Preparation And Properties Of The Organic Photovoltaic Materials Based On Oligothiophene Donors

In recent years, the power conversion efficiency (PCE) of the organic solar cellshave been greatly improved. The PCEs of solution-processed polymers and smallmolecules solar cells have reached9.2%and8%, respectively, and tandem deviceshave reached10.6%. Developing organic photovoltaic materials with novelstructures and desired performance is of great scientific and practical significance inthe field of organic electronics for commercial motives.In this thesis, we designed and prepared donor-acceptor structured organicphotovoltaic materials based on oligothiophene, and systematically studied the effectof energy levels and micro-morphology on photovoltaic performance. In detail, thedissertation is divided into three chapters:Firstly, we optimized the homopolymer of3,3’’’-didodecyl quaterthiophene(DDQT) PQT-12. Compared to P3HT, PQT-12has a similar absorption spectrumbut with better crystallinity, higher carrier mobility and lower HOMO level which isadvantageous for obtaining higher open circuit voltage and stability. Because of it’stendency to form nanoparticle dispersions in solution, the devices made from blendsof PQT-12and fullerene consequently contained pre-formed PQT-12crystallinenanoparticles, which caused mesoscopically demixed blends with lowcharge-separation efficiency. The optimal PCE of PQT-12/PCBM system is lowerthan1%. In an effort to tailor their energy levels and micro-morphology of PQT-12,we have synthesized two new QT-based copolymers, namely PQTF and PQTTSTby introducing fluorene and2,5-dithienylsilole units as strategic building blocks totune the HOMO and LUMO levels, respectively. GIWAXS tests found that thecrystallization behavior and crystal orientation of the copolymers were changedsignificantly. The PQTTST/PCBM system showed a PCE of1.49%higher than thatof PQT-12/PCBM system (1.03%).In an effort to reduce the aggregation of3,3’’’-didodecyl quaterthiophene based materials, in the second section we focused on the design and synthesis of solutionprocessed small-molecule bulk-heterojunction (BHJ) solar cells by reducingintermediate unsubstituted thiophene number. According to previous literature, theband gap of photovoltaic material based on TPD is1.7~1.9eV, which is notbeneficial to the absorption of sunlight. Particularly, the LUMO levels of theTPD-based D–A conjugated polymers are mostly higher than-3.6eV, which did notmatch optimally with the LUMO energy level of PCBM. In an attempt to tune theelectronic properties (band gap and HOMO/LUMO energy levels), we conducted astructural modification of the TPD unit by fusing with a naphthalene unit through apyrimidine to afford a perinone-type structure (TPN). By fusing naphthalenemonoimide with a thienoimidazole unit, we can obtain TIN monomer, which is theisomer of TPN. Here we synthesized a series of donor–acceptor–donor-typeoligomers3TTPD3T,3TTPN3T, and3TTIN3T based on TPD, TPN, TIN asacceptors and3,3’’’-dioctyl-2,2’:5’,2’’-terthiophene as donor. Optical andelectrochemical studies showed that structural modification of TPD exhibited aslight impact on the band gap and HOMO/LUMO energy levels, but played animportant role in determining the morphology of solid-state packing. The largerconjugation plane counterpart3TTPN3T and3TTIN3T showed an improvedstructure order than3TTPD3T.The structurally twisted compound3TTIN3Tshowed a strong crystalline nature with large crystalline domains of several hundrednanometers.3TTPD3T/PC71BM blends with a weight ratio of2:1showed a bestpower conversion efficiency of1.87%. Jsc=4.93mA cm-2, Voc=0.87V，FF=44.Finally, because the D–A–D-type oligomers have large bandgap which leads tonarrow absorption range and small short-circuit current. Thus, we designed tointroduce new receptors at both ends of the oligomers to enhance intramolecularcharge transfer, and, thus, narrow the optical bandgap and, equally important,improve solubility. Furthermore, we introduced acetylenic bond to enhanceconjugation effect, lower the HOMO level, and improve open circuit voltage and the charge mobility. The D-A-D core consists of a central aromatic alkyne (BDT,thiophene) unit flanked by two3,3’’’-dioctyl-2,2’:5’,2’’-terthiophene (donor) units.Octyl cyanoacetate units end-cap the molecule to increase intramolecular chargetransfer. Preliminary tests found photoelectric properties, the maximum absorptionpeak of oligomer11is469nm, and the compound12appears in468.5nm. TheHOMO energy level of two oligomers were-5.34,-5.29eV, respectively; LUMOenergy level were-3.37,-3.42eV; bandgaps were1.97,1.87eV.