Polymer based on difluorinated donor unit yield high-efficiency polymer solar cells

Abstract

In the development of high-efficiency organic solar cells (OSCs), fluorination is believed to be an efficient strategy for tuning energy levels and improving crystallinity, thus yielding better OSC performance. However, current research on fluorinated polymers focuses only on fluorination that occurs on the acceptor part, and seldom provides successful examples of fluorination on the donor part. The present study synthesized and characterized several fluorinated polymers with temperature-dependent aggregation (TDA) properties. Devices based on these polymers exhibited strong intermolecular aggregation, high crystallinity, high charge carrier mobility, and excellent OSC performance. In chapters 2 and 3, the synthesis and characterization of six different polymers based on the difluorinated bithiophene unit with TDA properties were discussed to study the effect of fluorination on polymers. With two fluorine atoms introduced, the polymers exhibited ideal domain sizes (20–30nm) and suitable crystallinity, thereby leading to high power conversion efficiencies (PCEs). Compared to PBT4T-2OD, both PffBT4T-2OD and PBTff4T-2OD exhibited dramatically improved PCEs (10.8% and 10.4%, respectively). Further fluorination with four fluorine atoms on each repeating unit resulted in a higher crystallinity and larger domain size, which are destructive to device’s performance. Herein, PffBTff4T-2OD can only yield an efficiency of 4.2%. Chapter 4 explored the synthesis of a novel TDA polymer (PTFB-O) with intentionally reduced lamellar stacking and crystallinity via the introduction of a less symmetric monomer unit. While conventional TDA polymers (PffBT4T-2OD or PTFB-P) performed better when combined with fullerenes, this new type of TDA polymer matched particularly well with small molecule acceptors (SMAs) to yield a high PCE of 10.9%. To understand why PTFB-O works particularly well with SMAs, we compared PTFB-O with an analog polymer (PTFB-P) with nearly identical chemical structures except for a minor difference in the fluorination position and thus the symmetry of the corresponding monomers. The structure-property relationship revealed in our work may also be applicable to organic materials in other optoelectronic applications.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology