摘要:近年來由於石油能源以及溫室效用造成的全球暖化問題,研發較節能的生活用品以及尋找無碳的替代性能源已成為全世界關注的焦點,因此高效率LED 與太陽電池就成了光電產業熱門的研究項目。LED 具備省電與高亮度的優點,而在太陽能電池方面,目前以結晶矽太陽能電池有著較成熟的發展,不過以上兩項發展都面臨著製作成本昂貴、複雜、不易大面積化的問題,相較於此,奈米半導體結構/有機合成薄膜光電元件更有競爭優勢,因為奈米半導體結構/有機合成薄膜結合了有機可彎曲、能大面積製作、成本低廉與無機半導體载子傳輸速度快、穩定性高的特性,所以不管在發展發光元件方面或是太陽能電池方面都相當具有潛力。
本計劃主要方向為奈米半導體結構/有機合成薄膜光電元件的研究。我們的研究範圍涵括以下內容:一為利用氧化鋅奈米粒子或奈米線結合有機傳輸材料發展紫外-藍光以及其他可見光的電激發光元件;二為使用半導體奈米線陣列結合有機感光材料製作薄膜太陽能電池,主要概念架構為製備具奈米結構之無機層作為增加接觸面積的作用以提高载子複合(發光元件)或载子接收(太陽能電池),其中半導體材料亦依照元件所需來選擇高载子傳輸與適當能階匹配的半導體,之後並輔以有機材料作為载子傳輸的媒介或吸收頻譜對應的载體(Matrix),如此互相搭配以期元件能夠同時獲得有機與無機的優點,以提高元件之光電效率。研究之簡要說明如下:
ㄧ. 電流激發發光元件研究部分
提出以分層結構的方式,來製作氧化鋅奈米粒子的紫外-藍光電激發光元件,我們將研究相分離的技術,在旋轉塗佈的過程中,達到氧化鋅奈米粒子與有機傳輸材料分離的結構,有效降低因奈米粒子堆疊所產生的孔洞缺陷,進而達到元件發光亮度的提高,另外我們亦研究氧化鋅奈米線的發光元件,利用低溫製程(<90 ℃)的溶液溶膠法的方式成長高品質的奈米線作為基板,再配合有機傳輸材料提高载子傳輸效率,製作成為一個有機無機異質接面的結構,其中氧化鋅奈米線的折射率約為2.0,高於大部份的有機材料(約為1.5),當發光層為氧化鋅奈米線,光子很容易被侷限在裡面形成共振,因此將有很大的機會製作出高電光轉換效率之雷射發光元件。
二. 太陽能電池研究部分
我們開發一種結合共軛高分子與無機半導體的奈米結構太陽能電池,做法為將製備出所需長寬之無機奈米線陣列嵌入共軛高分子層中。使用的無機材料為砷化鎵或是矽等半導體,我們並研究以各種蝕刻的方法來製作砷化鎵奈米線與矽奈米線,再搭配適當的高分子來補償無機材料中對於與太陽光譜不匹配的波長區段,來提昇太陽能電池的效率。無機半導體的奈米結構可提供形貌控制,吸收紅外光,以及做為電子傳輸之良好導電層等。預期此新的薄膜太陽能電池結構之光電轉換效率應可超越過去之薄膜太陽能電池。
Abstract: Because of the energy crisis and the global warming caused by the green-house effect,the researches on power-saving technologies and carbon-free renewable energy technologieshave been attracting significant attentions in recent years. Among them, high-efficient lightemitting diodes (LEDs) and photovoltaic devices thus become popular research items in the area of optoelectronics. Conventional methods of making LEDs require epitaxial-growth apparatuses, which are very expensive. Also, the most popular solar cells still use lots of Si materials. They both involve expensive cost either on fabrication facilities or material consumption. In comparison, using nano-structured semiconductors and organic polymers for the fabrication of light emission devices and solar cells has the prominent advantage of using less material, so cheap fabrication can be achieved. In addition, this approach utilizes the advantages of both organic polymer and the nano-structured semiconductors. One provides the flexibility on deformable surface, low cost, up-scalable and the other provides the stable,morphology-controllable, and fast transportation of electrons. Therefore, they offer the great potential to be used in both light emission devices and solar cells.
In this project, we propose the use of nano-structured semiconductor/organic composite film in optoelectronics applications. The scope of the research is as follows. First, we use ZnO nanoparticles or ZnO nanorods, combined with organic materials, to fabricate electrically-pumped light emission devices in the UV-blue and other visible-wavelength ranges. Second, we use semiconductor nanowire arrays, combined with light-sensitive organic materials, to fabricate thin-film solar cells. Our main concept is to utilize the nano-structures to increase the contact junction between the semiconductor and the organic materials for either enhancing the electron-hole recombination in light emission devices or increasing carrier generation in solar cells. Proper semiconductors and organic materials will be chosen to match the energy bands, carrier transportation, and solar-energy absorption. Hopefully, the advantages of both organic and semiconductors will be utilized to increase the efficiency of either electrical-to-optical conversion or optical-to-electrical conversion. They are briefly described in the following.
I. Electrically pumped light emission
We propose the layer separation between the ZnO nanoparticles and organics for UV-blue light emission. We will investigate the phase segregation that can separate the ZnO nanoparticles and organic transportation material to two layers over single-step spin-coating process. The conditions that can reduce the defects of the film will be studied. We will also use the low-temperature (<90 ℃) process to fabricate high-quality ZnO nanowires on the substrate. Then organic materials will be deposited on the nanowires to form ZnO-organic hetero-junction. In this way, large and scalable area is expected. In addition, the high refractive index of ZnO could easily form the light-confinement cavity and so even higher conversion efficiency of laser diodes can be possibly achieved.
II. Solar cells
We will explore a new architecture of thin-film solar cells in which conjugate polymers will be combined with the nano-structured semiconductors. The main concept is to fabricate the semiconductors of nano-structures with desired length and width. The semiconductors to be used will be Si, GaAs or others. Suitable processing steps like wet etching or dry etching that can form nano-structures on Si or GaAs will be explored. Those nano-structures will be
inserted into the conjugate polymer. The semiconductors and the organic will absorb different spectral ranges of the solar spectrum, so the overall absorption of solar energy will be increased. The nano-structured semiconductors can offer the advantages of morphology control, infrared absorption, and good electron-transportation. We expect the new architecture of thin-film solar cells will out-perform the conventional thin-film solar cells.