The Photophysical Properties of Magnesium Modified Alq3 for Solar Cell Applications

有機太陽能電池研究與發展至今已經有三十年的歷史，直到最近十年能源問題的浮現，它在科學與經濟上的利用價值才逐漸被科學家們所注意，元件因此在轉換效率上有了突破。為克服轉換效率問題，材料內部的載子傳輸能力、光激子分離及光激子擴散長度都是非常重要議題；一個優良的電子受體必須與施體材料間形成有效的能階落差才能使得光激子被分離，並且擁有良好的電子傳輸特性，就如同碳六十材料。由於目前廣泛使用的碳六十材料成本較高，而且其穩定性不盡理想，因此積極開發高穩定性且低成本的電子傳輸材料是一項重要的方向。們使用鎂原子來改善有機分子8-羥基奎林鋁（Alq3）的光電特性。在有機太陽能電池應用裡，鎂摻雜8-羥基奎林鋁能提供一個電子受體的角色並且擁有良好的電子傳輸能力；其能階增加0.64 eV以致於形成載子轉換的型態，提供較高效率的光激子分離。更從光譜中觀察到其單重態（螢光）的光激子之生命週期縮短從2 ns 到14 ns，而且其三重態（燐光）的光激子保持穩定的長生命週期0.5 ms，並且三重態光激子的擴散長度被預測為90 nm。這樣擁有長生命週期與擴散能力的有機材料能提供更多的光激子並且提升光電流的輸出。在太陽能電池元件的表現上，添加鎂金屬粒子的元件與尚未添加的元件相較之下，光轉換效率可以修正到0.15%，且其外部量子效率也驗證出其光電行為被改善。Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies in low-cost organic materials. To overcome this limitation of the photo-to-carrier generation, enhancements of electron conductivity, exciton separation and exciton diffusion have been suggested. A suitable electron acceptor in solar cells, which can replace the high-cost and air-instable fullerene C60, must has suitable energy to match p-type layer and cathode and owns high electron conductivity. n this thesis, the low molecular weight tris-(8-hydroxyquinoline)aluminum (Alq3) has been modified with magnesium (Mg) incorporation that altered the nature of electronic characteristics in it. This Mg modified Alq3 can provide an electron accepter and a good electron transporting layer in photovoltaic devices, which conduction enhancement of the current with variations of more than 3 orders of magnitude can be observed. The energy level of Mg-Alq3 shifts 0.64 eV and forms charge-transfer condition to promote the exciton separation at p-n junction. Detail optical and vibrational studies depict the quenching of the fluorescence, originating from singlet state transition of short lifetime (2-14 ns) excitons upon Mg incorporation. This is accompanied by enhanced phosphorescence, due to a triplet state transition, having longer lifetimes (0.5 ms) and hence an estimated excitonic diffusion length of ~90 nm to boost the performance of organic solar cell devices. Optimized Mg:Alq3 layer, when introduced in the device, improves the power conversion efficiencies by several orders to 0.15% compared to the pure Alq3 device. The improvement in the photovoltaic performance has been attributed to the carrier transport, high HOMO and the superior exciton diffusion length.Chapter 1 Introduction…………………………………………………………….. 1.1 The Story of Solar Cells……………………...………………………...…… 3.2 Organic Solar Cells…………………………………………………………. 4.3 Principle of Operation………………………………………………………. 5eferences………………………………………………………………………. 9hapter 2 Optical-Electronic Theory of Organic Material and Device Mechanis….18.1 Optical Properties of Organic Molecules……………………………...…... 19.1.1 Molecular Orbital and Excited States………………………………... 19.1.2 Excitons in Excited State……………………………………………. 22.1.3 Energy Transfer and Charge Transfer………………………………… 24.1.4 Dynamic Process and Intersystem Crossing………………………… 26.1.5 Exciton Diffusion……………………………………………………… 28.2 Electric Properties in Organic Molecules…………………………………. 29.2.1 Carrier Transport…………………………………………………… 29.2.2 Mobility in Amorphous Materials………………………………….. 31.2.3 Space-Charge Limited Current………………………………………. 32.3 Device Physics…………………………………………………………….. 33.2.4 Principle of Operation……………………………………………… 33.2.5 Power Conversion Efficiency………………………………………. 34.2.6 Open Circuit Voltage……………………………………………….. 34.4 Metal/organic Interfaces……………………………………………………35eferences……………………………………………………………………... 38hapter 3 Analysis of Electric and Optical Characteristics….…………………….. 56.1 Introduction……………………………................................................…... 56.2 Sample Preparation and Experimental Procedures………………………... 58.3 Ultraviolet Photoemission Spectroscopy Results…………………………. 59.4 X-ray Photoelectron Spectroscopy Results……………………………….. 60.5 Charge-Carrier Transport Characteristics…………………………………. 62.6 Absorption and Photoluminescence………………………………………. 64.7 Summary…………………………………………………………………... 68eferences……………………………………………………………………... 69hapter 4 Organic Photovoltaic Applications…………………………..…………. 95.1 Introduction……………………………..............................................…..... 95.2 Experiments………………………………………………………………... 96.3 Device Performance………….…………………………………………..... 96.4 Optimized Organic Solar Cells……………………………………………. 98.5 Summary………………………………………………………………... 102eferences………………………………………………………………….. 103hapter 5 Device Analysis……………………………………………..………... 118.1 Introduction……………………………..............................................…... 118.2 Experiments……………………………………………………………… 118.3 Electron Accepter Characteristics…………………………….………... 119.4 Carrier Transport………………………………………………………..…121.5 Summary…………………………………………………………………. 123eferences……………………………………………………………………. 124hapter 6 Conclusion…………………………………………………....………... 137ppendixes……………………………………………………………………….139.1 Electron-Only Device ……………………………………………………….139.2 FTIR and Absorption Analysis and Simulation …….……..………………….145.3 Temperature-Dependant PL and TR-PL…………………………………….153.4 Publications………………………………………………………………….16