近年來，有機半導體材料因為其價格低廉、製程簡易和可製作於塑膠基板上等優點，所以吸引著多數的研究人員投入研究，其在產品的應用上可製作成有機薄膜電晶體(OTFT)、有機發光二極體(OLED)、太陽能電池等。
本論文便著重於OTFT的研究，而主動層材料包含高分子與小分子材料，有機材料依分子量的差別又可分為高分子材料與小分子材料，在OTFT的電特性上，小分子OTFT優於高分子OTFT，但是在製程上，因高分子薄膜可以旋轉塗佈製程成膜，而小分子是以物理氣相沉積成膜，以難易度來說，高分子優於小分子。
在高分子OTFT方面，是以共軛高分子為主動層材料，在1970年代，希格、麥迪亞米德和白川英樹發現共軛高分子可透過參雜來增加導電度，也因這個研究而得到2000年諾貝爾化學獎，根據此參雜理論而製作出以共軛高分子(Poly(diphenylamine) (PDPA))為主動層、高分子電解質(poly(diallyldimethylammonium chloride) (PDDA))為緩衝層的增強型高分子OTFT，利用加於閘極與源極間的電場來推動高分子電解質中的氯離子而對PDPA做參雜，參雜的程度可以藉由電場強度來控制，就類似於增強型無機金氧半場效電晶體是利用電場誘發載子形成通道一樣，此增強型高分子OTFT是利用電場控制通道參雜程度，也確實呈現出電晶體特性，其場效遷移率為0.085 cm2/Vs，開關電流比為30。
而在證實可透過參雜共軛高分子來製作增強型高分子OTFT之後，也以透過去參雜共軛高分子來製作空乏型高分子OTFT，其主動層材料為poly(ethyleneimine) (PEI) 和以poly(styrenesulfonate) (PSS)做參雜的共軛高分子((poly(3,4-ethylenedioxythiophene) (PEDOT))，poly(ethyleneimine) (PEI)會與PSS連結再透過電場從PEDOT移除PEI，而PSS也一併被移除，也等同對PEDOT做去參雜的動作，結果也確實呈現出空乏型高分子OTFT之特性，其場效遷移率為1.94cm2/Vs，開關電流比為103。
氟化鋰通常當成是載子注入層運用於OLED，因其可以對銦錫氧化物(ITO)的功函數做調變，因此便應用氟化鋰做為電極緩衝層在以pentacene作為主動層的OTFT上，而加入緩衝層的OTFT也證實可比沒加入緩衝層的OTFT得到更佳的電性，而在1nm的氟化鋰插入在源極/汲極和主動層間上，其汲極電流和載子的場效遷移率分別增加超過6倍和4倍，而以7.5nm氟化鋰插入在閘極和隔離層間上，其汲極電流、載子的場效遷移率和開關電流比分別增加大約2倍、1.47倍及2.5倍。

In recent years, organic semiconductors have attracted many researchers because of their attractive advantages such as low cost, easy processing, and ability to be fabricated on plastic substrates. They are applied in organic thin film transistors (OTFTs), organic light emitting diodes (OLEDs), and organic solar cells, among others.
This thesis is focused on studies on organic thin film transistor based on both polymers and small molecular. Organic semiconductors are divided into two categories, polymer and small molecular, according to molecular weight. In electric characteristics, the small molecular OTFTs show better performance than polymer OTFTs. However, polymer OTFTs exhibit advantage in processing comparing to small molecular OTFTs. A polymer thin film can be deposited by solution process, but the small molecular films must be evaporated.
For the polymer OTFT study, the conjugated polymers are used as active materials in device fabrication. In 1970, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa discovered that electric conductivity of conjugated polymers can be enhanced by doping mechanism. They were awarded the Nobel Prize in Chemistry for this discovery in 2000. According to their discovery, the enhancement-mode (E-mode) polymer OTFTs that included an active layer, conjugated polymer (Poly (diphenylamine) (PDPA)), and a buffer layer, polymer electrolyte (poly (diallyldimethylammonium chloride) (PDDA)), are fabricated. The doping process was carried out by doping chlorine anions from the polymer electrolyte to PDPA. The doping level is driven by the electric field between the gate and the source. This is similar to the operation of inorganic E-mode metal-oxide-semiconductor field effect transistor where the induced carrier level is modulated by the electric field. The polymer OTFT then exhibits E-mode transistor behavior. The field effect mobility and on/off ratio of this device are about 0.085 cm2/Vs and 30.
The E-mode polymer OTFT has been demonstrated. The depletion-mode (D-mode) polymer OTFT is fabricated through dedop process of conjugated polymer, which is controlled by electric field. Poly(ethyleneimine) (PEI) and poly(3,4-ethylenedioxythiophene) (PEDOT) which is doped by poly(styrenesulfonate) (PSS), are used as the active material. In this mixture, PEI binds to PSS. PSS can remove PEDOT when PEI is removed by the electric field. This mechanism is similar to the dedop process of PEDOT. The polymer OTFT then exhibits D-mode transistor behavior. The field effect mobility and on/off ratio of this device are about 1.94 cm2/Vs and 103.
Based on previous studies, the work function of indium tin oxide (ITO) can be modulated by lithium fluoride (LiF). Based on the concept described, LiF is used as the electrode buffer layer in pentacene-based OTFT in this thesis. It has been demonstrated that the electric performance of OTFT with LiF is better than that of one without LiF. After inserting a 1 nm-thick LiF between the source/drain electrodes and the active layer, the drain current and field effect mobility are enhanced six and four times, respectively. After inserting a 5 nm-thick LiF between the gate electrode and the insulator layer, the drain current, field effect mobility, and on/off ratio are enhanced 2, 1.47, and 2.5 times.

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