Design and Synthesis of Fluorinated Conjugated Polymers Based on 3,3-Difluoro-2,2-bithiophene for High Performance Polymer Solar Cells

Abstract

Electronic energy level engineering of conjugated polymers including low-bandgap for harvesting a wide range of solar spectrum, deep highest occupied molecular orbital (HOMO) energy level for high open circuit voltage (VOC) and sufficient offset of lowest unoccupied molecular orbital (LUMO) energy levels between polymer donor and fullerene acceptor is a key strategy to achieve high performance polymer solar cells (PSCs) and several modification methods of chemical structure have been proposed to control energy levels of conjugated polymers. Among these methods, introduction of fluorine atom has attracted much attention for the past few years because high power conversion efficiencies (PCEs) over 7% have been reported with fluorine substituted polymer-based solar cells. However, the effects of fluorination on optoelectrical and photovoltaic properties of polymers have not been revealed clearly and have been studied in only few polymer systems. For further investigation, more fluorinated monomers and polymers for PSCs should be designed and synthesized. In this work, we synthesized difluoro-bithiophene as a new fluorinated building block in conjugated polymers for PSCs and various fluorinated copolymers based on difluoro-bithiophene are successfully designed and synthesized in order to clarify the effect of fluorination on the properties of polymers and its device performance of PSCs. First, fluorinated poly(3,4-dialkylterthiophenes) (PDATs) composed of difluoro-bithiophene and 3,4-dialkylterthiophene are synthesized. Fluorination on polymer backbone changes its electronic structure, leading to deeper HOMO energy level and enhances molecular packing of the polymers as evidenced by strong vibronic shoulder in UV-Vis absorption spectrum and pi-pi stacking pattern in GIWAXS. When bulky side chain (ethylhexyl) are introduced as a solubilizing group, fluorinated polythiophenes develop finer fibril structure and exhibit a high PCE of 5.12% with a VOC of 0.87 V and a short circuit current density (JSC) of 9.82 mA/cm2. Fluorinated DA type polymer, copolymerized by diketopyrrolo[3,4-c]-pyrrole (DPP) as an A unit and difluoro-bithiophene as a D unit, is also synthesized for investigating the fluorination effect on D-A type polymer. After introduction of fluorine atoms on DPP-based polymer, deeper HOMO energy level of polymer is observed without significant change of optical properties. Although fluorination increases fibril size and lowers JSC of DPP-based polymer, fluorinated DPP-based polymer exhibits higher PCE of 6.39% than non-fluorinated DPP-based polymer (PCE = 5.47%) due to large improvement of VOC. Furthermore, for investigating the effect of fluorination position on the properties of D−A type polymer, two types of fluorinated polymers are synthesized, HF with fluorination on D unit and FH with fluorination on A unit. Compared to non-fluorinated polymer, fluorinated polymers exhibit deeper HOMO energy levels without change of bandgap and stronger vibronic shoulder in UV−Vis absorption spectra, indicating that fluorination enhances intermolecular interaction. HF exhibits a high PCE of 7.10%, which is higher than the PCE (6.41%) of FH, with well-developed fibril network, low bimolecular recombination and high hole mobility. Finally, D−A polymers with different degree of fluorination are synthesized by using 2,1,3-benzothiadiazole (BT) unit substituted by different number of fluorine atoms. 3F with mono-fluorinated BT and 4F with di-fluorinated BT have deeper HOMO energy levels, stronger vibronic shoulder in UV-Vis absorption spectra, and narrower size of fibril in blend film with PCBM than 2F with non-fluorinated BT. Among fluorinated polymers, 3F exhibits the highest PCE of 7.92% with low bimolecular recombination, high hole mobility and well-developed interconnected network with nanoscale fibril. From these results, it can be concluded that the optoelectrical and photovoltaic properties of conjugated polymer are significantly influenced by introduction of fluorine atoms and fluorinated conjugated polymers are promising materials for achieving high performance PSCs.