三維氧化鋅奈米柱於鈣鈦礦太陽能電池之退火效應之研究

Abstract

近年來，由於工業興起，地球上的能源消耗逐年增加。石化燃料的使用導致全球溫室效應的加劇，而核能則是有核廢料存放的問題。因此再生能源的開發是必須的，其中太陽能電池引起了廣泛的興趣。鈣鈦礦係一種新興太陽能電池的主動層材料，於 2009 年開始被研究，在短短的幾年之內其轉換效率之成長幅度超越以往各類的太陽能電池，因擁有高吸光係數、高載子遷移率、載子再復合率低、製程簡單可大面積製備以及製程溫度低可使用於軟性基板等優點逐漸受到重視。除此之外。其製程十分便宜。 本論文利用溶膠凝膠法沉積氧化鋅晶種層並以低溫水熱法成長垂直排列之氧化鋅奈米柱製作太陽能電池之電子傳輸層，而主動層則利用兩階段溶液塗佈鈣鈦礦，形成鈣鈦礦太陽能電池，此方法運用氧化鋅奈米柱之奈米結構，大幅提升電子傳輸層與主動層之接觸面積進而提升光電轉換特性。首先探討不同長度之氧化鋅奈米柱對鈣鈦礦太陽能電池之光電轉換特性影響，鈣鈦礦之接觸面積與覆蓋率隨氧化鋅奈米柱長度的變化而影響，本文發現當氧化鋅奈米柱長度為 400 nm 之元件 iv 於 AM 1.5G 照射下(100 mW/cm2)具有最佳之能量轉換效率 8.59 %。再者，本文探討溶膠凝膠氧化鋅晶種層的轉速來調整氧化鋅晶種層的厚度和氧化鋅奈米柱之密度，因此提升接觸面積，本文發現當溶膠凝膠法晶種層之轉速為 6000 rpm 之元件，其成長的氧化鋅奈米柱長度為400nm時，於 AM 1.5G 照射下(100 mW/cm2)具有最佳之能量轉換效率 10.48 %。 另外，為去除氧化鋅奈米柱上的氫氧根和提升結晶性，本論文利用爐管退火奈米柱，並探討在氮氣及氮氣/氫氣下退火結晶性的不同。從實驗結果得知，在氮氣/氫氣環境下退火的結晶性更高，在上述條件下，當溶膠凝膠法晶種層之轉速為 6000 rpm 之元件，其成長的氧化鋅奈米柱長度為400nm時，此時得到的能量轉換效率為 11.50 %。在去除氫氧根之後，可避免鈣鈦礦主動層因退火而退化，因此可以進行鈣鈦礦之退火以增加晶粒大小和改善結晶性。本文發現在上述當溶膠凝膠法晶種層之轉速為 6000 rpm 之元件，其成長的氧化鋅奈米柱長度為400nm時且於氮氣/氫氣環境下退火氧化鋅奈米柱的條件下，退火鈣鈦礦100度五分鐘得到的能量轉換效率為 12.38 %。 總結，本研究所提出三維結構之氧化鋅奈米柱應用於鈣鈦礦太陽能電池，在退火處理後因具有優異的光電轉換效率、製程簡單，以及成本低廉等特性，於未來太陽能電池發展中深具潛力。

In recent years, due to the development of industries, the consumption of the energy raises severely. Burning the fossil fuel causes the global warming, while nuclear energy leads to the issue of nuclear waste. Development of the renewable energy seems to be inevitable and solar cells draw a broad concern. Perovskite is a brand new material as the active layer of the solar cells and it has been widely studied by scientists since 2009. The improvement of PCEs of perovskite solar cells is much more significant than the other kinds of solar cells. Due to the characteristics like high absorption coefficient, high carrier mobility, low photo-carriers recombination rate, easy fabrication, capable of large-area preparation, and low-temperature fabrication for the application in the flexible substrates and so on, the perovskite solar cells attract lots of attention. Besides, the lost-cost fabrication is another key factor of it. vi In this thesis, the vertically-aligned ZnO nanorods grown on the sol-gel ZnO seed layer via low-temperature hydrothermal growth process were used as the electron transfer layer (ETL) of the perovskite solar cells. Two-step spin-coating method to deposit the perovskite was used. The main idea of this thesis was utilizing the 3D structures of the hydrothermally-grown ZnO nanorods to increase the contact area between ETL and the active layer to improve the photovoltaic performance of the perovskite solar cells. In the beginning of this thesis, the length effect of the hydrothermally-grown ZnO nanorods on the power conversion efficiencies (PCEs) of the perovskite solar cells was discussed. The contact area and the surface coverage of the perovskite changed as tuning the length of the ZnO nanorods. It was revealed that the device with ZnO NRs of 400 nm long demonstrated the best PCE of 8.59 % under the AM1.5G illumination (100 mW/cm2). In addition, the effect of the spin speed of sol-gel ZnO seed layer on the power conversion efficiencies (PCEs) of the perovskite solar cells was also studied. With the change of spin speed, the thickness of ZnO seed layer was altered, and the density of hydrothermally-grown ZnO nanorods was accommodated. It was shown that the device with the spin speed of 6000 rpm demonstrated the best PCE of 10.48 % under the AM1.5G illumination (100 mW/cm2) for the ZnO nanorods of 400 nm long. Furthermore, in order to eliminate the OH groups in ZnO nanorods as well as enhance the crystallinity of ZnO nanorods, the ZnO nanorods were annealed at 400 oC in furnace with N2 and N2 / H2. From the result of the experiment, annealing with N2 / H2 had the better crystallinity and could achieve the PCE of 11.50% as the spin speed of sol-gel ZnO seed layer of 6000 rpm and the length of ZnO nanorods of 400 nm. After eliminating the OH groups on the ZnO nanorods which preventing perovskite from the degradation during the annealing. The annealing process to improve the grain size and the crystallinity of perovskite was conducted. It was vii revealed that the devices with annealing perovskite at 100 oC for 5 minutes demonstrated the PCE of 12.38 % under the AM1.5G illumination (100 mW/cm2) as the spin speed of sol-gel ZnO seed layer of 6000 rpm, the length of ZnO nanorods of 400 nm and annealing ZnO nanorods with N2 / H2. In this work, the perovskite solar cells with three-dimensional (3D) ZnO nanorods under the proper annealing process demonstrated the excellent photovoltaic performance, easy fabrication, and low cost, making it promising for the future developments in the solar cells.