Synthesis And Photovoltaic Properties Of Carbazole-based Conjugated Small Molecules And Polymers

Organic solar cells (OSCs) are devices comprised of the conjugated organic materials (conjugated polymers or conjugated small molecules) as donor and fullerene derivatives (such as PCBM) as the receptor with the structure of Al/Donor: Acceptor/PEDOT:PSS/ITO, and has been attracting considerable attention for their advantages of low cost, solution processibility, light weight, easy fabrication toward flexible large-area devices. There are three important factors determining the final performance of OSCs, including donor materials, acceptor materials and the morphology of each layer. It has been demonstrated that the low-bandgap conjugated molecules (polymers and small molecules) with low HOMO level and using additives to optimize the morphology of active layer can effectively improve the power conversion efficiency (PCE) of OSCs. In this dissertation, motivated by the the synthesis of new conjugated donor materials and double cable polymers (additives) to study their impact on the performance of OSCs, we carried out the following works:1. The acceptor-donor-acceptor (A-D-A) conjugated small molecules (Hexyl-Octyl-CZ-BDT, Octyl-CZ-BDT) were synthesized based on carbazole and benzothiadiazole with different alkyl side chains. Hexyl-Octyl-CZ-BDT, Octyl-CZ-BDT were characterized by thermal gravimetric (TGA), UV-Vis spectroscopy, and electrochemistry, indicating that both Hexyl-Octyl-CZ-BDT, Octyl-CZ-BDT have good thermal stability, absorption range and solubility. The bulk heterojunction OSC based on Octyl-CZ-BDT:PCBM (1/3, w/w) exhibited a highest PCE of2.43%without any post-treatment.2. We synthesized the two diblock copolymers (P1and P1-FF) based on the donor group (carbazole) and acceptor group (fluorinated quinoxaline and quinoxaline) by the Suzuki reaction.The physical and chemical properties of synthesized polymers (P1and P1-FF) were characterized by1H NMR, UV-visible spectroscopy, and electrochemistry, and the mobility of P1and P1-FF were measured by field effect transistor (FET). Their molecular energy levels were studied theoretically by DFT. Our results show that the introduction of a fluorine atom results in the enhancement of enhanced the intermolecular packing, lowering the HOMO level and improving the carrier mobility. The bulk heterojunction polymer solar cells (PSCs) fabricated from blends of P1-FF with PCBM (1/3, w/w) exhibited a PCE of0.66%. 3. Employing two-steps postpolymerization approach followed by a Steglich esterification reaction, we have successfully synthesized a novel double cable donor-acceptor P3HT derivative (PCBM-Ph-P3HT), in which the PCBM moiety as the side chain is closely linked to the thiophene unit via a phenyl group. This method avoids the complicated monomer polymerization reaction, and in the final step esterification reaction has the advantages of high conversion. The fluorescence intensity of P3HT/PCBM-Ph-P3HT is much more quenched compared to that of P3HT/PCBM blend, being～63%quenching. P3HT:PCBM BHJ-PSC devices doped by PCBM-Ph-P3HT show the enhanced PCE, with the maximum PCE being3.40%at the optimum PCBM-Ph-P3HT doping ratio of1wt%, which has a ca.12%enhancement compared to that of reference P3HT:PCBM device. The enhancement of PCE is evidently attributed to the increase of JSC, and the enhancement mechanism is proposed that the improvement on the bicontinuous interpenetrating networks of the P3HT:PCBM blend upon PCBM-Ph-P3HT doping leads to the increase of the charge carrier mobility.4. We synthesized two new alkyl sulfonated C6o derivatives by different routes, C60(O)14(OH)4(OCH2CH2CH2SO3Na)5(2) and C60(O),5(OSO3Na)5(OCH2-CH2CH2SO3Na)6(4) based on the fullerenol, with the advantage that the reaction was carried out in aqueous phase without any catalyst. The fullerene derivative4exhibits the highest solubility (118mg/mL) and dispersion with a mean aggregation size of49.2nm in0.1wt%aqueous solution. Our results suggest that more sulfonate groups on the fullerene surface result in the improvement of the dispersion and water solubility of fullerenol in water.