探討新穎非富勒烯小分子受體材料二環戊烷並苯二噻吩衍生物之分子結構設計對光電性質的影響

Structure Design and Photoelectric Properties of Novel Non-Fullerene Acceptors Based on Small Molecule Indacenodithiophene Derivatives

游天侑(Tien-Yu Yu)

指導教授 ： 黃慶怡

國立臺灣大學/工學院/高分子科學與工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201704186

摘要

本研究選擇新穎的非富勒烯A−D−A型小分子受體材料二環戊烷並苯二噻吩衍生物為研究對象，並採用全原子模型，結合分子動力學與量子力學二種模擬方法來探討不同末端受體單元對光電性質的影響。我們從分子構型出發，並分析一系列包括光捕獲效率、能階、能隙、吸光頻譜及電負度等光電性質，藉此瞭解末端電子受體單元對於A−D−A型二環戊烷並苯二噻吩衍生物光電性質的影響。
首先在構型模擬上我們針對3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2 b:5, 6-b′]dithiophene(IDTT_DCI)進行量子力學及溶液無序狀態下分子動力學兩種方法的模擬，基於兩種模擬方法的構型，進一步求得模擬UV-vis吸收光譜，與文獻實驗值比較後我們推測實際UV-vis吸收光譜可能是同時由主鏈構型較規整的IDTT_DCI(QM)及構型較扭曲的IDTT_DCI (MD)所貢獻，可分別對應到實驗主峰中的最大吸收峰與肩峰，當主鏈構型越扭曲時，主要吸收峰越藍移。我們認為考慮溶劑環境且經由等溫動態平衡後之構型是更加接近真實的情形，因此後續研究中IDT衍生物構型均利用分子動力學方法來取得。
進一步，我們探討一系列IDT衍生物之構型及光電性質。首先在構型部份，引入不同末端電子受體單元主要影響主鏈上扭轉角度，當末端電子受體與芳香核結構間存在較大的立體障礙時，會導致主鏈的扭轉；在側鏈部份則因空間中側鏈與末端電子受體立體障礙較小，影響的幅度並不大。在能階方面，隨著末端電子受體單元拉電子能力越強，IDT衍生物有越高的LUMO能階，當與予體材料P3HT匹配後隨之有越高的Voc，同時影響激子有效分離的參數〖∆E〗_HOMO及〖∆E〗_LUMO均大於0.3 eV，亦說明基於能階的考量上，IDTT_BT有最高的Voc與良好的激子分離效果。在電負度結果中，可發現在此改質IDT衍生物之末端電子受體單元相較於P3HT均有較高的電負度，說明其適合作為受體材料，且在結構類似的IDTT_DCI、IDTT_ID及IDTT_TZD中隨著拉電子能力越強，電負度越高；針對結構相似的IDTT_NTz及IDTT_BT也有同樣的趨勢。吸收頻譜方面，改質末端電子受體明顯改變IDT衍生物之主要吸收峰及吸收範圍；而末端電受體對於振子強度及光捕獲效率也有些微幅度的影響。在此Jsc考量因素較為複雜且並未觀察到固定的趨勢，因此初步未總結出何者會有較高的Jsc。此外，雖然在本研究中並未討論到主動層混摻型態甚至FF的影響，但我們探討了其中影響Voc及Jsc的因素，期許藉由本模擬所歸納出的趨勢與結果，能幫助實驗研究學者合成的設計方向與思考重點，發掘出潛在有應用性的非富勒烯受體材料。

關鍵字

非富勒烯小分子受體 ； 二環戊烷並苯二噻吩衍生物 ； 量子力學 ； 全原子分子動力學 ； 光電性質

並列摘要

A class of A−D−A type non-fullerene acceptor materials based on small molecule indacenodithiophene(IDT) derivatives used in organic solar cell has been designed and investigated by means of all-atom molecular dynamics(AAMD) and quantum mechanical methods(QM). To gain a better understanding of the effects of terminal acceptors on the modulation of photoelectric properties of these A−D−A molecules, the geometrical structure, light harvesting efficiency, energy level, bandgap, absorption spectrum, and electronegativity are analyzed.
The geometrical structure of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5, 6-b′]-dithiophene(IDTT_DCI) is simulated by AAMD and QM methods to get IDTT_DCI(MD) and IDTT_DCI(QM). The disordered IDTT_DCI(MD) exhibited a much larger degree of distortion out of coplanarity along the main chain direction, leading to a blue-shift absorption behavior compared with much ordered IDTT_DCI(QM). We speculated that the actual UV-vis absorption spectrum may be simultaneously contributed by the IDTT_DCI (QM) and the more distorted IDTT_DCI (MD), which can correspond to the wavelength of maximum absorbance and the shoulder at the lower wavelength. Also, we assume that the equilibrated molecular structure predicted from equilibrium dynamics of the solvent environment is closer to the real experimental conditions. Therefore, the MD simulation method is adapted for the geometries of the IDT derivatives.
Introduction of different terminal acceptors mainly affects the torsional angle on the main-chain due to the existence of larger steric hindrance between core structure and terminal acceptor. The calculated energy level results indicate that with the incorporation of stronger electron withdrawing building block, the IDT derivatives show higher LUMO level and higher Voc when paired with P3HT. Also, according to the energy matching relationship, the designed acceptor IDTT_BT is well-matched with P3HT, which not only guarantee the effective exciton dissociation but also maintain the higher Voc. The electronegativity results also show similar trends, compared to P3HT, IDT derivatives show higher electronegativity which are suitable to be as acceptor materials. In the absorption spectrum, terminal acceptors have strong correlation with main absorption peak and absorption range, and little effect on oscillator strength and light harvesting efficiency. Since factors affect Jsc are more complicated than Voc and inconsistent trend are observed from the results of optical properties of IDT derivatives; thus we can not surely say which material will have higher Jsc. Though effects of morphology or FF are not considered, still we investigate the factors influencing Voc and Jsc, hope these results can help researchers explore the potential candidates of non-fullerene acceptor materials for organic solar cells.

並列關鍵字

non-fullerene small molecule acceptor ； indacenodithiophene derivatives ； quantum mechanics ； molecular dynamics ； photoelectric properties

參考文獻

[1] J. Rogelj, M. Den Elzen, N. Höhne, T. Fransen, H. Fekete, H. Winkler, R. Schaeffer, F. Sha, K. Riahi, M. Meinshausen, Paris Agreement climate proposals need a boost to keep warming well below 2 C, Nature 534(7609) (2016) 631-639.

[2] IEA, World Energy Outlook 2015, OECD Publishing.

[3] G. Conibeer, Third-generation photovoltaics, Materials today 10(11) (2007) 42-50.

[4] M.A. Green, Photovoltaics: technology overview, Energy Policy 28(14) (2000) 989-998.

[5] G.v. Sauvé, R. Fernando, Beyond fullerenes: designing alternative molecular electron acceptors for solution-processable bulk heterojunction organic photovoltaics, The journal of physical chemistry letters 6(18) (2015) 3770-3780.