Conjugated Materials Comprising Hydrophilic Groups: Synthesis And Interfacial Modifications In Polymer Light-Emitting Diodes And Bulk Heterojunction Photovoltaic Cells

Conjugated polymers have been developed into a class of materials that provide structurally controllable properties ranging from chemical, physical and biological researches and applications. As one of the most fundamental category in conjugated polymers, conjugated polyelectrolytes and their neutral analogues comprise an electronically delocalized backbone with hydrophilic pendant groups. They combine the semiconducting properties of the neutral polymers with charge-mediated behavior of polyelectrolytes. Recently, these conjugated polyelectrolytes have attracted increasing attention due to their unique properties and potential applications in optoelectronic and biological devices.On the basis of our previous contribution, neutral poly(2, 7-carbazole) polymers (PC-N, PC-NOH, and PC-P) and polyelectrolytes (PC-NBr and PC-SO3Na) with hydrophilic pendant groups of ammonium, phosphonate, and sulfonate are synthesized as interlayers for cathode modifications in bulk-heterojunction photovoltaic cells (BHJ PVCs). UV-vis absorptions, light emisisons, energy levels of the polymers are particularlly investigated. PC-N, PC-NBr, PC-NOH, and PC-P are selected to construct ultra thin interlayers in BHJ PVCs with PFO-DBT35:PCBM=1:4 as the active layer. Compared with traditional Al cathode, bilayer cathodes show improvements of open-circuit voltages (Voc) and short-circuit currents (Isc) of the PVCs. PC-NOH is the best for the photovoltaic performances and over 20% increase of power conversion efficiency is achieved. These bilayer cathodes would have great potential to further elevate the power conversion efficiency of BHJ PVCs with other high-efficient active layers.In chapter 3, Alcohol/water-soluble 2, 7-carbazole copolymers PCP-NOH, PCP-EP and PCF-NOH comprising surfactant-like diethanolamino and phosphonate end groups on the side chains are synthesized respectively, and utilized as efficient electron injection layer (EIL) in polymer light-emitting diodes (PLEDs). The UV-vis absorption, photoluminescence properties and energy levels of PCP-NOH, PCP-EP and PCF-NOH are mainly determined by the conjugated main chain. Multilayer PLEDs with a device configuration of ITO/PEDOT:PSS/emissive layer/EIL/Al are successfully fabricated. With fluorescent PFO-DBT15 as the emissive layer, the PLEDs using the PCP-NOH, PCP-EP and PCF-NOH as the EIL display higher luminous efficiency (LE) than sole Al cathode and Ba/Al cathode. With a phosphorescent Ir(mppy)3 doped PVK blend as the emissive layer, the PLEDs with the EIL showed high maximum LE of 39.3 cd/A for PCP-NOH and 42.5 cd/A for PCP-EP, in comparison to 0.35 cd/A for sole Al cathode and 32.2 cd/A for the Ba/Al cathode. Inserting the polar alcohol-soluble polymers as interlayer can significantly increase built-in potentials of the PLEDs, from which electron injection barrier from the Al electrode is decreased. The results indicate that the PCP-NOH, PCP-EP and PCF-NOH are excellent electron injection polymers for high performanced PLEDs with high work function Al electrode.In chapter 4, oligothiophenes with thiols in the tail of the conjucated backbones are synthesized and utilized as surfactants in organic/inorganic hybrid solar cells. TEM images show these surfactants can avoid spherical CdSe nanocrystal forming large self-aggregates both in sole nanocrystals and the blend of MEH-PPV and spherical CdSe system. They obviously ameliorate the structural cooperation of an organic donor and an inorganic accepter, which inturn facilitate the excitons separation between the interface of MEH-PPV and CdSe nanocrystals.In chapter 5, new low band-gap copolymers PFO-DBT35-SH15 and PFO-DBT35-SH30, containing 2, 7-fluorene and 4, 7-bis(5-bromo-2-thienyl)-2, 1, 3-benzothiadiazole with thiols at the end of the side chains are synthesized to explore new polymer donors for polymer/nanocrystal hybrid solar cells. The UV-vis absorption and energy levels of the polymers are particularly investigated, which are mainly determined by the conjugated backbones, and similar with their neutral analogue PFO-DBT35. TEM images show the blend of each polymer and spherical CdSe nanocrystals are very uniform and well-distributed. Then, the optimization of synthetic route is also investigated to guarantee the purity quotient of the polymers, which is one of essential issue for high-efficient polymer donors in hybrid solar cells.