以堆疊串聯式結構提升固態近紅外光有機電化學元件效率

Abstract

和傳統的近紅外光有機發光元件相比，固態近紅外光有機電化學發光元件擁有以下幾個更優異的表現，例如簡單的元件結構、低操作電壓以及更好的載子注入平衡。然而，近紅外光染料在本質上的低發光效率，以及激子在發光層中的自我猝熄現象，限制了近紅外光有機電化學發光元件的元件效率。本論文所展示的堆疊串聯式結構，能有效提升磷光敏化螢光的近紅外光有機電化學發光元件效率。元件的兩層發光層均是由磷光主體與另一個螢光客體組成，來獲得同時具有單重態以及三重態的激子，經由一層傳輸薄層垂直地串聯起來，以得到相乘的光輸出。 近紅外光有機電化學發光元件的電激發光頻譜會隨發光層的厚度而改變，這是因為厚度變化所造成微共振腔效應的不同。利用模擬的電激發光頻譜對照量測到的電激發光頻譜進行匹配，穩定後的複合區位置在較厚的堆疊串聯式元件中，位置較為中間且遠離兩側摻雜層，可減少靠近摻雜層產生的激子猝熄現象。因此，擁有更佳元件效率及更長的壽命表現可望在較厚的元件中實現。 在這些堆疊串聯式的近紅外光有機電化學發光元件中，元件外部量子效率的最高值達到了2.75%，與同是使相同的紅外光染料文獻上，提升的數值超過了三倍。這些效率也是目前近紅外光有機電化學發光元件文獻上的最高值，並證實磷光敏化螢光系統與堆疊串聯式結構的結合，是一個有效實現高效率近紅外光電化學發光元件的方法。

Compared to near-infrared (NIR) organic light-emitting devices, solid-state NIR light-emitting electrochemical cells (LECs) could possess several superior advantages such as simple device structure, low operating voltages and balanced carrier injection. However, intrinsically lower luminescent efficiencies of NIR dyes and self-quenching of excitons in neat-film emissive layers limit device efficiencies of NIR LECs. In this work, we demonstrate a tandem device structure to enhance device efficiencies of phosphorescent sensitized fluorescent NIR LECs. The emissive layers, which are composed of a phosphorescent host and a fluorescent guest to harvest both singlet and triplet excitons of host, are connected vertically via a thin transporting layer, rendering multiplied light outputs. Output electroluminescence (EL) spectra of the tandem NIR LECs are shown to change as the thickness of emissive layer varies due to altered microcavity effect. By fitting the output EL spectra to the simulated model concerning microcavity effect, the stabilized recombination zones of the thicker tandem devices are estimated to be located away from the doped layers. Therefore, exciton quenching near doped layers mitigates and longer device lifetimes can be achieved in the thicker tandem devices. The peak external quantum efficiencies obtained in these tandem NIR LECs were up to 2.75%, which is over tripled enhancement as compare to previously reported NIR LECs based on the same NIR dye. These efficiencies are among the highest reported for NIR LECs and confirm that phosphorescent sensitized fluoresce combined with a tandem device structure would be useful for realizing highly efficient NIR LECs.