新穎有機染料共軛高分子之合成與鑑定及其在太陽能電池之應用

Abstract

本研究中，我們合成六個共軛高分子材料CPDT-co-TPADCN、CPDT-co-TPADTA、CPDT-co-TPATCN、CPDT-co-FADCN、CPDT-co-FADTA以及CPDT-co-FATCN。這些共軛高分子以Cyclopentadithiophene (CPDT) 為主體，並在其上導入含有分支的2-乙基己烷鏈，增加共軛高分子的溶解度，再搭配六個不同的以D-π-A結構為主的有機染料，這六個有機染料以兩種donors (D)，相同的π共軛系統 (π) ，styrylthiophene，以及三種acceptors (A) 組合而成，donors分別為triphenylamine-based donor (TPA) 以及fluorene-based donor (FA)，acceptors各別為DCN、DTA以及TCN；而donor的推電子強度為FA大於TPA，acceptor的拉電子強度是TCN最大，DTA次之，DCN最小。利用Stille coupling進行聚合反應得到前驅高分子，之後再以官能基修飾的方式在側鏈上接上acceptos得到這六個交錯型 (alternating) 共軛高分子材料。 在共軛高分子的吸收方面，CPDT-co-TPATCN與CPDT-co-FATCN的吸收範圍從400到900 nm，其能隙為1.34 eV，其餘共軛高分子的吸收範圍從400到700 nm，能隙在1.69-1.80 eV之間，從吸收的結果發現，隨著acceptor拉電子能力愈強，ICT的吸收就愈紅位移，能隙就愈小。在熱穩定性方面，所有材料的熱裂解溫度在305.3 ~ 425.6 oC之間，因此這些共軛高分子材料熱穩定性都相當好。在材料的能階位置方面，FA系列共軛高分子的HOMO都比TPA系列共軛高分子來的低，主要是因為fluorene分子比較剛硬 (rigid) 所造成的；而隨著acceptor拉電子強度愈強，共軛高分子的LUMO就愈低。由於TCN的拉電子強度太強，使得以TCN為acceptor的共軛高分子的LUMO太低，與PC71BM的LUMO相當靠近。 我們將這些共軛高分子材料製作成bulk heterojunction (BHJ) 太陽能電池的形式，其元件結構為：ITO/PEDOT:PSS/Polymer:PC71BM(1:4，w/w)/Ca/Al。其中以CPDT-co-TPATCN以及CPDT-co-FATCN的光電轉換效率最差，其原因主要來自於兩者的LUMO與PC71BM非常接近，使得激子分離不易，造成Jsc太低，導致效率不好；而CPDT-co-FADTA由於其有機染料之組成是以FA為donor，DTA為acceptor，因此其元件表現最好，Voc為0.82 V， Jsc為2.78 mA/cm2，fillfactor為0.47,光電轉換效率為1.07%。

We have synthesized six conjugated copolymers, CPDT-co-TPADCN, CPDT-co-TPADTA, CPDT-co-TPATCN, CPDT-co-FADCN, CPDT-co-FADTA and CPDT-co-FATCN. Each polymer consists of a 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene unit and a D-π-A organic dye in an alternating arrangement. Six organic dye monomers were designed and synthesized, triphenylamine-based (TPA) and fluorene-based (FA) units serve as the electron donors, while DCN, DTA, and TCN units are used as the electron acceptors. The electron-donating ability order of the donors is FA＞TPA, and the electron-withdrawing ability order of the acceptors is TCN＞DTA＞DCN. The conjugated precursor polymers were prepared via Stille coupling, followed by post-functionalization to introduce the acceptor groups on the side chains. The absorption spectra of CPDT-co-TPATCN and CPDT-co-FATCN cover from 400 to 900 nm, the optical band gaps are the same, 1.34 eV. The absorption spectra of the other polymers cover from 400 to 700 nm, the optical band gaps range from 1.69 to 1.80 eV. As the electron-withdrawing ability of acceptors increases, the maximum ICT absorbance peak becomes more red-shifted, leading to smaller optical band gap. The decomposition temperatures (Td) of all conjugated polymers range from 305.3 to 425.6 oC, indicating excellent thermal stabilities. For the energy levels of the polymers, the HOMO of the fluorene-based conjugated polymers are more low-lying than the triphenylamine-based polymers due to the rigidity of fluorene; as the electron-withdrawing ability of acceptors increases, the LUMO of the conjugated polymers becomes lower. However, the strongest acceptor, TCN, is too strong so that the LUMO of the conjugated polymers, CPDT-co-TPATCN and CPDT-co-FATCN, are too low and too close to the LUMO of PC71BM. We used these conjugated polymers to fabricate the bulk heterjunction (BHJ) solar cell devices with the configuration of ITO/PEDOT:PSS/Polymer:PC71BM(1:4, w/w)/Ca/Al. The worst two PCEs results are found for CPDT-co-TPATCN:PC71BM and CPDT-co-FATCN:PC7 1BM solar cells, the reason for this results is the relative LUMOs of the conjugated polymers and PC7 1BM, they are too close so that the exciton dissociation would be hindered, as a result, Jsc decrease and the PCEs are influenced. The best PCE of 1.07% was observed for CPDT-co-FADTA: PC71BM solar cell with a Voc of 0.82 V, a Jsc of 2.78 mA/cm2 and a fillfactor of 0.47. As we expected, the organic dye of CPDT-co-FADTA is composed of the best donor, fluorene-based donor, and the best acceptor, DTA, as a result, the highest PCE of 1.07% was obtained from CPDT-co-FADTA: PC7 1BM solar cell.