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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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How to choose a Monochromator/Spectrograph

Choose a monochromator/spectrograph based on

  • A system that will allow the largest entrance slit width for the bandpass required.
  • The highest dispersion.
  • The largest optics affordable.
  • Longest focal length affordable.
  • Highest groove density that will accommodate the spectral range.
  • Optics and coatings appropriate for specific spectral range.
  • Entrance optics which will optimize etendue.
  • If the instrument is to be used at a single wavelength in a non-scanning mode, then it must be possible to adjust the exit slit to match the size of the entrance slit image.

Note: f/value is not always the controlling factor of throughput.

For example, the light may be collected from a source at f/1 and projected onto the entrance slit of an f/6 monochromator so that the entire image is contained within the slit. Then the system will operate on the basis of the photon collection in the f/l cone and not the f/6 cone of the monochromator.

What are the criteria for choosing my monochromator?

Top criteria: Dispersion, resolution, bandpass

The dispersion shows the capability to disperse light. It gives the usable bandpass of a monochromator or indicates the spectral range of a spectrograph equipped with a multichannel array detector such as a CCD camera or InGaAs array. Changing the width of the slit aperture can adjust the bandpass.

Spectral resolution is inversely proportional to the linear dispersion of a monochromator. The resolution requirement of the experiment is often the key performance requirement of any application. For narrow structure analysis (resolution better than 0.1 nm in the visible range), large monochromators are the best choice because they offer increased spectral dispersion and thus a higher spectral resolution.

If the application’s most important requirement is to acquire a large spectral range in one shot, small spectrographs are better, as in a process application. In fact, single timonochromators with a focal length below 0.3 m are suitable where most stray light is not considered a major problem.

Laboratory researchers who don’t need high resolution and wide range at the same time, however, should choose larger spectrographs. These users can change gratings (i.e., to a higher or lower groove density) or acquire multiple spectra to achieve complete results.

Second criteria: Accuracy, speed

Monochromators work by scanning through the spectral features of the optical signals, step by step. As a result, the measurement process is generally slower than that of spectrographs with multichannel array detectors that operate in a fixed grating position and directly acquire a full spectrum according to their dispend mainly on the grating motor drive. Instruments with 0.5-m or higher focal length are usually equipped with sine bar mechanisms that give excellent accuracy (better than 0.1 nm) and repeatability (better than 0.01 nm). But the trade-off is speed: It can take several minutes to scan a large spectral range with high spectral resolution in monochromator mode. Smaller devices commonly use direct, or worm drives, because their resolution specifications are lower. In this case, they set the grating position within a few seconds.

Third criteria: Throughput, imaging quality

Most of the time, small monochromators and spectrographs have better throughput than large ones because of their larger numerical apertures (f numbers) and simpler design (often with fewer optical components).

However, the numerical aperture is not the final consideration in optical throughput. The linear dispersion is also important because it defines the input aperture sizes for a particular spectral resolution.

A more useful figure of merit for comparing monochromators for imaging applications is light-gathering capability. Manufacturers of small instruments usually find that to preserve spectral resolution, they can produce only a small image.

Producing a larger image, especially across a large spectral range, is very difficult because of the spatial corrections required.

The commercial introduction of imaging spectrographs has partially corrected the spatial imaging quality issues. These instruments use toroidal gratings or toroidal mirrors to correct for astigmatism in the image plane and to improve image quality, while keeping the numerical aperture at the same level as non-imaging devices. This correction requires complex calculations. The choice of the toroid, the optical incident angles of the device and wavelength optimizations shows the expertise of the manufacturer.

Well-corrected, fixed, compact spectrographs can provide excellent image quality. Some can discriminate three or four spectral channels over a 6-mm-tall focal plane. Interesting axial spectrograph configurations using prisms and lenses also offer excellent image quality over a few hundred nanometers in the visible range.

Some 30-cm or larger imaging spectrographs allow more than 10-channel analysis with a minimized channel overlap.

Final criteria: Stray light, design, focal length

Stray light relates mainly to the quality of the device’s optical components (mirrors and grating). The user is generally not aware of stray light or improper internal reflection, which can produce poor results.

Because of their slit/slit configuration, monochromators have less stray light or reentrant light than do spectrographs, which have no exit slit. However, when the stray light is important in an application, large-focal-length instruments, or double monochromators, are the best choice. Small devices present more risk of stray light than larger ones.

In terms of optical design, most large monochromators/ spectrographs use the asymmetric Czerny-Turner configuration. Smaller instruments tend to use an asymmetric “V” con-figuration as a compromise.

Weighing the trade-offs

No one device can cover all spectroscopic applications. However, a user who carefully analyzes the spectral and performance requirements of an application can weigh the tradeoffs involved in choosing between small- and large-focal-length monochromators and spectrographs.

If you need to analyse a short spectral range at low resolution, you can probably use an inexpensive, compact monochromator or spectrograph. Even if these devices have stray light, chemometric calibration methods can correct it without influencing the results. However, if you need high resolution, accuracy or versatility, large monochromators and spectrographs are often the safest buys. That is why these are generally the best instruments for research or high-technology industries.

HORIBA monochromators specifications

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Fluorescence Spectroscopy w A-TEEM for fast & precise wine authentication

​Wine is a multi-million dollar industry that is susceptible to fraud and the authentication of wines is an important aspect in the industry. Read more to find out how the team at University of Adelaide, South Australia uses fluorescence spectroscopy with A-TEEM, to achieve 100% accuracy in the classification of different wines according to its geographical origin.

Continue reading the rest of the article here.

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Water treatment using fluorescence spectroscopy & AI intelligence

Water treatment plants do not control the water quality of their source water. Rain events, temperature shifts and unique groundwater features create a lot of variability.

 
“For water quality, the cards they’re dealt is what they get,” he said. “It’s not like making oil and gas at an oil refinery, where they can control the raw materials. You have raw materials that you don’t have any control over.”

Temperature, turbidity, pH, alkalinity, hardness, and dissolved organic materials affect water quality. These variables come in different combinations and change over time. Treatment plants have to make treatment decisions on the back end and determine if they met multiple water quality targets that are chemistry driven.

The water sources are also changing due to climate.

“We get more intense rainstorms, so you get more turbidity (murkiness) coming in. You have different alkalinity now and the ability to manage these complexities is stressing the various plants’ technical capacities,” he said.

There are, however, chemicals and treatment processes operators can control, with multiple outputs and multiple objectives. Within limits, that is. For example, the objectives in the regulatory environment alone are increasing.

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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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Raman microscopy applied to Polymer characterisation

In the polymer field, Raman microscopy has become one of the most important characterization tools due to the large number of chemical and structural information which could be extracted from a single result. Thus, Raman microscopy can help from raw material characterization to genuine product
control, from synthesis process to defect investigation, covering the whole process of polymers manufacturing.

Continue reading the rest of the article from HORIBA here

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