利用溶液製程製備導電高分子混掺奈米材料太陽能電池

Abstract

本研究主要針對有機高分子混掺無機奈米材料製備太陽能電池進行探討，藉由移除奈米粒子表面絕緣的長碳鏈分子，我們發現可以有效提升元件之光電轉換效能。並在本研究中將奈米粒子表面接上更有利於電荷傳導的分子，以提高載子在元件內的傳導能力，並且避免電荷在傳導過程的損失。藉由這樣子的表面改質，我們可以將元件效能提升至2.2%. 在本研究中並應用具有週期性奈米結構的氧化鋅奈米柱與導電高分子P3HT混攙製備有機太陽能電池，實驗結果顯示此氧化鋅奈米結構除了可供應有效的電子傳導路徑外，對於製程中改變P3HT在空間中的型態也有助於提升電洞傳導能力.我們也發現此類元件的電荷遷移率以及高頻操作的性質與其非均向性的光學性質有相關聯性. 本研究中也嘗試提出新穎的製程以提高元件效能。研究中於製程中加入電場以改變高分子的排列性質。並在實驗結果中發現電場的施加可有效提升高分子排列特性。除了表面型態可獲得改善外，我們也發現元件的光吸收效果以及電荷遷移率也在施加電場後獲得了提升. 利用奈米碳管作為ITO透明電極的取代在本研究中詳盡的描述。我們利用化學方法改質碳管表面，並將碳管均勻塗布在基板上形成導電薄膜。並利用此導電薄膜作為電極製備有機太陽能電池。實驗結果顯示表面改質後的薄膜片電阻可大幅下降，並仍維持有高光穿透的材料特性。而本研究亦發現元件之開路電壓將受到碳管的表面改質而產生變化。 為了要提升元件對太陽光譜的吸收能力，本研究利用低能隙的硫化鐵奈米粒子與 高分子混掺製備光伏元件，實驗結果顯示添加硫化鐵確實可使元件吸收近紅外光的能量，然而對於最佳化製程和元件效能仍需要未來進一步的探討。

This thesis aims to explore an alternative for silicon based solar cell. The hybrid materials, which are a combination of conjugated polymer and inorganic nanomaterial, provide numerous promising device properties such as effective carrier transport, strong light absorption and flexibility. Compared with conventional silicon based solar cell, this hybrid material can provide a low cost, environmental friendly, light weight and easy to process possibility. Though the performance of polymer based solar cell is still too low for large scale application, it is still possible to increase device conversion efficiency by improving carrier transport and extending light harvesting range. In chapter 3, we focus on studying organic-inorganic hybrid bulk heterojunction solar cell based on conjugated polymer P3HT and TiO2 nanocrystal. Our result show the optimal device performance can be achieved by introducing 50 weight percent TiO2 nanorod into P3HT matrix. By TiO2 surface modification, the optimal device performance has a power conversion efficiency of 2.2%. Compared with CdSe/conjugated polymer hybrid, this material system not only provides comparable device efficiency, but also develops a nontoxic, environmental friendly solar cell. In Chapter 4, we demonstrate enhanced the performance of polymer solar cell based on poly(3-hexylthiophene)(P3HT)/ZnO nanorods array heterojunction hybrid. By infiltrating P3HT polymer chain along ZnO nanorods array nanostructure, carrier mobility has been found a increase from 8.2×10-5 cm2/Vs to 7.7×10-4 cm2/Vs, companied with polymer chain were aligned perpendicularly to substrate surface. The optical anisotropic measurement revealed that chain orientation of P3HT prefers align along ZnO (l0Ī0) surface. Our experiments also showed that device performance can be further improved by surface modified ZnO nanorod surface. A novel approach to improve polymer solar cell using electric field assisting process was proposed in chapter 5. Our results showed better device performance can be achieved by carefully applied electric field during thin film process. Atomic force microscopy measurement showed higher polymer chain organization properties of blend film. By changing the natural orientation of polymer order, the electrical properties, including device performance, carrier mobility in vertical direction can both be enhanced. The optical anisotropic measurement also showed the optical anisotropic ratio is as a function of the magnitude of electric field. A solution process single wall carbon nanotube (SWCNT) thin film as a transparent electrode for organic solar cell application was studied in chapter 6. By chemical modified SWCNT thin films using nitric acid and thionyl chloride treatments, a significant decrease of sheet resistance can be achieved. Photovoltaic devices based on P3HT and PCBM fabricated on surface functionalized SWCNT electrode shows a promising device conversion efficiency of 1.87% can be performed. The variation of open circuit voltage (Voc) in P3HT and PCBM bulk heterojunction organic photovoltaic with functionalized transparent SWCNT networks indicated that the change of surface potential of SWCNT thin films resulted in correlated change in short circuit current density and open circuit voltage of the photovoltaic devices. In previous chapters, we have proposed several approaches to improve device performance. In chapter 7, we use new material for organic IR harvesting solar cells application based on P3HT/FeS2 blend. The devices exhibited high photo-electric current conversion efficiency in infrared region (>700 nm).where the external quantum efficiency was 6.5% at wavelength 650nm and 1% at 700 nm. The photoresponsed measurement also indicated that onset of photogenerated edge was about 900nm, which is contributed by FeS2 NCs. These results also pointed out that FeS2 NCs: P3HT hybrid can provide a low cost, environment friendly and easy process organic solar cell. Finally, polymer solar cells that have been constructed by hybrid materials are very promising. This thesis mainly studied polymer solar cells and has provided some approaches to improve device performance. Our findings showed carrier transport properties and excitons dynamics are both directly influenced by photoactive layer morphology. In the future, we believe device performance can be further improve by optimized morphology of polymer based heterojunction solar cell with a good percolation of both phases to the respective electrode.