含3-hexylthiophene與2,3-diethylthieno[3,4-b]pyrazine之低能隙共聚高分子開發與研究

Abstract

本論文是利用不同的聚合方式(GRIM、Universal GRIM、Suzuki偶合)合成含不同電子予體-受體比例的低能隙共聚高分子，以調整共聚高分子的電子結構與光物理特性。共聚高分子的電子予體為3-hexylthiophene (3HT)，而電子受體單元為2,3-diethylthieno[3,4-b]pyrazine (ETP)；每一個單元皆具有長碳鏈碗基做側鏈修飾，以提升共聚物的溶解度。以GRIM與Universal GRIM的方法僅能有效聚合P3HT，但並無法有效活化含有ETP的雙溴修飾單體，以致於無法得到含有ETP的共聚高分子；而藉由Suzuki偶合則成功的合成出3HT：ETP = 1：1~4：1 (P3HTETP-1-1 ~ -4-1)及fluorene與ETP的共聚高分子(PBF)。共聚高分子皆可溶解於trichlorobenzene中，溶解度隨著3HT比例的增加而增加。各共聚高分子的裂解溫度(Td)在271 ~ 446oC，且隨著3HT的含量上升而較穩定。 各共聚高分子在溶液中的紫外光/可見光的最大吸收(λsol, max)位於580 ~ 730nm，相較於P3HT(440nm)有明顯的紅移，可歸因於主鏈內顯著的電子予體-受體效應。光學能隙(Eg, opt)在1.27eV~1.77eV，隨著ETP比例的增加而下降。共聚高分子的固態吸收則較溶液吸收有不同程度的紅移，取決於共聚高分子的組成。由循環伏安測得各共聚物的電子最高填滿分子軌域(HOMO)在-4.64eV ~ -4.90eV、電子最低未填滿分子軌域(LUMO)在-2.69 eV~ -3.11eV；隨著ETP在共聚高分子內的比例上升，HOMO與LUMO分別上升及下降，但LUMO下降較HOMO上升明顯，電化學能隙(Eg, CV)為1.52 eV~ 2.19eV。這些低能隙的共聚高分子預期將可應用於高分子太陽能電池以提昇太陽光吸收的效率。

In this research, we employed GRIM, Universal GRIM and Suzuki coupling to synthesize low band conjugated copolymers consisting of electron-donating units (3-hexylthiophene, 3HT) and electron-accepting units (2,3-diethylthieno[3,4-b] pyrazine, ETP) in different ratios in order to adjust the electronic structure and photophysical properties of the copolymers. Each unit was decorated with long alkyl chains as side substitutions to enhance the solubility. GRIM and universal GRIM only successfully synthesized poly(3-hexylthiophene) (P3HT) but failed to synthesize any copolymers consisting of ETP owing to the failure in activating di-brominated monomers containing ETP. Copolymers containing 3HT and ETP in 1:1 to 4:1 molar ratio (P3HTETP-1-1 ~ -4-1) as well as fluorene and ETP (PBF) were successfully obtained via Suzuki coupling. These copolymers showed reasonable solubility in TCB while the solubility increased with increasing 3HT to ETP ratio. Decomposition temperatures ranges from 271oC to 446oC and the thermal stability enhanced with increasing 3HT content. Absorption maxima of these copolymers in TCB solution were extended to longer wavelengths (565~690nm) compared to P3HT (440nm) due to strong intramolecular donor-acceptor interaction. Optical band gaps were 1.27eV~1.77eV, decreasing with increasing ETP ratio in the copolymers. The absorptions in solid state showed red shifts in different magnitudes in comparison with the absorptions in solution, depending on the composition of the copolymers. HOMO levels (-4.64eV ~ -4.90eV) and LUMO levels(-2.69eV ~ -3.11eV), obtained from cyclic voltametry, were raised and lowered respectively with increasing ETP ratio in the copolymer. The change in LUMO was more significant than that in HOMO. These ETP containing low band gap copolymers are potential candidates for polymer solar cells to enhance the light harvesting.