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Photoluminescence Spectroscopy Uncovers Photovoltaic Properties

Fluorescence spectroscopy is the key to new research into photovoltaic materials with the objective of developing more efficient, flexible and less costly solar cells.

​Generally, the better the luminescence of the materials, the better the efficiency of the solar cells, so researchers measure the luminescence of samples to gauge the potential semiconductor properties.
 
A team from Texas A&M University-Central Texas uses photoluminescence to gauge the quality of solar cells, materials that convert light to electricity. The luminescence of a solar cell can indicate the quality of the solar cell crystal. Semiconductors, which are the basis of solar cells, luminesce at a very specific wavelength.

​Generally, the better the luminescence of the materials, the better the efficiency of the solar cells, so researchers measure the luminescence of samples to gauge the potential semiconductor properties.

The research team is trying to develop new materials. Besides silicon, they do some work with mostly thin film photovoltaic materials like cadmium telluride, copper, indium, gallium and selenide.

The team looks at whether a cell is luminescing along with the uniformity of the luminescence. The individual materials should all have the same wavelengths of light and the same distribution.

Sometimes scientists see different spots that have dissimilar wavelengths of luminescence. That tells them the manufacturing process has introduced some level of variability in the photovoltaic materials.

The work encompasses both basic and applied research to want to understand the materials and how to process them so it can be manufactured effectively.

The team is part of a center with the long-term goal to help establish photovoltaic electricity as a major source of energy in the world.

The Texas A&M University group uses a HORIBA MicOS (a fully integrated microscope spectrometer) to do a broad array using photoluminescence across the entire sample. Its aim is to aid in the development of new materials.

The folks at Texas A&M are trying to find those poorer performing parts of the sample to understand it better, for basic research and applied applications. They are trying to understand how to get rid of the variations that are less efficient. Patterning, the deposit of poorly or non-photovoltaic materials, is something they want to eliminate. They do this through photoluminescence by engineering out the poor fluorescing components.

They will get a sample from a collaborator and take a rough scan of it with the MicOS, and do a broad array using photoluminescence across the entire sample. As a result, they have mapped the sample and might see some areas of interest.

If the team identifies one spot on a solar cell that’s more interesting, it can put it in the scanning electron microscope with the HORIBA CLUE (Cathodoluminescence Universal Extension), and zoom in on that area. Then the researchers can do micro-mapping in areas that are interesting to them.

The researchers can do micro-mapping this way in areas that are show unusual photoluminescence. Or they may be trying to understand how to get rid of the variations that are less efficient, like the way the photovoltaic materials were deposited. It lets them look at different ways to eliminate that patterning.

Researchers are trying to identify materials that can be much less expensive than silicon, the historical standard for photovoltaics. They also look for properties of substances that can do what silicon can’t do, like be lightweight, bend, and better integrate with our everyday lives. The paint mentioned earlier can be made on a solar cell on any low-cost material, like plastics and paper.  In labs around the world, novel materials include those made from semiconducting nanoparticles (quantum dots), perovskite crystals, semiconducting polymers, and even biological materials and materials made from plant-based extracts.  All of these are being investigated for better semiconducting properties. 

The key is to understand the efficiencies of the materials. Once achieved, the technology will become more commercial.

The hitch is this – the ability of these photovoltaic materials to convert light into electricity. These paints haven’t reached the efficiency of silicon – yet.

Silicon dominates the market, accounting for about 90 percent of solar cells. The remaining 10 percent is cadmium telluride. But silicon has severe limitations. It’s a poor absorber of light, requiring thick layers to absorb the energy. Researchers like those at Texas A&M are looking for more flexible less expensive alternatives.

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What is a A-TEEM spectroscopy?

A-TEEM spectroscopy refers to the ability to simultaneously acquire Absorbance, Transmittance and a fluorescence Excitation Emission Matrix (A-TEEM) of a particular sample. HORIBA pioneered this technique with the patented Aqualog and Duetta system, which combines A-TEEM spectroscopy with simultaneous multichannel CCD detection to provide extremely fast results.

A-TEEM spectrometers can be used for fluorescence EEMs or for absorbance measurements for multi-component analysis, but its real power is derived from the fact that the EEMs collected by the instrument are corrected for inner filter effect. This means they are true and accurate representations of the molecules of interest over a much broader and more useable concentration range (typically up to ~2 absorbance units).

Therefore, these EEMs allow for much more precise fingerprinting than is possible with an EEM collected from a traditional scanning fluorometer. A-TEEM spectroscopy is now bringing the fluorescence technique into the true analytical market and it has been demonstrated that it can, in some cases, replace traditional instruments like an HPLC or mass spectrometer as a simpler, faster and less expensive analytical tool.
​To access the true power of fluorescence A-TEEM spectroscopy, one needs to employ multivariate software methods such as Principal Components Analysis (PCA), Classical Least Squared (CLS) method and Parallel Factor Analysis (PARAFAC).

Most components of CDOM have broad overlapping fluorescence excitation and emission spectra in the UV and visible range. Many sample measurements are used to create a model and then use chemometrics to get scores of each component in an individual sample. The very unique thing about a fluorescence EEM is that it can be used as a molecular fingerprint. Changes in the emission spectrum, the excitation spectrum, or both can be tracked very easily using this 3D fluorescence method for water analysis as well as many other applications.

Applications using A-TEEM

One common application for EEMs, and especially for A-TEEM spectroscopy is for quality analysis of water, specifically for the study of chromophoric dissolved organic matter, also called CDOM. Dissolved organic matter includes amino acids, humic acids, fulvic acids, and other examples of decayed matter in natural water sources, or disinfection byproducts of water treatment processes. A-TEEMs are used to identify the presence of each at very low concentrations, typically in the ppb range.
Common fluorescent compounds in water include humic acids, fulvic acids, and amino acids (left) and a typical EEM of waste water during various treatment processes (right)
Fluorescence EEMs (A and B) and corresponding (C and D) Absorbance (OD) and %Transmittance spectra for an Italian wine sample before (A and C) and after (B and D) a one week oxidation treatment
Most components of CDOM have broad overlapping fluorescence excitation and emission spectra in the UV and visible range. Many sample measurements are used to create a model and then use chemometrics to get scores of each component in an individual sample. The very unique thing about a fluorescence EEM, corrected for IFE, is that it can be used as a precise molecular fingerprint. Changes in the emission spectrum, the excitation spectrum, or both can be tracked very easily using this 3D fluorescence method for water analysis.
EEMs are also used to study petrochemicals, drugs, proteins and food science, including wine, beer, and many other beverages. Below is an example of an Italian wine sample measured before and after a week-long oxidation treatment (exposure to air). The EEM and corresponding absorbance spectra give a fingerprint of the wine that show changes to the fluorescence spectral shape and intensity for components such as caffeic acid, flavanols, epicatechin, gentisic acid, and anthocyaninie.
 
By using chemometric analysis, the individual EEMs are also used to characterize single walled carbon nanotubes. (HORIBA App Note: Fluorescence Spectra from Carbon Nanotubes with the Nanolog, n.d.) The carbon nanotube, which is a rolled graphene sheet, can be single walled or multiwalled (multiple sheets rolled). (Dresselhaus, 2000) (O’Connel, 2002) Only single walled carbon nanotubes (SWCNTs) emit photons due to their semiconductor properties.

Helical Folding Angle

The emission wavelengths and absorption of the SWCNT can also be affected by its helical folding angle (Bachilo, 2002). In this case, the carbon structure would be different depending on whether the graphene sheet is rolled at an angle or horizontally.

In this instance, the specific geometry can be understood in terms of the wrapping vector containing the length – that is the tube’s circumference – and a helix angle ranging from 0 to 30 °. The two numbers are shown (n,m) are therefore used for SWCNT definition.
In the case of SWCNTs, the semiconductor absorbs between the c2 and v2 energy levels, where the electron hole is passed down as shown in Fig. 34. A photon is emitted at the band gap, or c1-v1 energy level. These conduction bands and emission bands depend on the diameter of a carbon nanotube, which can also be described as an exciton Bohr radius. The smaller the radius, the higher the energy (or shorter the wavelengths) of light absorbed and emitted. This is due to the quantum confinement effect, which dictates that the wavelength of light emitted is restricted by the size of the particle, or in this case, the diameter of the tube. (O’Connel, 2002) (Dresselhaus, 2000)
 

Monitoring cell culture media

With the rise of protein production using mammalian cell culture, it has become increasingly important to control the quality of the cell culture media for use in production processes.

Cell culture media are usually prepared as aqueous solutions, and should provide everything a cell line needs for optimal cell growth as well as product yield and quality.

In any given bioreactor process it is important to identify the proper type of cell culture medium and its quality, because even subtle variations in composition could have a noticeable impact on the growth rate of the cell culture and its yield.  Therefore, the composition and quality of cell culture media in bioreactors must be tightly controlled in order to maintain an optimal bioreactor process. As a result, methods of identifying and analysing the quality of cell culture media have become an important focus in this field

What is Synchronous scan?

The fluorescence excitation spectrum, emission spectrum, and synchronous scan (Da=0nm) for fluorescein in 0.1N NaOH(aq)

A synchronous scan is when the excitation monochromator scans at the same time as the emission monochromator and the fluorescence emission is read out. Typically, one can set an offset between the excitation and emission monochromators that matches the Stokes Shift (difference between excitation and emission peaks). These types of synchronous scans have been historically used for component analysis, but due to the more modern instruments for measuring EEMs with CCD detectors, the EEM gives more information and takes the same amount of time

An offset of 0 nm can be set so that the excitation and emission are scanning together at the same wavelengths. This is what is called right-angle light scattering, or RALS, and results in what is really a right-angle reflectance spectrum. This type of synchronous scan measures the reflected or scattered light from the excitation.

Click here for more information on the fluorescence spectrometers we provide here.

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Structural characterization of WS2 flakes by Photoluminescence and ultra-low frequency Raman spectroscopy on a unique multimode platform

2D materials are best characterised using both Photoluminescence (PL) & ultra-low frequency Raman spectroscopy. PL is most appropriate for band structures characterisation at the micron scale.  Additionally, Raman analysis very close to the laser line, allowing a precise characterisation of the number of layers of 2D materials as specific interlayer vibration modes are excited in the ultra-low spectral Raman range (< 50ms-1). Click below as we explore the application of the two techniques using one of the 2D materials, WS2 flakes. 

 
​By definition, photoluminescence is the most appropriate technique for band structures characterisation at the micron scale. Indeed, the luminescence band of a semiconductor informs directly about the bandgap energy. 

Furthermore, the Raman analysis very close to the laser line allows a precise characterisation of the number of layers of a 2D material. Indeed, specific interlayer vibration modes are excited in this spectral range. Being able to have both spectroscopy techniques, photoluminescence and ultra-low frequency Raman on the same instrument is a vital feature to characterise these materials as much as possible. 

Continue reading the rest of the article from HORIBA here.

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*New Product* LabRAM Soleil

One of our long-time partner, HORIBA has announced their latest microscope LabRAM Soleil.

LabRAM Soleil is part of one of Horiba’s most famous Raman series; LabRAM spectroscopy. LabRAM spectroscopy is a complete system that provides ultrafast imaging, advanced automation features, intuitive software and a robust design to supercharge researchers’ analyses.

Click below to download brochure, contact us for more information. 

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Standard Microscope Spectroscopy [HORIBA]

HORIBA is proud to introduce  the new Standard Microscope Spectroscopy (SMS) systems.  The SMS system is a turnkey system that allows microscope to multi-task by performing spectroscopy. With its unique set of accessories, the SMS system is designed to be extremely flexible as it is capable of turning any existing microscopes into a spectrometer. 

​The concept driving these systems is the ability to either leverage an existing standard microscope, or create a fully turnkey system that performs the microscopy function, and adds one or more spectroscopies as a complementary technique.  Multitask your microscope™ is the theme behind these Standard Microscope Spectroscopy (SMS) systems.

Features

  • Capable of performing a wide variety of spectroscopic techniques
    • Raman
    • Steady state & time-resolved Photoluminescence (PL)
    • Reflectance/Transmittance
    • Electroluminescence
    • Photocurrent
    • Dark Field Scattering
  • Highly compatible with any existing standard microscopes
    • Without compromising the imaging functionality

E-Brochure Catalogue

  • Raman
  • Steady state & time-resolved
  • Photoluminescence (PL)
  • Reflectance/Transmittance
  • Electroluminescence
  • Photocurrent
  • Dark Field Scattering

To know more about the SMS system, please request an e-brochure from us

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