界面工程與配位化學應用於有機無機混成鈣鈦礦太陽能電池之研究

Interface Engineering and Coordination Chemistry for Highly Stable and Efficient Organic and Inorganic Hybrid Perovskite Solar Cell

蕭凱起(Kai-Chi Hsiao)

指導教授 ： 林唯芳 ； 佳莉亞

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2021年)

https://doi.org/10.6342/NTU202100797

摘要

有機無機混成鈣鈦礦因其具有高可見光吸收係數、低載子束縛能、長載子擴散能力以及可藉由成分調控能隙的特性，使得鈣鈦礦成為一備受注目的新興光伏材料。鈣鈦礦太陽能電池之高效率以及可以溶液製程的特點使得此類新世代太陽能電池可望成為解決能源危機的新解答。為了將此新興的技術推向實際應用並商品化，如何提升鈣鈦礦太陽能電池之壽命以及光電轉換效率將成為是否能將研究成果推向實際應用端的至關重要的因子。因此本論文將聚焦於反式鈣鈦礦太陽能電池之界面以及其成分組成並以界面工程以及配位化學的方式提升鈣鈦礦太陽能電池之長時間穩定性以及效率表現。
於鈣鈦礦礦太陽能電池中，離子缺陷以及存在於界面間高維度的缺陷被視為是影響其穩定性以及效率表現的因素之一，為解決此問題有許多的表面鈍化技術因應而生。然迄今為止，關於如何同時鈍化陰離子缺陷和陽離子缺陷以及如何處理高維缺陷以及界面的能階匹配的知識仍然十分有限。為了近一步探究影響鈣鈦礦太陽能電池之穩定性以及效率表現之根源，我們分析存在其中的各種缺陷，從而針對不同的缺陷型態進行缺陷的鈍化。於微觀尺度而言，我們嘗試以兩性離子:2-噻吩乙基氯化銨鈍化以熱鑄法於大氣下製備之鈣鈦礦薄膜中的陰離子以及陽離子的缺陷，分析結果指出以兩性離子鈍化之鈣鈦礦薄膜，其中之陰陽離子之缺陷密度、非幅合載子再結合率以及能隙間的電子亂度皆有效地被抑制。此外因氯離子具有較強的電子親和力使得鈣鈦礦與電子傳輸層之界面能階更加匹配。於反式鈣鈦礦太陽能電池中，因無機的p-type半導體具有較優異的化學及物理穩定性，因此於電洞傳輸層方面我們以溶膠凝膠法製備之氧化鎳作為電洞傳輸層。然而溶膠凝膠法所製備之氧化鎳薄膜，其與鈣鈦礦之界面因缺陷的存在，使得鈣鈦礦於結晶過程中會產生不具光伏特質的死區並阻礙載子的傳遞，且能階的匹配能力亦不如有機之電洞傳輸層優異。因此我們以氯化鎳進行氧化鎳薄膜之缺陷鈍化，如此不僅可有效地降低氧化鎳與鈣鈦礦界面間的缺陷亦可藉由氯離子所賦予之能階梯度進一步提升氧化鎳薄膜與鈣鈦礦間的能階匹配度。
除上述所開發之界面工程技術可以有效的提升鈣鈦礦太陽能之穩定定性以及效率表現，進一步提升鈣鈦礦本身對於環境之耐受性仍需多加著墨。於電場作用下，鈣鈦礦中離子的遷移被視為是其劣化的初期徵兆，因較低的活化能以及較高擴散係數，使得甲基胺離子與碘離子於鈣鈦礦中的遷移非常容易發生，而其將進一步導致晶體結構的變化和薄膜的裂解並導致鈣鈦礦太陽能電池的效率衰退。為了抑制鈣鈦礦中離子擴散，我們將具有較大離子尺度、較高沸點以與鈣鈦礦中碘離子能產生較多配位鍵結之乙脒離子引入鈣鈦礦中並藉此穩定鈣鈦礦之晶體結構。藉由X繞射、X光電子光譜以及二次離子質譜的分析後可以得知，乙脒離子可進入鈣鈦礦中並抑制碘離子於鈣鈦礦中的擴散。配合上述之界面工程以及乙脒離子的配位化學，其效率表現可達20.68%，且未封裝之鈣鈦礦太陽能電池之效率在高濕度(大氣中60~80 %RH)環境下200小時及高溫(氮氣中85 °C)的環境下24小時仍可維持75%之初始效率；於封裝後可於高溫高濕(85 °C/85 %RH)的環境下維持80%之初始效率逾1300小時生命。界面工程以及配位化學所發展之鈣鈦礦太陽能電池技術不僅擴展了製程的操作範圍亦闡明了大規模生產以及進一步商品化可行的方向。

關鍵字

鈣鈦礦 ； 太陽能電池 ； 熱鑄法 ； 溶液製程 ； 兩性離子 ； 鈍化 ； 氯梯度 ； 能階匹配 ； 氧化鎳 ； 乙脒離子 ； 離子遷移 ； 穩定性

並列摘要

The emerging of organometal halide perovskite material (OHPVK) has created a great sensation to fabricate high efficiency solar cell owing to their high absorption coefficient in visible light, low binding energy of excitons, long diffusion length of carriers, and tunable bandgap from various compositions. The high efficiency and solution processable of organometal halide perovskite solar cells (OHPSCs) lead to a revolution of photovoltaic industry and become a solution for providing a low cost renewable energy. Improving the stability and enhancing the efficiency of OHPSCs are considered to bridge the gap between research and commercialization of this new technology. To realize the goal for practical application of OHPSCs, in this dissertation, we focus on the interfacial engineering of p-i-n OHPSCs and coordination chemistry of OHPVK lattice to improve the stability of materials and further on both the stability and efficiency of the solar cell.
Ionic defects and high dimensional defects at the interface between layer and layer of device are considered as the deterioration center for both stability and efficiency of OHPSCs. Various strategies of passivation have been adopted to overcome the issues to improve either photovoltaic performance or stability of OHPSCs. However, the knowledge of how to simultaneously passivate both negative and positive defects and to deal with the high dimensional defects along with band energy alignment at the interface is so far limited. In an aim to extend the understanding of improving the stability and efficiency of OHPSCs, we investigate the defects systematically. Anionic and cationic defects are considered as one of the crucial factors that affect carrier transport property and degradation of OHPSCs. To overcome the ionic defects in the OHPVK films, we demonstrate an advanced passivation strategy for crystallized air-processible OHPVK film by a dipolar ion of 2-thiophene ethyl ammonium chloride (TEACl). The passivated OHPVK films were characterized by space charge limit current model, photoluminescence spectroscopy, and time-resolved photoluminescence spectroscopy. The results reveal dipolar ion of TEACl can simultaneously passivate both cationic and anionic defects and reduce the defects density, non-radiative recombination, and electronic disorder. In addition to the defect passivation, the energy level alignment between OHPVK film and electron transport layer also improves due to a strong electron affinity of chloride ion from TEACl. Considering chemical and physical stability of carrier transporting materials, sol-gel NiOx was selected as hole transport layer (HTL) in this dissertation. However, such sol-gel NiOx film usually has defects at the surface and further induces photo-inactive layer, namely dead layer, of OHPVK. That impedes charge carrier transportation across such layer and reduces photovoltaic performance of OHPSCs. Additionally, energy level alignment between the sol-gel NiOx film and the OHPVK material is inferior to an organic HTL and an OHPVK material. That causes open-circuit voltage (Voc) loss of OHPSCs and shadows the contribution of Voc for OHPSCs composed of wide bandgap absorber layers. In order to resolve the problems, we established a technique of sequential passivation strategy of NiCl2 (SPS-NiCl2) for passivating defects of NiOx films and also talioring energy level by creating a chloride gradient in NiOx film. The energy level gradient renders such SPS-NiCl2 treated NiOx film to become an universial HTL for OHPSCs that comprised of narrow or wide bandgap OHPVK materials.
Although the advanced interfacial engineering was developed, improving the intrinsic property for environmental tolerance of OHPVK films still urgently needs. Generally, the electric field induced ion migration can be considered as the preliminary degradation step of OHPVK films. Ion migration in the OHPSC from ionic defects of MA+ and I- in perovskite lattice is considered as the trigger to destory the crystal structure of OHPVK films, to degrade the OHPVK films, and to further deteriorate the PCE of OHPSCs owing to the low activation energy and high diffusion coefficient of such ionic defects in OHPVK film. In an aim to retard the ion migration in OHPVK, a novel cation of acetamidinium (Aa+) with large ionic radii, high boiling point, and strong coordinated bond with I- than conventional cation, methylammonium, is induced into an OHPVK lattice to stabilize its crystal structure. The material was characterized by X-ray diffraction, X-ray photon spectroscopy, and time-of-flight secondary ion mass spectroscopy. The results validate that Aa+ ions can easily incorporate into perovskite lattice which was prepared by a hot casting process in air. The stabilized OHPVK lattice can retard the ion migration of I- as the OHPSC suffering external stresses of humidity or heat. By incorporating Aa+ ions into OHPVKs, the power conversion efficiency (PCE) of OHPSC without an encapsulation could maintain higher than 75% of its initial PCE after 200 h humidity (60~80% RH in air) or 24 h thermal stress test (85 °C in dry N2). The Aa-MAPbI3 device exhibits an outstanding efficiency of 20.68% and an over 80% of initial PCE is maintained after 1300 h damp heat (85 °C/85% RH) as encapsulated. It extends processing windows for OHPVK fabrication and sheds the light on mass production and further commercialization of OHPSCs.

並列關鍵字

perovskite ； solar cell ； hot casting ； solution process ； dipolar ion ； passivation ； nickel oxide ； chloride gradient ； band alignment ； acetamidinium ； ion migration ； stability

參考文獻

1. Kronmüller, H.; Parkin, S. S. P., Handbook of Magnetism and Advanced Magnetic Materials: Spintronics and magnetoelectronics. John Wiley Sons: 2007.

2. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 6050-6051.

3. De Wolf, S.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.; Yum, J.-H.; Ballif, C., Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5 (6), 1035-1039.

4. Zhang, H.; Du, L.; Wang, L.; Liu, J.; Wan, Q.; Kwok, R. T. K.; Lam, J. W. Y.; Phillips, D. L.; Tang, B. Z., Visualization and Manipulation of Molecular Motion in the Solid State through Photoinduced Clusteroluminescence. J. Phys. Chem. Lett. 2019, 10 (22), 7077-7085.

5. Sarritzu, V.; Sestu, N.; Marongiu, D.; Chang, X.; Wang, Q.; Masi, S.; Colella, S.; Rizzo, A.; Gocalinska, A.; Pelucchi, E.; Mercuri, M. L.; Quochi, F.; Saba, M.; Mura, A.; Bongiovanni, G., Direct or Indirect Bandgap in Hybrid Lead Halide Perovskites? Adv. Opt. Mater. 2018, 6 (10), 1701254.