側鏈對予體−受體交替導電共聚物的光電性質與自組裝行為之影響

Abstract

予體−受體交替導電共聚物有應用於低成本的場效電晶體、太陽能電池等軟性電子元件之潛力，但這類聚合物常具剛性而難以加工。我們透過側鏈工程的方法，不僅能改變共聚物的構型，也能調控其自組裝行為及光學能隙。 我們利用Stille coupling反應並變換側鏈的位向、類型及長度，合成出四個系列以terthiophene (3T)當作予體及thieno[3,4-c]pyrrole-4,6-dione (TPD)當作受體之予體−受體交替導電共聚物。四個系列的共聚物命名為P3T(RiN)TPD(R)、P3T(RiN)TPD(E)、P3T(RoN)TPD(R)及P3T(RoN)TPD(E)，其中R是烷基側鏈、E是寡醚基側鏈、i代表兩條R接在3T的3,3”位置、o代表兩條R接在3T的4,4”位置而N是R的碳數。共聚物的能隙以紫外−可見光光譜及循環伏安法研究；自組裝行為以掠角廣角度X光散射 (grazing incident wide angle X-ray scattering, GIWAXS)及原子力顯微鏡 (atomic force microscope, AFM)研究。 具Ro化學結構的共聚物表現出層板結構，具Ri化學結構的共聚物則表現出六角柱結構及較低的能隙。因為Ro的3T部分較Ri的3T部分共平面，使高分子鏈間的π-π作用力較強，所以具Ro化學結構的高分子鏈較易產生連續的堆疊而形成層板。具Ri化學結構的高分子鏈只有較短程之堆疊，並在側鏈的幫助下形成高分子束，然後高分子束以最密堆積的方式自組裝，產生長程有序的六角柱結構。高分子束有如將主鏈延長之效果，因此增加了有效共軛長度，得到較低的能隙。由GIWAXS的結果顯示，將TPD上的R換成E ，使共聚物具備雙親性的側鏈組合，受到側鏈相分離的影響，可以得到更高度有序的六角柱結構繞射峰值 (√21)。 共聚物在溶液態時，由於烷基側鏈對主鏈造成之電子給予效應，使其最高吸收度波長 (lambda maximum of solution, λmaxsol)隨著側鏈長度增加越紅移且與側鏈位向無關。然而在薄膜態時，Ro較Ri的化學結構能使共聚物的薄膜有更紅的最高吸收度波長 (lambda maximum of film, λmaxfilm)，因為具Ro化學結構的高分子鏈間作用力隨著側鏈長度增加而增強，並使λmaxfilm越紅移。我們透過改變予體−受體交替導電共聚物的側鏈，除了可以調控其能隙，並觀察到罕見的高度有序六角柱結構。所有共聚物皆不需經過熱退火即有自組裝結構，且有潛力應用於軟性電子元件的製作。

Donor-acceptor alternating conducting copolymers are potential materials for low cost flexible electronics such as transistors, solar cells, etc. These kinds of polymers are usually rigid and difficult to process. Through side chain engineering, we can not only tune the conformation but also optical bandgap and self assembly behavior of the copolymers. From Stille coupling reactions, we synthesized four series of donor-acceptor alternating conducting copolymers. They are basis on donor of terthiophene (3T) and acceptor of thieno[3,4-c]pyrrole-4,6-dione (TPD) by varying position, type, and length of side chain. They are named P3T(RiN)TPD(R), P3T(RiN)TPD(E), P3T(RoN)TPD(R), and P3T(RoN)TPD(E), where R is alkyl side chain and E is oligoether side chain, i represents the two R located on 3,3” position of 3T, o represents the two R located on 4,4” position of 3T, and N is the length of R. The bandgap of each copolymer is studied by the UV-Vis spectroscopy and cyclic voltammetry. The self assembly behaviors of copolymers are studied by grazing incident wide angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). It is noteworthy that the donor-acceptor conducting copolymer with Ro chemical structure performs lamellar structure. In contrast, the copolymer with Ri exhibits hexagonal cylinder structure and lower bandgap. Ro chemical structure has a more planar 3T building block structure as compared with Ri and thus provides stronger π-π interaction between the macromolecular chains, which results in easier production of continuous stacking and forming of laminates. Meanwhile, for the copolymer with Ri chemical structure, the stacking with short-range order forms macromolecular bundles with the aid of the side chain, afterwards long-range order hexagonal cylinder structure appeared due to the self assembly of macromolecular bundles with close packing. The macromolecular bundles have the effects of extending the length of the main chain, therefore increasing the effective conjugating length and obtaining a lower bandgap. According to the results of GIWAXS, the effect of phase separation leads to a higher degree of ordering in diffraction peak (√21) of hexagonal cylinder structure. This is mainly due to the advantage of replacing R with E on the TPD, which equips copolymer with amphiphilic characteristic. In solution, due to the donating effect of long alkyl side chain toward the main chain, the λmaxsol of copolymers is shifted to longer wavelength with increasing side chain length regardless the main chain structure of copolymers. In film, the extent of red shift for copolymer containing Ro structure is larger than that containing Ri structure. By tuning the side chains on the donor-acceptor alternating conducting copolymers, we can not only tune the bandgap but also observe rare case of highly ordered hexagonal cylinder structure. All of these copolymers have self assembly structure without thermal annealing, and have the potential to be applied in the fabrication of flexible electronic devices.