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UV-vis spectroscopy

Dechan Angmo

UV-vis spectroscopy is based on the principle of electronic transition in atoms or molecules upon absorbing suitable energy from an incident light that allows electrons to excite from a lower energy state to higher excited energy state. While interaction with infrared light causes molecules to undergo vibrational transitions, the shorter wavelength with higher energy radiations in the UV (200-400 nm) and visible (400-700 nm) range of the electromagnetic spectrum causes many atoms/molecules to undergo electronic transitions.

Figure 1. Left: schematic representation of electronic transition π-π* from the ground state ($S_0$) to an excited state ($S_1$). Right: Absorption spectra of the P3HT:PCBM blend in solution (black line) and as a thin film (blue line). Important feature differences are indicated.

The fraction of sunlight that can be absorbed is specific for each material and varies with its chemical structure. Band gap and molecular energy levels control are of crucial importance for device performance. The mismatch of the polymer absorption spectra and the solar irradiance spectrum is one of the reasons for low efficiencies of devices. Chemical modification of the semiconducting polymers structure is a common approach that leads to tuning of the band gaps. The extension of conjugation degree leads to an enhancement, in terms of intensity, and red-shift of the absorption spectra of conjugated polymers. As the collection of the radiation is broadened and more photons can be absorbed, thereby contributing to the energy conversion, the difference between HOMO and LUMO levels of the polymer becomes smaller, thus lowering the band gap. The energy levels also determinate the selection of electrodes and conducting materials. The alignment of the polymer on the film plays an important role in the solar cell device performance since it affects carrier mobility. Organic conducting polymers posses an anisotropic structure and the conductivity is higher along the chain direction, provided by π-π overlap between successive monomers, when the conjugated polymer films are macroscopically ordered. The maximum absorption wavelength, $\lambda _{max}$, of the polymer in the solid state is, in general, bathochromically shifted when compared to the solution spectra, due to major conformational order, resulting in different energy levels distribution. An evidence of the intermolecular packing in regio-regular P3HT, as a result of hexyl side chain and thiophene ring in the backbone contribution is shown in Figure 1. Another way to improve the absorption by reorganizing the intermolecular packing, therefore changing the properties of the material, is through the annealing process. The maximum absorption increases and broadens to a longer wavelength for corresponding transitions. This means that because the optical absorption corresponds to differences in energy states, it can be considered an indirect measure of the electronic structure. The optical band gap $E_{opt}$, expressed in electronvolts, depends on the incident photon wavelength by means of a Planck relation $$E_{opt} = h \nu = h \frac{c}{\lambda}$$ where $h$ is the Planck constant, $\nu$ is the wave frequency and $c$ lightspeed in vacuum. Experimentally, the optical band gap $E_{opt}$ of the polymer thin film is estimated by linear extrapolation from the absorption feature edge to $A=0$ and subsequent conversion of the wavelength (nm) into energy value versus vacuum (eV).

Transitions in molecules

To understand the energy levels in molecules, readers are encouraged to refresh their memory on molecular orbital theoryhttp://en.wikipedia.org/wiki/Molecular_orbital_theory. Briefly, chemical bonds in molecules are formed by overlapping of atomic orbitals that result in molecular orbitals of one of three types: 1) bonding 2) anti-bonding, or 3) non-bonding.

Figure 1. Four types of possible electronic transitions in molecules.

Typically, the electronic absorption is associated with transitions induced in electrons in bonding orbitals; and the atoms involved are generally those containing s+p electrons. Four types of electronic transitions are possible (See Figure 1). Both σ and π bonds have their corresponding anti bonding higher energy level while n is non-bonding orbital. The following transitions could take place in a molecule

  1. n to σ* (e.g oxygen, nitrogen, halogen, sulphur compounds)
  2. n to π* (e.g carbonyl groups)
  3. σ to σ* (e.g. alkene)
  4. π to π* (e.g. carbonyl, alkenes, alkynes, azo compounds, etc)
  5. σ to σ* and n to σ* transitions require relatively high energy and therefore occur in very short wavelength UV regions while n to π* and π to π* occur in relatively lower energy UV-visible region.

    Figure 2. An UV-vis spectrum of Tetraphenylcyclopentadienon (2).

    Determination of band gap from absorbance spectra

    Since the absorption spectrum reveal information on electronic transition, the onset of absorption is considered as the band gap of semiconductor or conjugated polymers. Many also consider the peak of the absorption spectrum as the band gap. Figure 3 gives an example of how the band gap is determined on a conjugated polymer (denoted as Polymer-X) in solution and film.

    Figure 3. Determination of band gap from a UV-vis spectrum.

    Generally, the color of a molecule in solution or film gives an indication of their band gap. The color of a film or solution that an observer perceives is usually the complementary wavelength of the electromagnetic spectrum that the molecule absorbs. For example, if a molecule in solution or film appears green in color, then the molecule is absorbing (and therefore has a band gap of) complementary color to green, which is red.

    Instrumentation

    A UV spectrometer which can look like Figure 4 consist of five essential parts

    1. A light source that can transmit both UV and visible light: The light source should be stable and should provide sufficient intensity over a large region of the electromagnetic spectrum. A combination of tungsten-halogen and deuterium (D2) arc bulbs provide visible and UV light, respectively.
    2. Monochromator setup to isolate the different wavelegnth of the light and to introduce each monochromatic light into the sample. It consists of 1) entrance slit (2) collimating device that produces parallel light (3) a wavelength selection or dispersing system (e,g. a prism or diffraction grating) (4) a focusing lends or mirror (5) an exit slit. These are not shown in Figure 4.
    3. Sample handling: Samples in solution or films can be analyzed in a spectrometer. Solution requires cuvettes made of quartz which have a large optical window and are solvent resistant.
    4. Detector: a means of detecting and measuring the light after passing through the sample. It could be photomultiplier detector, silicon diode detector, or a diode array.
    5. Processor: Each commercial UV-vis spectrometer is equipped with proprietary software that processes the data according to the input given by operator.
    Figure 4. UV-vis spectrometer is shown along with sample stage for films that comprise of a sample holder and a reference holder. The components and assembly of a UV vis spectrophotometer is shown below.

    Concentration and Absorption

    The combination of two laws describe the dependence of absorption of molecules in solution on their concentration. Lambert’s law says that the proportion of light absorbed by a transparent medium is independent of the intensity of light (provided there are no physical or chemical changes in the system). In other words, transmittance is directly proportional to the ratio of transmitted light intensity to the incident light intensity. This is often expressed as percentage transmittance (%T): $$\% T = \frac{I}{{{I_0}}} \cdot 100$$ Beer Law: The absorption of light is directly proportional to both the concentration of the absorbing medium and the thickness of the medium in the light path. Both these law when combined is known as the Beer-Lambert law. Beer-Lambert law defines the relationship between absorption and transmission and is given by: $$A = \log \frac{{{I_0}}}{I} = \frac{{\log 100}}{T} = \varepsilon lc$$ ε is the extinction coefficient / molar absorptivity with the unit of L mol-1 cm-1; l is the path length in cm, and c is the concentration in mol cm-3. From the absorption curve the $\varepsilon$ can be found at a given wavelength for a given material if the concentration of the material is known.

    Absorption and color

    The color of a sample is determined by the absorption spectrum. When white light passes through or is reflected by a colored substance, characteristic wavelengths are absorbed. The remaining light will then assume the complementary color to the wavelengths absorbed. In Table 1 you can see a summary of colors, wavelengths, and their corosponding complmentary color.

    Color of absorbed light Wavelenght [nm] Complementary color / Color of sample
    Violet 400 Yellow
    Blue 460 Orange
    Blue-green 500 Red
    Yello-green 530 Red-violet
    Yellow 550 Violet
    Orange-red 600 Blue-green
    Red 700 Green
    Table 1. Color of sample based on the color of the absorbed light.
    "Basic UV/Visible Spectrophotometry" www.biochrom.co.uk/download/72/ Accessed on 26-05-2014 http://www.chem.ucla.edu/~bacher/UV-vis/uv_vis_tetracyclone.html.html Accessed on 26-05-2014 http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm Accessed on 26-05-2014

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