有機半導體薄膜之光譜性質研究

Abstract

我們研究以熱蒸鍍法，在玻璃基板上成長並五苯(pentacene)和有機發光二極體薄膜(C47H32N2O2)的橢圓儀光譜性質。 首先，我們觀察到pentacene薄膜顯示一個1.87 eV的明顯吸收峰。我們認為這個吸收峰為最高分子佔據軌域到最低分子未佔據軌域的能隙。此外，其他光子能量大於2 eV的吸收峰屬於電子躍遷到較高軌域的貢獻。同樣的，我們也觀察到有機半導體薄膜能隙約為2.48 eV。 此外，我們也測量這兩個樣品的變溫(200 ~ 450 K)光譜。我們觀察到pentacene薄膜的戴維杜夫分裂(Davydov splitting)，隨著溫度由450 K降低至200 K時，分裂的情況會由0.08擴展至0.122 eV。我們認為這個現象與單位晶胞中分子和分子軌域重疊(overlap)的變化有關。另一個有機半導體薄膜也有類似的情況。

The pentacene and organic-light-emitting-device (OLED, C47H32N2O2) thin films were grown on glass substrate by thermal evaporation technique. The optical constants of these thin films are studied using variable spectroscopic ellipsometry. Room-temperature ellipsometric spectra of the pentacene thin film exhibit a main absorption peak at about 1.87 eV, which can be assigned to the HOMO-LUMO gap of pentacene. Moreover, other high-frequency peaks above 2 eV are likely due to the transitions of an electron excited to higher orbital or localized excitations. On the other hand, the HOMO- LUMO transition energy of OLED thin film moves toward 2.48 eV, indicating its more insulating character. Temperature-dependent ellipsometric spectra of both thin films were measured in the range from 200 K to 450 K. With decreasing temperature, the Davydov splitting of pentacene thin film increases from 0.08 eV at T = 450 K up to 0.122 eV at T = 200 K. This behavior is due to molecular reorientations that cause changes in mutual molecular overlap within the unit cell. A similar change of absorption bands is also observed for OLED thin film.