Control Of Polymer Band Gap And Its Application In Organic Photovoltaic Cells

According to the concept of "polymer band gap engineering", a series of soluble low band gap polymers were designed and synthesized, including polymers containing phenylene units among the backbone, polymers with electron- and hole- transport units as the side chains, and polymers grafted by porphyrin units. Meanwhile, a series of poly(phenylene vinylene)(PPV) derivatives were synthesized by chemically linked porphyrin units and multi-walled nanotubes(MWNT) as the side chains. They were characterized by FT-IR, 'H-NMR, UV-vis-NIR, PL spectra, cyclic voltammetry and etc. And their corresponding photovoltaic properties were discussed. In addition, MWNT modifying ITO electrodes in organic light-emitting devices were also investigated.With 3,4-dinitrothiophene units as electron-acceptors, thiophene and phenylene units as electron-donors, a low band gap polymer, PDTNTBQ, were synthesized. Due to its poly(heteroarylene methins) backbone, it also has alternating aromatic and quinoid thiophene segments in the main chain. PDTNTBQ has optical and electrochemical band gap of 1.46eV and 1.77eV, respectively.Meanwhile, with the introduction of phenylene units into polymer backbone, a series of poly(heteroarylene methine) polymers were synthesized. Their optical spectra and electrochemical analysis revealed that when the substituent groups of phenylene units charged from hydrogen, methyl, butoxyl to octoxy or decyloxy, their corresponding absorption bands among 400-800nm strengthened, and their corresponding band gaps decreased. While, when the substituent groups increased from octoxy to decyloxy or dodecyloxy, their corresponding band gaps improved due to steric effect. For polymer, PDTDOBQ and PDTTenBQ, their optical band gaps are 1.40eV and 1.45eV, respectively. Otherwise, soluble low band gap polymers could also be obtained by the introduction of double phenylene units into poly(heteroarylene methine) backbones.By controlling the feed ratio of monomers, a series of soluble low band gap polymers, PBTBQ-Oid-co-PBTBQ-An, were synthesized, with oxadiazole as electron-transport units, triphenylamine as hole-transport units and p-methylphenyl as soluble units. They have optical bands among 500-800nm, and their corresponding optical and electrochemical band gaps are among 1.34-1.44eV and 1.64-1.77eV,respectively. Cyclic voltammograms indicated that with the increase of the ratio of oxadiazole units among the side chains, their onset potentials (Eon) of oxidation and reduction generally increased, and thus their corresponding HOMO and LUMO energy decreased. For these polymer/MEH-PPV/PCBM devices, their corresponding short-circuit current(ISc) and energy conversion efficiency generally increased with the increase of the ratio of oxadiazole units among the side chains. For PBTBQ-Oid-co-PBTBQ-An(122)/ MEH-PPV/ PCBM(l/l/4, wt%) device, it has maximum energy conversion efficiency (ECE) of 0.0017%, and open-circuit voltage(Voc) of 0.25V, Isc of 0.0047mA/cm2, fill factor (FF) of 0.24.By porphyrin chemically linked to polymer side chains, a series of porphyrin-containing low band gap polymers, PBTBQ-OR-Porp, were synthesized according to two-step route. These polymers are soluble in common organic solvents, such as THF, Chloroform. They have optical band among 500-800nm. For PBTBQ-OR-Porp/ PCBM devices, their photovoltaic properties, such as Voc, Isc and ECE, increase with the increase of the ratio of porphyrin units among the side chains. While for their bulk heterojunction devices of PBTBQ-OR-Porp/MEH-PPV/PCBM, their corresponding photovoltaic properties generally decreased due to strong energy transfer from MEH-PPV backbone to porphyrin units. However, for PBTBQ-OR-Porp(ll)/ MEH-PPV/PCBM (1/1/8, wt%) device, it has maximum energy conversion efficiency ofO.013%.By porphyrin chemically linked to polymer side chains, two series of porphyrin-containing PPV derivatives were synthesized. They are soluble in common solvents, and are thermally stable before 340 °C. They have optical bands among 400-700nm, and have maximum PL and EL emission at about 650nm and 715nm. Due to strong energy transfer from PPV backbones to porphyrin units, their corresponding photovoltaic properties of bulk heterojunction devices have a decreasing tendence with the increase of the ratio of porphyrin units among the side chains. The maximum energy conversion efficiencies are for Porp-RO-PPV(31)/PCBM(l/4, wt%) and Porp-R-PPV(31)/PCBM(l/4,wt%) devices, with 0.325%and 0.361%, respectively.Seen from SEM images, MWNT-grafted PPV derivatives and MWNT-COO-PORP compounds show much wider diameters of MWNT than original, implying that PPV backbones or porphyrin units have an encapsulating effect on MWNT. And PL spectra revealed strong photoinduced electron (or electron) transfer process from PPVor porphyrin units to MWNT. hi addition, by modifying the ITO electrode of MEH-PPV light-emitting devices by functional MWNT mixture, composed of PSSNa:PVA:MWNT, their threshold voltages and the voltages with full brightness decreased with the increase of MWNT content among PSSNa: PVA: MWNT mixture.