使用水熱法成長氧化鋅奈米柱應用於低溫製備鈣鈦礦太陽能電池光電特性之研究

Abstract

近年來，由於地球上的能源消耗逐年增加，過量的燃燒化石燃料導致全 球溫室效應的加劇，再生能源的開發引起關注，其中太陽能電池引起了廣泛的興趣。鈣鈦礦係一種新興的主動層材料，於2009 年開始至今已被加強研究，在短短的幾年之內其效率的成長是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。本論文利用低溫水熱法成長垂直排列之氧化鋅奈米線其立體結構製作太陽能電池之電子傳輸層，配合兩階段溶液塗佈鈣鈦礦主動層，形成鈣鈦礦太陽能電池，此方法運用氧化鋅奈米線之奈米結構，大幅提升主動層接面面積進而提升光電轉換特性。本文首先探討不同長度之氧化鋅奈米線對鈣鈦礦太陽能電池之光電轉換特性影響，鈣鈦礦前驅溶液之滲透與覆蓋率隨奈米鋅氧化線長度的變化而影響，本文發現當氧化鋅奈米線長度為150 nm 之元件於AM 1.5 照射下(100 mW/cm2)具有最佳之能量轉換效率8.46 %。另外，為更進一步改善鈣鈦礦主動層之表面粗糙度，本論文利用基板預熱方式在旋塗碘化鉛溶液前加熱基板使碘化鉛結晶析出延緩、溶液黏滯性降低進而使 溶液分布更加平滑。從實驗結果得知，將基板加熱在越高溫的情形下塗佈碘化鉛溶液能夠有越平滑且覆蓋率越佳的表面且所得之碘化鉛結晶性更高，然而碘化鉛結晶性太高不利於轉化成鈣鈦礦，本文發現在基板預熱溫度達200度時雖能有平滑的碘化鉛表面但因其不易轉換成鈣鈦礦，因此將基板預熱溫度適當地調控在150度即可得到最佳的能量轉換效率10.34%。總結，本研究所提出之水熱法成長氧化鋅奈米柱應用於常溫常壓下製備鈣鈦礦太陽能電池因具有優異的光電轉換效率、低溫製備且製程簡單，以及成本低廉等特性，於未來應用於穿戴性軟性太陽能電池發展中深具潛力。

In recent years, the burning of the fossil fuel causes the aggravating of the global warming due to the increasing energy demand. Development of the renewable energy has attracted worldwide extensive attention, especially solar cells. Perovskite is an emerging material as the active layer of the solar cells, and it has been widely studied by scientists all around the world. The PCEs of perovskite solar cells has surpassed those for the other kinds of solar cells in a relatively short period. Due to the advantages 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 on flexible substrates and so on, the perovskite solar cells has gradually been paid attention to. In this thesis, the vertically-aligned ZnO nanowires grown via low-temperature hydrothermal growth process are used as the electron transfer layer (ETL) of the perovskite solar cells. Sequentially, the solution-processed perovskite using two-step method is collocated with the hydrothermally-grown ZnO nanowires to form the perovskite solar cells. The main idea of this thesis is utilizing the 3D structures of the hydrothermally-grown ZnO nanowires to increase the junction area to improve the photovoltaic performance of the perovskite solar cells. In the beginning of this thesis, the length effect of the hydrothermally-grown ZnO nanowires on the power conversion efficiencies (PCEs) of the perovskite solar cells is discussed. The infiltration and the surface coverage of the perovskite precursor solution changed as tuning the length of the ZnO nanowires. It is revealed that the devices with ZnO NW of 150 nm demonstrated the best PCE of 8.46 % under the AM 1.5G illumination (100 mW/cm2). However, the surface roughness of the perovskite film on the ZnO nanowires of 150 nm was still room for improvement. For the purpose to improve the surface roughness of the perovskite layer, the substrate preheating method which is heating the substrate to a temperature before dipping the PbI2 solution was carried out to solve the problem. The substrate preheating method prevents PbI2 crystallize too fast and lower the viscosity of the PbI2 solution so that the surface of the PbI2 solution can flow smoothly before the crystallization of the PbI2. From the result of the experiment, it is proved that the surface of the PbI2 spin-coated at a higher temperature might form a smoother surface, fewer pinholes, and higher PbI2 crystallinity. However, there is a trade-off between the crystallinity of PbI2 and the conversion to perovskite. This thesis demonstrated that the surface of PbI2 with the substrate preheating temperature at 200 oC is the smoothest, but the resulting high crystallinity inhibited the conversion of perovskite. Therefore, the best PCE of 10.34% was achieved for the substrate preheating at 150 oC. In this work, perovskite solar cells with hydrothermally-grown ZnO nanowires fabricated at low processing temperature demonstrated the excellent photovoltaic performance, easy fabrication, low-temperature process, and low cost, making it promising for the future developments in the solar cells on flexible substrates.