共軛高分子發光二極體中三重態激子與極化子之形成

Abstract

本研究使用光致發光注入吸收 (photo-induced absorption PA)與電致發光注入吸收 (electroluminescence-induced absorption EA)的光譜技術來研究共高分子發光二極體 (PLED) 中三重態激子 (Triplet exciton) 與極化子 (Polaron) 的形成及隨操作電壓改變的演化。本實驗是第一個能在室溫下量測出ＥＡ與ＰＡ光譜，藉以了解在室溫下操作的共軛高分子發光二極體中激發態 (excitations)的形成;包括三重態激子與極化子。在光譜中，我們成功地辨識出三重態激子與極化子地吸收訊號，結果顯示兩者的訊號在光譜上有著很顯著地重疊。在改變溫度與操作頻率的光譜量測，發現三重態激子的總數 (population)是很強的溫度函數，且其生命期 (lifetime) 較極化子長。在低壓的情況下，極化子的總數正比於外加的電壓，這樣的結果符合 Space-Charge-Limited Currents (SCLC) 的預測。在高電場時，我們觀察到EA光譜會有被抑制的現象，這個現象是三重態激子與極化子所組成的，其形成的原因有可能是三重激態子被抑制 (Triplet exciton suppression) 或者是極化子被瓦解 (Polaron dissociation) ，也可能兩者一起發生。

We use photo-induced absorption (PA) and electroluminescence-induced absorption (EA) spectroscopy to investigate the excitations of polymer light-emitting-diode (PLED), including excitons and polarons. Understanding the formation of the excitations in a PLED operated under voltage bias is the aim of this work. The EA and PA spectra are measured at room temperature for the first time. We identify the triplet exciton and polaron absorptions, and find that they have strong overlap in EA spectra. The triplet exciton population is sensitive to the temperature and its lifetime is longer than the polaron. At low bias the polaron population is proportional to the applied voltage due to the space-charge-limited currents. We observed the suppression of EA under high bias; it is probably due to debated phenomenon of polaron dissociation or triplet suppression or both.