利用量子力學計算探討電子予體─電子受體共軛高分子的結構設計對光電性質的影響

Abstract

電子予體─電子受體共軛高分子為近年來被廣泛研究的高分子太陽能電池材 料。如何藉由共軛高分子的結構設計來提升光電轉換效率是最重要的議題。共軛 高分子的能階、能隙是影響光電轉換效率的重大因素。因此在本研究中，我們利 用量子力學中的密度泛函理論研究電子予體─電子受體共軛高分子的光電性質。 我們選擇三種電子予體單體fluorene、CPT、BnDT 和三種電子受體單體BT、 DPP、TPD。我們計算這些交替共聚合物的HOMO 能階、LUMO 能階與能隙。 因為BnDT-DPP 的能階與能隙最接近理想值，所以BnDT-DPP 有最大的光電轉換 效率。在共軛高分子中，取代基也是影響能階與能隙的重要因素。我們在聚噻吩 接上推電子基和拉電子基形成交替共聚合物。推電子基包含羥基、胺基和甲氧 基。拉電子基包含氰基、醛基、羧基和硝基。我們發現羥基、甲氧基、氰基和醛 基使分子鏈構形較為共平面，因此OH-CN、OH-CHO、OCH3-CN 和OCH3-CHO 的組合有較低的能隙。最後我們探討取代基的數量和排列方式對共軛高分子的影 響。推電子基與拉電子基的組合會比只有推電子基或只有拉電子基更好。AT-NT 有最低的能隙。

Donor-acceptor conjugated polymers are polymer solar cells materials which are widely researched in recent years. How to promote the power conversion efficiency by structure design of conjugated polymers is the most important issue. Energy level and band gap of the conjugated polymers are major factors which affect the power conversion efficiency. Thus, in this study, we use density functional theory of the quantum mechanics to investigate the photoelectric properties of the donor-acceptor conjugated polymers. We choose three donor units, fluorine, CPT, BnDT and three acceptor units, BT, DPP, TPD.We Calculate the HOMO, LUMO and band gap of these alternating copolymers. Because the energy level and band gap of BnDT-DPP are the closest to ideal value, BnDT-DPP has maximum power conversion efficiency. In conjugated polymers, substituents are also important factors which affect the energy level and the band gap. We join electron-donating groups and electron-withdrawing groups to polythiophenes, becoming alternating copolymers. Electron-donating groups include hydroxy, amino and methoxy groups. Electron-withdrawing groups include cyano, formyl, carboxyl and nitro groups. We find hydroxy, methoxy, carboxyl and formyl make the chain conformation coplanar, so the combination of OH-CN, OHCHO, OCH3-CN and OCH3-CHO have lower band gap. Last, we investigate the effects of amount and rearrangement of substituents on conjugated polymers. The combination of electron-donating groups and electron-withdrawing groups is better than only electron-donating groups or only electron-withdrawing groups. AT-NT has the lowest band gap.