Design and Synthesis of Isoindigo-Based Conjugated Polymers for High Performance Polymer Solar Cells

Abstract

For high-performance polymer solar cells (PSCs), the molecular engineering of conjugated polymer is significantly important, because the chemical structure of the polymer directly influences on the optoelectronic properties including optical bandgap, extinction coefficient, frontier energy levels, molecular dipole moment, and charge carrier mobility. Optical properties such as bandgap and absorptivity are key factors in conversion process of solar energy to electrical energy. And electronical properties such as energy level, dipole moment, and mobility are related to generation and collection of the free charge carriers, which are separated to electrons or holes from photons. Moreover, physical properties such as crystallinity, solubility and miscibility with fullerene derivatives are conclusively determined by chemical structure of the polymer. To achieve the suitable properties of conjugated polymers, the most efficient strategy is the design of push-pull copolymer, which is copolymerized with alternatively electron-rich monomer and electron-deficient monomer. This method induces intramolecular charge transfer between two units in polymer backbone, resulting in redistribution of molecular orbitals (MO) of each unit. This hybridization of MOs generates new occupied and unoccupied MOs with a narrow bandgap of the polymer. Furthermore, the appropriate combination of electron-rich moiety and electron-deficient moiety can fine-tune other properties for achieving highly efficient PSCs. In this thesis, we studied the conjugated polymers containing isoindigo dye as electron-deficient moiety in push-pull copolymer system. For high-performance PSCs, we designed and synthesized three kinds of novel isoindigo-based copolymers by modifying bithiophene as electron-rich moiety and investigated the photovoltaic performance of the PSCs. First, a conjugated polymer consisting of isoindigo and thieno[3,2-b]thiophene (TT) was synthesized via the Stille coupling reaction in order to demonstrate the effect of molecular fusion from bithiophene to TT on the properties of polymers and those photovoltaic performance of PSCs. Incorporation of TT as electron-rich moiety on conjugated polymer manipulates its photophysical and electrochemical properties, including to lowered optical band gap (1.55 eV) in UV-visible absorption spectra and deeper HOMO energy level (5.44 eV) determined by cyclic voltammetry because of the larger resonance stabilization energy of the fused ring. Furthermore, the polymer with TT shows highly ordered crystallinity along  stacking direction, leading to high charge carrier mobility in photoactive layer. PiITT consisting of isoindigo and TT exhibits a power conversion efficiency (PCE) of 6.96% with a short-circuit current density (JSC) of 12.42 mA cm2, an open-circuit voltage (VOC) of 0.91 V and a fill factor (FF) of 0.67 when the PSCs were fabricated from the blend of the polymer and PC71BM. Secondly, highly -extended isoindigo-based copolymer consisting of isoindigo and thienylvinylene (TVT) was designed and synthesized for strong  interaction with large overlapping area between chain backbones due to rotational freedom between consecutive aromatic units. Although introduction of TVT in isoindigo-based polymer does not effect on optical band gap and electronic energy levels, the polymer PiITVT composed of isoindigo and TVT shows higher absorption coefficient and highly ordered crystallinity in photoactive layer. Moreover, in optimized condition for PSCs, PiITVT has preferential molecular orientation (face-on orientation) when it was demonstrated by grazing incident wide angle X-ray scattering measurement and the well-developed morphology with finer fibril of PiITVT when it was observed by transmission electron microscopy. PiITVT-based PSCs shows a high PCE of 7.09% with a JSC of 13.2 mA cm2, a VOC of 0.91 V and FF of 0.59. It is higher than that of PiI2T containing bithiophene (5.55%), because of the better coplanar structure of the TVT unit than the bithiophene unit. Third, the fluorinated isoindigo-based polymer was designed and synthesized by combining isoindigo with bulkier alkyl side chain and fluorinated bithiophene for high-performance PSCs. And the polymer are soluble in non-halogenated solvent such as o-xylene. The use of non-halogenated solvents such as o-xylene for fabrication of high performance PSCs has recently attracted much attention from academia and industry, because the halogenated solvents and additives cause serious environmental, health and safety problems. When the fluorinated polymer-based PSC is fabricated with o-xylene as processing solvent and diphenyl ether as additive, the cell exhibits a superior PCE of 8.80% with a VOC of 1.06 V, which are one of the highest values among PSCs processed with non-halogenated solvents. Our work successfully demonstrates that the combination of introduction of bulky alkyl side chain and substitution of fluorine atom on the conjugated polymer backbone is a promising strategy for eco-friendly device fabrication and highly efficient PSCs with high VOC. We revealed that three kinds of modification methods which are introduced in this thesis are promising design strategies for high-performance isoindgo-based PSCs and how each method achieves improved performance.