-based, and organic-based.

  • Silicon-based solar cells include monocrystalline silicon solar cells, which are expensive, but deliver a high level of performance, with excellent energy conversion efficiency.
  • Polycrystalline silicon solar cells are inexpensive to manufacture compared to monocrystalline silicon types and have good energy conversion efficiency. Thin-film silicon solar cells which are less efficient at energy conversion than crystal types, do have an advantage in that they are easy to manufacture in large quantities.
  • Compound-based solar cells include resource saving CIGS solar cells that have good energy conversion efficiency—close to that of a polycrystalline silicon type—and include CdTe solar cells. Organic solar cells include organic semiconductor and dye-sensitized solar cells. Research into dye-sensitized type solar cells are finding al advantages in that they are easy to manufacture using conventional roll printing techniques, they are semi-flexible and semi-transparent and low cost, compared to silicon types, and so can be used to develop lightweight products with high design quality.

Mechanisms of dye-sensitized solar cells

The basic mechanisms of dye-sensitized solar cells are as follows:
1. Under light irradiation, the dye in a battery is excited and emits electrons.
2. The electrons transfer to the transparent electrode via titanium oxide (TiO2) and flow to the outside.
3. In the counter-electrode, iodide ions (I-) are generated through a reduction of triiodide ions (I3-) in the battery electrolyte, by gaining electrons.
4. The dye, which loses electrons and becomes a cation, gains electrons from I- and returns to the original state.

Although TiO2 alone can generate electric power, TiO2 absorbs the UV light only. By adding a dye that absorbs light in the range of visible light wavelengths, light in a wide range of wavelengths can be used for electricity generation, thus improving photoelectric conversion efficiency. Dye-sensitized solar cells have advantages, such as greater flexibility in color and form, a light body compared to silicon-based solar cells, and simple construction, such that a large and complex manufacturing facility is not required, which allows low-cost mass production. However, the photoelectric conversion efficiency of dye-sensitized solar cells — even in high-end ones — is only around 10%, about a half that of a silicon cell. This is the issue currently driving development of more efficient dye-sensitized solar cells..
Types of dyes used in dye-sensitized solar cells and in ruthenium complexes

Many types of dyes with different chromophoric ligands, including transition metals such as polypyridine complexes, metalloporphyrin, and metallophthalocyanine, along with different types of donor-acceptor dyes that do not contain any metals, are being investigated regarding use in dye-sensitized solar cells.

The polypyridine complexes of ruthenium have been studied in the areas of photophysics and in the chemistry of photoredox reactions for many years. The transformation of the conjugated systems of polypyridine ligand facilitates the introduction of proper functional groups. Therefore, the development of highly efficient sensitizers is advancing.

Examples of the analysis of ruthenium complexes using UPLC/Xevo G2-XS QTof

Ruthenium complexes elute with difficulty from the column due to the features of adsorption and the irreversible interaction with silanol groups in the column packing materials used for reverse phase chromatography, such as ODS, and, as a result, show broad peaks in chromatogram. In the impurity analysis that estimates synthetic yield, the poor separation of the peaks can be a cause of overlooking impurity peaks, and the yield may be assessed inaccurately. Waters proposes to use a UPLC system and a BEH column that adopts hybrid particle technology, which enables the ability to analyze the ruthenium complex with sharp peaks. This combination allows the ability to assess the yield without overlooking the synthesized impurities. Through separation from the major component, more reliable mass information can be obtained, improving the structural elucidation of impurities. This enables the ability to understand the impurity generation mechanism in a synthesis process and helps find a better process with a smaller amount of impurity generation. The purity test using a PDA detector can be easily applied to the incoming inspection of the supplied dye, and this helps to conduct the stable quality control of the dye. Click for the application for the analysis of ruthenium complexes.

Systemic solution for the analysis of dye for dye-sensitized solar cells

UPLC/Xevo G2-XS QTof (UPLC/quadrupole, time-of-flight mass spectrometer)
Auto-calibration by IntelliStart

  • Precise accurate mass measurement by LockSpray
  • Precise elemental composition analysis by i-Fit
  • Optimum for ultra performance liquid chromatography (UPLC)
  • High-selectivity quantitation by QuanTof technology
  • Applicable to the ESCi, APCI/APPI, ASAP, APGC
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