Development of Novel Π-conjugated Polymer Semiconductors for Organic Solar Cells

Abstract

Bulk heterojunction polymer solar cell (PSC) is a promising photovoltaic technology for clean and renewable energy sources due to its superior advantages such as mechanical flexibility, light weight, low-cost fabrication, and large-area manufacturing compatibility. In this work, two classes of polymers, PzDP and TPT based polymers, were designed and synthesized for the application of PSC. Firstly, two copolymers PzDP24-T and PzDP16-BDT were prepared via Stille-coupling polymerization, which exhibited narrow bandgaps and low-lying energy levels. The PzDP24-T polymer showed n-type dominant charge transport properties with electron mobility up to 2.9×10-2 cm2 V-1, whereas PzDP16-BDT exhibited ambipolar charge transport behavior with balanced electron/hole mobilities up to 2.2×10-3 cm2 V-1 s-1/2.5×10-3 cm2 V-1 s-1 in OTFTs. These PzDP-based polymers were then used as acceptor with PTB7-Th as the donor in all-PSCs. The PzDP24-T/PTB7-Th devices showed relatively low power conversion efficiency (PCE) (~ 0.3%), which caused poor morphology because of insufficient solubility. After the optimization of the backbone structure to PzDP16-BDT, the PCE of PzDP16-BDT/PTB7-Th based devices improved to 1.57%. On the other hand, the inferior bulk charge transport properties for PzDP-based polymers limited the overall PCEs. Secondly, a novel pyridazine-based conjugated monomer TPT was synthesized and incorporated into D-A copolymer (TPT-BDT). Compared to the analogue pyridine-based polymers, the TPT-BDT polymer showed a similar or deeper energy levels of -3.66 eV/-5.55 eV, which suggested the utilization of pyridazine can efficiently modulate the HOMO and LUMO. However, the TPT-BDT showed no performance for hole/electron mobilities in OTFTs due to highly twisted backbone. In addition, as the pyridazine is a ligand of palladium catalyst, the molecular weight of the polymer is low (~ 10 kDa). The relatively low molecular weight is found to be the reason of the severe PCEs (~ 0.11%) for TPT-BDT/PC61BM solar cells.