二維-三維鈣鈦礦異質接面對鈣鈦礦太陽能電池效能之改善與研究

Abstract

鈣鈦礦太陽能電池發展至今短短幾年的時間，效率已從9.7提升至22.7％。然而，鈣鈦礦太陽能電池發展至今仍存在許多問題影響效能的提升，特別是在薄膜異質接面的能帶結構上，當兩層間的能階位置落差較大時，容易使載子再結合的機率增加，進而影響元件表現。因此，在本研究中我們透過同為鈣鈦礦之材料作為中間層，利用鈣鈦礦本身在結構及組成上的多樣性來改善異質接面。首先，我們藉由陽離子交換法將三維鈣鈦礦中的甲胺與反應溶液中的有機長碳鏈-正丁基銨進行交換，並透過有機長碳鏈離子濃度在薄膜內部的變化，製作出具有漸變層數的二維-三維鈣鈦礦，其在能帶結構上呈現梯形排列，因此當鈣鈦礦吸收光激發載子後，載子會自我分離，降低再結合率，使效率提升至15.25%，除此之外，二維鈣鈦礦中的疏水性有機長碳鏈，使二維鈣鈦礦中間層更做為保護層，降低水氣與鈣鈦礦反應降解的機率，提升元件的穩定度，使未封裝之二維-三維鈣鈦礦元件在經過12天後仍能保有原效率的84%。我們也透過反應溶液濃度的調控改變二維、三維鈣鈦礦之間的比例，同時利用光致螢光譜及X光繞射儀分析薄膜的光學性質，發現反應溶液的濃度對於二維鈣鈦礦的厚度及層數上都有影響，這會改變載子的轉移能力，影響元件的效能。最後，我們在表面形成最佳厚度之二維鈣鈦礦層並加入溴離子，使得在陽離子交換的同時，溶液中的溴離子會與薄膜內的碘離子進行交換，並調控反應溶液中溴的含量，觀察薄膜在光學性質、表面形貌、載子轉移能力上的變化，發現我們能藉由溴離子的摻入改變鈣鈦礦的結構並增加能隙，進而幫助在層數低於二的二維鈣鈦礦在價帶位置能更接近電洞傳輸層的價帶，幫助電洞的轉移，提升載子的轉移能力，使開路電壓有效提升至1.05V。從研究結果中，我們發現可以藉由溴混摻之二維-三維鈣鈦礦，改變能帶結構，使其具有較佳的載子轉移能力，並製作出轉換效能更佳的太陽能電池。

Organic-inorganic halide perovskite solar cells (PSCs) have pushed forward the photovoltaics progress since first reported in 2009 and their power conversion efficiency (PCE) has exceeded 22% in only a few years. However, there are some problems to affect the development of perovskite solar cells, especially the band structure at the heterojunction. So energy-level alignment at the interfaces is critical for the optimization of the solar cells. In this work, we use perovskite with different structure and composition as interlayer to improve the band structure. In the beginning, we form the graded 2D-3D perovskite through the cation-exchange, and change the band structure to the ordered band alignment between different perovskite components. Therefore, when the perovskite absorbs light and generates carriers, the carriers will self-separate, which reduces the recombination and increasing the efficiency to 15.25%. Moreover, the hydrophobic two-dimensional perovskite acts as a protective layer to reduce degradation of perovskite, and improve the stability of the device. We also change the ratio between 2D and 3D perovskite by adjusting the concentration of the reaction solution, and get the different optical properties by using the photoluminescence (PL) and X-ray diffraction. Then we discuss the relationship between the concentration of the reaction solution, thickness of 2D perovskite, and thickness of perovskite sheet, which affects the carriers’ transfer ability and the performance of devices. Finally, we doping different concentration of bromide ions into 2D/3D perovskite layer with the best ratio, which changes optical properties and morphology. In PL analysis that doping bromide will increase the band gap, and then decrease the valence band of 2D perovskite to be closer to HOMO of hole transporting layer, which improves holes’ transfer ability and effectively increases the open circuit voltage to 1.05V. Bromide-doped 2D-3D perovskite indeed improves the energy band structures at interface to get better carriers’ transfer ability, and high performance solar cells.