機械拉伸對高濃度共軛高分子光電量子效率提升之影響

The Impact of Mechanical Stretching on Optoelectronic Quantum Efficiencies of Conjugated Polymers in High Concentrations

盧仲廉(Lu, Chung Lien)

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/碩士(2015年)

摘要

本論文主要利用雙層薄膜拉伸法，進行機械應力與形變對共軛高分子光電性質影響之研究。所謂雙層薄膜拉伸法，乃指利用可進行局部頸縮形變的玻璃態高分子薄膜做為基材，上疊欲進行拉伸研究之共軛高分子薄膜，雙層薄膜經妥善界面接合後一起拉伸，利用合適的厚度比例，可使上層之高分子薄膜因下層薄膜的局部形變，而能進行大尺度的局部應力拉伸探討。實驗中，下層層薄膜為Polystyrene(PS)或是Polycarbonate(PC)，而上層薄膜為1.以不同濃度摻雜於PS之共軛高分子MEH-PPV，及2.具有不同側鏈的聚噻吩共軛高分子P3HT、P3EHT、P3BT，以探討分子間作用力對機械應力促進共軛高分子光致發光增益的作用。從實驗結果可以看到，共軛高分子薄膜的發光增益隨著局部應力上升而大約成指數上升。當1%MEH-PPV混摻99%PS的薄膜局部應力達220MPa時，纖化區內的發光增益可達89倍。P3EHT有著聚噻吩共軛高分子系列中最小的分子間作用力而在受力拉伸後有最高的26倍發光增益。應力的施加使得共軛高分子鏈段處於拘束的狀態，局部形變區內共軛高分子鏈的電荷捕捉能力下降，亦即應力抑制了charge-phonon coupling，因此我們可以看到發光增益。接著我們利用共軛焦微螢光光譜儀觀察受拉伸薄膜的各個區域，發現MEH-PPV和聚噻吩共軛高分子在局部形變區域中有著截然不同的行為。隨著MEH-PPV/PS比例變大，局部形變區相對於彈性形變區的藍位移也越大。我們則在聚噻吩系列的共軛高分子樣品中看不到此藍位移。最後我們利用飛秒時間解析上轉換系統，反推薄膜在不同區域中皮秒時間尺度的時間解析光譜。從重建的時間解析光譜我們可以看到不管100%MEH-PPV或是70%MEH-PPV/30%PS，發光峰隨時間紅移的速度皆隨著所受應力的增加而減緩。另外從delay time 對強度關係進行fitting後的結果發現MEH-PPV具有至少兩個能量逸散的機制，且較短時間的decay constant很明顯地隨著應力的增加而上升。上述兩點也呼應前面提到拉伸應力的施加使得高分子鏈處於拘束的狀態，進而抑制了電荷的捕捉和non-radiative relaxation的發生。

關鍵字

共軛高分子 ； 時間解析光譜 ； 雙層結構薄膜 ； 螢光上轉換

並列摘要

In order to have a better knowledge of the impact of mechanical stress and deformation on optoelectronic behavior of conjugated polymers, “two-layer structure stretching” is used in this thesis. In “two-layer structure stretching”, glassy state polymer is used as a substrate which can undergo local necking deformation. The substrate is then topped with polymers we are interested in. Not until these two layers are firmly attached to each other are we going to stretch this structure. With some adequate thickness ratios, top layer is greatly stretched due to the deformation of bottom layer. In our experiments, polystyrene and polycarbonate were used as a substrate. We top the substrate with (1) different ratio of MEH-PPV to PS (2) polythiophenes with different side chains. 89 times of PL enhancement was recorded as stress goes up to 220MPa for 1%MEH-PPV /99% PS case. P3EHT has the weakest intermolecular interaction among P3EHT, P3HT, and P3BT, thus it has the highest PL enhancement. It is the suppression of electron-phonon interaction of molecular segments that leads to the huge PL enhancements. A significant difference between MEH-PPV and polythiophene series is found under the confocal PL. We observe PL peak shifts in film and craze region for MEH-PPV cases, whereas we see no PL peak shifts for polythiophene series. We then use time-resolved up-conversion technique in probing different region of MEH-PPV/PS films, so that we can rebuild the time-resolved PL spectrum on a picosecond timescale. From the time-resolved spectrum, we observe a decrement of the rate of red-shift no matter in 100% MEH-PPV or 70% MEH-PPV/30% PS. We found at least two energy decay mechanism from the fitting curve of delay time versus intensity. The shorter decay time constants increase dramatically in highly stretched region comparing to longer ones. These two phenomenon are both the evidence of the suppression of charge trapping and non-radiative relaxation.

參考文獻

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