| 摘要: | 本論文主旨為利用Poly[2-methoxy-5-(3,7-dimethyoctyoxyl)-1,4-phenylenevi- nylene](MDMO-PPV)製作超強耦合有機發光二極體(organic light-emitting diode, OLED)並研究其物理現象。理論上，當光子與材料激發態(激子)在共振腔中進行強耦合作用，則產生偏極子(polariton)混成態和不同的能量色散分支，而拉比分裂為色散曲線上下支的最小能量差，當拉比分裂超過激子能量的20%以上則為超強耦合狀態。
在元件製作上，我們利用ZnO和polyethylenimine (PEI)的奈米複合層作為電子傳輸層，並優化PEI之摻雜比例以提升元件的外部量子效率。相較於未摻雜的OLED，優化PEI比例的元件外部量子效率可提高約70倍。我們進一步利用角度解析光譜技術和Hopfield Hamiltonian色散模型，研究超強耦合OLED發光和耦合強度，並探討改變出光側銀膜反射鏡厚度的影響。當銀膜厚度由20 nm增加至50 nm，元件發光亮度和效率降低但耦合強度隨之增加，而銀膜厚度最少需30 nm以上才能有效將出光限制在下支偏極子模態，呈現弱角度相關之能量色散且半高寬極窄的發光。而後我們調整不同MDMO-PPV與ZnO:PEI厚度優化元件效率，發現當兩者厚度相近比例為1:1時可獲得最高效率。
最終，在出光側銀膜厚度為30 nm和ZnO:PEI及MDMO-PPV厚度比例為1:1的條件下，我們得到最佳化之超強耦合電激發偏極子元件，其外部量子效率0.24%，最高亮度達到600 cd/m^2，且拉比分裂為860 meV，耦合強度為激子能量的33.2%。
;This thesis exploited Poly[2-methoxy-5-(3,7-dimethyoctyoxyl)-1,4- phenylenevinylene] (MDMO-PPV) as the emissive layer of ultra-strongly coupled organic light-emitting diode (OLED) and studied the physical phenomenon. In theory, strong coupling of exciton and photon in the cavity would form hybrid polariton states and different energy dispersions of upper (UPB) and lower (LPB) polariton branches. The minimum energy difference between two branches is called Rabi-splitting energy. When Rabi-splitting energy exceeds about 20% of exciton energy, the regime of ultra-strong coupling is reached.
In the device fabrication, we used ZnO and polyethyleneimine (PEI) nanocomposite layer as the electron injection layer, and optimized the PEI blend ratio to improve the external Quantum Efficiency (EQE) of OLEDs. Compared to the OLED without PEI, the optimized device shows 70 times higher EQE. We further employed the angle-resolved spectroscopy and Hopfield Hamiltonian dispersion model to study the emission and coupling strength of ultra-strongly coupled OLEDs, and investigated the effect of varying the thickness of silver mirror as the emission side. When the silver thickness is increased from 20 nm to 50 nm, the luminance and EQE are decreased but the coupling strength is increased. The silver thickness must exceed 30 nm to confine the emission in the LPB mode, revealing weakly angle-dependent dispersion and narrow emission. Later, we modified the thicknesses of MDMO-PPV layer and ZnO:PEI nanocomposite layer to optimize EQE, and obtained the highest EQE when the two layers with similar thicknesses.
Finally, when using the silver thickness of 30 nm and the 1:1 thickness ratio of MDMO-PPV and ZnO:PEI, we obtained the optimal polariton OLED with an EQE up to 0.24%, luminance of 600 cd/m^2, and the Rabi splitting of 860 meV, corresponding to 33.2% of exciton energy and demonstrating the operation of ultra-strong coupling. |
