Design, Synthesis, And Optoelctronic Properties Of Novel Poly(Para-phenylene Vinylene) Derivates (PPVs)

PPVs are the most intensively studied conjugated polymer materials which have been applied to organic light-emitting devices (OLEDs), solar cells and biosensors. However, it has been found that their holes mobilities (μ⁺) are one to three orders of magnitude higher than their electrons mobilities (μ^-). Concerning to OLEDs, many efforts have been made to address this issue. Among others, the most widely used practice is to induce an electron transporting material to block holes. Moreover, electron-withdrawing groups such as cyano, fluorine, and nitrogen-containing heterocycles may improve the ability of electron transport, balance electron and hole transportation so as to enhance the quantum efficiencies. It is well known that the molecular weight can be controlled by changing polymerization conditions. The introduction of bulky aromatic rings to conjugated polymers may also enhance the stability, especially the thermal stability, of conjugated polymers. In addition, materials generally favor moderate molecular weights and bulky substituent groups or long side chains on the polymers. Higher conductivity and suitable intermolecular aggregation are essential for solar cells, polarity and coplanation of molecules may increase intermolecular aggregation, and may improve the energy conversion efficiency. The polymer with large band gap, sufficient conductivity, small size of clusters is desirable as holes conductor in solar cells.Herein, twelve novel PPV derivates were designed and successfully synthesized through Gilch or Wittig reaction. The application of resulting polymers to OLEDs and solar cells was investigated. The main work is concentrated on the following aspects:1. Four novel PPV derivates (P1, P2, P3, P4) containing electron withdrawing fluorine atom and twisted terphenyl moiety were synthesized. Due to fluorine atom and terphenyl structure, the polymers exhibited good thermal stability withdecomposition temperatures higher than 400℃. Poly(2-(4'-(2''-ethylhexyloxy)-benzyl)-5-(2,4-di-fluoro-benzene)-1,4-phenylene vinylene, P2) and poly(2-(4'-dodecyloxy-benzyl)-5-(2,4-difluorobenzyl)-1,4-phenylene-co-2-methyloxy-5-2''-eth ylhexyloxy)-l,4-phenylenevinylene, P4 are soluble in common organic solvents. They exhibited bright fluorescence with higher absolute quantum efficiency over 33.2 and 18.1% in the solid state, respectively. The emission peaks of their solid-state photoluminescence (PL) were 499 and 585 nm, respectively. They are blue-shifted than those of PPV and MEH-PPV, respectively, which is attributed to their twisted terphenyl structure. Their lifetimes of transient fluorescence were all around 3 ns corresponding to the singlet fluorescence. The double-layer devices (ITO /PEDOT /Polymer /Ba /Al) were fabricated for P2, P4, and their maximum brightness was 598 and 200 cd/m² at 24 and 15 V with the current density of 100 and 80 mA/cm², onset voltage was 3.1 and 4.3 V, respectively.2. Four novel poly-(p-phenylene vinylene) (PPV) derivatives (P5, P6, P7, and P8) with pendent 2,4-difluorobenzene or fluorene moieties were designed and synthesized via Gilch reaction. The absolute quantum efficiency (η_a) of P5, P6, P7, and P8 as thin films were measured (36.7, 37.2, 20.3, and 17.5%, respectively). The products were thermally stable with decomposition temperatures above 400℃. Double-layer devices were fabricated with the onset voltage of 4.2, 4.8, 4.0, and 8.8 V for P5, P6, P7, and P8, respectively. The maximum luminance of 2700, 450, 4700, and 1700 cd/m² were obtained for P5, P6, P7, and P8, with the current density were 100, 24, 15, 55 mA/cm² at 7.8, 12, 7.0 and 13.8 V respectively, and then the maximum external quantum efficiencies were 0.56, 0.59, 1.16, and 0.38%, respectively.3. Two novel cyano-containing PPVs (CN-PPVs, P9, P10) with different solubilizing alkoxyl groups were synthesized through Gilch reaction from the monomer 2-cyano-5-(4'-alkoxyphenyl)-l,4-bisbromomethylbenzene. The results showed that the solubility, thermal stability, and fluorescence properties of CN-PPVs varies with the variation of molecular weights. The relationship between the molecular weight of the polymers and the amount of 4-methoxyphenol was consistent with anionic polymerization, of which the molecular weight is known to be inversely related to the amount of the initiator. The fluorescence emission maxima ranged from 457 to 520 nm, exhibiting appreciable red-shift with the increase of the molecular weight. These series of novel CN-PPVs also exhibited high thermalstability.4. Two polymers (P11, P12) containing phenyl and thiophene in main chain were designed and synthesized via Wittig reaction. The polymers held wide absorption extending from 300 to 650 nm. End-capped aldehyde enhanced intermolecular aggregation and fluorescene quenching was quite serious. The solar cells devices were fabricated with the structures of ITO/PEDOT:PSS/Polymer/Al and ITO /TiO₂/Dye /Polymer/Pt. The solar cells results are as follows. A). V_oc: 333 mV, I_sc: 3.86 mA/cm², FF: 29.0, η: 0.47%; B). V_oc 316 mV, I_sc: 2.85 mA/cm², FF: 34.3, η: 0.43%; C). V_oc: 348 mV, I_sc: 1.40 mA/cm², FF: 31.3, η: 0.83%; and D). V_oc: 618 mV, I_sc: 3.31 mA/cm², FF: 43.7, η: 0.28%, respectively.The results suggested that these series of polymers are promising materials for OLEDs and/or solar cells.