以高頻探測共軛高分子電激發光下單重態與三重態激子形成時差

Abstract

高分子發光二極體(polymer light-emitting-diode)乃是利用共軛高分子(conjugated polymer)當發光材料，將其塗成薄膜在ITO正極與金屬負極之間，其中正電極為電洞注入電極，負電極為電子注入電極，當施予電壓後，兩電極同時分別注入電洞(holes)和電子(electrons)在共軛高分子薄膜內，電子進入傳導帶(conducting band)，電洞注入價帶(valence band)，當電子與電洞相遇時，會互相結合形成一電子電洞對束縛態的激子(exciton)，所形成激子之衰變以能量光的形成散發出來，而不同能量間隙之共軛高分子，所發出相對應可見光的波長也就隨之不同。 假設電子電洞對結合成激子的過程中，自旋角動量是互相獨立的（spin-independent），那麼單重態激子與三重態激子的比例將會是1:3，這樣的結果將使得注入發光層的電子電洞對損失75％在不發光的三重態衰變過程。單重態與三重態1:3的這一個比例，會限制有激發光元件的發光效率最好只有25%，所以瞭解單重態激子的形成對改進LED發光效率而言是相當重要的。而我的論文主題就是針對以共軛高分子MEH-PPV為材料做成的發光二極體，在高頻下，量測電激發光下單重態激子（Singlet state exciton）與三重態激子（Triplet state exciton）吸收訊號的比例，觀察其對頻率的關係，再與理論預期結果做比較，探討是否有三重態聲子瓶頸效應（Phonon bottleneck）。

Polymer light-emitting diode uses conjugated polymers as light emitting materials. In electroluminescence(EL) process, PLEDs are operated by the injection of electrons and holes from negative(metal) and positive electrodes(ITO). Electrons and holes capture one another within the polymer film, and form neutral bound excited states(excitons). Conjugated polymers with different energy gap will have different emission wavelength. The total spin of the exciton , formed by two spin paricles, can be either singlet (S=0) or triplet (S=1). Spin-allowed radiative emission (luminescence) is from the singlet only. Assuming that the exciton formation process is spin independent, then the ratio of singlet and triplet state excitons will be 1:3. So it is important to understand the formation process of triplet excitons in PLED. My study is to measure the difference in formation time between singlet and triplet excitons in EL by observing their responses under high frequency . Then we compare it with our theoretical result, and decide whether there is a phonon bottleneck effect for the triplet excitons.