Synthesis And Properties Of Solution-Processed Optoelectronic Materials

Significant effort on using solar energy is important way for clean and renewable energysources. Compared with silicon-based solar cells, polymer solar cells (PSCs) have beenattracting more and more attention for their unique advantages, such as low cost, light weight,and the potential for making flexible large area devices by roll-to-roll manufacturing. Toachieve high performance PSCs, the electron-donating conjugated polymers need to havestrong and broad absorption for the solar light, a good hole mobility, reasonable highestoccupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO)level, good solubility and film-forming property, and so on.Organic light-emitting diodes (OLEDs) have drawn great attentions from both academicand industrial communities due to their potential applications in fullcolor flat panel displays,backing-lighting sources, and solid-state lighting. Traditionally, the classic materials ofOLEDs are small molecules and polymer. The device fabrication based on small moleculesrequires vacuum-deposition equipment, which is wasteful of materials and limit the displaysize. And the conjugated polymer emitters could be used by solution process such asspin-coating, ink-jet printing or roll-to-roll printing, which are the advantages of simplerfabrication and lower production cost, especially for the larger-area devices. Hence, it is veryimportant to develop new conjugated polymers for optoelectronic device applications.In this thesis, we present two aspects of ourthesis studies, first topic includes theimprovement of the copolymers structural planarity leading to the enhanced hole mobility andred-shifted absorption spectrum; second topic includes the supramolecular conjugatedpolymers based on the reversible host-guest interactions between the dibenzo-24-crown-8(DB24C8) and the dibenzylammonium salt (DBA) for application in light-emitting devices.In the chapter2, a series of narrow band-gap donor-acceptor (D-A) conjugatedpolymers, with thiophene substituted quinoxaline monomer (TTQx) or its cyclizedphenazine derivative monomer (TTPz) and fluorene or carbazole, were synthesized viaSuzuki coupling reaction. It was found that the copolymers based on thiophenesubstituted quinoxaline exhibit obviously red-shifted absorbance compared to previously reported D-A copolymers based on phenyl substituted quinoxaline. Their analogouscopolymers based on the cyclized acceptor TTPz show more pronounced red-shiftedabsorption spectra with a significantly decreased band gap, due to the enlarged planarpolycyclic aromatic ring of TTPz. Moreover, compared to the copolymers based onTTQx, the TTPz based copolymers’ mobilities are also significantly increased due to thereduced steric hindrance and improved structural planarity among the copolymers.Bulk-heterojunction polymer solar cells based on the blends of the copolymers with afullerene derivative as an acceptor exhibit promising performance and the best deviceperformance with power conversion efficiency up to4.4%was achieved.In the chapter3, the9-alkylidene-9H-fluorene unit is inclined to take on a planarconformation, due to the sp2-hybridized carbon at the9-position. The copolymers based on9-alkylidene-9H-fluorene unit and quinoxaline or2,1,3-benzothiadiazole were beensynthesized and investigated for the photovoltaic properties. The coplanar copolymersfacilitate the polymer backbones forming close packing in solid atate to enhance chargecarrier transportation. PSCs based on on the blends of the copolymers with PC71BM as anacceptor exhibit promising performance, the highest power conversion efficiency is6.85%with a high fill factor of72.6%.In the chapter4, we designed and synthesized the supramolecular copolymers based onthe host-guest interactions between DB24C8and DBA, which is used as host materials inphosphorescent OLEDs. The two homoditopic monomers ended with DB24C8or DBAself-organize into a linear supramolecular polymer at high concentration. The formation of thesupramolecular polymer was confirmed by1H NMR, differential scanning calorimetry (DSC),viscosity and optical properties studies. The formation supramolcular polymer maintains thesame ETof the monomers. It is a novel approach for design of solution-processed hostmaterials with high triplet level by host-guest interactions.In the chapter5, we reported the first use of host-guest interaction-based supramolecularlight-emitting polymers (SLEPs) for solution-processed electroluminescent devices. TheDB24C8functionalized blue-emitting conjugated oligomer and green-emitting conjugatedoligomer were used as the host materials, and the DBA functionalized blue-emittingconjugated oligomer was used as the guest material. The resulting linear SLEPs were obtained from the self-organization of the host and guest oligomers, which was confirmed by1H NMR,viscosity and DSC studies. Highly fluorescent SLEPs nanofibers can be easily obtained bydrawing or electron-spinning from the equimolar solution of the host and guest oligomers.The photophysical and electroluminescence properties of the resulting SLEPs were fullyinvestigated. It was found that the SLEPs’ emission colors can be well tuned from blue togreen with significantly enhanced photoluminescent efficiencies by using host green oligomeras the dopant, which is due to the efficient energy transfer caused by the exciton trapping onnarrow band gap host green oligomer in the SLEPs. As a result, the designed SLEPs showedcomparable electroluminescence device performances to those analogous traditionalconjugated polymers. Considering the precisely defined starting monomers and catalyst-freepolymerization process for the designed SLEPs, combining the good device performances, thepresent study provide a promising alternative route to develop solution processedsemiconductors for optoelectronic applications.In the chapter6, we designed and synthesized the supramolecular copolymers based onhost-guest interactions and applied it to white OLEDs. The DB24C8functionalizedblue-emitting, green-emitting and red-emitting conjugated oligomers were used as the hostmaterials, and the DBA functionalized blue-emitting conjugated oligomer was used as theguest material. The white OLEDs supramolecular polymers obtained by doping differentcontents of green and red-emitting monomers. The photophysical and electroluminescenceproperties study showed that this supramolecular copolymer is a promising material for whiteOLEDs.