Structure-Property Interrelations in Non-Crystalline Semiconducting Polymers for Organic Electronics

1 Structure-Property Interrelations in Non-Crystalline Semiconducting Polymers for Organic Electronics May 2024 Written by Nicolás Ramos Gómez Structure-Property Interrelations in NonCrystalline Semiconducting Polymers for Organic Electronics A dissertation submitted to the University of the Basque Country (UPV/EHU) presented by Nicolás Ramos Gómez In partial fulfillments of the requirements for the degree of DOCTOR IN APPLIED CHEMISTRY AND POLYMERIC MATERIALS Research conducted under the supervision of Dr. Jaime Martín Pérez (UPV/EHU) Donostia, May 2024 (cc) 2024 Nicolás Ramos Gómez (cc by-nc-sa 4.0) I Iturri zaharretik edaten dut, ur berria edaten, beti berri den ura, betiko iturri zaharretik. De la vieja fuente bebo, bebo el agua nueva, el agua que siempre es nueva, de la fuente que siempre es vieja. Joxean Artze I Acknowledgements I want to thank all the people that stay with me during this years and support me in this new knowledge field for me. The made me improve myself and expand my research career. This thesis it is not only the fruit of my work, it is made thanks also to all the people that was around me help me in and outside the job. En primera instancia querría dar las gracias a mi director Jaime Martín, ya que entre los dos hemos sacado este trabajo adelante a pesar de todas las inclemencias debido a los cambios producidos durante estos años por la pandemia, traslados, etc. Ha sabido darme espacio pero también meterme caña cuando era necesario. También quiero dar las gracias al resto del grupo. Muchas gracias a Sara por acogerme y ayudarme cuando llegué a una ciudad completamente nueva y empecé en un campo totalmente diferente a todo lo que yo había estudiado. Gracias a ella le cogí el gustillo a las noches de sincrotrón y a tratamiento de datos con el maldito Fit2D. Por otro lado quería darle las gracias a mi postdoc Edgar. Entró casi igual de perdido que yo en la técnica de flash DSC pero conseguimos entenderla y buscarle truquitos juntos. Nos mejoró mucho la calidad de vida automatizando procesos tanto del flash como del sincrotrón poniendo en peligro su pelo al hacer una rasta inmensa! También quiero darle las gracias a Valentina por estar ahí y apoyarnos mutuamente en los peores momentos y ayudarme a centrarme y no dispersarme. Gracias también a Daniele Cangialosi y Valerio, que desde el CFM me han enseñado lo complicado y bonito que puede llegar a ser el estudio de la fase vítrea de los polímeros. Gracias también a todos mis compañeros de la universidad y de polymat por acogerme cuando quedé huérfano de jefe aquí en Donosti. Gracias a Elena, Miryam, Marta, Xabi, Ainhoa, Paula, Aritz, Giulia, Lucas, los Iones, Emelin, Fermín, Álvaro… Gracias a todos los que han estado y están ahora por todos los buenos momentos que hemos pasado y nos quedan por pasar!! También a David por acogerme en su grupo y enseñarme a trabajar happy pero también hard. A todo el personal de la de la universidad como Maite o el personal de la cafetería y de limpieza que hacen que todo funcione como un reloj suizo. Muchísimas gracias a mi novia Raquel que ha estado estos últimos dos años aguantándome y soportando todas las horas que he echado en casa haciendo la tesis. Ha conseguido que me sienta en San Sebastián como en casa y ha sido mi fuente constante de apoyo y aliento a lo largo de esta ardua travesía académica. Este logro no solo es mío, sino nuestro, y quiero expresar mi gratitud por su amor incondicional, su apoyo inquebrantable y su contribución invaluable a mi vida, tanto personal como académicamente. Gracias por todo (menos por ayudarme en la tesis jejeje) a todos mis amigos y familia tanto de Salamanca, como mis abuelos Abue y Cani, mis tías Sonia y Belén y las primas. De Torrecaballeros a todos (que sois muchos) y los amigos de mi peña. También a los de toda la vida con los que he compartido todos los años de mi vida y mi educación Desde la infancia como Mateo, Diego, Javi, Ana… hasta la universidad y el master como Carlitos, Ana, Eva, Samu, Maya, Silvia, Andres, Rulo, Pretus, Ferni… Y también a los nuevos de San Sebastián como Gabriel, Luna, David y Álvaro y mis colegas del Atlético de San Sebastián. Y por último, muchas gracias a toda mi familia en especial a mis padres Jose Manuel y Susana y a mis hermanos María y Miguel. En vacaciones me han echado la bronca y me han hecho avanzar en la tesis cuando menos me apetecía. Nos hemos ido de viajes y me han criado y han conseguido un (mini)doctor en la familia. III Summary Nowadays, all the semiconductor industry is governed by the silicon technology. The increasing of the energy consumption and the use of electronic devices force to develop and found new materials with less energy consumption, less harmful and new properties. In the last decades the organic semiconductors (OSC), including semiconducting polymers, appear as a new, promising alternative. Flexible and transparent devices can be developed with these new materials. The scalability and the easy process from solutions compared to molecular beam epitaxy (MBE) or atomic layer deposition (ALD) used for the common semiconductor devices make the most important difference and make the OSC as a new promising technology. However, organic electronic technologies are still at their initial steps and the community is still focused on understanding these important materials. This thesis presents 3 timely studies on materials science aspects of polymeric semiconductors. The idea is to understand and control the morphology of the polymers to improve and tune their optoelectronic properties. The first chapter (chapter 3) deals with the study of the active layer of an organic solar cell. The glass transition of the bulk heterojunction is studied to determine the composition of the intermix phase on the active layer. The second chapter (chapter 4) is focused on the study of the solid-state microstructure of the high-mobility polymer IDTBT, which has been claimed to be “nearly amorphous” polymer and thus confronts the general idea that good electrical properties stem from high crystallinity. In the third experimental chapter (chapter 5) I stablish that, like crystalline regions, glassy regions can be also manipulated to optoelectronic properties in semiconducting polymers. IV 3 1) Introduction A semiconductor is a material that is capable of acting as a conductor or insulator depending on the external stimuli. Normally the temperature, pressure, radiation, magnetic or electric field are the most common factors that change the semiconductor properties1. This type of materials is very interesting to use as detectors, sensors or amplifiers because their electrical conductivity can be tune changing, controlling or knowing the environment where is the device and vice versa. Semiconducting materials revolutionized the electronic technology obtaining smaller devices (switchers and amplifiers) than with thermionic valves. The latter relies on the flow of electrons from a heated cathode to a positively charged anode in a vacuum-sealed glass tube. In contrast, semiconductors rely on the properties of materials like silicon and germanium to control the flow of electrons and can be much smaller2. The most common and well-studied semiconductor is the silicon (Si). The abundance of this material, together with its intrinsically good semiconductor properties are the reasons why silicon dominates the industry. Further widely used semiconducting materials are among others: germanium (Ge), sulfur (S), selenium (Se), gallium arsenide (GaAs), etc3. Semiconductors are divided in two types: intrinsic materials which possess inherent semiconductor properties and extrinsic ones which need to be doped with other atoms to obtain semiconductor properties. Doped semiconductors can be either N-type or P-type depending on whether the free charges are electrons (e.g. with boron (B), indium (I) and gallium (Ga) as dopants) or holes (e.g. with phosphorous (P), arsenic (As) and antimony (Sb) as dopants)1.Combining Nand P-type semiconductors (e.g the invention of the p-n junction) the humanity has been able to create a wide range of electronic devices including diodes, transistors, sensors, solar cells, etc. Nevertheless, the above inorganic semiconductors have shown some disadvantages. The most significant one is the complexity of fabrication because they need very expensive techniques such as molecular beam epitaxy (MBE), atomic layer 4 deposition (ALD), etc. These techniques require a high technology level like high vacuum or high intensity. Moreover, the classic semiconductors have not too much flexibility and ductility so they are not able to use for example in flexible devices. Currently, the growth of the semiconductor industry is exponentially growing. The size of the transistors evolved from centimeters to nanometers in few decades. Moore developed a theory stating that the size of the transistors in an integrated circuit will be doubled every two years4. Nowadays, experts predicted that in this decade the physical limit to decrease the size of devices is almost reached due to the physical challenges associated with miniaturization. Figure 1 Milestones of the optoelectronic industry. The first scientific publication on organic semiconductor materials dates from 1954, in which the electrical conductivity of organic polysulfones was described5. In the 70s, the first semiconducting polymer, i.e. doped polyacetylene (PA), was reported by Chiang et al. These development rises the possibility to break the barrier of the classical semiconductors and obtain better mechanical properties, non hazardous and biocompatible devices that allow to link the electronic world and biology , i.e. 1870s 1887 1904 1947 1954 2010s 2000s 1990s 1980s 1962 1960 Thermionic valve Semiconductor device Transistor Solar cell Organic semiconductor Laser Integrated circuit LED OLED Blue LED light Flexible and transparent organic device 5 bioelectronics67. During the 80s and 90s, different types of organic semiconductors were developed as small molecules or polymers. The common and most used commodity polymers are insulators. These include. e.g. polyethylene (PE), polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), etc. These families are composed of repeating units connected by single covalent bonds. These sp3 hybridization are strong, non-polar, and have low electron mobility. As a result, commodity polymers have limited electrical conductivity and are mainly used as insulators, films or for mechanical purposes8. If the polymers have aromatic rings or double bonds, a sp2 hybridization occurs. With this configuration, the electron that is not linked with the adjacent atom is forming a π-bond instead of a σ-bond. The electrons that are in the π-bond are weaker and delocalized so the electrons can move more freely. If this explanation is moved to the electronic band structure, in the insulator polymers, the space between the valence band or highest occupied molecular orbital (HOMO) and the conduction band or lowest unoccupied molecular orbital (LUMO) is large so the electrons cannot climb to the conduction band to move freely9. In the case of conductor polymers, that space does not exist so the electron can move at will. Finally, in the case of the semiconducting polymers that space, named bandgap, is less so, applying some energy to the system the material can conduct the electricity10. 6 Figure 2 a) Hybridization sp2 of the orbitals, b) Link between two sp2 orbitals and c) Chemical structure of the polyacetylene. As described above, semiconducting organic molecules or conjugated polymers owe their optoelectronic and magnetic properties to the sp2 bonds but also to the conjugation. Conjugation is the delocalization of multiple π-bonds that form larger and more stabilized molecular orbitals. It is achieved by alternating single and double bonds along the backbone. The orbital p that is not linked can either be in-phase or out-ofphase with neighboring orbitals, forming bonding (π) and antibonding (π*) interactions respectively. The planarity of the conjugated molecule is important to maximize the orbital interactions1112. The bandgap of these materials is usually in the range of visible light. This means the material can be excited optically offering new solutions in the field of optical sensors or photovoltaic applications13. Doping in these polymeric materials refers to chemical oxidation in the case of the p-type polymers and reduction in the case of the n-type. When the p-type polymers are excited the Fermi level is displaced to the valence band so electrons move to lower level of the bandgap and the transport of the charge occurs in the valence band (the positive hole that the electron left can move). If the material is n-type occurs otherwise and the σ bond sp 2 orbitals p orbital π bond a) b) c) σ π 7 electron (negative charge) can move through the conduction band14. This process is the same as the exciton pair electron-hole in the classic semiconductors15. One molecule is not enough for the charge transport so some π-orbitals have to be overlapped with their contiguous to have a continuous path for the conductivity. Intrinsically, the materials with π-bonds try to heap them with the π-bonds of other chains to stabilize all the system so all these conjugated polymers have π-π stacking that also contributes to enhance the carrier mobility16. The traditional conjugated polymers have poor stability, which limits their efficiency and performance so, to overcome this issue, push-pull polymers were developed. This new family of conjugated polymers has two parts integrated in the same molecule. It has a donor and an acceptor part balancing the electron distribution and enhancing its stability. Also, this results in efficient charge separation and improved electrical conductivity. Figure 3 Indacenodithiophene-co-benzothiadiazole (IDT-BT) push-pull polymer. In green the donor part and in gray the acceptor part. Keeping this in mind, the mobility of the electrons on the conjugated polymers not only depends on the formulation of the polymer. It depends also on the orientation of the backbone and the side chains, the planarity, the packing, the distribution… In other words, the microstructure. There are several advantages of conjugated polymers with respect to inorganic materials. The fabrication of the devices is cheaper because they do not need complex 8 techniques like MBE. This is related to the easy and low-cost techniques to deposit the polymers like spin coating or blade coating and the facility to scale these techniques. Polymeric ones have, also, less fragility and more flexibility. This gives the option to develop curved displays or flexible microprocessors17. They are transparent or translucent so there is a possibility to develop transparent solar cells for windows for example. And one of the most promising advantages is the biocompatibility of the conjugated polymers to expand the devices to medicine and biological research due to the interaction with the environment which was previously unavailable7. With organic polymers actually, there are devices like transistors, diodes, integrated circuits, memory devices, solar cells, sensors and screens and displays based on them. To understand properly how the optoelectronic properties vary in a semiconductor polymer is essential to study their microstructure. On the solid state, matter could have some microstructures depending on the order of their atoms. In the case of polymers, it depends on the order and conformation of the chains. If a material has a disordered molecular structure where the polymer chains are randomly arranged, it is referred to as amorphous. This microstructure has singular qualities as transparency or flexibility between others. If the polymer was completely ordered, with the same periodicity between their atoms it would be a crystalline material. It is impossible in polymers to obtain a 100% of crystallinity so the polymers that crystallize are denominated semicrystalline. If the material has some degree of order but not enough to call it crystals (it could be crystals with a lot of defects) they are named paracrystalline polymers. They are characterized by the presence of a lot of structural defects that give them localized random domains with different structural properties but a certain periodicity in the system18. Between both models of organization of the polymer chains there are other one. The semiparacrystalline model shows some degree of long-range order and regularity like the paracrystals but with a more limited degree of structural order. It has the dense arrangement of very small paracrystallites coexisting with more disordered sites in an amorphous matrix19. This opens the field to more flexible and adaptable devices than semicrystalline materials. Also, the last year, our group developed a new theory of the possible structuration of the polymers. The semi-para-cristallinity19. 9 The last microstructure is the liquid crystal. This is a distinct phase in the liquid state that exhibits properties of liquids and crystalline solids. It has an intermediate state between solid and liquid, where their molecular structure aligns to some degree while still retaining fluid-like behavior. Liquid crystal polymers possess chains that can order themselves into a partially crystalline arrangement under certain conditions. This state is characterized by their anisotropic nature, meaning that their properties can vary depending on the direction. These polymers can exhibit different degrees of molecular alignment or orientation, which can be influenced by external factors. Focusing on the transition between the states of solid matter, polymers have certain temperature named glass transition temperature or Tg. If the polymer is above this temperature, it can move and if the polymer is below Tg is freeze the system. This parameter is very important because the material experiences huge changes in the free volume, density, specific heat, mechanical properties, etc. Figure 4 Scheme of the polymer thermal transitions. T c T m T g T LCST 10 There are four transition temperatures. The Tg that was explained before, the crystallization temperature (Tc), liquid crystal-liquid transition and the melting temperature (Tm). The crystallization temperature is the point when the polymer chains move from a disordered state to an ordered state. When the system equalizes and goes above Tc, the free energy of the polymer system is minimized and the polymer chains begin to arrange themselves into an ordered structure. This crystals are composed by a repeating, long-range structure and are a function of the polymer's chemical composition, molecular weight, and processing conditions2021. The melting temperature is the point when the polymer goes from solid to liquid state. At this temperature, the system takes enough energy (heat) to surpass the intermolecular forces that maintain the crystals linked. The last temperature, less knowledgeable and it has only a few polymers have it is the liquid crystal-liquid transition (LCST). The LCST is the temperature at which the polymer changes from liquid crystal to isotropic liquid. This is an important transition talking about processing due to its special mechanical properties being a viscoelastic liquid in a range of temperatures. The state of matter named liquid crystal has properties between liquids and solids. Is a partially ordered liquid with long-range arrangement who got it anisotropic properties like birefringence, polarizability or dichroism22. Figure 5 Classic solid-state structural models for polymers. The classical theory of conjugated polymers said as many crystals, as much conductivity. This is because, normally, the defects and lattice disorder affect as traps reducing the conductivity23. For this reason, the theory of the conductivity in semicrystalline or paracrystalline polymers is through ordered domains interconnected. The most common “links” between ordered regions are the aggregates and the tie chains. Aggregates are clumpings of polymer chains with some order and tie chains are Temperature Amorphous Semi-crystalline Para-crystalline Liquid Liquid Liquid T g T g T m T m T c 11 flexible chains that connect crystals or ordered regions in a polymer network acting as wires through which electricity is conducted. Therefore, the high mobilities in polymers are not dependent only on the crystallinity. They have many complex structures (some of them even considered amorphous) well connected that conduct electricity. The charge transport always increases the temperature of the system like the Joule effect in the conductor materials. Due to this, optoelectronic devices based on polymers have to have high thermal stability. To be sure that the material will not change its structure and properties high Tg and Tm values are preferred. On the other hand, low Tg and Tm confers to the system more flexibility so there has to be a balance. This work is focused on how control the optoelectronic properties in sundry promising devices for applications in transistors solar cells and sensors. In summary, conjugated polymers are promising materials for advanced applications, such as flexible electronics and bioelectronics. To get the most out of conjugated polymers is necessary to understand, control and know how to tune their structure. Normally, classical studies are focusing on understanding the crystallinity but, nowadays, the vitreous phase is taking more importance. Knowing and switching the vitreous phase the optoelectronic properties also could change significantly. 12 19 down to -80 or -90 Celsius (the minimum temperature is reached by the FSC) and immediately heated up to the higher temperature. Finally, the material is cooled with the same cooling rate and it is made another heating ramp to use it as a reference (heating without the isothermal step). Figure 7 FSC protocol with Ta (annealing temperature) and ta (annealing time). The aforementioned overshoot can be explained with the own definition of the Tg. The glass transition temperature (Tg) is defined as the temperature (or range of temperatures) where the glassy phase undergoes a change between an immobilized state (out-of-equilibrium glass) to a molecular mobility state.30 In the case of calorimetry, if the material in the immobilized state evolves through a less energy level (reducing its enthalpy) an endothermic overshoot can be seen at temperatures close to the Tg. This process is called physical ageing. In conclusion, the glass transition temperature and the metastable equilibrium of an order glassy phase below that temperature can be studied by researching the endothermic overshoots formed by ageings below Tg. Tmax Tmin Temperature (ºC) Time Annealing Reference Ta, ta 20 Figure 8 Temperature dependence of the enthalpy for a typical glass former cooled down at a given cooling rate β. Figure 9 Effect of an ageing in a DSC experiment. In green the sample aged and in grey the sample unaged. When the annealing time is changed (at the same temperature all de annealings), studying the evolution of the endothermic overshoot, the kinetics of crystallization and ordering can be studied to know if the peak is due to the glassy phase, nucleation or T g Enthalpy Temperature Cooling rate = β T a < T g physical ageing Ta > Tg No physical ageing H ref =H aged H a H ref H ref =H a W Temperature 21 growth of crystals. The total area of the endothermic overshoots are fitted with a sigmoidal-like law (e.g. Kohlrausch-Williams-Watts, Avrami, etc.). The parameter in which we are interested is β, the Avrami exponent. ∆𝐻=𝑒(𝑘𝑡)𝛽 Equation 1 Kohlrausch-Williams-Watts or Avrami-like law. Where ΔH the maximum enthalpy value measured in the experiments, k is a constant associated with the rate of the advance of the the calorimetric process, t is time and β, is a parameter related with how the advance of the process depends on temperature and it is typically referred to as the Avrami constant. 2.2.1.1.2) Isochronous “annealing” Developed by Cangialosi et all, it is used to identify the Tg of the samples by FSC.31 The same protocol as in the isothermal Figure 7 is used but, instead of varying the time of annealing, is varying the temperature of the annealings. In this way, when the temperature is below the Tg, in the isotherm the vitreous phase is ordered (every time the materials try to reach the low energy state) that is translating into an endothermic overshoot in the heating curves of the DSC. The point when the overshoot becomes 0 can be extrapolated when some measurements are made increasing the annealing temperature. At this point, the vitreous phase cannot evolve. Therefore, this means that we have reached the early part of the Tg region called Tg onset32. The evolution of the crystalline phase above Tg can also be studied with this method. 2.2.1.1.3) Modulated DSC The idea of this protocol is, instead of using a linear heating ramp, to use a periodic temperature modulation (step response or sinusoidal usually) of a certain amplitude and frequency. 22 Thanks to this protocol, we can separate processes due to reversible Cp and irreversible Cp. In this way, for example, cold crystallization processes and reversible processes such as the glass transition can be separated. Figure 10 Schematic scheme of one pseudo-wave of the modulated DSC. This protocol in FSC consists in a pseudo wave that simulates an harmonic heating protocol. One heating ramp of two degrees, a small isotherm, one cooling ramp of 1 degree and another isotherm with the same time of the previous one. Repeating this four steps x cycles a modulated DSC is obtained with a step of 1 degree33. The maximum velocity of the equipment for the data acquisition with this small heating and cooling ramps is 1000 K/s. Changing the time of the isothermal steps we are able to change the modulated frequencies (typically between 1 and 20 Hz). It is necessary to use the Fourier transform to treat the data and separate the Cp reversible from the non reversible34. 2.3) X-ray techniques with synchrotron radiation A synchrotron is a light source emitted by electrons moving very fast (almost light speed) inside a ring. The spectrum of synchrotron light reaches from the far infrared to near γ-rays. The radiation is emitted tangentially and forward from the particle orbit due to the nature of the relativistic particles. The angle of the collimated light corresponds to 1/γ where γ is the Lorentz correction factor for the particle relativistic motion: γ = (1 − β2)−0.5, where β = v/c and v is the electron velocity and c the speed of light in vacuum. The wavelengths obtained from the synchrotron radiations include from the far 23 infrared to near γ-rays.3536 We leverage the hard X-rays of this light thanks to the monochromators. Due to the wave-particle duality, the electrons can be considered “wave packages” and, therefore, particles named photons in the case of the light. Also, they can have all the wave properties such as diffraction, reflection, etc. 37 This is because they can be considered “matter waves” with their own wave function so knowing the mass and the velocity, the wavelength can be known thanks to the de Broglie equation λ=h/mv where lambda is the wavelength, h is the Plank’s constant, m is the mass of the particle and v the velocity.38 The synchrotron has 5 differentiate parts. The electron generator or “Electron gun” where the electrons are generated by heating a metal up to 1000 Celsius (tungsten with barium oxide normally), a linear accelerator (Linac) where the electrons were accelerated up to 100 MeV using radio frequency cavities. Then, the electrons accelerated up to 3 GeV in a booster ring with electromagnetic fields. Finally, the electrons are delivered into the storage ring. Maintaining under vacuum to minimize the scattering of air, it keeps the electrons spinning until the extraction of them tangent to the ring in the beamlines with the help of magnets. Then, in the beamline, they are used some slits, mirrors and monochromators to obtain the desired wavelength.3940 24 Figure 11 Synchrotron graph. a) Electron gun b) Linac c) Booster ring d) Storage ring e) Beamline. The synchrotron radiation light source is used because the intensity is higher than in the conventional X-Ray tubes and the light is very polarized and collimated. 2.3.1) Grazing incidence wide-angle and small-angle X-ray scattering (GIWAXS and GISAXS) X-Ray techniques are good to study the structural and morphological aspects of the semiconductor polymers in a broad range. In our case, the maximum range we use is between 0.006 and 25 nm-1 approximately. All the distance data are expressed in scattering vector, momentum transfer or wavevector transfer scattering vector, momentum transfer or wavevector transfer (q) which is inversely proportional to the real distance (d). Moreover, the Bragg law41 relates the distance between adjacent periodic planes (d), the wavelength (λ) and the angle between the scattered wave and the incident wave (𝜃). If this law is fulfilled, the periodic distances of the material can be studied. a) b) c) d) e) 25 𝑞=2𝜋 𝑑 𝑛𝜆=2𝑑sin𝜃 Equation 2 a) q-d relationship and b) Bragg's law. In the specific case of GISAXS and GIWAXS, instead of knowing the interplanar distances, the shape, texture and orientation can also be shown. Figure 12 GIWAXS and GISAXS geometries. The detector where the intensity values of the scattering are collected is planar so it is necessary to make some corrections in GIWAXS. In the case of GISAXS, the detector is very far from the source so it is considered that the image in the detector is veracious. The scattering of the sample is isotropic (vectorially speaking) so, having a planar detector, the distances in the planar detector are not the real value of q. A spherical detector is needed to obtain the real distances in the reciprocal space to have all the detectors equidistant to the detector. With the planar detector, the Ewald sphere correction is used4243 as you can observe in Figure 13. q R q R q Z q Z GIWAXS GISAXS a) b) 26 Figure 13 Ewald sphere correction scheme. Moreover, as much ordered the material (more perfect is the periodicity), the peak observed in the detector is sharper. Knowing the width at the halfway up the peak (FWHM or full width half maximum) and the position in the q range, can be determined the coherent length (Lc) thanks to the Scherrer equation (Equation 3)44 where k is the shape factor (between 0.9 and 1)45 and Δq is the FW M. 𝐿𝑐=2𝜋𝐾 ∆𝑞 Equation 3 Scherrer equation. This parameter is directly related with the paracrystalline parameter (g). With this parameter is measure de statistical fluctuation of individual lattice spacings45. For highly disordered systems, the g parameter can be extracted from the width of the first order diffraction peak19 only if the peak is entirely described by cumulative disorder and if paracrystallinity is more significant than lattice parameter fluctuation46. If these two conditions are fulfilled 𝑔=√∆𝑞 2𝜋𝑞 Equation 4 g parameter equation. 27 where Δq is the FWHM and q is the maximum peak position. The g parameter can oscillate a lot between 0.01 (1%) and more than 0.20 (2%) using as scale-based material the a-SiO2. Ig g > 1% the material is highly crystalline, if 1% < g < 10% the material has imperfect crystals and/or mesophases and, if g < 10% the material is amorphous30. For GIWAXS the incidence angle used was 0.12º (little below the Si angle) but the measurements were carried with 4 different angles between 0.08 and 0.2 to obtain the best data. 3 different types of integrations were carried out. Complete (in the whole directions), in-plane taking the 45 degrees below to see the order in the chains that are parallel to the substrate and out of plane to study the perpendicular substrate direction planes. The temperature experiments were conducted in a Linkam® in a N2 atmosphere. All the experiments were performed at the BL11 NCDSWEET beamline at ALBA Synchrotron Radiation Facility (Spain). The incident X-ray beam energy was set to 12.4 eV using a channel cut Si (1 1 1) monochromator. A Rayonix® LX255HS area detector was used. The setup was calibrated using Cr2O3 as reference. Figure 14 GIWAXS pattern and scheme of a) Low disorder out of plane, b) Totally disorder, c) Out of plane and d) In plane. For GISAXS the horizontal integration was used at the Yoneda peak to obtain the maximum range of intensity. The experiments were developed at Alba and Elettra a) b) c) d) 28 synchrotrons. In both cases the calibration sample was AgBH (Silver Behenate) and the detectors were the Pilatus 1M detector from Dectris®4748. For the temperature experiments a Linkam® in a N2 atmosphere was used at Alba and an Anton Paar model DHS1100 was used at Elettra48. 2.3.2) NEXAFS Understanding the molecular arrangement and orientation of polymer chains is pivotal in tailoring the properties of advanced materials. In this pursuit, Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was employed as a powerful tool to elucidate the intricate details of polymer electronic structures. The unique advantage of utilizing synchrotron radiation as the X-ray source further enhances the precision and sensitivity of NEXAFS, offering unprecedented insights into the orientation of polymer chains. NEXAFS is based on the principle that X-rays incident on a sample can be absorbed by core-level electrons, causing them to transition to unoccupied states. Near the absorption edges of specific elements, such as carbon or nitrogen, the absorption spectrum exhibits fine structures that are sensitive to the local electronic and molecular environment. The orientation of polymer chains can be inferred by exploiting the polarization dependence of NEXAFS. The absorption cross-section is sensitive to the orientation of the absorbing molecule with respect to the polarization vector of the incident X-rays. Analysis of the NEXAFS spectra involves comparing the intensities and shapes of spectral features for different polarization directions. This information can be used to deduce the average orientation of the polymer chains in the sample. 35 "La ciencia y la tecnología revolucionan nuestras vidas, pero la memoria, la tradición y el mito nos agrupan como seres humanos." Michael Crichton 37 3) Using the vitreous phase to unravel the morphology in bulk heterojunctions 3.1) Summary Organic solar cells are the future for sustainable and environmental friendly energy. The study of how is the behavior of the active layer (Bulk eterojunction) is key to improve this technology. Thanks to the Flash DS , X-ray scattering and other techniques is possible to understand it. The study of the mixture of PBDBTl (donor) and ITI -Th1 (acceptor) is used for this work. Thanks to the previous work of Liang et al the better concentration of the mixture is 50:50 %wt with a solvent annealing. With this knowledge and studying the vitreous phase of the mixture, the proportion between the mixed phase and the single component phase can be determined in the same development conditions of the real solar cell. 39 3.2) Introduction The energy consumption in the world is increasing exponentially in the last decades. The petroleum reserves are not infinite and, nowadays, nuclear energy has the potential to be the best option but from the point of view of the population it is very dangerous. Due to these handicaps, renewable energies are taking the lead. The renewable energies are taken from the wind, the sunlight, the biomass, the movement of the water and the geothermal heat56. Focusing on the sun’s energy, there exist three ways to extract energy. eating a fluid to boil it and produce energy with a turbine, heating water or some spaces like a pool or a restaurant and directly convert the solar energy into electricity57. To extract electricity from the sun is necessary to have a material or materials that are able to absorb the energy of the sunlight and generate free charge carriers (electrons and holes). This mechanism is called the photovoltaic effect. When the photons of the sunlight arrive at the solar panel are absorbed by the atoms of the semiconductor material. If the energy of the photons is enough, an electron of the atom can “jump” from the balance band to the conduction band and become free from their atom. The released electrons move through the semiconductor material towards an electrode, creating an electric current585960. There are several types of solar cells depending on the materials and the deposition of the active material techniques. The most common are perovskite solar cells, silicon solar cells and organic solar cells61. As for organic solar cells, they could not be viable without the discovery of Alan Heeger in the 1980s who later received the Nobel Prize in Chemistry for his work62. The first viable organic solar cells or organic photovoltaics (OPVs) consist on a single layer of organic material between two electrodes. In the next decade more complex structures started to develop like multi-layer or tandem but without good results. Since 2000, the combination of improved materials (polymers and fullerene or non-fullerene acceptors), 40 device architectures and understanding the fundamental physics of them had improved significantly their efficiency. The solar cells that are being studied are primarily composed of two main materials. A donor that “repels” electrons and an acceptor that attracts electrons. These two materials are typically blended, forming a so-called bulk heterojunction (BHJ) morphology., so that the blended material contains three well-defined regions: donor domains, acceptor domains and intermix face between both where the charge separation occurs. Nowadays, organic solar cells continue having less efficiency than silicon ones but they have the potential to dethrone them thanks to the easier and cheaper manufacture. This makes them ideal for large-scale applications reducing a lot the energy consumption in the process of develop them. Also, they are lighter and, thanks to their flexibility, they can be integrated into a wide range of applications like clothing, building materials or portable electronics. They need to improve their lifespan because they lose properties when the time exposure is too long or in some environments. One of the most auspicious advantages is the tuneability of the materials. They can be chemically or physically altered to have specific properties. The absorption spectra of the materials can be modified by selecting the light wavelengths depending on the country or region, the season or the things that are below it. For example, if you want to have a solar cell in the ceiling (transparent or semitransparent) of a greenhouse and you are planting lettuces, you can make a device that absorbs in a range of solar light but avoiding the wavelengths that the lettuce needs to grow63. Organic solar cells have the potential to be a key technology in the transition to a low-carbon economy. They use a clean and renewable energy source and can be cheap and easily processable. With their transparency and flexibility they can be used, not only on the top of buildings or solar farms, but also in areas where it was previously impossible such as windows, greenhouses, or irregular surfaces among others. 41 Of course, there are a lot of variables to have into account. The angle of incidence of the sunlight, the resistance of the materials, the joule effect, etc. This work is focused on how the microstructure of the bulk heterojunction affects the photovoltaic properties. 3.2.1) Evolution of donors As was mentioned in the introduction of the thesis, the traditional conjugated polymers are homopolymers composed by only one repeating unit. In the 70s polyacetylene was discovered. Afterward, in the 80s, thanks to the incorporation of ring structures in the backbone, 2D structures, more stable and better soluble polymers than the precursors polyacetylene-based could be made. This is the case of a few specific structural polymer families such as polyfluorenes, polyphenylenes or polythiophenes like well-studied P3HT (poly(3-hexylthiophene-2,5-diyl))646566. These new materials allowed for better control over the polymer's electronic properties, resulting in high efficiencies in organic solar cells. In the next decade, the donor-acceptor or push-pull polymers were developed. As it was briefly explained in the introduction, this type of polymers is composed by two different structures alternated so it is possible to say that the monomer is made by two components blended. One compound is electron-rich trying to catch an electron. They usually have a high concentration of electron-donating groups such as nitrogen or sulfur to increase their mobility and electron density. The other compound is the opposite, electron-poor that decreases the electron mobility thanks to electron-withdrawing groups like carbonyl67. These new polymers emerged as the most promising materials in the field of OLEDs and OPVs since the 90s to nowadays. The most used polymer for solar cells actually is the PBDB-T. The poly[(2,6-(4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] was first reported in 2012 by Qian et al and an era in the polymer solar cells started thanks to it and it derivates68. PBDBT has a lot of names but the most known one is PCE12. This 42 material has a high power conversion efficiency (PCE) due to its broad absorption spectrum, high charge carrier mobility, and good morphological stability. The derivates consist on add atoms to the principal structure to improve its properties as the PBDBTCl, PBDBT-2F, etc. Figure 18 a) P3HT molecular structure and b) PBDBT. Actually, the ongoing research has focused on developing new conjugated polymers with higher processability, stability and solubility to be a promising rival in the solar energy career. In summary, while homopolymers are composed of identical repeating units, pushpull polymers have a more complex chemical structure with both electron-donating and electron-withdrawing groups. This structure leads to improved charge transfer and broader absorption spectra, resulting in higher efficiency and performance of organic solar cells. a) b) 43 3.2.2) Evolution of acceptors The first acceptors developed for OPVs consist on phthalocyanines and perylenes. Created in the 70s, these organic dyes conferred low efficiencies due to their limited absorption and electron mobility. During the 90s the perylene diimide (PDI) was synthesized as a better acceptor69 and, during the 2000s, PDI derivates obtained better and strong absorption in the sunlight range and upgraded the hole mobility. It was not until 2002 that fullerene derivatives were developed. These structures composed by carbons offer excellent compatibility with conjugated polymers resulting in a good power conversion with a relatively good efficiency. The inconveniences are the relatively low hole mobility compared with the conjugated polymers, the aggregation, low solubility and the limited range of absorption70. The most important fullerene acceptors are [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), [6,6]-phenyl-C71-butyric acid methyl ester (PCBM70) or [6,6]-phenyl-C61-butyric acid butyl ester (PCBB) between others. In 2009 the non-fullerene acceptors (NFAs) began to be explored owing to the limited sunlight absorption range. These acceptors consist on small molecules synthesized and designed with precise control over their properties71. The pioneering were the quinolines and diimide derivates (more complex). In 2012 the development of fused-ring electron acceptors (FREAs) incorporating more electron-deficient aromatic rings in the core like fused thiophene or benzothiadiazole marked a breakthrough. The most important problem is the aggregation, but, changing their chemistry or structure, it is possible to tune the absorption range and the energy levels maintaining the high mobility and stability72. 3 years later, in 2015, Zhang et al synthesized the milestone of the NFA. The indacenodithieno[3,2-b]thiophene or ITIC73. The core structure of ITIC is formed by fusing two thiophene rings to an indacene core conferring a planar and rigid structure. It is a push-pull acceptor with a donor part between two acceptor parts. Also, the electron mobility and the charge separation increase thanks to the sulfur atom of the thiophene and the fused ring. A wide range of derivatives is obtained by changing the side chains 44 or substituents that are attached to the core structure. Some derivates were developed by the introduction of an electron-rich group like methyl into the end-capping units (ITICM)74, by the incorporation of electron-deficient atoms such as fluorine (ITIC-2F)75 or chlorine (ITIC-2Cl)76 or by the substitution of the phenyl units by thienyl groups (ITICTh)77. Another promising type of small molecules in this field were the acceptor-acceptor (A-A) type polymers like poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2)787980 and its derivates. In the last years of the 2010s, Y6 or ITIC-4F (a derivate of the ITIC) increase the PCE up to 13% (combined with a donor polymer) consolidating the NFA as a viable alternative against the fullerene acceptors8182. O-IDTBR or IDIC were developed also. Figure 19 Chemical structures of the most important small molecules: a) PCBM, b) PDI, c) ITIC and d) Y6. Nowadays, in this decade, the tendency is to continue with the ITIC derivates developing new small molecules more symmetric as N3 (derivate of the Y6) for example. a) b) d) c) 51 is possible to extrapolate a calibration curve to know the Tg of one unknown intermix composition but knowing its Tg. By the isochronous annealing method, it was impossible to determine the Tg of the intermix phase due to the overlap of the cold crystallization on the heating ramp of the small molecule and the ageing overshoot of enthalpy recovery of the PDCBT-Cl. This issue can be clearly seen in the Figure 24. It can be observed also how in this 75:25 chip of polymer and small molecule respectively at the annealing of 70 or 80 Celsius for example, the crystallization and the peak due to the physical ageing are superposed. For this reason, the modulated method has been used. With the temperature modulated DSC, the non-reversible processes, such as cold crystallization, are ridded. All the systems were studied by this method to maintain the same criteria with all the samples. Figure 25 Temperature modulated DSC of all the systems (a) and their derivates (b). To estimate the Tg of the intermix phase in all the measurements, 10 Hz were used in the experiments. -50 0 50 100 150 200 250 300 TMDSC CP rev (a.u.) Temperature (ºC) PDCBT-Cl 10 Hz 75:25 10Hz 50:50 10Hz 25:75 20Hz ITIC-Th1 10Hz 020 40 60 80 100 120 140 160 180 200 220 240 Derivate od the TMDSC experiments d(Cp rev)/dT(a.u.) Temperature (ºC) PDCBT-Cl 10Hz 75:25 10Hz 50:50 10Hz 25:75 10Hz ITIC-Th1 10Hz a) b) 52 After the Fourier-transform of the data, it is more visible that the cold crystallization is deleted and can be clearly the glass transition and the melting of the neat materials. To have a common criterium with all the results, the 1st derivate was develop. The maximum of the peak of the derivate in the region corresponds with the Tg. To not make mistakes, the peak was fitted to a Gaussian function and extracted the maximum. Intuitively, in the panel b of the Figure 25, a clear observation emerges: the Tg of the intermix phase is displaced from the Tg of the polymer to the Tg of the small molecule when the weight percentage of the last one is increased. It is appreciated also the melting of the two neat materials, making sure that the measurements are well done. In the next figure, it can be observed a table with the extracted Tgs from de modulated experiments and their fitting to a Gordon-Taylor equation to have a calibration curve of the system PDCBT-Cl:ITIC-Th1. % wt PDCBT-Cl:ITIC-Th1 Tg (°C) 1:0 (PDCBT-Cl) 73 75:25 87 50:50 105 25:75 122 0:1 (ITIC-Th1) 140 Figure 9b shows that the experimental data can be adequately fitted to the GordonTaylor equation. Once the calibration curve is done, the solar cell with the best electronic properties can be studied. To study the solar cell a thin film of 50:50 % in weight was deposited in a silicon wafer with the conditions of Liang et al. Immediately, a solvent annealing in CS2 at room temperature 2 minutes. The Tg of this system only can be measured in the first heating because the SA confers to the solar cell better properties. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 70 80 90 100 110 120 130 140 150 Calibration curve Tg (ºC) % wt of ITIC-Th1 Model Gordon-Taylor Equation (x*T1)+(K*(1-x)*T2)/x+(K*(1-x)) T1 140.18653 ± 0.87813 T2 72.55195 ± 0.85846 K1.12155 ± 0.06563 Figure 26 Tgs of the different intermix phases and Gordon-Taylor fit calibration. 53 If we use the second heating to extract the Tg the thermal history of the system was deleted so we are not seeing the real Tg. For that, the thin film was deposited in a silicon wafer. Having this, we are able to scratch this film and take a small part to measure. We can take many samples from the same film to measure many first heatings of the same sample. Figure 27 Tg of the solar cell improved by Liang et al. and the extracted composition of it. This result show how, with the solvent annealing, the percentage in weight of the small molecule in the intermixed domains decreases considerably to 37.5 %. This can be rationalized in terms of migration of the ITIC-Th1 from the intermixed phase to a compositionally pure phases, for example into the crystalline domains. Cp rev 050 100 150 200 250 300 d(cp rev)/dT Temperature (ºC) 96 0.0 0.2 0.4 0.6 0.8 1.0 70 80 90 100 110 120 130 140 37.5 Tg (ºC) % wt of ITIC-Th1 96 54 Figure 28 GIWAXS patterns of 50:50 %wt of PDCBT-Cl:ITIC-Th1 without (a) and with (b) solvent annealing of 2 minutes. Indeed, in the Figure 28 it can be observed how the solvent annealing improve the crystallization of the small molecule conferring to the system more PDCBT-Cl in the intermix phase. This is congruent with the results of the flash DSC because the Tg is lower than in the case 50:50 without solvent annealing. To see the morphology the AFM technique was used. An image of the best system was analyzed to see the domains. In the phase image of the AFM it is clear that there are separated domains and between them a visible boundary were the intermix phase is located. The domain size is approximately 85 nm of diameter and it was measure by image J software. a) b) 55 Figure 29 Height and phase AFM images of 50:50 with solvent annealing thin film. To support the AFM images a small GISAXS study was carried on. The mixtures have more scattering than the neat materials due to the higher electronic contrast in the film. When the film does not have solvent annealing, the domains have a plane disc shape with 30 nm of radius approximately. Then, after the SA, the domains increase their size to 45 nm of radius and the discs swell until they acquire the shape of a fractal sphere. a) b) c) d) 56 Figure 30 GISAXS integrations of the Yoneda peak of a) different systems and b) the thin film 50:50 with and without solvent annealing. Knowing the structure, it is important to check the optical properties. For this, the UV-Vis absorption of the system was analyzed. In the next figure, it is shown how with the 50:50 mechanical mixture is obtained the highest range of absorption of the light in all the range between the neat materials. In addition, with the solvent annealing, it is demonstrated that the absorption range is increasing a little bit increasing also the total area of the light absorption but decreasing the maximum of absorption. Figure 31 UV-Vis spectra of all the system. 0.1 1 1E8 1E9 1E10 Intensity q (nm-1) PDCBT-Cl 7525 5050 2575 ITIC-Th1 0.1 1 1E8 1E9 1E10 Intensity q (nm-1) No SA SA 2 min 350 400 450 500 550 600 650 700 750 800 850 900 Intensity normalized Wavelength (nm) PDCBT-Cl 75:25 50:50 25:75 ITIC-Th1 350 400 450 500 550 600 650 700 750 800 850 900 Intensity normalized Wavelength (nm) 50:50 50:50 SA 2min a) b) 57 Figure 32 External quantum efficiency of the reference solar cell. Finally, in the Figure 32 we can observe that it has the same shape of the absorption spectra. This indicates that the electronic current generated is along all the absorption of the photons corroborating the good solar cell properties. 3.4) onclusions In summary, the utilization of the TMDSC (Thermal Modulated Differential Scanning Calorimetry) has enabled precise reproduction and compositional analysis of solar cells. By faithfully replicating the solar cell using a flash DSC chip, we can investigate the morphology of the real active layer through calorimetry. An intriguing finding is that the Intermix phase of the solar cell, when combined with the SA (Solvent Annealing) process, deviates from the 50:50 composition expected from the precursor solution. Instead, it comprises 37.5% of the weight of ITIC-Th1, attributed to its favorable crystallization behavior during solvent annealing. 300 400 500 600 700 800 900 0 10 20 30 40 50 60 70 80 90 100 EQE (%) Wavelength (nm) 58 Additionally, the examination of the vitreous phase allows for a detailed analysis of the solar cell's composition. This comprehensive approach sheds light on the intricate aspects of solar cell structure and behavior, providing valuable insights for further advancements in solar technology. 59 "El descubrimiento consiste en ver lo que todos han visto y en pensar lo que nadie más ha pensado." Albert Szent-Györgyi 67 paradigm (much order, better properties) exhibiting high mobility while lacking of crystalline-like long range order109–116. IDTBT is a push pull polymer composed by indacenodithiophene and benzothiadiazole. Its backbone has a very low torsion angle conferring it a rigid and planar structure115117. These characteristics give to the polymer a low disorder structure but not crystalline. Also, in the free carbon of the thiophene groups, aliphatic chains are located to give solubility to the system. Figure 35 IDTBT chemical structure. In summary, the FETs have revolutionized modern electronic. Their low power consumption, ease to integration and precise control of the current make them an invaluable tool for new complex integrated circuits. The ability to manufacture the organic active layer on flexible and transparent substrates using low-temperature deposition techniques open the door for new emerging applications. The objective of this part of the work is to understand the thermal behavior of the IDTBT family to determine why is one of the best organic materials for transistors and is called amorphous. 4.3) Results and discussion In this chapter the IDTBT and derivates were studied. An exhaustive analysis of the IDTBT (two different molecular weights) was made and a superficial one for the 68 derivates. After several experiments, the dichlorobenzene was decided as the best solvent to made films. One of the most important results is that the mobility does not change with the thermal treatments given to the transistors. For the thermal treatment of the transistor, it is important to say that it was made when the active layer (IDTBT in this case) is on the top. Then, after the treatment, the insulator layer and the top gate was grown so the annealing was made with the same conditions of the samples of the other techniques. Figure 36 Mobility data of the IDTBT transistors. The charge carrier mobility values are more or less the same in each sample. The small differences might be associated with changes in the ideality of the I-V curves, which do not seem to be microstructure-related. If there are changes in the mobility due to the annealings, the results are shielded by the intrinsic error related to the extraction of parameters from the I-V curves of transistors. RT 100 °C 140 °C 200 °C 260 °C 300 °C 1 2 m sat (cm2/Vs) Annealing temperature (º ) Mobility saturation (cm2/Vs) Standard deviation RT 1.17674528 0.07222518 100 1.21399166 0.21989346 140 1.21949628 0.35537115 200 1.22493352 0.19324101 260 1.19403097 0.33225271 300 1.23069779 0.23940238 69 Figure 37 I-V data IDTBT transistors. In order to understand the electrical behavior, it is important to know if there are changes on the solid-state microstructure. On the as cast GIWAXS pattern of the IDTBT they are observed essentially three diffraction peaks. One thick in the out of plane (010) at q = 14 nm-1 related with the stacking of the π-π bonds or π-π stacking and the refection in the in plane (001) due to the periodicity of the backbone of the polymer. It is curious that the IDTBT does not have the typical out of plane refection (100). This means that it does not have periodic alternation of aliphatic and aromatic regions contrary to what usually happens with conjugated polymers. The molecular orientation was also measured by NEXAFS showing less energy peaks on the π-π* bonds. This means that the chains are preferably horizontal, although there is a small proportion of chains in a vertical orientation, sustaining the GIWAXS results. -60 -40 -20 0 10-10 10-9 10-8 10-7 10-6 10-5 IDS (A) VGS (V) RT 100 ºC 140 ºC 200 ºC 260 ºC 300 ºC a) b) 70 Figure 38 a) GIWAXS pattern and b) NEXAFS of the IDTBT. To determine the thermotropic behavior, two different temperature-resolved experiments were taking. In situ taking GIWAXS patterns every 5 degrees and ex situ images (annealings of 10 minutes) every 20 degrees. The data recorded was integrated along the in plane and out of plane directions as a function of the scattering vector (q). The structural behavior of this polymer at temperatures between room temperature and 300 ºC where it is totally melted. Increasing the temperature, the polymer acquires more energy so it is able to move easily. Thus altering the short-range molecular order and promoting the supramolecular one. In other words, switching to les q the π-π stacking reflection and appearing the (100). This changeover can be explained like the transition from crystal to liquid crystal in other conjugated polymers like PBTTT. 270 280 290 300 310 320 330 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Normalized intensity E (eV) Vertical Horizontal 285 286 287 288 289 290 71 Figure 39 Integrations of IDTBT in situ experiment. Thermotopic materials, in the field of semiconductor materials, are semicrystalline or semiparacrystalline (partially ordered) in the low-temperature region but in this case it happens the opposite. At low temperatures it is disordered but, increasing the temperature, it becomes more ordered until 260 ºC approximately. The most rational thing of why it becomes more order is the formation of crystals but by GIWAXS it is demonstrated that no. There most accurate alternative is the undercooled liquid. In this case, the molecules tends to self-assemble spontaneously into a more-or-less ordered structures. In addition, the order increase during the heating reorganizing and compacting the lattice. Consequently, when semiconducting polymers are heated up (below the order-disorder transition temperature, TO-D) both (100) and (010) diffraction peaks tend to increase and/or become narrower. When this temperature is surpassed, the structural order disappears. To study this we used the parameter g118119. In the Figure 40 is represented the evolution of this parameter in the three representative reflection on the GIWAXS experiments. In the (001) and the π-π stacking reflections, the order increase modestly 510 15 20 300 260 220 180 140 100 60 qZ (nm-1) 20 510 15 20 300 260 220 180 140 100 60 Intensity qR (nm-1) 20 510 15 20 Out - of - plane qZ (nm-1) 510 15 20 20 60 100 140 180 220 260 300 Temperature (ºC) In - plane qR (nm-1) min max (001) (010) (100) 72 from 150 ºC approximately that correspond exactly with the formation of a clear (100). This process means that the chains are more near between them and the periodicity of the backbone becomes more regular. Also, in the same way, the decreasing of the (100) g parameter with the temperature indicates the appearance and arrangement of the lamellar stacking. Figure 40 Evolution of the g parameter in the in situ GIWAXS experiment. Based on these findings, a more comprehensive investigation into the thermal characteristics of this material is warranted, prompting the application of Flash DSC. The first experiment was carried out using the isochronous annealing explained in the materials and method chapter. The time isotherm time was 1h each annealing, the maximum temperature 400 Celsius and the minimum -80. All the coolings were at 4000 K/s forcing the material to be freeze in the state of the ageing. In the next chart are represented the heat flow against the temperature for each ageing (the heating ramp just after the ageing and a reference). 20 40 60 80 100 120 140 160 180 200 220 240 260 280 10 12 14 16 18 20 22 24 26 28 g parameter (%) Annealing temperature (ºC) p-p 100 001 73 Figure 41 Isochronal annealings of 1h from -60 ºC to 180 ºC. To clarify the information of the Figure 41, the excess of the heat flow (ΔHF) was represented versus the temperature. ΔHF was calculated subtracting the reference line to the measurement. With this simple operation, any peaks (not near zero values) evidence calorimetric features. In this case, IDTBT reveal 3 different endothermic peaks which means three different thermal transitions27. The third process is easy to determine. It is strange because the endothermic peak is located at 10 ºC, very low temperature having in account that the minimum ageing temperature where it appears is 80 ºC. Therefore, this process cannot be associated with something physical that occurs in the ageing like physical ageing or crystallization during the annealing. During the cooling it cannot be also a crystallization because the material does not have enough time to order so the peak (for rejection) is the melting of crystals. In this thermal region, the IDTBT tries to self-assemble into an ordered structure composed by layers of aromatic and aliphatic nanodomains. Then, the aliphatic regions are able to crystallize. The peak of the process 3 is due to the melting of this crystals. 74 The first process (blue) is due to the physical ageing of a region of the IDTBT. It occurs at -10 Celsius and we think that is associated to the Tg of the lateral chains. We are not able to link this transition with the lateral chains but observing the shape, the position and the behavior we can say that it is a vitreous transition. We can discard the degradation and the melting of crystals due to the low temperatures and the shape of the endothermic peak. The second process is the most intricated one. It encompasses a very wide range of temperature so is important to explain properly this. It is important also to mark that appears above what we associate as the Tg of the lateral chains. When this Tg is exceeded, the system starts to gain mobility. It could lead in three different reasons: a melting of the ordered domains in the system, structural relaxation of the glassy phase or a transition from liquid crystal to isotropic liquid. The liquid-liquid transitions the endothermic peak is not shifted so much so this theory can be discarded. Also, the appearance of the peak at an annealing at 20 Celsius starts at 60 degrees and, visually, at room temperature the material seems solid. As if this were not enough, at this annealing temperature, the π-π stacking begins to deteriorate in the GIWAXS analysis. It is inferred that this disruption is associated with the disorder of intermolecular aggregates characterized by a limited short-range order. This transition is very broad suggesting a very big heterogeneity due to the large range in which this process appear. It is also agreed with the (100) GIWAXS peak. It is visible until room temperature (when the process 2 in Flash DSC starts to contribute) but starts to acquire importance at 140 Celsius approximately and disappear at the same temperature in both techniques (temperature order-disorder or TO-D = 260 ºC) when the domains become disordered. 75 Figure 42 Integration of the ΔHs values and extrapolation of the thermal transitions. Last of this, the third process seems to be related with the side chains only. It is interesting because the overshoot appears when the ageing temperature is above 80 degrees but the peak is located at 10 degrees approximately. At this temperature, the (100) peak in the GIWAXS starts to take importance. That means that in this thermal region the backbone and the alkyl chains of the IDTBT increases its order into a layered structure. When the nanodomains are formed, maybe the linear chains can crystallize or physically aged during the cooling step. It is possible because this endothermic peak disappears when the material does not exhibit more peaks (in the rest of the ageings) and not before. The process 2 explained before is only a theory. This experiment does not bring light to clarify totally this. It is not clear if this process is due to the melting-like transition of the ordered regions or if it is related with the glass transition. To disclose this paradigm, a sequence of 2 or 3 consecutive isothermal steps in an ascending or a descending way were developed. 76 Figure 43 Descending (a) and ascending (b) isochronal annealings. The physical ageing was finally discarded because in the ascending sequence with the annealings at 20 and 100 Celsius and the annealing at 100 does not have the same result. If this process were due to physical ageing, the annealing at 100 after the annealing at 20 should delete the thermal history generate by the less temperature ageing. Furthermore, when 100 and 20 degrees of annealing are done, three peaks appear due to the three processes but if the same sequence is done in ascending order only the peaks at 20 and 200 ºC approximately pop up. This implies that, when there was an annealing at 100 Celsius, it effectively erases the thermal changes that occurred during the prior isothermal treatment at 20 ºC. However, when the order is reversed, the annealing process at 20 ºC induces further modifications in the material, resulting in the appearance of an additional peak. This discrepancy between the results from ascending and descending sequences explain the compatibility with the ordered regions theory. When there is an annealing of 100 preceded by another of 20, the melting temperature increase due to the improve of the growth and arranged of the ordered regions. Conversely, when the 100 ºC is followed by the 20 ºC annealing, with the first step ordered regions are created that melt at 200 ºC approx. During the subsequent annealing at 20 ºC, these crystallites formed at 100 ºC remain unaltered, maintaining their melting point at around 200 ºC due to their origination under more thermodynamically favorable conditions. However, it's important a) b) 83 5) Glassy Phase Engineering for Tunning Photoluminescence in PFO 5.1) Summary The chapter analyzes vitreous polymers' thermodynamic state, focusing on the concept of fictive temperature (Tf). Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) is scrutinized, revealing crucial insights into its glassy behavior connected to optoelectronic properties. It is established that the control of Tf is key (like crystallinity) in shaping the optoelectronic properties of semiconducting polymers. 85 5.2) Introduction Light emission and detection are important milestones for scientists, aiming to understand how to make efficient emitters and photodetectors. They catch environmental light and transform it, thanks to the cones and rods, into electricity and an image is interpreted by the brain. Nowadays, a wide quantity of different sensors is developed to cover an extensive wavelength range. This type of sensors is used in diverse applications, from measuring the light radiation in the desert to adjust the time exposure in a camera. Also, they are important in microscopy and astronomic observations but the most important one is the data transmission. To use light pulses (faster) than electric pulses to send information. In recent decades, technological advances have led to significant innovation in the field of light sensors, particularly in the development of organic light sensors (OLD). These new materials based on organic materials have provided significant advantages in terms of flexibility, production costs, and energy efficiency. 5.2.1) Organic light sensors The organic light sensors (OSD) are one of the most innovative materials in the field of light detection and optic properties. Modifications on the conjugated polymers or small molecules properties are used to detect light changes in the environment. They have also high sensitivity and a very wide range in the light spectrum acting from UV to near infrared wavelengths. This makes them an attractive option for applications such as digital cameras, lighting control systems, medical devices or motion sensors. The light detection process of these sensors is almost the same as the work of each component in an organic solar cell. Firstly, the light that is going to be detected hits the OSD. Then, the energy of the photons is absorbed and, if this is enough, an electron of the material is excited and jumps to a high energy state level. When electrons have energy in excess, they can return to the basal energy and emit light with another 86 wavelength or separate from the atom and move freely. In the latter case, they can be collected (applying a potential to help them to move) and used to extract electrical current or be recombined with a hole and also emit the energy. With respect to inorganic sensors, OSD present the advantage of being flexible and light, therefore exhibiting the possibility of being integrated in clothes or bandages. Changing the chemical structure, they are able to sweep a broad spectrum of wavelengths, thus providing specific sensors for particular purposes. Furthermore, their efficiency is very high, it means that almost all the photons that fall into the sensor are absorbed by the OSD. Last but not least, one of the most important advantages is their scalability. Normally, the way to produce them is by solution casting on a substrate. By this method, large quantities of OSD can be made with low costs and in a sustainable way. In contrast, there are some obstacles that need to be overcome. The first one is the competitiveness with traditional semiconductors based on the silicon technology. They are very well established in the market and the whole niche currently is covered by them. Related to it, they must fulfill some standards and regulations that nowadays are set up for conventional inorganic semiconductors. To be competitive with the actual sensors, OLDs have to improve their quantum efficiency and, more importantly, their stability and durability. These materials should be very sensitive to the humidity because the water acts as tramps for the hole and electrons. Also, they have to overcome their resistance to the oxidation, sharp temperature changes… All of these factors can affect their stability, durability and, therefore, their usability. These materials find their most crucial applications in the realm of organic lightemitting diodes (OLEDs) for image-related devices. In the market, we can witness a proliferation of televisions and smartphones equipped with OLED screens, primarily due to their exceptional ability to deliver deep blacks and high-resolution displays. Moreover, the adaptability of these materials has given rise to flexible screens and sensors for digital cameras, contributing to their widespread adoption. 87 Another significant application area is the domain of data transmission via optical fiber, where these materials shine due to their high efficiency and cost-effectiveness, making them ideal for addressing a wide range of bandwidth requirements. Beyond this, they play a pivotal role in proximity and gesture/movement recognition technologies, enhancing user interfaces and interaction with various devices. In the context of image-related devices, these materials extend their importance to the field of biomedical detection, serving as near-infrared (NIR) detectors for the analysis of tissues and the diagnosis of diseases. This diverse range of applications underscores the versatility and significance of these materials in our modern technological landscape. Normally, the tuning of the principal characteristic of OLDs are achieved by two main strategies, that are the development of new materials by changing the chemical structure and by changing the crystalline structure. However, the glassy phase is also emerging crucial to tune material performance. This work will be focused specifically on the characterization of the glass thermodynamic state via the fictive temperature, whose definition will be introduced in the next subsection. 5.2.2) Fictive temperature The fictive temperature (Tf), originally formulated by Tool120 and associated with the 'frozen-in' liquid structure in the glassy state, serves as a conceptual tool for characterizing the non-equilibrium state of a glass previously cooled from its hightemperature, molten state121. Tf is defined as the temperature at which a glass with given thermodynamic state would be at equilibrium. Hence, Tf quantifies the extent of deviation from equilibrium that persists in the glassy state122. If the glass has reached equilibrium, its Tf is identical to the actual temperature. Given its definition, Tf is identified as the point where the glass line intersects the extrapolated equilibrium line at a specific enthalpy. In calorimetric measurements, where the first derivative of the enthalpy is characterized, Tf is typically derived from heat flow curves during a heating scan after cooling. When employing identical heating and cooling rates, with the heating scan immediately following the cooling scan, the resulting Tf is 88 referred to as the limiting fictive temperature, denoted as Tf′123. The fictive temperature (Tf) is generally determined by using Moynihan’s method124. This defines Tf as the temperature at which the area of the aged sample and that of the unaged sample are the same. The equation is the next: ∫(𝐶𝑝𝑙−𝐶𝑝𝑔)𝑑𝑇= ∫ (𝐶𝑝−𝐶𝑝𝑔)𝑑𝑇 𝑇>𝑇𝑔 𝑇<𝑇𝑔 𝑇>𝑇𝑔 𝑇𝑓′ Equation 5 Moynihan's method to calculate Tf. Where, Cpl and Cpg are the heat capacity of the liquid and glass, respectively, and Cp is the apparent heat capacity of the sample at each temperature. It is noted that the differences in Tf between the same material having two different thermal histories are related to the differences in their enthalpy overshoots124. Tf exhibits a decreasing trend with prolonged ageing time or diminished cooling rate. As the glassy structure approaches equilibrium, Tf converges toward the value of the ageing temperature itself. In essence, the glass transition temperature (Tg) marks the shift of an amorphous material from a supercooled liquid to glassy state. Determined by observing changes in heat capacity or thermal expansivity during cooling, often using dilatometry, Tg is highly sensitive to the cooling rate, yielding distinct values for the same material under different rates. Conversely, the fictive temperature (Tf) defines the structural state of a glass during heating. It is the temperature at which specific properties, like specific volume or enthalpy, intersect with the equilibrium liquid line when extrapolated along the glass line. Both, Tg and Tf, depend on the cooling rate, with a common assumption of their approximate equality under similar conditions. Tg signifies a fundamental transition impacting mechanical properties, while Tf reflects the structural aspects of the glass, capturing the degree of relaxation during cooling and heating. 89 Together, these temperatures provide insights into the dynamic behavior and structural transformations of glass-forming materials, enhancing our understanding of their thermal properties and practical applications125. Figure 46 Fictive temperature scheme. In this chapter, the Tf of poly(9,9-di-n-octylfluorenyl-2,7-diyl), a renowned polymer with a very intense photoluminescence, is characterized following a wide variety of thermal histories. 5.2.3) PFO Poly(9,9-di-n-octylfluorenyl-2,7-diyl) or PFO was synthetized in the 80s as parrot of the emerging field of organic electronics, particularly in OLEDs and organic photovoltaic cells. It was developed to exhibit strong electroluminescence properties. This made it a promising material for new technologies for flat-panel displays and lighting. PFO and its derivatives played a crucial role in the development of OLED 90 technology. OLED displays offered advantages over traditional liquid crystal displays (LCDs) in terms of better color reproduction, faster response times, and flexibility126–130. It is flexible, almost transparent, low weight and all the properties related to the polymers and, also, it can be dissolved in the common organic solvents giving it the advantage in the industry field. Figure 47 PFO chemical structure. PFO solid state shows all the morphological behaviors mentioned in the Chapter 2) . In particular, PFO has reported α phase, β phase and liquid crystalline, all influenced by alterations in the surrounding solution environment131132. The α-phase of PFO is characterized by a well-ordered hexagonal lattice structure. The repeating units align in a way that maximizes π-π stacking interactions, leading to good charge transport properties with high crystallinity and stiffness which contributes to PFO's favorable optoelectronic properties133. This crystalline structure tends to be the most stable and dominant crystalline phase in PFO at room temperature129. The β-phase is another crystalline phase of PFO, typically observed at lower temperatures than the α. The exact structure and properties of the β-phase can be influenced by factors such as cooling rates during solidification and the presence of additives or impurities126. The β-phase structure may involve a different lattice arrangement compared to the hexagonal lattice found in the α-phase130134. Compared to the α-phase of PFO, the β-phase often exhibits lower charge carrier mobility135128. 91 In the nematic phase, the long flexible polymer chains of PFO tend to align along a common axis, but they lack positional order. This means that, while the chains have orientational order, they are not arranged in a regular pattern as they are in a crystalline solid136137. The objective of this part of the work is to tune the photoluminescence changing the physical behavior of the vitreous phase of the PFO. In summary, the flexibility, lightness, and easy processability make organic light sensors a pertinent successor to inorganic sensors, creating a new frontier in flexible sensor technology. The current focus is on enhancing efficiency, stability, and versatility, while maintaining their affordability, sustainability, and potential for large-scale production. 5.3) Results and discussion This polymer can explore all thermodynamic states mentioned in the chapter 2) . It may be either in the amorphous, crystalline or liquid crystal phase. It has relatively low thermal transition temperatures well defined. Therefore, suitable thermal protocols can be designed to minimize the risk of significant degradation issues. In addition to (at least) two crystalline forms, PFO exhibits a nematic liquid-crystalline mesophase (here after referred to as the NEM state) in the temperature range immediately above the crystalline phase(s) along with an isotropic liquid phase (here after referred to as ISO state) at higher temperatures. To understand the thermal behaviour of the polymer, samples were aged for 30 min over a wide temperature range between -80 and 280 Celsius, followed by heating scans at 4000 K s-1 that are showed in Figure 48. Following the development of calorimetric features resulting from ageing, this procedure allows unveiling the thermal events triggered by a given thermal protocol. In this way, three regions can be distinguished: isothermal annealing at or below 70 ºC results in the arise of an endotherm at low temperatures (gray areas of Figure 48), between -20 and 100 °C depending on 92 the annealing temperature, testifying the physical ageing of the glassy polymer and the subsequent recovery in proximity of the glass transition. Then, between 70 and 110 ºC crystallization to the  form takes place during the annealing step, highlighted by a sharper and more intense melting endotherm taking place at 130-140 °C (purple areas, Figure 48). Annealings between 110 °C and 220 °C allow the formation of the liquid crystal phase, that eventually disrupts during the weak endothermic transition at 260 °C (red area, Figure 48). Figure 48 Isochronous method applied to the PFO polymer. To provide a quantitative picture, the endothermic peaks were integrated to assess the enthalpy variation resulting from each thermal event. The outcome of this analysis is plotted in Figure 49. As in the Figure 48, the grey data are associated to the physical ageing of the glassy polymer, the purple one to the crystals and the soft red one to the liquid crystal. Extrapolating the high temperature part of the enthalpy variation by a straight line, the upper temperature limit of each thermal event can be determined. This procedure delivers the upper limit of Tg about 70 ºC, that of the crystallization process of 110 ºC and that of the liquid crystal formation at 220 Celsius. 99 There is a clear tendency for both peaks of the spectrum to shift towards higher wavelengths regardless of the initial state of the polymer (isotropic liquid or liquid crystal). With the decreasing of the Tf, the first peak is shifted to higher wavelengths allowing us to tune the light emission. Figure 54 a) Photoluminescence spectras of the selected samples and b) position of the most relevant peak vs the fictive temperature of the samples. With this first overview of the possibility of controlling the thermodynamic state of the vitreous phase to tune the photoluminescence new possibilities for optoelectronic devices are opened. Not only improving the crystallization better devices are obtained. With this approximation, also amorphous or near amorphous polymers can be used for new electronic technologies. 100 5.4) onclusions The thermodynamic state of the vitreous polymers is a difficult field of knowledge. Thanks to the flash DSC, we are able to distinguish and understand how the Tf behaves. In this case, the LC form appears to have a greater temperature sensitivity and a tendency for faster relaxation and vitrification compared to the LQ form. The behavior of PFO in a 2D structure is the same as in bulk so active layers on thin film devices are possible. Also, a wide range of Tf were obtained controlling only the cooling rate. To enlarge the range the cooling rate control were developed from the isotropic liquid state and liquid crystal state. But the most important and relevant conclusion is that the control of the Tf is also determinant to control the optoelectronic properties of organic semiconductor polymers. 101 "La ciencia no consiste en acumular datos, sino en unirlos." Sir William Lawrence Bragg 103 6) Conclusions As main conclusions, the utilization of the TMDSC (Thermal Modulated Differential Scanning Calorimetry) has enabled precise reproduction and compositional analysis of solar cells and the active layer morphology on sensors and transistors. By faithfully replicating the solar cell and the transistor/sensor active layer using a flash DSC chip, we can investigate the morphology of the real active layer through calorimetry. Also, the GIWAXS and GISAXS provide a lot of information understanding the direction and the range of order. These techniques, along with other complementary approaches, allow us to understand and modify nano and microstructures at our discretion. To know the real composition on the bulk heterojunction in a solar cell as in the Chapter 3) discovering that a 50:50 weight mechanical mixture has, in real, 37.5% of the small molecules and the rest polymer in the intermix phase. To study a “near amorphous” polymer as active layer on a thin film transistor like in the Chapter 4) In this case is demonstrated that IDTBT is, at a glance, amorphous because there are no order at microscale but has a high order at nanoscale with 4 key temperatures. -10 and around 50 Celsius associated to the aggregation of the backbone and the sidechains, one at 150 when the backbone mobility starts and the order increases a 250 °C associated to the melting of the material. To understand and tune the amorphous phase of the polymers modifying the cooling rates and the start state (isotropic liquid or liquid crystal) to control the changes on the optoelectronic properties. In Chapter 5, the fictive temperature (Tf) of PFO was adjusted to manipulate its optoelectronic properties, achieving a range of approximately 30 K within which Tf could be modulated in a timely manner (not requiring extended durations). Although stabilization of Tf remained elusive, it became evident that prolonged cooling times are necessary to gradually decrease it. Now, I want to rewrite the conclusions in Spanish for my family and the people that does not know English and want to know what is about this work. 104 Conclusiones Como principales conclusiones de este trabajo se encuentran la utilización de la TMDSC (Calorimetría Diferencial de Barrido Modulada Térmicamente) que ha permitido reproducir con precisión y analizar la composición de las células solares, la morfología de la capa activa de los transistores y sensores. Al reproducir fielmente la célula solar o la capa activa de un transistor o sensor utilizando un chip de flash DSC, podemos investigar la morfología de la capa activa real mediante calorimetría. Además, gracias al GIWAXS y GISAXS, comprendemos la dirección y el rango de orden en la muestra. Estas técnicas, junto con otros enfoques complementarios, nos permiten comprender y modificar a nuestro antojo la nano y microestructura del material. Para entender la composición real de una hetero-unión de una placa solar como en el capítulo 3) En este apartado se analiza una mezcla mecánica de 50:50 % en peso de donor y aceptor con un recocido en atmósfera saturada. Se determina que, en realidad, la fase entremezclada tiene sólo un 37,5 % de aceptor. Para estudiar los polímeros denominados “casi amorfos” como capa activa en transistores. En el capítulo 4) se demuestra como el IDTBT que hasta la fecha es considerado de esta familia, en realidad tiene un alto orden a escala nanométrica aunque no se aprecie ningún orden a escala micrométrica. Éste tiene 4 temperaturas clave. Asociadas a la formación de agregados de la cadena principal y a las cadenas laterales son -10 y 50 ˚ . A los 150 grados la cadena principal comienza a adquirir movilidad y ordenarse y a 250 ˚ el material funde. Para modificar el estado termodinámico de la fase amorfa a antojo modificando la velocidad de enfriamiento y el estado desde el que se enfría (líquido isotrópico o cristal líquido). En el Capítulo 5, la temperatura ficticia (Tf) de PFO se ajustó para manipular sus propiedades optoelectrónicas, logrando un rango de aproximadamente 30 K dentro del cual Tf podría modularse de manera oportuna (sin requerir tiempos muy largos). Por otro lado, la estabilización de la Tf no se consigue debido a que se necesitaría un enfriamiento excesivamente lento. 105 "En ciencia, algunos experimentos tienen éxito y otros, enseñanza." John C. Polanyi 107 7) Bibliography 1. Semiconductores - Qué son, tipos, aplicaciones y ejemplos https://concepto.de/semiconductores/ (accessed Jan 12, 2023). 2. Jenkins, R. et al. A Review of Thermionic Cathodes. Vacuum 1969, 19 (8), 353–359. 3. Segal, M. et al. Material History: Learning from Silicon. Nature 2012, 483 (7389), S43– S44. 4. Moore, G. E. et al. 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