| With the exhaustion of fossil energy resources and the increase of the environment consciousness of human beings, solar energy as a real sense of renewable and green energy sources has been paid much attention. Photovoltaic cells are one of attractive methods for harnessing inexhaustible clean energy from the sun. Polymeric solar cells (PSCs) have become a hot research topic over the past decade for their potential application in large-scale power generation based on materials that provide the possibilities of being flexible, light in weight and cheap in price. However, the power conversion efficiency (PCE) of PSCs needs to be improved for future commercial applications. The factors limiting the PCE of the PSCs include the low exploitation of the sunlight due to the narrower absorption band of the absorption spectra of polymers in comparison with the solar spectrum and the mismatch of the two spectra, and the low charge transport efficiency in the devices due to the low charge carrier mobility of polymer pbtotovoltaic materials. Therefore, the design and synthesis of polymers with broad absorption and high charge carrier mobility was an effective way to improve the power conversion efficiency.Polyimides have been proven to be very stable to thermal, chemical and photophysical actions. Polyimide chains consist of alternating electron acceptor diimide fragments (A) and electron donor arylene rests of starting diamines (D). Aiming to prepare photovoltaic materials with broad absorption, good thermal stability and high charge carrier mobility, we have proposed two stratagies in this paper. One of our efforts is to introduce ruthenium metal complex to extend the absorption in the visible region into polyimide main chain or side chain. And the other one is to introduce the cardo4,4'-(9H-fluoren-9-ylidene) bisphenylamine (FBPA) or 4,4'-diaminotriphenylamine (tpa) as hole transporting moieties (HTM) by copolycondensation to improve the solubility, thermal stability and charge carrier mobility into polyimide main chain. With such a molecular design, the light harvesting ability of ruthenium complexes would be combined with the excellent thermal stability of polyimide in one molecule. Firstly, two monomers M1 (bpy) and M3 [tpa(bpy)] containing bipyridine ligand (L) were synthesized. Then, the three series of polyimides, which were named LHPI {LHPI-1 (bpyx-FBPA-PI) (x:mol%of diamine containing bipyridine ligand), LHPI-2 (bpyx-tpa-PI), and LHPI-3 [tpa(bpy)x-tpa-PI]}, were prepared by the cololymerization of 4,4'-(Hexafluoroisopropylidene)-diphthalic anhydride (6FDA) with two diamine monomers in different ratio. These polyimides were synthesized by a one-step polymerization in m-cresol with isoquinoline as catalyst at 200?and purged with argon flow. Then, the polyimides containing ruthenium complexes DHPI (DHPI-1, DHPI-2, and DHPI-3) were synthesized by the reaction LHPI with ruthenium complex using a "one-pot" method. The difference of DHPI-1, DHPI-2 and DHPI-3 is that the Ru-complexes of the former is in the main chain of polyimides, the later is on the side chain. The chemical structures of the monomers and polymers were confirmed by1H NMR, IR, and elemental analysis. The Mn of the ligand polymer LHPI was measured by GPC.LHPI and DHPI showed good solubility in common aprotic organic solvents such as DMSO and DMF. While, DHPI showed somewhat poorer solubility than LHPI, especially in low boiling point solvents, such as CHCl3 and THF. And, the solubility were increased with increasing the FBPA or tpa unit content. These copolyimides containing ruthenium complexes were thermaly stable as measured by TG analysis. UV-Vis measurements revealed that DHPI exhibited very broad absorptions in the range 350-750 nm due to the introduction of ruthenium complexes. Such absorption enhancement would enable the polymer to harvest solar light in the visible region. The fluorescence spectra of all polyimides solution were investigation. The results showed that the emissions from polyimides containing Ru-complexes in the main chains were quenched signigicantly compared with metal-free polymers. This may be due to the presence of an energy transfer process from the main chain to the ruthenium complex with lowlying energy levels, which could reduce the loss of polymer absorption sunlight energy, facilitate separation of excitons, and improve photoelectric conversion efficiency. While, the emissions from polyimides containing Ru-complexes on the side chains had the same fluorescence intensities with their corresponding to metal-free polyimides. The onset oxidation potential of DHPI-1 and tpa(bpy)100Ru-PI were lower than that of LHPI-1 and tpa(bpy)100-PI as measured by cyclic voltammetry. This result suggested that the introduction of Ru-complexes made the DHPI easier to oxidize than the ligand polymer LHPI. The band gaps of DHPI-1 and tpa(bpy)100Ru-PI were 1.55-1.77 eV, which were narrower than that of polyimides of metal free, LHPI-1 and tpa(bpy)100-PI.Polymer photovoltaic cells were fabricated by using DHPI-1 and tpa(bpy)100Ru-PI or the blend of DHPI-1 (or tpa(bpy),ooRu-PI) and PCBM as the photoactive materials. Current density-voltage (J-V) measurement of the devices showed a typical rectifying behavior under a 55 mW cm-2 compact white arc lamp. The results showed that pure polymers in all cases exhibited very poor photovoltaic performances, which may be result from the molecular internal lower carrier mobility of polyimide. The structure, ITO/PEDOT:PSS/DHPI:PCBM(1:1)/Al, showed better Photovoltaic Properties. Especially, when the blend tpa(bpy)100Ru-PI and PCBM (1:1) (w/w)as the photoactive material, the performances of device were best compared with other same structure devices. The Voc, Jsc, FF, and?e were 0.30 V,100.1 uA/cm2,0.31 and?e 1.71×10-2%, respectively. Optimization in device structure and molecular design will be a part of our future work for the improvement of device performance with materials combining polyimide and ruthenium complexes. |
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