Desigen, Synthesis And Properties Of Polymer Solar Cells Based On Diketopyrrolopyrrole Conjugated Polymers

With the development of human society and the gradual depletion of fossil resources, more and more attentions are taken to search for alternative energy sources. As a clean energy resource, the solar energy has attracted widespread interest. Polymer solar cells has become a hot research area in recent years due to their fascinating characters such as easy processing, low cost, and flexibility in comparing with traditional inorganic solar cells. Conjugated polymers based on electron-deficient diketopyrrolopyrrole(DPP) units have been intensively explored and applied in solution-processed organic electronic devices. Due to their good planarity and strong intermolecular ?-? interactions of the conjugated backbone, DPP polymers show high carrier mobilities in field-effect transistors(FETs), exceeding those of amorphous silicon. The strong electron-withdrawing character of DPP units also causes DPP polymers to absorb near infrared light and their absorption spectra cover a significant part of the solar spectrum. As a result, DPP-based polymer solar cells(PSCs) can provide high photocurrents and achieve excellent power conversion efficiencies(PCEs). Compared to other high performance polymer semiconductors, DPP polymers show significantly less synthetic complexity. All these merits indicate that DPP polymers have great potential for large-area industrial application.This dissertation focuses on the design, synthesis, and the study of the relationship between chemical structure and organic electronic properties of polymer materials, and the specific contents are as follows:1. We plore a conjugated polymer consisting of three acceptors: diketopyrrolopyrrole(DPP), benzothiadiazole(BT), and naphthalenediimide(NDI) and investigate its charge transport properties. The new regioregular polymer PDPP2T-BT-co-NDI is found to have an ultra-small band gap of 1.21 eV, deep LUMO level at-4.28 eV and a HOMO level at-5.49 eV. The polymer is used to fabricate polymer nanowire FETs, which show high hole and electron mobility of 1.61 and 0.98 cm2 V-1 s-1 respectively in inert atmosphere and 1.64 and 0.14 cm2 V-1 s-1 in ambient air. These results demonstrate that the design strategy of incorporating multi-electron acceptors into one conjugated polymer is an effective method to finely tune the energy level for high performance ambipolar OFETs.2. We show the first example of asymmetric DPP polymers by incorporating two different aromatic substituents. The new DPP polymers, PDPPTT-2T and PDPPMT-2T, possess a high molecular weight(> 100 kg mol-1) and small band gap, and show good solubility in toluene, but at the same time afford highly crystalline thin films. Processing either of the two polymers from toluene with diphenyl ether(DPE) as additive provides a hole mobility as high as 12.5 cm2 V-1 s-1. In addition, PSCs based on the two polymers as electron donor, processed from a toluene/DPE solvent mixture, show PCEs of 6.5 %. The results demonstrate that incorporating structural asymmetry into the main chain via different aromatic substituents is an efficient strategy to achieve high performance FETs and PSCs of DPP polymers using non-chlorinated solution-processing.