大氣噴射電漿製作染料敏化太陽能電池及銦錫氧化物之熱處理

Abstract

本研究利用噴射式大氣電漿(Atmospheric pressure plasma jet, APPJ )進行薄膜材料快速熱退火以及奈米氧化物燒結製程。論文內容包含兩大部分，第一部分為利用大氣噴射電漿快速熱退火缺氧銦錫氧化物；第二部分利用大氣噴射電漿燒結二氧化鈦，並利用食鹽溶液混合二氧化鈦漿料進行燒結具有微米孔洞的奈米二氧化鈦光電極，微米孔洞可增加光散射，而增加染料敏化太陽能電池的光電轉換效率。 實驗一利用大氣壓噴射式氮氣電漿處理缺氧之銦錫氧化物(Indium tin oxide, ITO)，進行缺氧銦錫氧化物的氧化製程，大幅降低熱處理時間及熱預算，達到製程上節省能源成本及縮短製程時間的目的。未經過處理的室溫電子鎗蒸鍍(未補充氧氣)之ITO薄膜顏色為黑褐色，而經大氣噴射電漿處理之ITO薄膜隨著處理時間的增加而逐漸變透明。本實驗將有側孔和無側孔之石英套管安裝於大氣噴射電漿的底部，由於有側孔之石英管可引入空氣冷淬電漿的高能量分子及離子，造成電漿火炬變得較微弱，而電漿氣體溫度(於基板位置量測)也由580˚C下降至385˚C。空氣中的氧氣與ITO薄膜產生反應，黑褐色ITO薄膜在較短的處理時間下有較高的透光度。藉由側孔引入空氣之大氣電漿處理90秒之100 nm的ITO薄膜，在波長550 nm的穿透率達到87%，相較於未處理的ITO薄膜穿透率僅有7.2%。由於原本較接近金屬相的ITO在經過快速退火後形成結晶的氧化物，因此在APPJ的退火後電阻率大幅下降，之後隨退火時間增加有微微提升的現象。 實驗二為利用大氣噴射電漿快速燒結摻有食鹽晶體的二氧化鈦光電極，並研究不同食鹽水濃度摻入二氧化鈦漿料中對染料敏化太陽能電池效率的影響。我們使用40 ˚C去離子水潤洗，將食鹽晶粒從二氧化鈦中溶解去除，並從掃描式電子顯微鏡分析中可得知，當食鹽水的濃度提高時，附著在二氧化鈦中的食鹽晶粒/孔洞也就越大。這些微米孔洞可以造成光散射，使太陽能電池的電流密度增加並提升電池的效率。其中最好的製程參數為1 wt%的食鹽水摻入二氧化鈦漿料中，所製作出的光電極使電池具有最高的光電轉換效率6.5%，相較於標準式片的效率5.9%，提升11.5%，其二氧化鈦表面的微米孔洞大小約為0.5 ~ 1 μm。然而隨著食鹽水的濃度提升，微米孔洞的數量和尺寸增加會使二氧化鈦的量及表面積下降，造成N719染料吸附量不足，導致電池的電流密度下降進而影響電池的效率。由於食鹽的成本低廉，本研究以低成本的製程提升了染料敏化太陽能電池的效率。

We use the atmospheric pressure plasma jets (APPJs) for the rapid thermal annealing processes of ITO thin films and for rapid sintering processes of nanoporous oxide materials. The first part of this thesis focuses on rapid annealing processes of oxygen-deficient indium tin oxide (ITO) thin films by atmospheric pressure plasma jets. In the second part of this thesis, we fabricate dye-sensitized soalr cells (DSSCs) with microcavity-embedded nanoporous TiO2 photoanodes sintered by atmospheric pressure plasma jets. In the first experiment, we annealed oxygen-deficient ITO thin films using atmospheric pressure plasma jets (APPJs) with/without air-quenching. The as-deposited oxygen-deficient ITO thin films were dark in color and gradually became transparent after N2 APPJ treatment. Quartz tubes with and without side holes were installed downstream of the APPJ to control the introduction of air into the plasma jets. Air-quenching reduces the plasma jet temperature from 580 to 385 °C but enhances the reactivity and renders faster conversion of dark ITO to transparent ITO despite the lower APPJ temperature. With air-quenching, the transmittance of 100-nm-thick ITO thin film on glass substrate reached 87% after 90 s APPJ treatment, compared to 7.2% in the case of the as-deposited ITO thin film. The resistivity decreased dramatically at the beginning of the APPJ treatment owing to crystallization and oxidation processes that reduced the defect density in the material. Subsequently, the resistivity increased slightly because of the reaction of oxygen and ITO that reduced the oxygen vacancies. Our results demonstrated that APPJ treatment can be used as a rapid thermal annealing process for ITO thin films. The second experiment is regarding the fabrication of microcavity-embedded nanoporous TiO2 photoanodes for DSSCs. The embedded micrometer-size cavities enhanced the light scattering to improve the cell efficiencies of DSSCs. Sodium chloride solutions with various concentrations were mixed into nanoparticle TiO2 pastes for screen-printing. The printed pastes were sintered by APPJs. The sodium chloride crystallized after APPJ treatment and was washed away by rinsing the sintered photoanodes in DI water. Abundant micrometer-size cavities were created in nanoporous TiO2 after the rinsing processes. These microcavities could enhance the light scattering to improve the efficiencies of DSSCs. When the concentration of sodium chloride solution became too high, the total amount and surface area of nanoporous TiO2 reduced such that the amount of anchored dye decreased, which thereby reduced the efficiencies of DSSCs. The best cell performance was achieved with TiO2 pastes mixed with 1 wt% sodium chloride solution. The short current density was 12.75 mA/cm2 and the conversion efficiency was 6.57%, which showed the enhancements by 9.2% in short circuit current density and by 11.5% in cell efficiency. Owing to the low cost of sodium chloride, this technique can enhance the efficiencies of DSSCs economically.