Bulk-Heterojunction Polymers Solar Cells: Donor Materials Engineering, Synthesis And Properties

As the storage of fossil type energy sources (i.e. coal, oil and natural gas) decreases and their detrimental long-term effects on the natural balance on our planet, it's urgent to develop new energy supply for replacement. Exploiting and developing the solar energy as a substitute energy is an effective route. Comparing with the traditional organic solar cells, although the inorganic solar cells exhibit higher efficiency, the high cost for manufacture still limits them to be widely used. Due to low-cost, highly flexible properties of organic solar cell, they are playing more an more important role in the field of solar cell.In this thesis, we firstly discussed principle, structure, development condition of bulk-heterojunction polymers solar cells, then designed a serials donor polymers. We modulated the band gaps, energy levels, carrier mobility, surface morphology and photovoltaic properties of the synthesized polymers by changing the conjugated length and structures. The detail results of research were described as follows:1. We have synthesized a series copolymers with good solubility and thermal stability by GRIM copolymerization method. In order to low the HOMO level of the copolymers, the triphenylamine was introduced into the side chain of the polythiophene and obtained the high open circuit voltage (Voc). Meanwhile, the absorption spectrum and energy levels of the copolymers can ben controlled by increase the thiophene units on the main chain of the copolymers. When the ratio reached 1:4.8 between side chain and main chain, the band gap and absorption spectrum of the copolymer is lower and broader than P3HT. In addition, the polythiophene with alkoxylation triphenylamine showed a intramolecule energy transfer between side chain and main chain. All of them guarantee that the copolymers can absorb enough photons of the solar spectrum. For the photovoltaic cell of copolymers, with the increase of thiophene units, the PCE of copolymer devices gradually increased, owing to the reduce disorder degree of copolymerization, broader absorption, ??? stacking and aggregation.2. We have synthesized a series polymers with good solubility and thermal stability by Suzuki copolymerization method. Optical property and molecular orbital distribution calculations investigations unequivocally indicate that these new copolymers exhibit enhanced ICT bands in solid state by with electron-donating ability increasing of the bridged-D, which lead to an extension of their absorption spectral range. Meanwhile, the varied electron-donating ability increasing of the bridged-D can modulate the energy level, miscibility between polymers and PC61BM, ??? stacking of solid film and photovoltaic properties. The highest PCE value of 0.52% was obtained from the device based on PFPMP with a strongest electron-donating ability bridged-D, which was more than 12 times higher than that of the device based on PFTMT with a lowest electron-donating ability bridged-D (0.04%).3. We have synthesized a series D-A copolymers with high molecule, good solubility and thermal stability by Suzuki copolymerization method. Optical property investigations unequivocally indicate that these new copolymers exhibit enhanced ????stacking and ICT bands in solid state by changing the macromolecular architecture, which lead to an extension of their absorption spectral range. The copolymers obtained low optical band gap 1.76 eV. The HOMO and LUMO energy levels of resulting copolymers can be fine-tuned as demonstrated from the investigation of electrochemical study. The relatively low HOMO energy levels promised good air stability and high Voc for photovoltaic cells application. The highest PCE value of 0.99% was obtained from the device based on PDTTMT with a strongest electron-donating ability D*, which was more than 23 times higher than that of the device based on PFTMT (0.04%).4. We have synthesized a series oligothiophene-based polymers with good solubility and thermal stability. Optical property investigations unequivocally indicate that these new copolymers exhibit strong ????stacking and the long wavelength ICT absorption bands, which lead to an extension and broad absorption with the optical band gaps in the range of 1.72-1.76 eV. Both the electrochemical properties and molecular orbital distribution calculations revealed that the HOMO and LUMO energy levels of the resulting copolymers can be fine-tuned by increasing the conjugated length of comonomer OThn, and can also be used to adjust the open circuit voltage. Meanwhile, the morphology of the blend films containing copolymers and PC61BM can be controlled by varying the conjugated length of OThn. Hence, the Jsc and PCE can also be improved.5. We have synthesized a series copolymers containing two different electron accepting moiety (TVM and BT) by Stille copolymerization method, which showed high molecule, good solubility and thermal stability. Optical property investigations unequivocally indicate that these new copolymers exhibit strong ????stacking, the long wavelength ICT absorption bands and red-shift absorption spectrum with the increasing of BT moiety, which lead to an extension and broad absorption with the optical band gaps in the range of 1.70-1.84 eV. Meanwhile, the varied ratios between TVM and BT moieties can modulate the energy level, miscibility and self-assembly between polymers and PC61BM. The copolymer PM50 with copolymerization ratio of 50% showed the best photovoltaic performance, the active layer (PM50:PC61BM) showed Voc = 0.78V, Jsc= 5.47 mA/cm2, FF=0.40,PCE=1.67%, and the active layer (PM50:PC71BM) showed Voc = 0.82V, Jsc= 8.32 mA/cm2, FF= 0.42,PCE=2.89 %.