透過摻雜自組裝小分子提升無電洞傳輸層鈣鈦礦發光二極體之性能

Abstract

近年來，有機無機混摻鈣鈦礦由於具有易製備、高溶液相容性、可調節能隙及窄發光頻譜等特性，在全世界引起廣泛的關注與研究。其中，準二維鈣鈦礦(Quasi-2D perovskites)由於增強的量子侷限效應(Quantum Confinement Effect)和能量傳輸效應，具有顯著提升的光致發光效率，使其成為適合製備發光二極體之候選材料。經過幾年來的努力，鈣鈦礦發光二極體(PeLEDs)的效率已大幅提升。至今，綠光鈣鈦礦發光二極體的外部量子效率(EQE)已超過28%，顯示了極佳的應用潛力。 為了進一步實現商業化進程，降低生產成本和簡化製造過程為提升實際應用競爭力的關鍵。因此，部分研究開始致力於開發無電洞傳輸層(HTL-free)鈣鈦礦發光二極體，以簡化元件製程提升製造效率。然而，由於ITO和鈣鈦礦發光層之間存在較大之功函數勢壘，導致無電洞傳輸層之元件因電荷注入的不平衡而效率不彰。 在本論文的第二章中，我們提出了利用摻雜自組裝小分子來提升無電洞傳輸層鈣鈦礦發光二極體的效率。我們透過將自組裝小分子摻入鈣鈦礦前驅溶液中，在旋塗製備薄膜及加熱退火的過程當中，我們發現這些小分子會自組裝於ITO基板表面，形成類似電洞傳輸層之沈積層，從而成功調節ITO基板表面之功函數，有效將地鈣鈦礦發光層及ITO之間的能障，大幅提升電洞注入效率與載子之輻射複合。此外，我們亦發現這些小分子能與鈣鈦礦產生互相作用，其磷酸端之P=O基團中的氧原子可通過其孤對電子鈍化鈣鈦礦中未配位的鉛離子缺陷，而溴末端基則可鈍化鈣鈦礦薄膜中的鹵素空位。因此，元件的外部量子效率及亮度皆有顯著提升。由此可見，自組裝分子摻雜策略可為製備高性能無電洞傳輸層鈣鈦礦發光二極體之提供一可能的發展方向。 在第三章中，我們提出了實驗室未來可能的研究方向，包括表面鈍化策略和開發可撓式鈣鈦礦發光二極體。表面鈍化策略廣泛應用於減少鈣鈦礦表面的缺陷，透過此方法得以改善薄膜特性，從而提高元件性能。我們亦討論了發展可撓式鈣鈦礦發光二極體(flexible PeLEDs)的可能性，相較於傳統PeLEDs，可撓式鈣鈦礦發光二極體具有柔韌性及較輕量等優勢，適合可穿戴和便攜式設備之應用。由於鈣鈦礦的低剪切係數，其特性非常適用於可撓式元件。通過選擇合適的材料並改進各層的設計，可以提升可撓式鈣鈦礦發光二極體的耐久性和效率。

In recent years, organic and inorganic hybrid perovskites have attracted considerable interests and research around the world due to the properties such as easy preparation, high solution compatibility, tunable bandgap, and narrow emission spectrum. Among them, quasi-2D perovskites are suitable for the development of light-emitting diodes (LEDs) due to their enhanced quantum confinement effect and efficient funneling effect, which significantly increase the photoluminescence efficiency. After several years of efforts, the efficiency of perovskite LEDs (PeLEDs) has been greatly improved. To date, the external quantum efficiency (EQE) of green PeLEDs has exceeded 28%, demonstrating excellent application potential. In order to realize commercialization, reducing production costs and simplifying the manufacturing process are the keys to enhancing the competitiveness of practical applications. Therefore, some researches have started to develop hole-transporting layer free (HTL-free) PeLED in order to simplify the device manufacturing process and improve the manufacturing efficiency. However, due to the large work function barrier between ITO and the perovskite layer, the efficiency of HTL-free PeLEDs are not satisfactory due to the imbalance of charge injection. Herein, in Chapter 2, we proposed the use of doped self-assembled small molecules to enhance the efficiency of PeLEDs. By doping the self-assembled small molecules into the perovskite precursor solution, we found that these small molecules would self-assemble on the ITO substrate during the spin-coating preparation of the thin film and the annealing process, forming a deposition layer similar to the hole-selective layer, which successfully regulate the work function of the TIO surface and effectively reduce the energy barrier between the perovskite layer and ITO, and greatly improve the efficiency of the hole injection and radiative recombination of the carriers. In addition, we also found that these small molecules interact with perovskite The oxygen atoms in the P=O group at the phosphate end can passivate uncoordinated Pb2+ ions in perovskite through its lone electrons, while the bromine head group can passivate the halide vacancies in the perovskite film. As a result, the external quantum efficiency (EQE) and brightness of the device is significantly improved. Thus, the self-assembled molecular doping strategy can provide a possible direction for the fabrication of high-performance HTL-free PeLEDs. In Chapter 3, we proposed possible future research directions for our lab, including surface passivation strategies and the development of flexible PeLEDs. Surface passivation strategies are widely used to minimize the defects on the perovskite surface, through which the film properties can be improved to enhance the device performance. We also discussed the possibility of developing flexible PeLEDs (flexible PeLEDs), which have the advantage of flexibility and light weight over rigid PeLEDs, making them suitable for wearable and portable device applications. Due to the low shear moduli and mechanical stiffness of perovskite, its characteristics are very suitable for flexible devices. By choosing the right materials and improving the design of each layer of the device, the durability and efficiency for flexible PeLEDs can be improved.