Design, Synthesis And Characterization Of Soluble Oligothiophene And Derivatives Based Organic Semiconductors

Soluble oligothiophenes possess well-defined structures that are relatively easy to modify and purify, and by design they can show a strong tendency to self-assemble into ordered crystalline domains with superior charge transport properties during solution processing. Moreover many of endcapped oligothiophenes exhibit excellent air stability, which have been investigated extensively for applications in organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs). In this context, this thesis has focused on synthesis, characterization and properties of oligothiophene derivatives.In chapter 2, we describe a new rapid and convenient microwave-assisted synthesis of oligothiophenes. The three soluble oligothiophene derivatives C₁₇PT₄, C₁₇NaT₄ and NaHT2 were prepared by homocoupling of iodide precursors, using Pd(dppf)Cl₂ as the catalyst. Compared with the known synthesis of similar compounds, our approach has greatly facilitated purification and appears to be more environmentally friendly. All the target products are identified by 1H NMR, element analysis and MALDI-TOF. Cyclic voltammetry reveals that the HOMOs of these oligothiophenes are lower than -5.1 eV and TGA reveals that their decomposition temperatures Td are higher than 340 oC.In chapter 3, asymmetrical M1 and symmetrical M2 were synthesized via Pd(PPh3)4 catalyzed Stille-coupling reactions with yields of 81% and 90% respectively. We utilized non-polar toluene and polar DMF as reaction solvents for preparing M1 and M2 separately, in order to decrease self-coupling byproducts. After identification, M1 and M2 were characterized and analyzed by the UV-vis, DSC, TGA and CV. We also study the photovoltaic properties of the two oligothiophenes with devices of ITO/PEDOT:PSS (40 nm)/M1(M2):PCBM (70 nm)/Al (120 nm) under AM1.5G solar illumination at 94 mW/cm². The best performance was obtained for devices with the ratio of molecules to PC71BM at 1:2 (wt/wt), Voc = 0.65 V, Jsc = 2.59 mA/cm², FF = 0.33%, PCE = 0.59% for M1 and Voc = 0.65 V, Jsc = 4.82 mA/cm², FF = 0.35%, PCE = 1.17% for M2, respectively.