新型二維共軛性導電高分子之合成與性質分析

Synthesis and Characterization of Novel Two-Dimensional Conjugated Polymers

郭震宇(Cheng-Yu Kuo)

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(2013年)

https://doi.org/10.6342/NTU.2013.01624

摘要

在第一部分，我們設計並合成出三種新型具有terthiophene共軛側鏈的二維聚噻吩高分子，Poly{3-(5”-hexyl-[2,2’:5’,2”]terthiophenyl-5-vinyl)-thiophene-alt-
thiophene}, P1, Poly{ 3-(5,5”-dihexyl-[2,2’:5’,2”]terthiophenyl-3’-vinyl)-thiophene
-alt-thiophene}, P2 以及Poly{3-(4,4”-dihexyl-[2,2’:5’,2”]terthiophene-3’-vinyl)-
thiophene-alt-thiophene}, P3，利用共軛側鏈擺放位置的變化，來控制高分子鏈的結晶行為，並探討其對於高分子光學性質以及電化學性質的影響。在terthiophene的存在下，P1、P2及P3在固態時的吸收圖譜均具有非常廣的涵蓋範圍，由300延伸至700 nm附近。另一方面，從P1、P2以及P3的實驗或是模擬數據中都可發現到，當共軛側鏈水平於主鏈，且長烷鏈擺放位置不造成立體障礙的狀況下，其高分子主鏈之共平面性可大為改善，進而增加分子鏈堆疊的規則度。再者，由於具有垂直側鏈之M1，其重複單元間扭轉角度的銳減，因此M1有著比M2以及M3更長的共軛長度以及高的HOMO能階，使得聚合後的P1、P2以及P3之HOMO能階和能隙也因為單體所帶來之能階組態的不同而有顯著的改變。綜合以上結果，可知共軛側鏈的取向以及長烷鏈取代基團的位置對於高分子光電性質以及固態堆疊有著重要的影響。
從第一部分的結果得知，水平放置的側鏈對於二維高分子的結晶性質以及光學性質方面有著正面的影響，此架構下的共軛分子具有廣吸收以及低HOMO能階等之優點，非常適合利用於高分子太陽能電池中，但是在此之前，我們希望可以再進一步改善P3高分子主鏈的吸收強度以及固態結晶性。因此在第二部分，我們將第一部分P3主鏈的共聚單體thiophene改為bithiophene來增加共軛側鏈間距離，使其側鏈間的立體障礙減小，驅使主鏈共平面程度上升而增加鏈間作用力。從實驗結果得知，在相同M3單體架構，但共聚合單體改為bithiophene後，P4除了比P3有更強的主鏈吸收強度外，在XRD繞射圖譜中更可以清楚的觀察到一個比P3窄且強的繞射峰出現在2θ=4oC的位置，經由公式換算後，其數值亦符合主鏈與主鏈之間藉由側鏈所隔開的距離22A晶格大小的距離，表示高分子鏈之間是以更為規則且緊密的lamellae晶體結構排列。除此之外，為使二維高分子可更有效的利用太陽光譜，我們基於P4的分子模型，另行設計P5以及P6等具有更長共軛側鏈的高分子，兩者之側鏈分別以水平於主鏈，或是垂直主鏈方向增長，藉此了解延長共軛的取向對於分子結晶結構之影響。實驗結果顯示，往水平方向增加長度的高分子P5可以在不犧牲原先P4優越之結晶以及吸收性下，將其共軛側鏈之吸收紅位移至可見光區域，但是P6則無法，再次證明了側鏈保持水平的重要性。
第三部分，為測試我們所架構之二維高分子所精進的結晶性對於太陽能電池效率是否有幫助，我們將前述長烷鏈為直鏈型的P4、P5、P6上改變為支鏈型的P7、P8、P9來增加溶解度使其更適合應用在太陽能電池元件上。結果表示，由於P7與P8固態的分子排列結構規則度高，因此他們的載子移動率高於結構雜亂的P9，此項結果與電池效率的測試結果相當吻合，由P7所製作之高分子電池，其開路電壓(Voc)為0.79、填充因子可達67.44%，使其具有最高的光電轉化效率3.87%，另一方面，雖然P8與P7相較其HOMO較高而使Voc上升，但由於其不遜於P7的載子移動率以及紅位移後側鏈所貢獻的吸收助益下，由P8所製作的元件依然可以展現出3.29%的效率以及優於P7 的8 mAcm-2的短路電流(Jsc)。而P9由於其載子移動率過低，吸收強度過小，所以由其所製作之元件效率僅達0.67%，遠低於P7，P8。從以上結果我們可以知，高分子的排列規則度對於元件的效率的確有決定性的影響，因此P7、P8所展現的載子移動率以及元件效率皆遠優於P9，表示水平的側鏈擺放方式才是二維高分子應用於太陽能電池的最佳構型。此外，由於P8側鏈的紅位移，使其在350~500 nm此範圍間光電流貢獻遠優於P7，因此P8展現了比P7更高的短路電流，表示藉由側鏈共軛的延伸，的確可以有效地增加高分子對於太陽光光譜的利用度。
前三部分的實驗中，我們利用具有水平側鏈之二維高分子，改善了普通一維聚噻吩高分子650 nm之前的光譜涵蓋範圍並使其具有相當規則的固態堆疊程度，最後在第四部分，為了進一步減小二維高分子的能隙大小來增加高分子對於650 nm之後的吸收範圍，我們有系統地合成了六種具有噻吩共軛側鏈的二維高分子，藉由此架構來了解主鏈芳香環電子親和力大小對於高分子能階與組態的影響以及高分子在不同共軛側鏈組成，其固態下構型的改變。從實驗結果發現，在弱電子提供者-強電子接受者之間電荷轉移作用力的影響下，由benzo[1,2-b:3,4-b]dithiophene (BDT) - benzothiadiazle所架構的高分子(P13、P14 及P15)比採用benzo[1,2-b:3,4-b]dithiophene (BDT) - bithiophene的高分子(P10、P11及P12)，其涵蓋範圍更廣由300 nm至650 nm進一步擴大至800 nm，此外，其能隙以及LUMO也更低，但HOMO在這兩類的高分子中卻沒有顯著的改變，表示HOMO能階主要是由BDT此芳香環所決定，且軌域主要集中在此單體上，因此HOMO能階沒有太大的變化。在這兩系統的共軛高分子中，我們又另外將三種不同長度以及長烷鏈密度之噻吩共軛側鏈連結於BDT分子上，利用不同的二維結構來探討共軛側鏈對於主鏈共平面性的影響。結果顯示，當我們增加共軛長度或是長烷鏈的位置與密度，這些高分子間的作用力會因為立體障礙的增加、主鏈平面性降低而減小，而顯現在P10~P12吸收光譜中的570 nm、和P13~P15中640、712 nm這些源自高分子聚集區塊的波峰強度上，越強代表分子間作用力越大而聚集程度越明顯，從螢光光譜中我們也觀察到一樣的現象。最後，我們再度利用X射線繞射儀來觀察分子鏈在固態下的堆疊規則度，實驗結果發現，由於共軛鏈短，長烷鏈的密度小，P14的分子鏈具有最高的平面性以及規則的分子排列。總結以上，在二維共軛高分子中，共軛側鏈的結構與結晶排列性質是一體兩面，了解其中相互關係，對於往後光電性質的預測以及高分子太陽能電池的應用是非常重要的。

關鍵字

聚噻吩高分子 ； 二維共軛性高分子 ； 太陽能電池 ； 高分子固態排列 ； 分子動力學模擬 ； 時間相關密度泛函理論 ； 高分子合成 ； 分子設計

並列摘要

In first part, we report here synthesis of three novel polythiophene derivatives with conjugated terthiophene-vinylene side chain, Poly{3-(5”-hexyl-[2,2’:5’,2”]terthiophenyl-5-vinyl)thiophene-alt-thiophene}, P1, Poly{ 3-(5,5”-dihexyl-[2,2’:5’,2”]terthiophenyl-3’-vinyl)thiophene-alt-thiophene}, P2 and Poly{3-(4,4”-dihexyl-[2,2’:5’,2”]terthiophene-3’-vinyl)thiophene-alt-thiophene}, P3 were synthesized via stille coupling reaction. The terthiophene side chain provides the ability to control the molecular organization which further impacts the photo-physical and electrochemical properties. These polymers show a broader absorption ranges from 300 to 700 nm, significantly broader than the absorption of pure poly(3-hexylthiophene); especially, comparing to their the absorption spectrum in solution, P3 displays most red shifted in the wavelength range between 450 to 700 nm , which is attributed to the extension of conjugation resulting from the linear conformation and preferred chain packing, as manifested in the X-ray diffraction and simulation results. Moreover, we have determined the HOMO and LUMO of these three polymers through the cyclic voltammetry study; HOMO of the three polymers following the order, P1>P2>P3, a result suggests that the energetic level is regiochemistry dependent, and/or related to the linked position between backbone and conjugated side chain.
From the results getting from first part, we figured out the two-dimensional polythiophene with parallel conjugated side chain was a plausible way in developing solar-active material, due to its broad absorption and relatively low HOMO level. However, the featureless XRD pattern and low absorption intensity of the main chain, which were mainly resulted from the lose packing of the polymer chains, were still needed to be amended. We suspected the reason was due to the steric hindrance existed among the conjugated side chain, which would twist the polymer backbone and further lowered the π-π interaction between the polymer chains. To resolve the problem, here in second part, we choose the bithiophene as the monomer instead of thiophene to copolymerize with the two-dimensional thiophene monomer. By the extra spacing providing from the bithiophene, the steric hindrance of the side chain could be eased. Compared to previously synthesized polymer, P3, the P4 showed stronger π-π* transition absorption and red-shifted on the absorption edge, moreover, from the XRD study, a steep and sharp diffraction peak was clearly discovered at 2θ =4oC, which was representing the lamellar spacing 22A between the polymer chains. Besides, in order to better utilize the solar spectrum, we increased the conjugation length of the side chain vertically (P6) or horizontally (P5) to the main chain by introducing two more thiophene rings into the side chain, interestingly, the results revealed that the P5 polymer with parallel conjugated side chain has the better optical properties without scarifying the crystallinity.
In third part, based on the results odtained from last part, we synthesized series of alternative copolymers P7 、P8 and P9 with the same conjugated backbone but differed in the alkyl chain, by varying the linear alkyl chains in P4~P6 to branch ones in P7~P9, the solubility was improved to be further applied in solar cell. The hole mobility of P7、P8 and P9 determined from space-charge-limited-current (SCLC) method was 1.23x10-4 cm2 V-1s-1、 1.54x10-4 cm2 V-1s-1 and 6.33x10-5 cm2 V-1s-1, respectively, confirming that the mobility is strongly relative to the degree of packing regularity in solid state. Bulk heterojunction devices fabricated from P7 and PC60BM showed a relative higher power conversion efficiency (PCE) of 3.87 % with a high open circuit voltage (Voc) of 0.79 V 、Jsc of 7.3 mAcm-2 and Fill factor of 67.44 %, under the illumination of AM 1.5G, 100 mWcm-2 as compared to P8 and P9, which was due to its excellent mobility in the well-percolated blend film and low-lying HOMO measured from cyclic voltammetry. In comparison with P7, benefitting from the increase of the absorption from 350 to 500 nm which provided by elongation of conjugation length of the side chain and the comparable mobility to that of P7 , the device fabricated from P8 possessed a PCE of 3.29 % with higher Jsc of 8 mAcm-2. The current contribution of the side chain in P8 was further verified by the IPCE spectra. Finally, owing to the random stacking and poor light-harvesting properties, the power conversion efficiency based on P9 was only 0.67 % with a low FF of 33% and small Jsc of 2.68 mAcm-2.
In last part, we have systematically synthesis two series of alternative copolymers, which individually have benzo[1,2-b:3,4-b]bithiophene-bithiophene (donor-donor) and benzo[1,2-b:3,4-b]bithiophene-benzothiadiazle (donor-acceptor) building blocks in the main chain, through the electron-donating /accepting ability difference between bithiophene and benzothiadiazole units, a drastically 0.4 eV downshifting existed both in the energy band gap and LUMO level among the donor-acceptor type polymers, however, the HOMO level remained unchanged around -5.4 eV, indicating the HOMO level was essentially decided by the benzo[1,2-b:3,4-b]dithiophene (BDT) moieties and the LUMO level and band gap could be fine-tuned by varying another co-monomer. Moreover, in these two series of polymer, we attached three kinds of oligo-thiophenyl subtituent, differing in the alkyl chain density and the numbers of side chain thiophene unit in the range of 2~3, onto the BDT to examine the structure-property relationship. The results showed these two-dimensional polymers all have broad absorption character covering from 300 to 650 nm in bithiophene-based polymers (P10 to P12) and 300 to 800 nm in benzothiadiazole-based polymers (P13 to P15). Furthermore, as the numbers of thiophene or alkyl chain increased, the intermolecular π-π interaction was reduced through enhanced steric hindrance, which could be evidently found by comparing the vibronic band intensity of the absorption spectra at 570 nm in P10 to P12 and 640, 712 nm in P13 to P15, which was originated from strong aggregated species in the polymer matrix. To further testify the effect arising from geometrical difference in the solid state, the XRD techniques were employed as the tools to trace the π-stacking properties of these polymers. As a result, the studies revealed a consistent trend, the P14 with lower numbers of alkyl chain and shorter length of conjugated side chain has planar chain conformation and shortest π-π stacking distance around 3.6 A. In conclusion, the side chain geometries strongly influence the polymer properties. Obviously, the aggregation tendency (π-interaction), which gives rise to absorption or emission bands at longer wavelengths are responsible for this observation. This tendency is also validated from the XRD results. Importantly, A good exemplification with ordered packing structure, wide absorption coverage (300 to 800 nm), low-lying level (-5.41 eV) and narrow band gap (1.56 eV) was achieved in P14 by side chain engineering.

並列關鍵字

Polythiophene ； Two-dimensional conjugated polymer ； Solar cell ； Crystalline properties ； Molecular dynamics simulation ； TD-DFT ； Polymer synthesis ； Molecular design.

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