Characterization

Liquid crystal materials are known to interact strongly with the polarized light. Their refractive index depends on the polarization and propagation direction of light and this is responsible of their birefringent property ^[110]. Polarization optical microscopy (POM), is a high sensitive, contrast-enhancing technique widely used in analyzing birefringent materials, and yields generally high quality images. In case of liquid crystal materials, this technique allows the observation of phase transitions of the mesophase. This is usually performed under in-situ conditions, with the analyzed specimen gradually heated up, on a host stage linked to the polarized microscope while the change in the mesophase texture is observed.

To observe the phase transition temperature of FPPTB, in-situ measurement was done, using a Zeiss Axio 100 POM microscope equipped with Linkam hostage controlled by a Linkam TMS 94 temperature and cooling stage. Measurement was done under an inert environment for the sample, mounted inside a sealed holder connected to a nitrogen flow and heated at the rate of 10 °𝐶/𝑚𝑖𝑛 and cooled down at a rate of 20 °𝐶/𝑚𝑖𝑛. For steady state measurement of FPPTB, 2,6-FFPTB & 2,3-FFPTB thin films, an Olympus U-CMAD3 microscope was used for analysis of the microstructure of thin films. In both cases, the microscopes were equipped with polarizers, positioned in the light path at the bottom of the sample, and analyzers (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes. To practically observe birefringence, the direct transmitted light going through the sample, was blocked with a polarizer oriented at 90 degrees to illumination.

Birefringence gives indication of large ordered domains in the thin films. With both

38 microscopes, images with maximum resolution of 10 𝜇𝑚 could be digitalized, and for the results discussed in this thesis, the selected resolution is 60 𝜇𝑚.

X-ray diffraction (XRD)

X-ray diffraction (XRD) is one of the fundamental experimental techniques used to analyze atomic structure of new materials. Typically, X-Ray diffraction occurs when a beam of monochromatic X-rays of wavelength λ, collide with crystalline atoms, resulting in a strong scattering which occurs in specific directions only. In the case of this study, X-rays are emitted by bombarding metal targets with high velocity electrons accelerated by strong electric field in the range of 40.0 kV and 40.0 mA, using a Cu K𝛼 source. While POM is used as a qualitative tool for analysis of LCs, XRD offers a more quantitative analysis giving information about the crystal structure of materials such as lattice parameters, or crystalline orientation and molecular packing on film.

In-situ measurements were performed by means of a solid anode X-ray tube from a X’Pert Pro PANalytical X-rays diffractometer, equipped with a high temperature chamber linked to a temperature controller unit TCU 1000N Anton Paar, as well as multiple stages (Eulerian Cradle, Sample spinner) for measurement at standard conditions. The steady state samples were annealed up to different phase transition temperatures, and subsequently cooled before XRD pattern being recorded in a (2scan configuration using an Eulerian cradle stage.

For in-situ measurements, the sample as spun, was placed in a hot stage, and mounted in a high temperature chamber, monitored with a temperature controller. Under an argon flow, the sample was gradually heated up or cooled down at the rate of 30 °C/min while the XRD pattern was recorded also in a (2scan at specific temperatures. For the data analysis discussed in this thesis, the software High Score plus, combined with Data Viewer from PANanalytical was first used for background subtraction, and the extraction of crystal lattice parameters such as the peak positions, the relative intensity of the reflections and the full width at half maximum (FWHM). The XRD patterns as displayed in the results section was plotted and treated using Origin 2016.

Optical Spectroscopy (Absorption and Photoluminescence)

UV–visible spectra in solution and on films were recorded at room temperature using a Cary 500 UV–visible spectrometer connected to a computer unit. Photoluminescence (PL) in

39 solution as well as on film was measured with a Fluorolog Horiba, Jobin Yvon linked to a computer unit and equipped with excitation and emission spectrometers, as well as an infrared compound, an iRH 320 spectrometer. Samples were usually excited at the respective wavelengths of 375 nm, 450 nm, and 530 nm. For more accuracy and minimization of scattering effects, the same measurements both for absorption and PL were repeated using analogous setups, equipped with an integrating sphere.

Impedance

For this work, impedance measurements were performed using a Metrohm-Autolab electrochemical setup (PGSTAT302N) equipped with a FRA32M module. This was used in the frequency range from 1 MHz down to 10 Hz for FPPTB and PTzs samples and from 1 KHz down to 10Hz for 2,6-FFPTB and 2,3-FFPTB samples, with an AC voltage amplitude 𝑉_𝐴𝐶 = 20 mV or smaller used to probe the device. The offset DC voltage applied 𝑉_𝐷𝐶, was usually in the range of 0 to 0.5 V in the step of 0.1 V. To avoid or to cancel any background noise, sample’s holder built as a Faraday cage for low frequencies, is connected with the instrument’s ground plug.

Data extraction for electrical analysis and equivalent circuit modelling were recorded and extracted using the software package NOVA-1.10.3.

Raman Spectroscopy

Raman measurements (transient and Steady-state) for this work, were recorded by Dr. Simon Boehme at the VU Amsterdam. I used Origin 2016 for plotting and analysis of data discussed in this thesis.

Current - Voltage (I-V) measurement

I-V measurements for the single carrier devices in dark were performed under a nitrogen environment inside a glove box using a Keithley 2400 source meter, connected to a computer interface and controlled by a LabVIEW program. Usually for this, a voltage range from -1 V to 1 V or -3 V to 3 V was applied to the diode and the resulting current was measured.

40 Simulations

All the calculations on the LC dyes used in comparison with the experimental results in this work were provided by Merck. These calculations include the HOMO/LUMO orbitals for all LC dyes, the transfer integrals for holes (J+) and electrons (J-) for the molecular pairs in the FPPTB crystal and simulated XRD pattern of FFPTB; the electrostatic potential maps of 2,3-FFPTB & 2,6-2,3-FFPTB as well as the simulated Raman spectra of 2,3-2,3-FFPTB and 2,6-2,3-FFPTB.

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