Charge and Energy Transfer in MEHPPV/TiO2 Hybrid Materials

本研究主要是藉由量測MEHPPV/TiO2混摻材料的光學性質觀察混摻材料界面上所發生的電荷及能量轉移現象。我們觀察到在不同激發波長下TiO2奈米柱對於MEHPPV螢光發光均有淬息(PL quenching)的效果，這代表著電荷轉移有效的發生。同時我們發現當激發波長小於TiO2奈米柱的吸收邊界時，淬息的效率(quench rate) 發生巨大的改變，我們假設這個有趣的現象意味著由能量轉移現象的發生。我們使用低溫螢光激發光譜儀(Low temperature photoluminescence excitation spectroscopy)以及頻率範疇載子生命期量測技術(Frequency-domain lifetime spectroscopy)來證實由TiO2奈米柱到MEHPPV的能量轉移的確發生於此混摻材料的界面。此外，我們使用化學方法對TiO2奈米柱的表面進行改質並觀察不同表面型態對於電荷及能量轉移的影響。同時，我們也觀察物理方法(退火)對於此混攙材料的影響，在此實驗中可發現TiO2奈米柱的存在對於高分子的鏈段移動具有阻礙的效果。除此之外，我們比較不同長短的奈米柱對於電荷及能量轉移的貢獻程度。最後，我們做了初步的電性質分析，證明了MEHPPV/TiO2混摻材料應用在太陽能電池之中的可能性。In this thesis, optical properties of MEHPPV/TiO2 nanorods hybrid materials have been examined to realize the charge and energy transfer in MEHPPV/TiO2 nanorods hybrid materials. The photoluminescence quenching after blending TiO2 nanorods into the polymer matrix is observed at all excitation wavelengths. An interesting change in the quench rate occurs when the excitation wavelength is shorter than the absorption edge of TiO2 nanorods. Subsequently, the possible energy transfer from TiO2 nanorods to MEHPPV is proved by the low temperature PLE measurements and the frequency-domain lifetime spectroscopy. Furthermore, influences of chemical and physical modifications to the hybrid materials are discussed. We compare two different TiO2 nanorods, with and without the cover of a surfactant, in their charge and energy transfer abilities. Besides, thermal treatments change the property of polymer physically. The inclusion of TiO2 nanorods will hinder the re-stacking of polymer chains; thus, a smaller change in spectral feature is observed. We also discuss the size effect to the charge and energy transfer. In the end, preliminary electrical properties show the potential MEHPPV/TO2 nanorods hybrid materials for the photovoltaic application.Chapter 1 Introduction……………………………………………….1 1.1 Organic semiconductors……………………………………………………..1 1.2 History of photovoltaic application…………………………………………2 1.3 Motivations…………………………………………………………………...5 1.4 References…………………………………………………………………….6 Chapter 2 Background………………………………………………..7 2.1 Electronic transitions in molecules………………………………………….7 2.2 Charge transfer in organic/inorganic hybrid materials…………………..11 2.3 Energy transfer in organic/inorganic hybrid materials…………………..13 2.4 Reference…………………………………………………………………….22 Chapter 3 Experimental procedures………………………………..23 3.1 Experimental techniques…………………………………………………...23 3.1.1 Introduction of fluorescence detection………………………………..23 3.1.2 Continuous-wave photoluminescence spectroscopy…………………24 3.1.3 Photoluminescence excitation spectroscopy………………………….26 3.1.4 Time-resolved photoluminescence spectroscopy……………………..28 3.1.5 Frequency domain lifetime measurement technique………………...32 3.2 Sample preparations………………………………………………………38 3.3 References………………………………………………………………….39 Chapter 4 Materials………………………………………………….40 4.1 Characterizations of MEHPPV…………………………………………….40 4.1.1 Introduction…………………………………………………………….40 4.1.2 Characterizations………………………………………………………41 4.1.2.1 Molecular weight and structures………………………………..41 4.1.3 Fundamental optical properties of MEHPPV………………………..42 4.2 Synthesis and characterizations of TiO2 nanorods………………………..44 4.2.1 Introduction…………………………………………………………….44 4.2.2 Synthesis methods……………………………………………………...48 4.2.3 Characterizations of as prepared TiO2 nanorods…………………….50 4.2.4 Fundamental optical properties of TiO2 nanorods…………………...55 4.3 References…………………………………………………….……………..56 Chapter 5 Charge transfer in organic/inorganic hybrid materials.59 5.1 Fundamental optical properties of hybrid materials……………………..59 5.1.1 Examinations on hybrid materials……………………………….……59 5.1.2 Temperature dependence………………………………………….…...68 5.2 Ligand-dependent effects………………………………….………………..70 5.3 References…………………………………………………………………...80 Chapter 6 Energy transfer in organic/inorganic hybrid materials.81 6.1 Possible energy transfer processes observed by PLE spectroscopy……...81 6.2 Carrier lifetime measurements…………………………………………….85 6.3 Low temperature PLE examinations………………………………….…...90 6.3.1 Low temperature PLE measurements for pristine MEHPPV………90 6.3.2 Low temperature PLE measurements for hybrid materials………...94 6.4 Ligand-dependent effects……………………………………………….….96 6.5 References……………………………………………………….…………..98 Chapter 7 Other factors influencing the charge and energy transfer processes………………………………………….…………………….99 7.1 Thermal treatment………………………………………………………….99 7.2 Size dependence……………………………………………………...…….106 7.2.1 Charge transfer phenomenon………………………………………...106 7.2.2 Energy transfer phenomenon………………………………………...110 7.3 References………………………………………………………………….111 Chapter 8 Preliminary electrical properties………………………112 8.1 Time-of-flight mobility measurement…………………………………...112 8.2 Device performances……………………………………………………..116 Chapter 9 Conclusions……………………………………………...11