Finding the low-energy vibrational modes of polymer-fullerene blend using Raman spectroscopy On building a Raman setup to measure the low modes of regiorandom P3HT:PCBM and to determine the influence of post-deposition annealing

Daniël van Grinsven 10800662 Bachelor project Bèta-Gamma, major Physics: 15 EC Department of Physics and Astronomy VU University - Faculty of Sciences Conducted between 01-04-18 and 01-07-2018 Date of submission 28-08-18 Coordinators Elizabeth von Hauff

Freek Ariese Simon Böhme

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ABSTRACT A new type of solar cell has emerged, the organic solar cell. A very well studied combination of materials for an OPV cell is regiorandom P3HT and PCBM in a bulk heterojunction. The molecular structure and conformation of these materials has a big influence on the photovoltaic properties. To analyse these properties, Raman spectroscopy is used. The main goal of the research was to construct a Raman setup for measuring low energy vibrations. The setup was used to investigate the low energy modes in the Raman spectra of P3HT:PCBM as a function of annealing. By using an existing Raman setup the measurement parameters were improved. With the new setup it was possible to measure high energy Raman modes in the reference sample Paracetamol. However lower energy peaks were not observed. Still a lot of improvements need to be made on the setup for measuring the low modes of P3HT:PCBM, but since a Raman signal has been measured, a good foundation for further research has been established.

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TABLE OF CONTENTS ABSTRACT 2 CHAPTER 1 INTRODUCTION 4

1.1 Organic Photovoltaics 4 1.2 Raman Spectroscopy 7 1.3 Raman Setup 9 1.4 High & Low Frequency Region 11

CHAPTER 2 METHOD 12

2.1 Film Preparation 12 2.2 Absorption Measurement 14 2.3 High Frequency Raman Setup 14 2.4 Low Frequency Raman Setup 15

CHAPTER 3 RESULTS & DISCUSSION

3.1 Raman Spectrum Of P3HT Samples 16 3.2 P3HT:PCBM Samples 17

3.2.1 Absorption Spectrum Of P3HT:PCBM Samples 173.2.2 Raman Spectra Of P3HT:PCBM Samples 18

3.3 Low Mode Setup 21 3.3.1 Raman Spectrum Paracetamol 21

3.4 Optimization Raman spectra 23 3.4.1 Slit Width 23 3.4.2 Signal To Noise Ratio 24 3.4.3 Additional Explanation Of The Low Mode Setup 25 3.4.4 Bragg Filter Performance Check 26 3.4.5 Monochromatic Laser Source 28

CHAPTER 4 CONCLUSION 29 LITERATURE 30

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CHAPTER 1 INTRODUCTION 1.1 Organic Photovoltaics In the last two decades organic photovoltaic (OPV) cells have emerged on the chart of best research-cell efficiencies published by the National Renewable Energy Laboratory (NREL) in 2013. They have seen an increase in interest as an alternative for inorganic photovoltaics. Despite having lower energy conversion efficiencies than inorganic photovoltaics, they offer promising properties. These are lower production and installation costs, as well as flexibility and lightweightness (Mazzio, 2014). OPV cells are based on conjugated systems. Conjugated systems have alternating single and a double bonds between carbon atoms. Another characteristic of a conjugated system is that every carbon atom also needs to have a p-orbital available. P-orbitals contain one valence electron. In conjugated systems the p-orbitals overlap between the carbon atoms in the molecule, forming 𝝅-bonds. The 𝝅-bonds are delocalized over the molecule. An example would be a benzene molecule, shown in figure 1.

Figure 1: The chemical structure and π orbitals of benzene. (Source:Solar Energy: The physics and engineering of photovoltaic conversion, technologies and systems. UIT Cambridge Limited.) Two p-orbitals can form a molecular 𝝅-orbital. The HOMO and LUMO are the bonding/antibonding orbitals from the pi-orbitals. HOMO refers to the highest occupied molecular orbital and LUMO to the lowest unoccupied molecular orbital. The energy difference between the two levels is the bandgap of the system. When a photon with sufficient energy is absorbed by the system this results in the creation of a exciton, this is an electron-hole pair which is still in a bound state, due to the Coulomb force between the particles. If two conjugated systems with different HOMO and LUMO bands are brought together an interface is formed. An electrostatic potential is created due to the difference in the electron affinity. The electrostatic potential provides a direction for charge separation in the system, the electron will flow to the acceptor material, the material which has a lower LUMO

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band. The hole will stay behind in the donor material. This separation of the electron and hole at the acceptor-donor interface is shown in figure 2 (Jäger , 2016).

Figure 2: Illustrating the separation of electrons and holes in an exciton at the acceptor-donor interface. (Source: The physics and engineering of photovoltaic conversion, technologies and systems. UIT Cambridge Limited.) The difficulty with OPV cells is that the created excitons have a diffusion length in the order of only 10 nm. The thickness of the OPV cell is therefore strongly limited. To solve this problem bulk heterojunctions are used, in which the two conjugated systems form a blend and therefore get domain sizes equal to the diffusion length. A promising combination of conjugated systems for OPV cells is Poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM). Figure 3 and 4 show the chemical structures of P3HT and PCBM, respectively. In this combination P3HT operates as as the donor material and PCBM as the acceptor material. This combination of P3HT and PCBM is well studied, therefore a reference system for new measurements. In this research the combination between P3HT and PCBM as a bulk heterojunction will be investigated.

Figure 3: Schematic representation of P3HT and PCBM. (Source: https://www.sigmaaldrich.com/nederland.html) It is well-known that there is a link between the morphology of the active layer and the efficiency of an OPV cell (Campoy-Quiles , 2011). The conclusion was drawn that a higher degree of molecular order due to better 𝝅-𝝅 stacking, leads to a higher charge mobility, thus a better power conversion efficiency (Brown, 2003). In this research regiorandom (RRA) P3HT is used. The regioregularity of P3HT is defined as the ratio monomers which have a head-to-tail (HT) coupling relative to

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head-to-head (HH) coupling (Mazzio, 2014). In Figure 4 the two types of coupling are shown. The more HH couplings the P3HT polymer has, the less structured the material is. RRA P3HT has a 1:1 ratio of HT and HH couplings, therefore RRA P3HT has no tendency to crystallize. However, when PCBM is blended in with the P3HT, the PCBM fills the chain voids of the P3HT which leads to a higher degree of molecular order. Besides this, due to the interface which is formed between donor material P3HT and acceptor material PCBM the induced excitons in P3HT can now dissociate into electrons and holes. (Quiles , 2009). A way of further enhancing the crystallization is post-deposition annealing. By post-deposition annealing the substrate to temperatures above the glass transition temperature of the materials, the materials have time to reorient themselves into lower energy conformations.This means less entropy in the system, hence a more controlled structure. It was found that post-deposition annealing the combination P3HT and PCBM resulted in a bicontinuous network with crystalline structures on the nanometer-scale (Chen, 2011).

Figure 4: Schematic representation of the Head to Tail (HT) and the Head to Head (HH) couplings for P3HT. (Source: Mazzio, 2014) The morphology of possible combinations of these two materials need to be well understood for optimal application in potential OPV systems. One way of measuring the morphology of materials is through Raman spectroscopy.

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1.2 Raman Spectroscopy When light interacts with molecular matter, the incident photons interact with the material and are either absorbed or scattered. Absorption occurs when the incident photons equal the energy gap between the ground state and the excited state of the static molecule. This promotes the electrons or the molecule itself to an excited energy and vibrational state. This excited state is unstable and when the electrons lose energy in the shape of vibrational energy, they return to the excited singlet state. From there the electrons return to the ground state and will emit light, which is called fluorescence. Scattering occurs when the incident light promotes the molecule to a so-called virtual state. When the incoming light waves interact with the molecule, it causes the electrons to polarize and the electron geometry changes. Hereby energy is transferred from the incident light to the molecule. This brings the molecule to a high energy form, the so called virtual state. Because this virtual state is very unstable, the molecule relaxes and the energy is released immediately in the form of scattered radiation. The scattered radiation leaves the material in all directions. The changed electron geometry is different from any of the electron geometry found in the static molecule, therefore the electrons are not promoted to any of the excited states of the static molecule. The virtual state is therefore no real state of the static molecule. The energy of the virtual state is dependent on the energy of the incoming laser (Smith, 2013). There are two types of scattering, Rayleigh and Raman scattering, which are shown in figure 6. Rayleigh scattering is the most common scattering of the two. It occurs when the electron geometry changes without any nuclear geometry change. Rayleigh scattering is therefore an elastic process. Raman scattering on the other hand, is far less likely to occur and happens only with one in the - scattered01 6 01 8 photons. With Raman scattering, the nuclear geometry changes with the changed electron geometry. in this case the process is inelastic, some energy of the excitation energy gets transferred into vibrational energy. Therefore the energy of the scattered photon differs from the incident photon with one vibrational unit. When the molecule prior to interaction is situated in the ground vibrational state, the incident photon lifts the molecule to an excited vibrational state, which leaves the scattered photon with a lower energy than the incident photon. This is called “Stokes scattering”. On the other hand, if the molecule prior to interaction is situated in an excited vibrational state, the incident photon drops the molecule to its ground vibrational state. As a result the scattered photon has a higher energy that the incident photon. This is called “anti-Stokes scattering”. At room temperature molecules at rest are most likely to be in the ground vibrational state, therefore most of the Raman scattering is Stokes scattering (Smith, 2013) .

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Figure 5: Diagram of the Rayleigh and Raman scattering processes. (Source: Smith, E., & Dent, G. 2013) Raman spectroscopy measures Raman scattering from a sample by analysing the difference in the energy of the excitation source and the scattered radiation. This difference corresponds to the energy of vibrations of the material. Raman spectroscopy can therefore be used to assess which chemical bonds are located within the material. Because Raman spectroscopy relies on the polarization of the electron cloud, only symmetrical vibrations can be analysed. The energy difference between the excitation radiation and Raman scattering is also called “Raman shift” and is expressed in units of wavenumbers cm-1. The Raman shift is independent of the incident radiation, because it is an relative value. In a Raman spectrum the intensity of the Raman scattered radiation is plotted as a function of the Raman shift. Each peak in the spectrum correspond to a characteristic vibrational mode. The bigger the Raman shift, the bigger the energy of vibrations (Smith, 2013). Two examples of bonds for P3HT that could be found due to Raman spectroscopy are the C=C intramolecular bond, which is situated around 1450 cm-1 and the C-C intramolecular bond in the 1350-1400 cm-1 region (Tsoi, 2011). What already was mentioned earlier is that post-deposition annealing enhances the crystallization of P3HT:PCBM samples. This change in morphology can also be observed in the Raman spectra. Post-deposition annealing the P3HT:PCBM samples could have an influence on their Raman spectra in the following 3 ways, which indicate a more ordered structure of the P3HT:PCBM samples (Tsoi, 2011):

1. The C=C intramolecular bond in P3HT will shift to a lower Raman shift. 2. The FWHM of the C=C intramolecular bond in P3HT will decrease. 3. The ratio between the C-C intramolecular bond relative to the C=C

intramolecular bond increases.

With Raman spectroscopy the distinction is made between resonant and non-resonant Raman spectroscopy. Resonant Raman spectroscopy is when the wavelength of the excitation laser coincides with and absorbance band (of the

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material which is investigated). Therefore vibrational modes related to the electronic transition are enhanced, which lead to more Raman scattering to the non resonant Raman scattering, where the wavelength of the excitation laser is not in the absorbance spectrum of the material. However, with resonant Raman spectroscopy background fluorescence becomes a problem. The fluorescence contaminates the Raman spectrum because its intensity is much higher than that of Raman scattering. To avoid this contamination, non-resonant Raman spectroscopy could be used (Yanga, 2013). The disadvantage of using a longer wavelength is that the efficiency of the Raman scattering is proportional to the inverse fourth power of the wavelength (Smith, 2013). As a result, a longer wavelength will lead to a lower amount of fluorescence but also to a lower efficiency of Raman scattering. 1.3 Raman Setup To measure a Raman spectrum a Raman setup is used. An example of a Raman setup is shown in figure 8. For this setup a monochromatic laser beam is used. It is important that the laser beam is monochromatic because therefore the incoming light can be filtered out very precisely and, since the Raman shift is relative to the energy of the incoming light, monochromaticity is important to achieve sufficient spectral resolution. A neutral density filter can be used to weaken the excitation laser. It also prevents damaging the charge-coupled device (CCD) due a too high laser intensity. The CCD will be discussed later on in this section. From here, the beam is directed to a 50/50 beam splitter. The beam splitter transmits 50% of the incoming light and reflects the other 50% in a 450 angle. The beam splitter therefore needs to be placed at an 45⁰ angle with the incoming laser light. The reflected light hits an objective or microscope, which focuses the light on the sample. Due to the objective/microscope the light is focussed on a smaller area of the sample, which increases the intensity of the scattered radiation. The advantage of using a microscope is that you can collect more signal and precisely determine of which part of the sample you want the laser light to be focussed on. The scattered light coming of the sample will go through the objective and passes the 50/50 beam splitter for the second time and hits a notch filter. This filter will leave most of the wavelengths unaltered, but attenuates the transmittance of the wavelength region of the laser beam by reflecting it. Hereby attenuating the Rayleigh scattering that overshadows the Raman signal. Only if collimated scattered light interacts with the notch filter, it will attenuate the Rayleigh scattering. The transmitted light is eventually focussed with a lens on the entrance slit of the monochromator. The function of a monochromator is to disperse the incoming scattered light. First, the incoming scattered light hits a mirror which collimates the light. A diffraction grating is installed inside the monochromator. The grating consists of many closely spaced grooves. When the collimated light hits the grating it acts like a multi-slit source. Because each incoming wavelength undergoes constructive interference at

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a different angle, the wavelengths get spatially dispersed. This is also shown by formula 1, where the angle of the reflected light is dependent of the angle of theβ incoming light , the wavelength of the incoming light and the groove density d.α λ The dispersed light can eventually leave the exit slit of the monochromator. By choosing a grating rotational angle, a light with a restricted wavelength range can pass the exit slit. The groove density d determines the width of this wavelength band (Ingle, 1988).

in(β) in(α)s = λ

d − s (1) Attached to the monochromator is a CCD. The CCD is a two dimensional array of pixels. Each pixel can be seen as a well. When electrons within the CCD get excited by incoming photons coming off the excitation laser, these excited electrons end up in the pixel well. Every photon can excite one electron. These electrons get accumulated over the measurement time and converted to a digital number, referred to as counts. The dispersed light is projected onto the pixels that are ordered on the horizontal axis. The first pixel detects light with the lowest Raman shift, and the last pixel the highest Raman shift of the spectrum. With the use of full vertical binning all the light on the same horizontal pixel gets added up (.The monochromator and CCD are connected with a computer which projects the Raman spectrum. However, the Raman spectra are disturbed by a number of external factors. These disruptions can be broken down into two general categories: dark current and noise. Dark current is an internal disruption observable within the CCD and originates from thermal excitation of electrons. Even when the slit of the monochromator is closed and no scattered light falls onto the CCD, the electrons in the CCD may become excited. These excited electrons form a dark current (Belloir, 2016). In attempts to mitigate experimental disruption from the dark current the CCD is cooled. Despite attempts at cooling the CCD, Dark Current continued to generate a disruption termed dark noise. Dark noise represents one of three sources in the second category of disruption, noise, which appears in the Raman spectra as random fluctuations over time. The second noise source is a stable light source in the form of the excitation laser. Stable light sources generate fundamental noise which is formally called shot noise. The third noise source stems from the CCD. The CCD generates persistent unavoidable background noise during read out, this noise is termed read-out noise. Read-out noise is a fixed noise which is independent of the signal intensity. For large acquisition times this read-out noise is insignificant, but the noise increases to significance as exposure times decrease, because the ratio of the read-out noise to the intensity of the Raman signal increases (Source: www.Ibsen.com).

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An important aspect of Raman spectroscopy is the signal to noise ratio (S/N ratio). This is the ratio of the absolute intensity of a Raman peak to it’s total noise. The relevance of this S/N ratio is shown in the fact that a larger S/N ratio provides a clearer Raman spectrum and therefore more meaningful results. 1.4 High & Low Frequency Region In Raman spectroscopy generally a distinction is made between two Raman shift regions, the low frequency region and the high frequency region. In this thesis the arbitrary border between low and high frequency regions will be set at 200 cm-1. The low frequency region is more difficult to detect because this region is close to the wavelength of the intense excitation laser. Therefore very sharp filters are needed to block out the excitation laser very precisely. The goal of this research is to build a low mode setup to find the low frequency region of P3HT in combination with PCBM. The research question holds:

Can we use low mode Raman spectroscopy to investigate the effect of annealing on organic semiconductors?

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CHAPTER 2 METHOD 2.1 Film Preparation Regiorandom P3HT and PCBM are both purchased from Aldrich. The P3HT has a 1:1 ratio of HH:HT couplings. All these materials are used as received and the sample processing is done in a nitrogen glovebox. Glass substrates are used. They are first cleaned in the Ultrasonic bath, with Milli-Q water (10 min), with Acetone (10 min) and Isopropanol (10 min). Next, the glass substrates are put under the UV-ozone cleaner (20 min). In this research two independent batches of samples are made, 12 samples in total. The first batch consists of 16.9 mg regiorandom P3HT dissolved in 1.12 ml dichlorobenzene. The solution is placed on a stirring hot plate of 60⁰C for 48 hours. Next the solution is distributed over 6 glass substrates. These are then spin coated (Polos spin coater). The used duration of the spin coating process is 30 s and consists of 2 steps. The used speed for step 1 is 800 rpm with an acceleration of 500 rpm/s , with a duration of 30 s . The second step has a speed of 4000 rpm and an acceleration of 1200 rpm/s, with a duration of 20 s. Half of the samples are post annealed at 140 ⁰C for 10 min. The second sample batch consists of a mix between regiorandom P3HT and PCBM. 16.8 mg P3HT with 8.4 mg PCBM (1:2) and are dissolved in 1.12ml dichlorobenzene. Like the first batch, the solution is then placed on the stirring hot plate at the same temperature as the first batch and then spin coated on 6 glass substrates. This time the speed of step 1 is changed, to achieve a thicker material on the substrates. The subsequent steps stay the same. The spin speed in step 1 for 2 samples was changed to 600 rpm with an acceleration of 400 rpm, and for 2 other samples the spin speed was changed to 500 rpm with an acceleration of 300 rpm. The last two samples are dropcasted and dried. Per set (two samples) one is post annealed at 140 ⁰C for 10 min. The samples are named RRA 1,2,3,4,5,6 respectively and are shown in figure 6. Table 1 summarizes all the different preparation parameters used for the second sample batch.  

 

 

 

 

 

 

 

 

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RRA1: - spin coating: 600 RPM w/ 400 RPM/s acceleration for 30 seconds.

RRA 3: - spin coating: 500 RPM w/ 300 RPM/s acceleration for 30 seconds.

RRA 5: - dropcasted

RRA 2 - spin coating: 600 RPM w/ 400 RPM/s acceleration for 30 seconds. - annealed on 140 degrees Celsius for 10 minutes

RRA 4: - spin coating: 500 RPM w/ 300 RPM/s acceleration for 30 seconds. - annealed on 140 degrees Celsius for 10 minutes

RRA 6: - dropcasted - annealed on 140 degrees Celsius for 10 minutes

Table 1: of the different preparation parameters used for the RRA P3HT:PCBM samples

Figure 6: Photo of the RRA P3HT:PCBM samples. The top samples are non-annealed and the bottom samples are post annealed. From left to right: Spin coating speed: 600 RPM, 500 RPM and drop casted.

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Figure 7: Side shot of the RRA P3HT:PCBM samples. The left sample is the RRA 2 sample, and the right sample is the RRA 4 sample. 2.2 Absorption Measurement Absorption spectra were recorded using a UV visible spectrophotometer (Varian). 2.3 High Frequency Raman Measurements The pure P3HT and P3HT:PCBM samples are investigated under an existing Renishaw Raman setup. This setup is linked to WiRE software. A Leica microscope is attached to the setup. In all cases the measurements are done with an invictus 532 laser, an 1800 l/mm grating, a 50x objective and 1% of the laser intensity. The CCD is cooled at 550C. For the P3HT samples an acquisition time of 1 s is used with 10 accumulations. For the P3HT:PCBM also an acquisition time of 1 s is used, but with only 1 accumulation. All measurements are done in a dark room to avoid any additional ambient light entering the slit of monochromator. The Renishaw Raman setup gives a Raman spectra in the high frequency region.

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2.4 Low Frequency Raman Setup figure 8 shows a schematic representation of the low mode setup. An Invictus 532 laser (1) is used. A neutral density filter (2) is placed to decrease the intensity of the excitation laser signal up to a factor of 104 if needed. Two mirrors (4) are placed to direct the light beam to the height and angle of the slit of the monochromator. the 50/50 beam splitter (5) reflects 50% of the incoming light in a 450 angle. The sample (7) is placed vertically upward in the focal length of the 12x objective (6). Two BragGrate notch filters (8) are installed (Optigrate). If placed at an angle of 6.1 degrees, both filters decrease the intensity of the excitation laser by 104. This theoretically allows us to reach Raman signal 5 wavenumbers (cm-1) from the excitation laser. Pinholes (3) are placed through the setup to block all non collimated light that will fall onto the notch filter. A lens (9) with a 5 cm focal length focuses the scattered light into the Andor Shamrock monochromator (10), which contains three gratings: 300, 1200,2399 l/mm. An Andor newton CCD (11) with 1024x1024 pixels is attached to the monochromator and is operated at -50⁰C all all times. Andor Solis software projects the Raman spectrum on the computer.

figure 8: Schematic representation of the low mode setup. 1:Laser beam, 2:neutral density filter, 3: pinholes, 4: two Mirrors, 5: 50/50 beam splitter, 6: Objective, 7: sample, 8: notch filters, 9: lens, 10: monochromator, 11: CCD.

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CHAPTER 3 RESULTS AND DISCUSSION 3.1 Raman Spectrum Of P3HT Samples Before using the low mode setup, the existing Renishaw Raman setup was used to measure if RRA P3HT would give a useful Raman spectrum in the high frequency region. If this is not the case, it is not useful to analyse them under the low mode setup. Unfortunately only the post annealed samples gave a weak Raman signal relative to the strong background. With the non-annealed samples no signal could be distinguished from the noise. This is probably due to a too high spin coating speed in step 1, which caused the solution to spin off the glass substrate. The spectrum of the post annealed P3HT sample is shown in figure 9. The Raman peak is weak relative to the strong background. The Raman peak is situated at a Raman shift of around 1450 cm-1. This peak is attributed to the C=C intramolecular bond of P3HT (Tsoi, 2011). Figure 9 shows a large amount of background fluorescence, Which builds up with increasing Raman shift. The large amount of background fluorescence can arise due to the absence of an acceptor material like PCBM, which dissociates the excitons in electrons and holes. Due to the this absence the excitons fall back to their ground state and emit fluorescence. The reason why this background fluorescence increases with increasing Raman shift is because it follows the shape of the PL spectrum of P3HT.

Figure 9: Raman spectrum of post annealed RRA P3HT sample. An Invictus 532 nm excitation laser is used.

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The conclusion was made that the pure P3HT samples gave a weak Raman signal relative to the high amount of fluorescence in the spectra. Therefore new samples are made containing PCBM, in the hope the PCBM will lower the fluorescence in the spectra by dissociating excitons in electrons and holes. 3.2 P3HT:PCBM Samples 3.2.1 Absorption Spectra Of P3HT:PCBM Samples Optical absorption spectroscopy is used to detect the absorption spectrum of the P3HT:PCBM samples. The normalized absorption spectrum are shown in figure 10. The normalized absorbance is plotted as a function of the wavelength. The absorbance is normalized relative to a blank sample, a clean glass substrate. Only the spin coated samples were used for these measurements, because a high transmission is required. The green vertical line indicates the wavelength of the excitation laser, situated at 532 nm. As shown in figure 10, the excitation wavelength lies in the tail of the absorbance spectra of the P3HT:PCBM samples. This implies that we are partially in resonance during the Raman measurements.

Fig 10: The normalized absorption spectra of the RRA P3HT:PCBM samples RRA 1,2,3,4.

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3.2.2 Raman Spectra OF P3HT:PCBM Samples After obtaining the absorbance spectra of P3HT;PCBM, we analysed the P3HT:PCBM samples under the existing Renishaw Raman setup to measure signal in the high frequency region and to observe if post-deposition annealing indeed influences the morphology of the samples. For all the measurements an acquisition time of 1 s is used with 1 accumulation. 1% of the intensity of the laser light is used. First 10% of the intensity of the laser light was used, but that damaged the samples. Visible burned marks were found on the samples. The Raman spectra of the P3HT:PCBM samples are shown in the figures 13,14 and 15. The spectra are all normalized. Table 4 summarizes the 3 characteristics to quantify the degree of molecular order of the P3HT:PCBM samples relative to each other. By comparing these parameters between the P3HT:PCBM samples we can determine if the post annealed samples indeed have a higher degree of molecular order, compared to the non annealed samples. First, the samples RRA 1 and RRA 2 are compared to each other. The FWHM of the C=C bond for sample RRA 2 has decreased by 4 cm-1 relative to sample RRA 1. Also the ratio C-C/C=C has increased from 0.00335 to 0,04 for the post annealed RRA 2 sample. The FWHM of the RRA 4 sample has increased by 2 cm-1 with respect to the non-annealed RRA 3 sample. This could be due to impurities of the RRA 4 sample, or by not properly focussing the Leica microscope on the material. The Ratio C-C/C=C increases with 0,01 for the RRA 4 sample. For the drop casted samples, the post annealed RRA 6 sample shows a decrease of 1 cm-1 in the FWHM and an increase of 0,02 in the C-C/C=C ratio relative to the RRA 5 sample. For the C-C/C=C ratio an overall change has been observed, which implies a more ordered structure of the P3HT:PCBM samples. But no significant changes have been observed for the peak position of the C=C bond. Also the increased FWHM for the RRA 4 sample relative to RRA 3 indicates that post-deposition annealing does not change the molecular structure and conformation of the P3HT:PCBM samples.

RRA1 RRA2 RRA3 RRA4 RRA5 RRA6

peak position C=C (cm-1)

1465 1466 1465 1461 1465 1465

FWHM (cm-1)

44 40 38 40 39 38

Ratio C-C/C=C

0,00335 0,04 0,04 0,05 0,04 0,06

Table 4: The 3 characteristics to quantify the degree of molecular order of the P3HT:PCBM samples.

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Figure 13: Normalized Raman spectra for the RRA1&2 P3HT:PCBM samples.

Figure 14: Normalized Raman spectra for the RRA1&2 P3HT:PCBM samples.

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Figure 15: Normalized Raman spectra for the RRA1&2 P3HT:PCBM samples. To observe the low modes for P3HT:PCBM using the low mode setup, first a control sample of paracetamol is used to verify the functioning of the low mode setup.

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3.3 Low Mode Setup 3.3.1 Raman Spectrum Paracetamol The measured Raman spectrum of the control sample paracetamol using the low mode setup is shown in figure 17. In figure 16 the Raman spectrum of paracetamol in the 1300-1800 cm-1 region is shown. The observed Raman peaks situated in the high frequency region, correspond to the C-N-H, the C-C and the C=O bonds. As shown in figure 20, the Raman signals are still very weak relative to the high intensity of the laserlight still passing the filters. Therefore no signals are observed in the low frequency region.

Figure 16: Raman spectrum of paracetamol in the 1300-1800 cm-1 range measured by the low mode setup. The observed Raman peaks correspond to the C-N-H, the C-C and the C=O bonds.

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Figure 17: Raman spectrum of paracetamol in the 0-2000 cm-1 range measured by the low mode setup.

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3.4 Optimization Raman Spectra Before measuring the Raman spectra of the P3HT:PCBM and the control sample paracetamol, some pre-work was done with an Argon source. This was to optimize the measurement parameters, thereby optimizing the measured Raman spectra. Besides this, some extra explanation on optimizing the components in the low mode setup is given, which further enhanced the Raman spectra. 3.4.1 Slit Width First of all, an Argon source is used to measure the influence of a changing slit width of the monochromator on the signal intensity and Full width half maximum (FWHM) of the Argon spectrum. The highest peak of the Spectrum is used. This peak is situated at 415,86 nm. The measurements are done for the slit widths 10 until 100 μm in steps of 10 μm. The grating 2399 l/mm is used with an acquisition time of 0.2 s. The results are shown in table 2. The conclusion can be made that the signal intensity is proportional to the slit width. This is because more light enters the monochromator and gets detected by the CCD. The FWHM however decreases when the slit width increases. A smaller slit width gives a sharper peak in the spectrum. The slit widths 10 till 30 μm have the same FWHM. However the signal intensity still decreases with a decreasing slit width. Therefore it is not useful to decrease the slit width under 30 μm because it will only worsen the spectrum. By analysing these results a slit width of 80 μm is chosen for the measurements.

slit width (μm) peak counts FWHM (cm-1)

10 3051 3,47

20 5514 3,47

30 7519 3,47

40 8804 4,62

50 9930 4,62

60 10602 5,20

70 11867 5,78

80 12709 5,78

90 13185 7,51

100 13325 8,09

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Table 2: The influence of a changing slit width of the monochromator on the signal intensity and FWHM. An argon source is used for these measurements. The measurement are done for the slit widths 10 until 100 μm in steps of 10 μm. A grating of 2399 l/mm is used, with an acquisition time of 0.2 s.

3.4.2 Signal To Noise Ratio Secondly, the Argon source is used to analyse the S/N ratio and how this ratio is related to the number of accumulations used in a measurement. To observe how these parameters are related, three measurements are done with each a different number of accumulations. To compare the measurements with each other the same measurement time is used. The following three measurements are done:

1. 1 accumulation of 1 s 2. 10 accumulations of 0.1 s 3. 100 accumulations of 0.01 s

The results are shown in table 3. An increase from 1 accumulations to 10 and 100 accumulations leads to a 7,7 and 75 times higher baseline respectively, while the corrected peak intensity stays unchanged. Therefore the S/N ratio for 10 and 100 accumulations decreases to 73% and 26% from the original S/N ratio, respectively. This worsened S/N ratio is due to the read-out noise that increases to significance due to the low acquisition time. Therefore, 1 accumulation is used for every measurement of the P3HT:PCBM samples, to contain the best S/N ratio.

1 x 1 sec 10 x 0.1 sec 100 x 0.01 sec

peak 8262 13197 63925

baseline 750,7364341 5806,244186 56381,81008

corrected peak 7511,263566 7390,755814 7543,189922

STDev baseline 9,831409448 13,22583765 38,49154207

S/N ratio 764,0067893 558,8119261 195,970063

baseline relative change 1 7,734064743 75,1020032

S/N relative change 1 0,7314227229 0,2565030386 Table 3: S/N ratio measured for an Argon source. Three measurements are done with the same measurement time, but different number of accumulations. After using these chosen parameters for our measurements we came to the conclusion that we perhaps made the wrong decision. By increasing the acquisition time to 1 sec, the read-out noise will be insignificant. The increase in accumulations would then not worsen the S/N ratio.

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3.4.3 Additional Explanation Of The Low Mode Setup To key point in building the low mode setup is to optimize every component at the height of the slit, which is 12,5 cm. Thereby the scattered light could precisely fall into the slit of the monochromator. To check if the 50/50 beam splitter was optimized on the height and angle of the slit, the laser light coming of the 50/50 was then projected onto the wall a few meters away, to determine if no divergence occurred. There was some trouble optimizing the objective. At first a 16x objective was used, but the scattered light coming of the objective kept on giving strange interference patterns. Therefore we changed the objective to a 12x objective, which solved the problem. To install the Optrigrate notch filters we first placed them at an angle perpendicular to the incoming laser light. Then we changed the angle until a shadow band appeared in the transmitted light. Using the x,y,z component the notch filters could be adjusted so that the shadow band could be straightened. The lens was optimized by using a liquid that gives high amount of fluorescence in the yellow region. By using the x,y,z components of the lens the intensity of the fluorescence peak could be optimized. This gives a good indication that the maximum amount of scattered light falls through the slit, onto the CCD. At first, the low mode setup provided no Raman signal using the paracetamol sample. This was due to the fact that still a lot of laser light signal passed the slit onto the CCD, which clouds the Raman spectra. Further troubleshooting was required in order to remedy the Raman signal. Extra pinholes are placed to block the non-collimated light even more. Also the optimal angle of the notch filters was precisely determined. Neutral density filters are used to decrease the intensity of the excitation laser, so that the Central width (CW) of the Raman spectrum could be changed to the wavelength of the excitation laser. Because of this the signal of the excitation laser could be projected in the spectrum with no chance of damaging the CCD. By adjusting the angle of the notch filters the intensity of the excitation laser could be minimized. Also by adjusting the tilt of the notch filters, using the x,y,z components, this intensity could further be minimized. we also isolated the notch filters and laser source in attempt to find the source of the issue.

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3.4.4 Bragg Filter Performance Check First the notch filters are isolated to check if their performance corresponds to the product specifications given by Optigrate. In figure 18, obtained from the Optigrate site, the filter transmission curve of the Braggrate notch filter is shown. In figure 18, The y-axis (log-scale) should read ‘Transmittance’ instead of ‘Optical density’. This figure shows that if the notch filter is placed at the optimal angle, namely 6.1 degrees, the transmittance of the wavelength of the excitation laser drops to less than 0,001 of its original transmittance. The angular selectivity of the notch filter is stated to be 0.2 degrees, which makes this filter a very sharp filter.

Figure 18: Product specifications of the Braggrate notch filters used in the low mode spectrum. Source: http://www.optigrate.com/product_briefs/Low-frequency-Raman-filters-OptiGrate.pdf) We examined if this theoretical performance of the notch filters corresponds to the observed performance of the notch filters installed in the low mode setup. By using the neutral density filters, the CW is placed on the wavelength of the excitation laser, so that the signal of the excitation laser is visible in the spectrum. The transmittance is calculated for an angular displacement of 0 degrees up to 0,50 degrees, in steps of 0,02 degrees. An angular displacement of 0 degrees corresponds to the notch filter being placed on the optimal angle. The observed performance of the notch filters are shown in the figures 19 and 20. Table 5 summarizes the theoretical performance and observed performances of the installed notch filters.

Lowest transmittance

Relative Change degrees T=1

relative change

OptiGrate 0,001 1 0,2 1

Bragg filter 1 0,055 55 0,38 1,9

Bragg filter 2 0,026 26 0,44 2,2

Table 5: Theoretical and observed performance of the Braggrate notch filters.

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Figure 19: Transmission spectrum of first Optigrate notch filter (532 nm) used in the low mode Raman setup.

Figure 20: Transmission spectrum of second Optigrate notch filter (532 nm) used in the low mode Raman setup.

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As summarized in table 5, When placed at the optimal angle, the first and second notch filter resulted in a 55 times and 26 times higher transmittance with respect to the theoretical performance, respectively. Besides this, the observed spectral selectivity of the notch filters is about two times as broad with respect to the theoretical spectral selectivity. The observed performance of the notch filters installed in the low mode setup does not match the product specifics given by Optigrate. There are some possible explanations for this disparity in performance. This disparity can be caused by the alignment of the laserlight. If this is not perfectly done, non-collimated light will form. Non-collimated laser light could pass the notch filters and fall onto the CCD. This disparity can also be caused by a production error of Optigrate, which of course will be unusual. This installed notch filters decrease the transmittance of the excitation laser to 0,14%. Taking into account that the Rayleigh intensity is 106 to 10 8 higher than the Raman scattering, this decrease in transmittance will be not enough to measure a worthy Raman spectrum. 3.4.5 Monochromatic Laser Source Another source could be the monochromatic laserlight. The notch filters used in the low mode setup have a spectral selectivity of <0.3 nm. So the laserlight must be at least as narrow as this spectral selectivity to be filtered out. To check if this is the case for the laserlight which is used in this research, the FWHM of the laserlight was measured for the three gratings, with each a different spectral resolution. This measurement is done with two slit widths, namely 10 μm and 80 μm. The results are summarized in table 6. The FWHM of the laserlight signal obtained by grating 2 and 3 and for both slit widths are all smaller than the spectral selectivity of the notch filters. This is not the case for grating 1, there the FWHM is around 0.15-0.20 nm higher than the spectral selectivity, However the spectral resolution of grating 1 equals the spectral selectivity of the notch filters. There can be concluded that the laserlight is monochromatic enough to be filtered out by the notch filters.

Grating 1 (10 μm)

Grating 1 (80 μm)

Grating 2 (10 μm)

Grating 2 (80 μm)

Grating 3 (10 μm)

Grating 3 (80 μm)

FWHM (nm)

0,4670 0,49446 0,176694 0,1703835 0,0528 0,1452

Table 6: FWHM of the Invictus 532 nm laser for the 300,1200 and 2399 l/mm gratings. Two slit widths are used: 10 μm and 80 μm.

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CHAPTER 4 CONCLUSION In this research a low mode Raman setup was built to measure the influence of annealing on the low modes of RRA P3HT:PCBM. First pure P3HT samples were used, but due to high fluorescence in the spectra we altered the sample preparation by adding PCBM to the solution and decreasing the spin coating speed. For the P3HT:PCBM samples high mode peaks were found using the Confocal Raman setup. No significant change was found due to post-deposition annealing. With the low mode setup only weak high mode peaks of the C-N-H, C-C and C=O bonds were observed for the reference sample paracetamol. This weak signal is due to the high amount of laser light intensity that still enters the slit of the monochromator, clouding the spectrum. This could be due to the mismatch between the observed and the theoretical performance of the notch filters. Another reason could be the imperfectness of the outlining of all the components, causing non-collimated light to interact with the filters. No Raman signal could be detected using the low mode setup. A future improvement that could be made is installing a bandpass filter, which gives an assurance of a monochromatic excitation laser. Furthermore, in future research a laser light with a lower wavelength could be chosen, which lies more in the absorption spectrum and less in the fluorescence region of RRA P3HT:PCBM. Therefore reducing the amount of fluorescence in the spectrum. Also the signal to noise measurement done in this research could be repeated, but now with a constant acquisition time of 1 s. To check if accumulations indeed worsen the S/N ratio. More pinholes could be placed to reduce the amount of non-collimated light even more. Lastly, the stability of the component holders could be improved, to maintain the quality of the spectra. Unfortunately the end goal of this research is not reached, but a good foundation for further research has been established.

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Websites https://www.nrel.gov/pv/assets/images/efficiency-chart.png (Data accessed: August 2018: ) http://www.optigrate.com/product_briefs/Low-frequency-Raman-filters-OptiGrate.pdf (Data accessed: July 2018:) http://www.optigrate.com/Raman/Low-frequency-THz-Raman-filters-OptiGrate.pdf (Data accessed: July 2018:) https://ibsen.com/technology/detector-tutorial/noise-in-detectors/# (Data accessed: July 2018:) https://www.utsc.utoronto.ca/~traceslab/Raman%20correlation%20table.pdf (Data accessed: July 2018:)

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