含苯并噻二唑及雙氟噻吩之共軛高分子之合成、性質及光伏打應用

Abstract

在D-A共軛高分子主鏈上，引入氟原子是很常用來改善高分子太陽能電池光伏效率的方法之一。主鏈氟化帶來的優點包含：降低高分子HOMO及LUMO能階而不改變能隙、增加光子吸收範圍及數量、增進分子鏈共平面性、改善混摻後異質接面型態、提升載子遷移率以及抑制再結合的發生，因此可提升開路電壓(Voc)、短路電流(Jsc)、結晶性以及太陽能電池光伏效率等。 本篇論文利用3,4-二氟噻吩單元，在主鏈上引入氟原子。3,4-二氟噻吩單元在分子設計上位於電子予體單元三噻吩之中心，故氟原子亦位於電子予體單元中心。相較於3,3’-二氟-2,2’-聯噻吩，3,4-二氟噻吩單元不僅也具有氟原子的優點，3,4-二氟噻吩上的兩個氟原子可各自和另一個單元形成F-S或F-H作用力，而不像3,3’-二氟-2,2’-聯噻吩大多只和同一聯噻吩單元內之另一噻吩作用；除此之外，3,4-二氟噻吩單元之兩個C-F鍵形成之偶極距亦大於3,3’-二氟-2,2’-聯噻吩，故分子極化特性亦較好。 本實驗利用3,4-二氟噻吩單元為電子予體，2,1,3-苯并噻二唑為電子受體，再引入3-己癸基噻吩作為增加溶解度之π橋，設計出含氟之高分子BT3T-2F；而不含氟之高分子BT3T-2H則作為對照組。在光電效率上，BT3T-2F:PC61BM元件之Voc為0.81 V、Jsc為12.50 mA/cm2、FF為68.55 %以及PCE為6.91 %；而BT3T-2H:PC61BM元件之Voc為0.73 V、Jsc為8.02 mA/cm2、FF為63.5 %以及PCE為3.74 %。故BT3T-2F:PC61BM之元件在所有方面皆有明顯提升。為了確認數據正確性，進一步用IPCE量測，得到BT3T-2F:PC61BM在吸收光譜全範圍內皆有較高的EQE；再用SCLC模型量測各載子遷移率，得到BT3T-2F:PC61BM之各遷移率均高了一個數量級；最後用TEM觀測異質接面型態，得到BT3T-2F:PC61BM具有較良好的微觀相分離。由此可知利用3,4-二氟噻吩單元在主鏈上之電子予體單元中引入氟原子可對光電效率帶來良好效果。

The introduction of fluorine atom on the backbone of conjugated D-A polymers is a common approach to improve the photovoltaic properties of polymer solar cells due to down-shifted HOMO and LUMO levels, enhanced light absorption, improved backbone coplanarity, increased carrier mobilities and suppressed recombination. In this work, a new donor-acceptor structured conjugated polymer, BT3T-2F, is synthesized by coupling benzothiadiazole acceptor block with terthiophene donor block, in which the middle thiophene unit is 3,4-difluorothiophene. Compared with 3,3’-difluoro-2,2’-bithiophene, the 3,4-difluorothiophene has advantages of simple synthesis route and low cost. Moreover, the two fluorine atoms can interact with S and/or H on both neighbor units, forming a coplanar three-member block. For comparison, an analog without fluorine atoms, BT3T-2H, is also prepared. Experimental results from photoelectron spectroscopy, UV-vis spectroscopy, X-ray diffraction and space charge limited current measurements indicate the replacement of the middle thiophene in the terthiophene with 3,4-difluorothiophene results in deeper HOMO and LUMO levels, enhanced absorption coefficient in solid film, better crystallinity and higher hole mobility, respectively. Consequently, the bulk heterojunction solar cells based on BT3T-2F:PC61BM exhibited a promising power conversion efficiency (PCE) of 6.91 % with a Voc of 0.81 V, a Jsc of 12.50 mA and an FF of 68.55 %, while the BT3T-2H:PC61BM cell has a PCE of 3.74 % with a Voc of 0.73 V, a Jsc of 8.02 mA and an FF of 63.5 %.