| Organic photovoltaics (OPVs) have emerged as a promising alternative technique for producing clean and renewable energy due to their potential for fabrication of low cost, large-area devices on flexible substrates by simple means of painting or printing from polymer solutions. Hybrid polymer-inorganic nanocrystal (NC) solar cells consisting of a hole-conducting conjugated polymer and inorganic semiconducting nanocrystal, like CdS, CdSe and ZnO have attracted considerable research attention, since the advantages of two classes of materials can be effectively combined, such as low fabrication costs due to easy solution-based processing, high optical absorbance and flexible substrate manufacturing of the conjugated polymer matrix and high electron conductance, tunable optical bandgap, and carrier multiplication due to the semiconducting NCs.The conventional methods for preparing nanocrystal-conjugated polymer composite materials rely upon synthesizing nanocrystals separately, and then mixing them with the conjugated polymer. This approach has some significant drawbacks: firstly, a surfactant must be used to control nanocrystal size, shape and distribution. Although the incorporation of the surfactant into an organic/inorganic composite can improve distribution of nanocrystals in a conjugated polymer, most surfactants are insulating which will impede the photo-induced electron transfer and the charge transport in the blend and thus reduce the efficiency of the devices. Secondly, the mixing approach requires the use of co-solvents. It is hard to find a solvent that can accommodate both nanocrystals and conjugated polymers and, hence, the processing window for such combinations of materials is often quite narrow. In addition, actual control over the morphology of the hybrid bulk heterojunction is extremely difficult.In this dissertation, firstly, a novel method to in-situ synthesis of one-dimensional rodlike ZnO nanocrystals directly in the presence of a self-assembling diblock copolymer-poly(3-hexylthiophene)-b-poly (zinc methacrylate acetate)(P3HT-b-PZnMAAc), where the P3HT-b-PZnMAAc is acting as a molecular template for geometrical manipulation of rodlike ZnO nanocrystals and, meanwhile, as a precursor for ZnO nanoparticles. HRTEM and SAED reveal that rodlike ZnO nanocrystals are assembled by ZnO nanoparticles with almost the same orientation due to the dipole-induced interaction between adjacent ZnO nanoparticles. SEM images show that the rodlike ZnO nanocrystals are homogenously dispersed in the polymer matrix without obvious macrophase separation and its length can be controlled by adjusting the hydrolysis time. In the nanocomposites, as rodlike ZnO nanocrystals synthesized in a well-defined morphological confinement from the self-assembly of a diblock copolymer dispersed closely to P3HT chains with a high interface area, the photo-generated excitons are easy to be separated into electrons and holes at the interfaces, resulting in the strong photoluminescence quenching of70%observed in the P3HT/ZnO nanocomposite film hydrolyzed for1h. These results indicate that this type of P3HT/ZnO nanocomposite films can be promising candidates for photovoltaic applications. The device based on P3HT/ZnO nanocomposite films hydrolyzed for1h yields a power conversion efficiency of0.19%under AM1.5G illumination from a calibrated solar simulator with an intensity of100mW/cm2.Then, a cross-linked block copolymer poly(3-hexylthiophene)-b-poly(zinc dimethacrylate)(P3HT-b-PZn(MA)2), which acted as precursor for the preparation of poly(3-hexylthiophene)/ZnO (P3HT/ZnO) hybrid film by in-situ hydrolysis, was rationally designed and synthesized via nitroxide-mediated in-situ polymerization of zinc methacrylate (Zn(MA)2) using poly(3-hexylthiophene) alkoxyamine (P3HT-TIPNO) as macroinitiator for the purpose of stabilizing the P3HT/ZnO hybrid solar cells. The cross-linking was confirmed by the insolubility of the film in organic solvents and Fourier-transform infrared experiment. With the function of the cross-linked template, the diffusion of ZnO nanoparticals prepared by in-situ hydrolysis could be lowered to suppress the formation of large aggregations, which favored the formation of a better and more stable interpenetrating network and provided more heteroj unction interfaces for exciton dissociation. As a result, the inverted device based on cross-linked P3HT/ZnO hybrid film obtained by in situ hydrolyzing P3HT-b-PZn(MA)2block copolymer yielded a power conversion efficiency of0.45%under AM1.5G illumination from a calibrated solar simulator with an intensity of100mW/cm2, and the deterioration of the photoconversion performance was suppressed in the hybrid solar cells with the cross-linked P3HT/ZnO compared to cells with non-cross-linked P3HT/ZnO obtained by in situ hydrolyzing P3HT-TIPNO/Zn(MA)2blend film.Lastly, a novel approach to improve the performance of P3HT/ZnO hybrid photovoltaic devices by binding the4,7-diphenyl-2,1,3-benzothiadiazole-based liquid crystal (LC) with a mono-thiol end group onto the surface of ZnO nanoparticles (LC-ZnO). The attachment of LC onto ZnO nanoparticles’ surfaces can improve the dispersion of ZnO nanoparticles and can endow the ZnO nanoparticles self-assembled behavior upon annealing at LC state temperature (160℃). Using of the LC-ZnO as electron acceptors in hybrid solar cells enhances the order and crystallinity of P3HT chains and evolves the microstructure of P3HT/LC-ZnO blend, enabling short-circuit current density (Jsc) to be increased. More interestingly, the order of the P3HT/LC-ZnO blend morphology is significantly enhanced after thermal treatment at160℃, indicating that the spontaneous assembly of the LC-ZnO pushes P3HT chains to form oriented nanodispersing structure with highly oriented channel layers upon only heating at liquid-crystalline states, leading to the improved power conversion efficiency (PCE) by1.8fold compared with the device based on P3HT/ZnO, demonstrating that the described self-assembled LC-ZnO hybrids represent a promising strategy toward nanoscale controlled bulk heterojunction solar cells. |
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