Synthesis And Optical Characterization Of Narrow Band-gap Conjugated Polymers Based On Multi-parallel Aromatics

In recent years, the conjugated polymers for solar cells have been attracted considerable attentions. Although the copolymer photovotaic solar cells possess the advantages of large-area, flexible, low-cost et al., the stability and the power conversion efficiency of solar cells should be greatly enhanced if it would be widely used in the market. Increasing the absorption range and improving charge mobility are two main approaches to improve the overall power conversion efficiency. Since the polymers based on indolo[3,2-b]carbazole and indenofluorene have good thermal and chemical stability, P-type transport properties and good absorption spectrum. Moreover, indolo[3,2-b]carbazole and indenofluorene units have greater rigidity and conjugated diphenyl structures, and it can be enhanced the polymers' solubility in organic solvents by introducing long alkyl chains to indolo[3,2-b]carbazole and indenofluorene backbones. On the other hand, the conjugated polymers based on indolo[3,2-b]carbazole/indenofluorene and narrow-band gap monomers not only possess the better absorption matching with the near solar spectrum, but also increase the Packing effects in polymer molecules for the greater rigidity and conjugated diphenyl structures of indolo[3,2-b]carbazole and indenofluorene units. Thus, it is very beneficial for the formation of fine structure layer and charge mobility. Furthermore, the narrow band-gap polymers with wide spectral response also have attracted extensive research and applications in photodetectors. Therefore, the studies focus on these.In Chapter 3, the alternating narrow band-gap conjugated polymer PICZ-DTBT (Eg=1.83eV) is synthesized, which derived from 5,11-di(9-heptadecanyl)indolo[3,2-b]carbaz-ole and 4,7-di(thieno[3,2-b]thien-2-yl)-2,1,3-benzothiadiazole by Suzuki coupling reaction. The resulted copolymer is soluble in common organic solvents and exhibits high molecular weight and good film-forming. In o-dichlorobenzene solution and solid thin film, PL emission peaks of the copolymer are at around 650nm and 690nm respectively. The values of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are-5.18eV and -3.35eV respectively. Under AM 1.5 simulator (80mW·cm-2), the photovotaic devices based on PICZ-DTBT:PC71BM(1:2) blend ITO/PEDOT:PSS/PICZ-DTBT:PC71BM/ Ba/Al show power conversion efficiency(PCE) of 2.62%, an open circuit voltage(Voc) of 0.75 V, a short circuit current (Isc) of 7.87mA/cm2 and a fill factor (FF) of 42%. By taking place Ba with PFN, the devices ITO/PEDOT:PSS/PICZ-DTBT:PC71BM/PFN/A1 show power conversion efficiency of 3.1%, an open circuit voltage of 0.9V, a short-circuit current of 6.24 mA/cm2 and a fill factor of 44%.In Chapter 4, two narrow band-gap alternating copolymers derived from 6,6',12,12'-tetraoctylindenofluorene (IF), 4,7-di(2,2'-bithiophene-5-yl)-2,1,3-benzothiazole (DDBT) and 4,7-di(thieno[3,2-b]thiophene-2-yl)-2,1,3-benzothiadiazole (DTBT), are synthesized by Suzuki coupling reaction, named PIF-DTBT and PIF-DDBT respectively, and the optical properties are characterized. In chloroform solution and solid thin films, the PL emission peaks of copolymer PIF-DDBT are at around 700nm and 720nm respectively, while PL emission peaks of copolymer PIF-DTBT are at around 680nm and 710nm respectively. The HOMO and LUMO values of the copolymer PIF-DDBT are -5.5eV and -3.7eV, while those of the copolymer PIF-DTBT are -5.48eV and -3.71eV, respectively. Under AM 1.5 (80mW/cm2) simulator, the photovoltaic devices based on PIF-DDBT:PC61BM(1:4) blend ITO/PEDOT:PSS/PIF-DDBT:PC61BM/Ba/Al show power conversion efficiency (PCE) of 1.54%, an open circuit voltage (Voc) of 0.85 V, a short circuit current (Isc) of 3.13mA/cm2 and a fill factor (FF) of 0.44. The photovoltaic devices based on PIF-DTBT:PC61BM(1:4) blend ITO/PEDOT:PSS/PIF-DTBT:PC61BM/Ba/Al show power conversion efficiency of 1.77%, an open circuit voltage of 0.88V, a short-circuit current of 3.03mA/cm2 and a fill factor of 0.50.In Chapter 5, the polymerization of PDDTT-C10 reported on Appl. Phys.Lett. by Xia. et al. in 2006 is optimized at C. Brite's financial support. The results show that the molecular weight of the polymer increases from 6500g/mol to 12000-19000g/mol, which is 2-3 times of the reported polymer PDDTT-C10. Furthermore, with the cooperation of C. Brite Inc. of the United States, Cao Yong Fellow of South China University and University of California, Santa Barbara, 2000 Nobel Prize winner Professor Alan. Heeger research team, the photodetector based on polymer PDDTT-C10 and PC61BM is prepared, which is of high detectivity (greater than 1012cm-Hz1/2/W at room temperature) and wide spectral responses from 300-1450nm. Recently, the flexible alkyl-chain of thiophene unit in PDDTT-C10 polymer is optimized by exchanging C10 to C12. The result shows that not only the larger molecular weight of polymer is achieved, but also the solubility is improved and the absorption spectra onset is red shifted to 1700nm. Besides, the characterizations of devices based on PDDTT-C12 polymer are ongoing.