以笏為主體並具有長碳鏈側鏈之七環梯狀分子 於高分子太陽能電池之應用

Abstract

本論文將噻吩引入笏分子，利用共價鍵將兩者綁在一起，成功合成一個七環梯狀分子fluorine-dicyclopentathiophene (FDCT)。而利用一個包含Friedel-Crafts 環化、Wolff-Kishner 還原及烷化之三步驟合成路徑，能夠成功在FDCT中之環戊二烯中導入四支八個碳之長直碳鏈。 藉由改變單體Br-FDCT-C8、4,7-dibromobenzothiadiazole、以及2,5-trimethyltin thiophene三者之組成比例，以Stille coupling聚合方式合成兩個隨機共軛高分子r-PFDCTBT11及r-PFDCTBT12。此外Br-FDCT-C8也與4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-(2,1,3-benzothiadiazole)用Suzuki coupling聚合而得交替型共軛高分子PFDCTBT-C8。 以混摻異質接面 (ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al) 製作元件，其中r-PFDCTBT11及 r-PFDCTBT12 之效率分別可達2.95%及2.50%。PFDCTBT-C8/PC71BM混摻 (1:3.5 in wt%) 之元件效率高達5.01% (Voc = 0.80 V, Jsc = 10.86 mA cm-2, FF = 57.6%)。PFDCTBT-C8在有機薄膜電晶體 (OTFT) 之元件下具有相當高之電荷遷移率，可達0.03 cm2 V-1 S-1，此數值明顯提升是由於FDCT結構之側鏈為長直碳鏈，能夠增強分子間之作用力，使得堆疊能力上升而增強電荷遷移率，其結果也驗證了太陽能電池之高電流密度與高電荷遷移率有相當之關係。 由於反結構元件較正結構元件具有較高之穩定性，共軛高分子PFDCTBT-C8以元件結構ITO/ZnO/C-PCBSD/PFDCTBT-C8:PC71BM (1:3 w%)/PEDOT:PSS/Ag，得到光電轉換效率4.8% (Voc = 0.83 V, Jsc = 10.25 mA cm-2, FF = 55.7%)。文獻中證實添加劑為一有效用來控制薄膜之形態之方法，因此我們添加2.5% 1-氯萘後能夠改善其主動層形態，在相同之反結構元件中效率高達6.7% ，開路電壓為0.83 V，短路電流提升至12.29 mA cm-2，FF 值為65.6%，這個效率在含笏分子之共軛高分子中是相當高的。 由於低溫元件製程，我們嘗試以可撓式polyethylene naphthalate (PEN) 軟基板取代ITO玻璃，也得到一具有高效率5.6%之可撓式之太陽能電池。

In this research, we have successfully developed a ladder-type heptacyclic fluorene-dicyclopentadithiophene (FDCT) unit in which two outer thiophene subunits are covalently fastened to the central 2,7-fluorene core. A three-step synthetic approach including Friedel-Crafts acylation, Wolff-Kishner reduction and alkylation was designed to successfully incorporate four octyl chains on the cyclopentadiene rings embedded in the conjugated backbone. By varying the ratio of corresponding monomers, the Br-FDCT-C8 building block was copolymerized with 4,7-dibromobenzothiadiazole and 2,5-trimethyltin thiophene units by Stille coupling to obtain two random donor-acceptor copolymers, r-PFDCTBT11 and r-PFDCTBT12, respectively. On the other hand, Br-FDCT-C8 also reacted with 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-(2,1,3-benzothiadiazole) to afford an alternating copolymer PFDCTBT-C8 by Suzuki polymerization. Bulk heterojunction devices (ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al) were fabricated to evaluate these polymers. The devices based on r-PFDCTBT11 and r-PFDCTBT12 exhibited moderate power conversion efficiencies of 2.95% and 2.50%, respectively. Encouragingly, the device incorporating PFDCTBT-C8/PC71BM blend (1:3.5 in wt%) showed a Voc of 0.80 V, a Jsc of 10.86 mA cm-2, a FF of 57.6%, delivering an impressive PCE of 5.01 %. The PFDCTBT-C8 also showed a high mobility of 0.03 cm2 V-1 S-1 measured from the field-effect transistor device, which is in good agreement with its high current density. This improved performance is associated with the modification of aliphatic side chains on FDCT structure to optimize the interchain interactions for enhanced charge transporting. Considering that the solar cells with inverted configuration possess much enhanced stability, performance of PFDCTBT-C8 copolymer in the inverted device was also investigated. The device based on ITO/ZnO/C-PCBSD/PFDCTBT-C8:PC71BM (1:3, w%)/PEDOT:PSS/Ag exhibited a Voc of 0.83 V, a Jsc of 10.25 mA cm-2, a FF of 55.7%, delivering a high PCE of 4.8%. Additive processing is known to be an effective strategy to control the optimized morphology. By incorporating small amount of 1-chloronaphthalene into the active layer solution (2.5%, v/v) as the additive, the corresponding device exhibited much improved performance, showing a high PCE of 6.7% with a Voc of 0.83 V, a Jsc of 12.29 mA cm-2, a FF of 65.6%. This value represents one of the best performance among fluorine-based conjugated polymers in the literature. By taking advantage of low temperature device fabrication process, a flexible polymer solar cell was fabricated by replacing the ITO glass substrate with the flexible polyethylene naphthalate (PEN) substrate. This device exhibited a high PCE of 5.6%.