在高電場下發光共軛高分子內電子與電洞相互捕捉的抑制與發光

Abstract

在發光共軛高分子裡, 電子與電洞藉庫侖吸引力相互捕捉結合成激子而放出綠光的機率會大大的被外加的強電場所抑制. 共軛高分子的遷移率很低, 為了使元件產生足夠的電流, 外加的強場是必須的. 這庫侖吸引力捕捉的機制是電子電洞不斷放出聲子, 使電位能和動能的和（總能量）下降, 而強電場卻是不斷的加熱電子和電洞, 使動能上升. 當電場變大, 電子和電洞直接結合產生紫外光的效應卻不甚受影響, 因此在強電場下, 電子電洞結合發出紫外光變成主要的發光機制. 當場的強度大於10^8V/m, 電子電洞直接結合的機率就比電子電洞慢慢結合成激子的機率大了. 結合衝擊離子化, 我們的理論模型可以定量的解釋最近觀察到的高場下電流, 可見光與紫外光在元件崩潰前後的效應.

The probability for the Coulomb capture of an electron-hole pair to form an exciton in conjugated polymers is found to be significantly suppressed by the high electric field, which is usually required for electroluminescence due to the low mobility. The Coulomb capture is a continuous descent of electronic energy through emitting phonons, and the high field prevents theis descent by heating up the carriers. Ultraviolet emission by direct interband transition is found to be much less affected by the field, and become the dominant radiative decay channel as the field increases. The probability for direct interband transition(ultraviolet emission)is higher than the probability for excition formation(with subsequent visible emission)when the field is beyond 10^8V/m. Combined with impact ionization, our model provides a complete quantitative explanation of the high-field behaviors of current, visible and ultraviolet emission recentlyobserved in conjugated polymer, both before and after the breakdown.