The Study Of Regioregular Thiophene-Benzothiadiazole Alternating Polymer And Oligomer

Recently, organic solar cells have attracted much more attention due to its facile preparation process, low cost, structure-designable at molecular scale, and facile fabrication as large flexible devices. However, organic solar cells suffer from the issue of low efficiency and short life. The device efficency were mainly depended on the light absorbed ability and microstructure (i.e., their packing structure in film) of the active layer materials. Therefore, in this thesis, we mainly focused on the active layer material of organic solar cell for the purpose of developing polymers with low bandgap and regioregularity structure and expected to obtain organic solar cell with excellent performane. The main contents are listed as follows:(1) A common way to reduce the polymer band gap is alternative copolymerization of an electron-deficient acceptor unit and an electron-rich donor unit. Here, we also use this approach, in which the acceptor unit is benzothiadiazole, while the donor unit is 3-hexylthiophene, and the orientation of hexyl substituents determines the polymer regioregularity. Using a step-growing approach, we prepared fully regioregular alternative co-oligomers monomer (M₁) to heptamer (M₇). For these oligomers, we have analysised the ultraviolet-visible (UV-vis) spectra, and found that the UV-vis visible absorbed gradually red-shift when the side chain longer, but the wavelength gap of border upon oligomers decreased. Meanwile we have also carried out the electrochemical properties for the seven oligomers, and found the band gap between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are getting smaller.(2) We synthesized the corresponding polymer (rrP) with a completed regioregularity using the classical Suzuki coupling method. And a non-regioregular polymer (urP) was also prepared. We studied UV-vis absorption spectroscopy and electrochemical properties for the obtained polymers, and used the starting absorption wave-length to calculate the band gap between HOMO and LUMO. Then we compared the band gap of polymers with the theoretical band gap of oligomers (1.76 eV), and discovered the band gap are same. In contrast, due to the low molecular weight of urP, we calculated the band gap of urP, and this band gap is larger than that of rrP, about 1.82 eV.(3) we fabricated solar cell device with rrP and PCBM. We received the property parameters of this solar cell as follows: an open circuit voltage (V_OC) of 0.72 V, a closed-circuit current density (J_SC) of 3.49 mA/cm², and a fill factor (FF) of 0.24, giving power conversion efficiency (PCE) of 0.6%.