含三噻吩共軛側鏈之聚噻吩高分子:合成、性質及其光伏應用

Novel Polythiophene Derivatives with Terthiophene-vinylene (TTV) Conjugated Side Chains: Synthesis, Properties and Their Application in Photovoltaics

蕭全佑(Chuen-Yo Hsiow)

指導教授 ： 邱文英

共同指導教授 ： 王立義

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02721

摘要

本文主要設計、合成及分析一系列以含三噻吩烯基為共軛側鏈之二維聚噻吩高分子，著重探討分析高分子結構與性質，以開發新型高性能二維共軛高分子為目標。
首先，第一部分設計不同連結方式的含三噻吩烯基為共軛側鏈二維聚噻吩高分子，PBTTTV-h與PBTTTV-v，比較共軛側鏈的不同聯結方式，對於其光學、電化學、分子自組裝及光伏等性質影響。PBTTTV-h以類平行方式聯結共軛側鏈，其吸收光譜呈現雙吸收峰形式且形成固態薄膜時具有結晶性；而PBTTTV-v以類垂直方式聯結共軛側鏈，則呈現單吸收峰形式，形成固態薄膜時則無結晶性。兩二維共軛高分子均都具有低HOMO能階及優異的溶解度。進一步應用到有機高分子太陽能電池元件上，兩高分子電池均展現高開環電壓，而PBTTTV-h與PC71BM有較佳的相容性及電荷遷移率，其光電轉換效率達4.75 %；而PBTTTV-v與PC71BM亦可達4.00 %。
第二部分，根據第一部分實驗結果發現含三噻吩烯基為共軛側鏈以類平行方式連結之二維聚噻吩高分子，PBTTTV-h，具高結晶性此一特性結果，設計烷基鏈於不同側鏈位置，PBTTTV-h2，及提供不同π-spacer之稠環結構，PTTTTV-h與PDTTTTV-h，與PBTTTV-h相互比較其對於結晶性的影響及變化。根據XRD繞射實驗顯示，當重複單元中的主鏈提供足夠共軛側鏈堆疊排列的空間，該系列高分子便可產生結晶的可能，同時亦影響高分子排列的緊密程度及d-spacing大小。進一步應用到有機高分子太陽能電池元件上，PBTTTV-h2與PC61BM混摻之有機太陽能電池元件有最佳光電效率4.62 %。
第三部分，為增強二維共軛高分子的光學吸收能力，透過引入拉電子基2, 1, 3-benzothiadiazole （BTD）以無規共聚合方式於含共軛側鏈之共軛高分子PBTTTV-h主鏈中，促使吸收光譜紅位移且增廣，並探討電化學、結晶性、光電性質等變化。紫外光-可見光吸收光譜顯示共軛主鏈吸收確因BTD分子存在而明顯紅位移，並增加650-750間的吸收範圍，光學能隙有效地下降至1.68 eV。透過共軛主鏈結構的修飾，PBTTTV-h-r-PBTDBT-12%可進一步提升光電轉換效率達5.31% (Voc = 780 mV, Jsc = 10.82 mA/cm2, FF = 62.92%)。由IPCE結果顯示，短路電流增強主要來自於650-750 nm間的貢獻，高分子PBTTTV-h-r-PBTDBT-23%混摻PC71BM相較於PBTTTV-h/PC71BM元件的短路電流提升13%。
第四部分，在PBTTTV-h共軛側鏈引入雙庚酮基，使短波長區域吸收紅位移；再透過無規共聚合方式，引入BTD單元於高分子主鏈中，促使大幅提升400-700 nm間的光學吸收。結晶性部分，雖引入雙庚酮基使主鏈扭轉角度增加而無結晶性，但進一步以少量BTD的取代下，減少高立障的二維共軛單元，反可進一步使高分子恢復結晶性。透過一系列共軛側鏈與共軛主鏈結構的修飾，PBTTTVDK-r-BTD/PC61BM光電轉換效率達4.17 %。
綜合以上實驗，我們對於含三噻吩烯基為共軛側鏈之二維聚噻吩高分子的結構設計將有更進一步認識與了解，這將有助於未來設計高性能二維共軛高分子材料。

關鍵字

有機高分子太陽能電池 ； 共軛高分子 ； 共軛高分子合成 ； 二維共軛高分子 ； 共軛側鏈

並列摘要

In this thesis, a series of novel polythiophenes with terthiophene-vinylene moieties as conjugated side-chains were designed and synthesized by Stille polycondensation. The correlations between the chemical structure and properties of such polymers and their effect on the performance of solar cells were intensively investigated.
In the first part, two isomeric polythiophenes with terthiophene-vinylene (TTV) as conjugated side-chains, PBTTTV-h and PBTTTV-v, were designed, synthesized, and characterized. Although both polymers show low-lying HOMO energy levels and good solubility, the architecture of conjugated side chain significantly affects the optical, electrochemical, molecular self-assembling and photovoltaic properties. The PBTTTV-h displays two distinct absorption peaks in UV-Vis spectrum and a crystal polymer as verified by XRD; on the other hand, the PBTTTV-v displays a single broad absorption peak and is an amorphous material. Moreover, the BHJ solar cell fabricated from PBTTTV-h/PC71BM exhibited a higher power conversion efficiency (PCE) of 4.75 % under AM1.5G illumination at 100 mWcm-2 than PBTTTV-v/PC71BM of 4.00 %.
Based on the results of the previous part that the two-dimensional polythiophene with parallel TTV conjugated side chains has good crystallinity, we further varied the anchoring site of alkyl groups on the conjugated side chain and substitute fused thiophenes in different sizes, TT and DTT, for BT as a spacer to control the spatial distance between bulky terthiophene-bearing monomers. The results clearly indicate the length of π-spacer is a crucial factor to the crystallinity of these 2D-polythiophenes as well as the anchoring site of alkyl chains and the coplanirity of main chain apparently influence the degree of molecular packing and the d-spacing of crystallites. Under AM1.5G illumination at 100 mWcm-2, the cell based on PBTTTV-h2/PC61BM exhibits the best PCE of 4.62%.
In the third part, the 2D conjugated copolymers, PBTTTV-h-r-PBTDBT, containing 12% and 23% of 2, 1, 3-benzothiadiazole (BTD) were synthesized by Stille random-copolymerization and their optical, electrochemical, charge transport and photovoltaic properties were investigated. Both copolymers show a broader absorption spectrum, covering the spectral range from 300 nm up to 750 nm, and a reduced optical bandgap of 1.68 eV. The low content of BTD unit in PBTTTV-h-r-PBTDBT allowed preservation of many important properties of PBTTTV-h, including low-lying HOMO and high crystallinity. Under AM1.5G illumination at 100 mWcm-2, the polymer solar cell based on PBTTTV-h-r-PBTDBT-12%/PC71BM exhibits the best PCE of 5.31% with an open-circuit voltage (Voc) of 780 mV, a short-circuit current density (Jsc ) of 10.82 mAcm2, and a fill factor (FF) of 62.92 %. The IPCE measurements revealed a strong photo response from the random copolymers up to 750 nm, with IPCE values above 40% from 300 nm to 650 nm for PBTTTV-h-r-PBTDBT. These results indicate that the incorporation of acceptor units into a 2D-polymer by random copolymerization provide a simple and effective route toward polymers with a broad photocurrent response in PSCs.
In last part, two new copolymers were designed, synthesized, characterized and applied in polymer solar cells (PSCs) as donor materials. Copolymer PBTTTVDK attaching two electron-withdrawing heptanoyl groups at TTV conjugated side chains showed a deeper HOMO energy level, better solubility, and red-shifted absorption of conjugated side chains. By introduction of strong electron-withdrawing BTD unit, PBTTTVDK-r-BTD possessed further red-shifted absorption band with absorption onset at 743 nm and effectively improved the coplanity of polymer backbone and ability of self-assembling to enhance carrier mobility. The bulk-heterojunction PSCs based on PBTTTVDK/PC61BM and PBTTTVDK-r-BTD/PC61BM displayed PCE of 1.52% and 4.17%, respectively, under the illumination of AM1.5G, 100 mW cm-2.
We have successfully designed, synthesized and characterized a family of polythiophenes with a terthiophene-vinylene (TTV) as conjugated side-chains and applied them in PSC device. The achievement of structure-property correlations will be benefit of developing high performance 2D-conjugated polymeric semiconductor materials.

並列關鍵字

polymer solar cells ； conjugated polymer ； synthesis of conjugated polymers ； two-dimensional conjugated polymer ； conjugated side chains

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