吲哚基[3,2-b]吲哚為新電子予體之合成及其於有機太陽能電池之應用

Abstract

藉由兩個吲哚單元融合構成的吲哚基[3,2-b]吲哚，由於它的分子平面、對稱與共軛延伸結構特性，在D-A材料中是具有潛力的多電子單體。我們將吲哚基[3,2-b]吲哚引入D-A的π共軛系統中，進而研究組成2,7或3,8位置向外延伸所造成之電子與立體效應。本研究中我們成功發展出新的合成途徑去得到2,7-雙溴吲哚基[3,2-b]吲哚。不幸的是，當我們依照文獻中企圖得到3,8-雙溴吲哚基[3,2-b]吲哚時，卻發現吲哚基[3,2-b]吲哚在溴化時出乎意料地傾向發生在2,7位置上。這個發現證實了，吲哚基[3,2-b]吲哚的2,7位置擁有更強的親核力去進行親電子芳香環取代反應。2,7-雙溴吲哚基[3,2-b]吲哚在相對位置上置換上硼酯基，得到單體M1。當M1分別與單邊與雙邊溴化dithienodiketopyrrolopyrrole (DPP) 進行 Suzuki 耦合反應，得到A-D-A型小分子2,7-bis(dithienoldiketopyrrolopyrrole)indolo[3,2-b]indole (2,7-BDPPII) 與D-A共聚高分子PDPPII。此外，M1還可與benzothiadiazole (BT) 進行聚合得到隨機型共聚高分子PIIDTBT11與PIIDTBT13。所有的材料都有好的熱穩定性，熱裂解溫度在386到486 oC之間。而從薄膜X光繞射圖中，發現2,7-BDPPII是一結晶性材料。另外，2,7-BDPPII在固態下比在溶液態下有更為紅移且更廣的吸收。這個結果指出吲哚基[3,2-b]吲哚單元的平面性有利於提升分子間作用力。PDPPII得到1.45 eV的能隙大小，展現較2,7-BDPPII (1.66 eV) 低的能隙，因為PDPPII有較長的共軛。我們使用一般構成是 ITO/PEDOT:PSS/D-A material:PC71BM/Ca/Al的BHJ型元件來製作這些新材料。2,7-BDPPII在進行7分鐘150 oC的熱處理後，得到元件參數：Voc是0.72 V，Jsc是6.88 mA/cm2 ，FF是49.6 %，最後可得元件效率為2.45 %。這個效率在小分子溼式製程的BHJ型元件中已是相當優良的。在更進一步的元件製程之改進優化後，2,7-BDPPII的效率預期將可再提升。然而，PDPPII在元件上因為嚴重的相分離現象，造成效率相當低落。而PIIDTBT11和PIIDTBT13之元件則分別得到0.65 % 與0.59 %的元件效率。

Indolo[3,2-b]indole, containing two fused indole units, is an promising and attractive electron-rich monomer for constructing donor-acceptor materials due to its planar, symmetric, and extended conjugated structure. It is of interest to incorporate indolo[3,2-b]indole unit into a donor-acceptor □-conjugated system via its 2,7 or 3,8 positions to investigate the electronic and steric effects. In this research, we have successfully developed a new synthetic scheme to prepare 2,7-dibromo-indolo[3,2-b]indole. Unexpectedly, in an attempt to make 3,8-diromo-indolo[3,2-b]indole by following a procedure in the literature, we found that the bromination of indolo[3,2-b]indole turns out to occur at 2,7 positions rather than 3,8 positions. This finding confirms that 2,7 positions of indolo[3,2-b]indole actually possess stronger nucleophilicity to undergo electrophilic aromatic substitution reaction. 2,7-dibromoindolo[3,2-b]indole was then converted to the corresponding boronic ester monomer M1. The Suzuki coupling of M1 with monobrominated and dibrominated dithienodiketopyrrolopyrrole (DPP) led to an acceptor-donor-acceptor small molecule 2,7-bis(dithienoldiketopyrrolopyrrole)indolo[3,2-b]indole (2,7-BDPPII) and an alternating donor-acceptor copolymer PDPPII, respectively. On the other hand, M1 was also copolymerized with the benzothiadiazole (BT) unit to prepare two random copolymers PIIDTBT11 and PIIDTBT13. All the materials exhibited good thermal stabilities with the decomposition temperature ranging from 386 to 486 oC. The thin-film X-ray diffraction pattern of 2,7-BDPPII exhibited a crystalline structure. In addition, the absorption spectrum of 2,7-BDPPII in the solid state also showed red-shifted and band broadening behaviors compared to the solution state. These results indicated that highly planar structure of indolo[3,2-b]indole unit can enhance the intermolecular interaction. PDPPII showed the lower optical band gap of 1.45 eV than the small molecule 2,7-BDPPII (1.66 eV) as a result of longer conjugation length. The bulk heterojunction devices using the configuration of ITO/PEDOT:PSS/D-A material:PC71BM/Ca/Al were fabricated to evaluate these new materials. After thermal annealing at 150 oC for 7 min, the device incorporating 2,7-BDPPII as the active layer exhibited a Voc of 0.72 V, a Jsc of 6.88 mA/cm2 and a FF of 49.6%, leading to a PCE of 2.45%. This value is rather promising for small molecule-based solution processing BHJ solar cells. It is anticipated that the efficiency can be improved by further optimization of device fabrication. However, the PDPPII–based device exhibited very low performance due to the poor morphology associated with severe phase separation. The devices using PIIDTBT11 and PIIDTBT13 delivered the efficiencies of 0.65% and 0.59%, respectively.