利用層間高分子擴散及電場感應提升共軛高分子薄膜之發光量子效率

Quantum efficiency enhancements of pristine conjugated polymer by interlayer polymer diffusion and electropoling

陳柏村(Po-Tsun Chen)

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/博士(2021年)

摘要

半導體共軛高分子在光電及為電子應用上雖然有著低成本、優秀的機械性質以及彈性的製程方式等優點，但共軛高分子的量子效率提升仍然關鍵點。有鑑於近期的報導中提到提高共軛高分子中鏈段應力，可以得到巨大的發光增益，我在這篇論文中以發展出使共軛高分子薄膜自身發光效率增加之方式並同時釐清其機制為目的，進而提出了兩種加強純共軛高分子旋塗薄膜機械性質的方法，就是分子層間擴散以及電場極化兩種方式。
在分子擴散的方式中，純MEH-PPV旋塗成膜在基材上後，再覆蓋一層光學惰性的polystyrene (PS)形成雙層結構，在退火前後由螢光放光光譜(PL)測得樣品量子效率(QEs)，且隨著退火時間變化紀錄其改變狀況。PS的鏈段應力將個別由除潤初期的應力釋放量測，而基材將選用矽基板以及蓋玻片玻璃基材，在矽基材上因為內建電場的關係，MEH-PPV的光激發態在異質介面上會有焠熄效應，而在玻璃基材上並沒有此效應。在溶劑退火後雙層樣品最終會產生除潤現象，但在不穩定性開始前，層間擴散不只造成層間高分子混摻區域的量子效率上升，同時也提高了未混摻之純MEH-PPV區域的量子效率。我們發現純的MEH-PPV的量子效率隨著共軛高分子的鏈段應力提升同時提高，而共軛高分子鏈段應力的提升是藉由在層間擴散時PS的注入且擴散同時涉及應力傳遞的過程。應力傳遞與誘導出的PL發光增益會隨退火時間持續增加，到除潤現象開始前的那一刻，在矽基材上的樣品量子效率大約有6倍的增加同時鏈段應力增加了1.5 MPa。之後退火時間再增加除潤現象隨之產生，會移除一層固定膜厚的MEH-PPV，這層膜厚相當於剛旋塗完的MEH-PPV薄膜上層中的膚層。隨著除潤現象的進行量子效率會持續增加，因為高分子的大規模流體流動會持續拉伸共軛高分子鏈段。然而當基材換成沒有異質介面焠熄效應的蓋玻片時，我們也發現因為層間高分子擴散與除潤高分子流動剪切引起鏈段應力的上升，造成顯著的抑制焠熄現象，進而使得樣品的發光的量子效率增加。應力誘導抑制焠息效應甚至有能力將異質介面焠熄效應完全抑制，比較不同基材間上殘留薄膜的螢光放光光譜就證明了這一點，而且與之前刊登文章上的單層共軛高分子薄膜除潤的現象一致。總結以上，我們找到層間高分子擴散會造成巨大的鏈段應力施加在純共軛高分子上，進一步使得共軛高分子的發光量子效率有所增益。這個增益是來自於分子鏈段拉伸抑制了電子電洞的非放光衰退與異質介面的焠熄效應。
第二個方式是利用電場極化，在這個方式裡我們將共軛高分子薄膜夾在上下兩個金屬電極中間，再利用絕緣的玻璃墊片將樣品與電極隔開一段固定距離，之後將樣品置於2k V的電壓下并施予溶劑退火，做電場極化處理。將電場極化解除後，樣品會量測其螢光放光光譜，並觀察其隨著極化時間(t)的變化。我們將使用不同的共軛高分子進行研究包括純的MEH-PPV、側基區域規則度高的rr-P3HT、側機不規則的rra-P3HT以及利用PS稀釋的共軛高分子薄膜。我們發現在電場極化後會造成分子排列的現象，使螢光光譜強度會因為退火後應力釋放先下降，接著分子排列後會造成螢光強度的上升。然而透過分子排列產生的螢光發光增益會受阻於強烈分子間作用力形成的聚集現象，以至於螢光強度在應力釋放造成下降後無法回到電場處理前的強度水準。在rr-P3HT的實驗中，電場極化造成不大的結晶度增加，但卻沒有產生相較於極化前樣品的螢光增益。相反的在rra-P3HT的實驗中，電場極化後84小時卻造成2倍的螢光強度增加。總結這部分我們得到了一個結論就是電場極化造成的分子鏈排列與僵直化共軛高分子鏈段類似，都會造成發光量子效率的增加。
綜合以上所述，不管是利用層間擴散或電場極化來增加共軛高分子鏈段上的機械應力都可以增加共軛高分子的螢光量子效率。這篇論文中所發展的引發旋塗薄膜高分子自身發光增益的方式可能給製造高效率共軛高分子元件的方法開發一些啟示。

關鍵字

共軛高分子 ； 量子效率 ； 光致發光 ； 鏈段應力

並列摘要

Quantum efficiencies remains key to opto- and micro-electronic applications of semiconducting conjugated polymers (CPs) despite their clear advantages on cost, mechanical properties, and processing flexibility. In light of the newly reported dramatic enhancement effects from elevated segmental stresses, I in this study explored two approaches modifying the molecular mechanics in the as-spun films, i.e., via interlayer molecular diffusion and electric poling, in an attempt to develop the in-situ enhancement methods for pristine CP films and at the same time to clarify the mechanisms.
In the diffusion method, the pristine MEH-PPV thin film was juxtaposed underneath an optical inert polystyrene (PS) film on a substrate, and the quantum efficiencies (QEs) were measured from the photoluminescence (PL) before and after solvent annealing, as a function of the annealing time. The segmental stress in the PS was measured separately from stress release during incipient dewetting, and the substrate used for the diffusion study was either a Si-wafer, which quench the photo-excited states in MEH-PPV via operations of the heterojunction built-in field, or a non-quenching glass cover slip. The solvent annealing ultimately led to dewetting of the bilayer sample, but before the onset of instability, interlayer diffusion caused the QE to rise not only in the intermixing zone but also in the non-pervaded pristine region of MEH-PPV. We found that the QE of the pristine MEH-PPV rose with the CP segmental stress that was increased along with the PS infusion during the interdiffusion, implicating a stress transfer process under the interlayer diffusion. The stress transfer and the induced PL enhancements continued with the interlayer diffusion until the onset of dewetting, at which point the QE had increased ~6 fold on the Si-wafer with a segmental stress increase of 1.5 MPa. The dewetting that followed, removing a fixed layer of MEH-PPV of a thickness commensurate to the skin layer of the spin-coated MEH-PPV, caused the QE to rise continuously due to segmental stretching of the CP molecules in the hydrodynamic flows. Moreover, by changing the substrate to the non-quenching glass cover slip, we found the QE rise under both the interlayer diffusion and dewetting hydrodynamic shear in fact contained significant contributions of de-quenching due to increased segmental stresses. The stress-induced de-quenching was found even capable to reset completely the heterojunction quenching, as shown by comparison of the PL from the remaining layers on both the substrates, consistent with that reported elsewhere for dewetting of the single-layer CP films. In summary, we have found that interlayer diffusion causes substantial increases of segmental stress of pristine CP molecules to result in large QE enhancements, an outcome contributed from the operations of segmental stretching in suppressing not only the non-radiative decays but also the heterojunction quenching.
In the second method of electric poling, the CP film, sandwiched between two metal electrodes spaced by the thicknesses of the sample and the two spacers that insulated the sample from the electrodes, was under electric poling by a voltage of ~2 KV in a solvent annealing environment. The PL of the CP film was then measured, after retreating from the poling station, as a function of the poling time (t). The polymers investigated were pristine MEH-PPV, regio-regular (rr-) P3HT, and regio-random (rra-) P3HT, and the CP films diluted with PS. We found that electric poling created chain alignment, resulting increased PL after a decline of PL from annealing-induced stress relaxation. The PL enhancement via molecular alignment, however, was strongly obstructed by molecular aggregation in the CPs of robust intermolecular interactions such that the PL never returned from that of the relaxed stress to above the level before poling. In the rr-P3HT, the poling induced a modest increase of crystallinity but failed to produce an enhanced PL relative to that before poling. In contrast, in the rra-P3HT, the electro-poling resulted ~ 2-fold increase of PL after t=84 h. Summarizing for this part, we conclude that the induced molecular alignments under electro-poling have the similar effect of rigidifying the CP chains to result in QE enhancements.
In conclusion, increasing the mechanical stresses of molecular segments of CPs, either by interlayer diffusion or electro-poling, can result in increased QEs of CPs. The in-situ methods developed in this thesis for as-spun polymer films may shed some light to the development of new methods of manufacturing high-efficiency devices based on CPs.

並列關鍵字

Conjugated polymers ； Quantum efficiencies ； Photoluminescence ； Segmental stress

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