All Polymer Solar Cell Materials: Structure−Property Relationship For Low Bandgap P(Ndi2od-T2) Solutions And Blends

Abstract

Nowadays, energy crisis and environment pollution are two big challenges that restrict the development of society. Energy is a very important driving force to improve the standard of living and develop a country. The most versatile material class used in the field of organic photovoltaics is called π-conjugated polymers. Solar energy is a sustainable, environmentally friendly, unlimited energy from the sun and renewable energy source. The morphology of the active film is important for the efficiency of the solar cells. The most important branches in materials science is called polymer blend which has gained considerable attention to meet multifunctional need. Blends of two different polymers are likely to form a large phase separated structure; this is an inherent characteristic of polymers with a long main chain. Phase diagrams display specific information in terms of when the phase separation occurs and which phase separated structure can be formed and therefore can be a suitable guidance to the phase separation of polymer blend. In this study, the phase diagrams of n-type low bandgap P(NDI2OD-T2) solutions and blends were constructed. To this end, we employed the Flory Huggins lattice theory for qualitatively understanding the phase behavior of P(NDI2OD-T2) solutions as a function of solvents (chlorobenzene, chloroform, and p-xylene). Herein, the polymer-solvent interaction parameter was obtained from a water contact angle measurement, leading to the solubility parameter. The phase behavior of these P(NDI2OD-T2) solutions showed both liquid-liquid and liquid-solid phase transitions. However, depending on the solvent, the relative position of the liquid-liquid phase equilibria and solid-liquid phase equilibria (i.e., two-phase co-existence curves) could be changed drastically, i.e., LLE > SLE, LLE ≈ SLE, and SLE > LLE. Finally, we studied the phase behavior of the polymer-polymer mixture composed of P(NDI2OD-T2) and r-reg P3HT, in which the melting transition curve was compared with the theory of melting point depression combined with the FH model. The FH theory describes excellently the melting temperature of the r-reg P3HT/P(NDI2OD-T2) mixture when the entropic contribution to the polymer-polymer interaction parameter (ꭓ =116.8 K/T−0.185, dimensionless) was properly accounted for indicating an increase of entropy by forming a new contact between two different polymer segments. Understanding the phase behavior of the polymer solutions and blends affecting morphologies plays an integral role towards developing polymer optoelectronic devices. We report the phase behavior of amorphous/semicrystalline conjugated polymer blends xvii composed of low bandgap PCPDTBT and P(NDI2OD-T2). As usual in polymer blends, these two polymers are immiscible because 0 and 0 m m     S H , leading to 0   Gm , in which m S ,H m , and Gm are the entropy, enthalpy, and Gibbs free energy of mixing, respectively. Specifically, the FH interaction parameter for the PCPDTBT/P(NDI2OD-T2) blend was estimated to be 1.26 at 298.15K, indicating that the blend was immiscible. When thermally analyzed, the melting and crystallization point depression was observed with increasing PCPDTBT amounts in the blends. In the same vein, the X-ray diffraction patterns showed that the π-π interactions in P(NDI2OD-T2) lamellae were diminished if PCPDTBT was incorporated in the blends. Finally, the correlation of the solid-liquid phase transition and structural information for the blend system may provide insight for understanding other amorphous/semicrystalline conjugated polymers used as active layers in all-polymer solar cells, although the specific morphology of a film is largely affected by nonequilibrium kinetics. The thesis is organized into six chapters. Chapter 1 gives a brief general introduction. Chapter 2 focuses on a literature review on the topic of the study. The third chapter focuses on the experimental methods and materials adopted for the present work. The fourth chapter deals with the phase diagrams of n-type low bandgap naphthalenediimide-bithiophene copolymer solutions and blends. Chapter 5 focuses on the phase behavior of amorphous/semicrystalline conjugated polymer blends for which PCPDTBT and P(NDI2OD-T2) was chosen as a model system. Last chapter 6 includes the general discussion of the investigations, overall messages, strengths and limitations, conclusions drawn from the works, and recommendations relates to the outlook for future work. In each case, the goal is to understand the phase behavior of the polymer solutions and blends affecting morphologies that plays an integral role in developing polymer optoelectronic devices.