鍺架橋的七環熔合低能隙高分子之合成與有機太陽能電池之應用

Abstract

本論文成功地設計與合成出一個有鍺元素架橋的七環熔合電子施體dithienogermolo-carbazole ( DTGC ) ，DTGC是由鍺原子架接起中心咔唑 ( 2,7-carbazole ) 與外邊兩個噻吩 ( 2-thiophene ) 的結構，只需經由一步產率高達88%的親核基環化反應即可得到DTGC，因碳－鍺 ( C-Ge ) 鍵相對較低的極性，DTGC可進行較多元的官能基化反應。電子施體單體DTGC與五個不同電子受體單體benzothiadiazole ( BT ) 、dithienylbenzothiadiazole ( DTBT ) 、 difluorobenzothiadiazole ( FBT ) 、difluorodithienylbenzothiadiazole ( FDTBT ) 、thienopyrrolodione ( TPD ) 進行聚合，形成一系列交替型電子施體－受體的共聚高分子poly(dithienogermolo-carbazole-alt-benzothiadiazole) ( PDTGCBT ) 、 poly(dithienogermolo-carbazole-alt-dithienylbenzothiadiazole) ( PDTGCDTBT ) 、 poly(dithienogermolo-carbazole-alt-difluorobenzothiadiazole) ( PDTGCFBT ) 、poly(dithienogermolo-carbazole-alt-difluorodithienylbenzothiadiazole) ( PDTGCFDTBT ) 與poly(dithienogermolo-carbazole-alt-thienopyrrolodione) ( PDTGCTPD ) 。鍺架橋的PDTGCBT，與矽架橋的PDTSCBT、氮架橋的PDTPCBT、碳架橋的PDTCCBT進行比較，探討不同架橋原子所造成的影響。鍺架橋的五個低能隙高分子，熱裂解溫度均高於440 oC，有利於太陽能電池的實質應用。鍺架橋的PDTGCBT，相對氮架橋PDTPCBT、碳架橋PDTCCBT有較低的HOMO能階，有益於較高Voc值的表現。此外，鍺架橋PDTGCBT與矽架橋PDTSCBT，因結構上的silole與germole有相似的δ* - π*軌域作用，因此實驗與理論計算均存在相近的光學、電化學性質。從薄膜態吸收計算出的光學能隙大小順序為：PDTSCBT ( 1.83 eV ) ＞ PDTGCBT ( 1.82 eV ) ＞ PDTCCBT ( 1.64 eV ) ＞ PDTPCBT ( 1.50 eV ) ，此順序可推測出各電子施體對電子受體BT供給電子的能力大小為：DTPC ＞ DTCC ＞ DTGC ＞ DTSC 。

PDTGCFDTBT因結構上較PDTGCFBT多了π電子易非定域化的thiophene重複單元，PDTGCFDTBT有較低的光學能隙值；PDTGCFDTBT也因結構上較PDTGCDTBT多了拉電子效應強的氟原子，PDTGCFDTBT有較低的LUMO、HOMO能階。最後，五個不同電子受體的低能隙高分子當中，由PDTGCFDTBT取得最好的光電轉換效率 ( Voc = 0.84 V，Jsc = 9.87 mA/cm2，FF = 48.8%，PCE = 4.05% ) 。添加3% ( v/v ) 1-chloronaphthalene可有效改善主動層的形態，使PDTGCFDTBT電池元件的Jsc、FF值獲得提升。最後，電池元件使用較高分子量的H-PDTGCFDTBT，呈現出最好的表現，Voc = 0.84 V，Jsc = 11.19 mA/cm2，FF = 47.7%，光電轉換效率提高至4.50%。

We have successfully designed and synthesized a new germanium-bridged heptacyclic arene, dithienogermolo-carbazole (DTGC), in which two outer thiophene subunits are covalently fastened to the central 2,7-carbazole cores by germanium bridges. The DTGC core structure is constructed by one-step nucleophilic cyclization in a high yield of 88%. Due to the relatively low polarity of carbon-germanium bonds, the DTGC unit can survive in basic conditions, rendering its more versatile functionalization. This DTGC monomer was polymerized with five different acceptors, benzothiadiazole (BT), dithienylbenzothiadiazole (DTBT), difluorobenzothiadiazole (FBT), difluorodithienylbenzothiadiazole (FDTBT) and thienopyrrolodione (TPD) respectively to yield a series of new alternating donor-acceptor copolymers, poly(dithienogermolo-carbazole-alt-benzothiadiazole) (PDTGCBT), poly(dithienogermolo-carbazole-alt-dithienylbenzothiadiazole) (PDTGCDTBT), poly(dithienogermolo-carbazole-alt-difluorobenzothiadiazole) (PDTGCFBT), poly(dithienogermolo-carbazole-alt-difluorodithienylbenzothiadiazole) (PDTGCFDTBT) and poly(dithienogermolo-carbazole-alt-thienopyrrolodione) (PDTGCTPD). The germanium-bridged effect on PDTGCBT was also systematically investigated by comparing with the corresponding carbon-bridged PDTCCBT, nitrogen-bridged PDTPCBT and silicon-bridged PDTSCBT copolymers. The germanium-bridged polymers exhibited high decomposition temperatures above 440 oC, which is beneficial for photovoltaic device applications. Compared to N-bridged PDTPCBT and C-bridged PDTCCBT, Ge-bridged PDTGCBT showed deeper HOMO energy level, which is beneficial for greater Voc. As a result of the *-* orbital interaction in germole and silole units, the experimental and theoretical data demonstrated that Ge-bridged PDTGCBT has similar optical and electrochemical properties with Si-bridged PDTSCBT. The optical bandgaps ( Egopt ) deduced from the absorption edges of thin film spectra are in the following order: PDTSCBT ( 1.83 eV ) ＞ PDTGCBT ( 1.82 eV ) ＞ PDTCCBT ( 1.64 eV ) ＞ PDTPCBT ( 1.50 eV ). This result suggested that the electron donating strength of the heptacyclic arenes is in the order: DTPC ＞ DTCC ＞ DTGC ＞ DTSC.

Owing to the two more thiophene rings in the repeating units on the backbone to facilitate -delocalization, PDTGCFDTBT showed a lower optical band-gap than PDTGCFBT. PDTGCFDTBT also showed lower-lying LUMO and HOMO energy levels than PDTGCDTBT due to the electron-withdrawing fluorine atoms. As a result, the bulk heterojunction solar cell incorporating PDTGCFDTBT exhibited the highest performance with Voc of 0.84 V, Jsc of 9.87 mA/cm2, FF of 48.8%, PCE of 4.05%. By adding 3 % 1-chloronaphthalene to tailor the morphology, the solar cell using H-PDTGCFDTBT with higher molecular weight delivered the best performance with a Voc of 0.84 V, a Jsc of 11.19 mA/cm2 and an FF of 47.7%, leading to an efficiency of 4.50%.