Design And Synthesis Of Conjugated Polymer Based On Hexa-Peri- Hexabenzocoronene For Polymer Solar Cells

Hexa-peri-hexabenzocoronene (HBC) is a discotic-shaped conjugated molecule consisting of 42 carbon atoms and thirteen fused six-membered rings. It can be seen as a minimum building block of graphene, and has strong π-π stacking driving force between molecules. Functionalized HBC have shown to self-assemble into columnar and nanotube structures in solution and in solid state. HBC nanowires demonstrated extremely high charge transporting properties along the axial direction. These properties make HBC have great potential applications in designing organic electronics materials. But till now, the organic photovoltaic solar cells based on HBC do not give satisfied performance. The best photovoltaic conversion efficiency (PCE) of HBC solar cells is 2.6%. We believe that the PCEs of HBC-based solar cells still have a plenty of room to improve.From our point of view, there are two main problems on previously reported HBC based OPV devices.First, HBC compounds exhibit really narrow absorption spectra and wide band gap.This means that there are only minor part of sunlight could be absorbed and converted to electrical energy. Second, HBC demonstrate strong self-assembly abilities, and this properties make HBC derivatives strongly self-aggregated together in active layer thin films. The domain size of HBC and acceptor molecules blend films are considerably larger than 10-20nm and this makes the exciton hard to dissociate at D/A interface. In order to solve to these problems, this dissertation firstly introduced HBC to conjugated polymers. We believe that through rational designing on HBC macromolecules, taking the advantage of its self-assembly properties and high charge transporting abilities, and avoiding its shortcomings like absorption and morphology of solar cells active layer, the PCEs of HBC PSC devices can be improved.(1) We designed and synthesized the three component conjugated terpolymers based on PCDTBT and HBC as third component, which named as PCDTBT-5HBC and PCDTBT-10HBC.The DA conjugated dipolymer PCDTBT was also synthesized to investigate the effect of HBC units on conjugated polymers. The HBC-containing polymers exhibited higher thermal stabilities, broader absorption spectra and lower highest occupied molecular orbital (HOMO) energy levels.Particularly, the field-effect mobilities were enhanced by more than one order of magnitude after the incorporation of HBC into polymer conjugated backbone, for the reason of increased interchain π-π stacking interactions. The bulk heteroj unction (BHJ) polymer solar cells fabricated with the polymers as donor and PC71BM as acceptor demonstrated gradual improvement of open-circuit voltage (VOC) and short-circuit current (Jsc) with the increase of HBC content. As a result, the PCEs were improved from 3.21% for PCDTBT to 3.78% for PCDTBT-5HBC and then to 4.20% for PCDTBT-10HBC.(2) We designed and synthesized HBC and PTh4FBT based three component terpolymers and the molar contents of HBC are 0,2.5,5 and 10%, in order to examine how HBC as a third component affect the different properties of the conjugated polymers which give high photovoltaic performance. We found that incorporating HBC have little affect on the thermal, optical and electric properties of the PTh4FBT. However, with the HBC contents increase from 0% to 2.5% then to 5%, the Jsc, fill factor and PCEs improved simultaneously. The PCEs of PTh4FBT、PTh4FBT-2.5HBC、PTh4FBT-5HBC and PTh4FBT-10HBC are 6.07,6.81,7.56 and 7.20%, respectively. The PCE of PTh4FBT polymer was improved by 24.5% after 5% HBC incorporation.(3) We designed and copolymerized a series of polymers based on HBC and strong electron-withdrawing diketopyrrolopyrrole (DPP) unit through Suzuki coupling reaction. We also investigated the influence of different alkyl side chains and π-bridges to these polymers. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowed the band gaps. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated most broad absorption for its extensive π-conjugation and relatively more planar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave hold mobilities of 1.35×10-3,2.31×10-4, 2.79×10-4 and 8.60×10-3 cm2 V-1 s-1,and the blend films with PC71BM displayed power conversion efficiencies (PCEs) of 2.12%,2.85%,1.89% and 2.74%. To our knowledge,2.85% is the highest PCE of HBC derivatives till now.(4) We designed and synthesized two conjugated polymers based on HBC and quaterthiophene, which named as PHBC-T4 and PHBC-T4-DPP, in order to solve the problem that HBC materials domenstrate too large phase separation domain size in active layer blend film. The absorption specta of the polymers are broadened (600 to 750nm) and the band gaps are narrowed (2.04 to 1.64eV) by incorporating DPP to PHBC-T4. At the same time, the HOMO levels were also enhanced. Two polymers both demonstrated well P-type organic field effect transistor (OFET) properties. The hole mobilities of PHBC-T4 and PHBC-T4-DPP are 8.8×10-3 and 8.6×10-3 cm2 V-1 s-1, respectively. This result is among the highest performance of HBC based OFET devices. GIXRD result showed that PHBC-T4 thin films adopt edge-on arrangement, and better crystallization property. The PCEs of PHBC-T4 and PHBC-T4-DPP are 1.24% and 2.74%. PHBC-T4-DPP gave higher PCE for its broader absorption.