Design, Synthesis And Properties Of Conjugated Copolymers/molecules With Thermally Removable Side Chains

Organic solar cells(OSCs) have attracted considerable attention due to their unique advantages of light-weight, flexibility and being solution processable for large-area devices. Nowadays, various novel conjugated polymers/molecules have been developed and thus OSCs with power conversion efficiency(PCE) over 10%have been achieved, which is closed to the demand for their commercialization. When we are designing the conjugated polymers/molecules, however, various long alkyl side chains were attached on the backbone of the molecules to improve their solubility,which might adversely affect the intermolecular stacking and therefore decrease the charge transport properties in their thin films. Addressing at this point, a novel molecular design strategy was proposed in this dissertation to simultaneously achieve the contradictory features of good solution processability and strong packing of conjugated polymers/molecules, which might lead to significantly improved charge transport property in their thin films.In chapter 2, anthracene unit with good planarity was used to construct donor-acceptor conjugated copolymers. And we have successfully developed a series of anthracene-based copolymers with different side chains appended on 9,10-positions of the central phenyl ring in anthracene unit. After the careful observation on the opto-electronic properties of the obtained copolymers, we found the photovoltaic properties of the copolymers could be significantly affected by the side chains in athracene unit. Finally, the copolymer with phenyl side chains exhibited the highest PCE of 4.34% with a Voc of 0.98 V. This result has demonstrated that anthracene could be a good donor unit for constructing donor-acceptor conjugated polymers with high photovoltaic efficiencies.Based on the conclusions in chapter 2, we have proposed the molecular design stratety to develop anthracene-based conjugated polymers with thermally removable groups for polymer solar cells(PSCs). The Diels-Alder cyclic adduct of anthracene has been successfully developed and was utilized to contruct donor-acceptor conjugated copolymers. Due to the solubilizing effect of the side groups, the obtainedIV copolymers exhibited good solubility in common organic solvents and thus the uniform films can be easily formed. And after the thermal annealing treatment on the polymer films at 250 o C, the side groups could be successfully removed and the molecular planarity of the copolymers was effectively enhanced, resulting in the improved carrier transport properties in thin films. Finally, we have used these copolymers to fabricated polymers solar cells and achieve a relatively high PCE of2.15%.In chapter 4, we have utilized such a molecular design stratety to develop electron-withdrawing molecules with thermally cleavable side chains and successfully synthesized thermally cleavable indigo, isoindigo and DPP units, which could be used to construct donor-acceptor copolymers. Due to the solubilizing effect of the thermally cleavable side chains, the obtained copolymers exihibited good solubility in common organic solvents and uniform thin films could be easily fabricated. After the thermal treatment on the polymer thin films, the side chains could be successfully removed from the polymer main chains, leading to the enhanced coplanarity of polymer backbone and the emergence of intermolecular NH…O=C hydrogen interaction due to the lactam structure of acceptor units. Accordinally, the hole mobilities of the polymer thin films was successfully improved. We have also fabricated polymer solar cells with these polymers, but the PCEs have not been improved as expected. Through the careful study on the morphology of the blend films of photovoltaic devices, we conclude that the serious aggregation of polymers and PC71 BM is the main reason for such decreased PCEs.In order to effectively control the change of the films morphology during the thermal annealing treatment process, thermal cleavable indigo and isoindigo-based small molecules have been developed in chapter 5. After the thermal cleavage in the solution of isoindigo-based molecule, the emerged intermolecular NH…O=C hydrogen bonding would promote the self-assembly of the thermally cleaved molecules and the nanowires have thus been successfully obtained. Due to the ordered organization of isoindigo molecules in the nanowires structure, the charge transport property has been significantly improved and the organic field effect transistor(OFET) hole mobilityhas been measured to be 1.3×10-3 cm2V-1s-1.In chapter 6, in order to reduce the adervese effect of thermal cleavage on the backbone of the molecules, we introduced the thermally removable ester chain into the side chains of the polymers. After the thermal annealing treatment on the polymers,it was found that the orbital energy levels have been slightly changed, while a significant red-shifted UV-vis absorption could be observed for the polymer thin films,which indicating the the intermolecular stacking has been enhanced after the thermal cleavage of the ester side chains. We have used these copolymers to fabricated polymer solar cells, and the decrease of photovoltaic performances have been eased at some extent. We surmise the performance of the devices might be improved after the careful control of the morphology change of the blend films during the thermal treatment.