堆疊串聯式固態有機發光電化學元件

Abstract

有機發光電化學元件（light - emitting electrochemical cells, LECs） 相較傳統有機發光二極體（organic light - emitting diodes, OLEDs），具有更簡易的單層結構、溶液製程、可使用非活性金屬與低電壓操作等優點。然而有機電化學元件效率，通常低於傳統結構的多層有機發光元件，而且固態發光電化學元件的效率和壽命會隨亮度與電流密度增加而降低。本論文提出新型堆疊串聯式有機發光電化學元件，並研究不同厚度之連接層對元件特性的影響，堆疊串聯式固態發光電化學元件在相同電流密度下，可以獲得近二倍的光輸出（μW/cm-2），由於光學特性受微共振腔效應的影響，單層結構之頻譜波鋒位於660 nm，而串聯結構頻譜之波峰移置625 nm，更接近光度函數（luminosity function）的波峰，亮度（cd/m2）在微共振腔效應幫助下有效的提升至三倍之多。此外，適當厚度之連接層有效改善載子平衡而使外部量子效率提升至5.83 %，是單層結構的兩倍以上。堆疊串聯式與單層元件具有相同亮度時，堆疊串聯式元件能同時改近元件效率與壽命。

Compared to organic light-emitting diodes (OLEDs), solid-state light-emitting electrochemical cells (LECs) exhibit simple single-layered structure and low operating voltages due to in-situ electrochemical doped layers. However, device efficiencies of LECs are usually lower than those of sophisticatedly designed OLEDs. Furthermore, device efficiencies and lifetimes of LECs degrade significantly as brightness increases. In this work, we demonstrate tandem LECs to obtain nearly doubled light outputs (μW cm-2) in comparison with single-layered LECs under similar current densities. Since the output EL emission is modified by microcavity effect of the device structure, the EL spectra of tandem LECs exhibit EL emission peak at ca. 625 nm while the EL spectra of single-layered LECs center at ca. 660 nm. Better spectral overlap between the EL spectrum of tandem LECs and the luminosity function results in further enhanced candela values, rendering a tripled brightness (cd m-2). The device efficiencies can be optimized by adjusting the thickness of the connecting layer between the two emitting units of the tandem devices. The peak external quantum efficiency achieved in tandem LECs is up to 5.83%, which is higher than twice of that obtained in single-layered LECs due to improved carrier balance. When single-layered and tandem LECs are biased under higher voltages to reach similarly higher brightness, tandem LECs show higher EQEs and longer lifetimes simultaneously. These results indicate that device efficiencies and lifetimes of LECs can be improved by employing a tandem device structure.