延長有機金屬鹵化物鈣鈦礦太陽能電池壽命之研究

Enhancement in Operation Lifespan of Organometal Halide Perovskite Solar Cells

黃信祥(Hsin-Hsiang Huang)

指導教授 ： 林金福

共同指導教授 ： 王立義(Leeyih Wang)

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/NTU202101239

摘要

以有機銨碘化鉛作為光敏材料的鈣鈦礦太陽能電池(Perovskite solar cells, PSCs)具有較高的光電轉換效率和較低的加工成本，因此被認為具有商業化的潛力，但如何提升PSCs的元件使用壽命仍然為目前最重要的研究課題。蒙脫土(Montmorillonite, MMT)是一種天然的近晶粘土，具有多層堆疊的奈米片(厚度約為1 nm，橫向長度約為300 nm)，脫層完的MMT (exfoliated MMT, exMMT)就像屏障一樣，可以阻止水分滲透到膜中。除此之外，exMMT還可以通過陽離子交換與有機碘化胺相互作用產生作用力(氫鍵)。本論文發現在鈣鈦礦層旋塗成膜之前，將exMMT參雜進MAPbI3的鈣鈦礦前驅溶液中，可以在鈣鈦礦的晶粒表面上形成一保護殼，使得未封裝的PSCs抗濕能力大幅提升，在相對濕度50%的環境下儲存半年後，效率仍然保持在17.29%以上。值得注意的是，MMT可以在水中溶脹後再利用超聲波的方式來達到脫層的效果，其方法簡單且成本低廉，整個脫層過程也受到Derjaguin-Landau-Verwey-Overbeek的理論支持。
接著，我們引入一種新的氟化富勒烯，用作鈣鈦礦(CsFAMA)光伏電池中的電子傳輸層(Electron transporting layer, ETL)，效率可達21.27%，並顯著的提高了濕度，熱和離子遷移的耐久性。因氟化富勒烯的疏水特性使得未封裝的PSCs在相對濕度85%的條件下，仍然超過1,400個小時以上的溼度穩定性，且該設備在水下浸泡了600秒後仍保持其原始性能的70%。此外，我們也發現氟化的富勒烯可以固定鈣鈦礦中的有機離子並使表面缺陷鈍化，提升元件的光熱穩定性。結果對長期的光浸泡以及高溫老化表現出極好的耐受性，在氮氣的環境下持續的光照(1-sun)或在85oC加熱1,000小時，可保持其超過95%的初始效率。由於氟化富勒烯的電子傳導層在PSCs中的成功可以激發進一步的研究，為未來的光伏技術帶來真正的動力。
我們進一步利用原位同步加速器X射線表徵來明確揭示鈣鈦礦薄膜在不同相對濕度的退火過程中材料轉變和結晶過程，水的結合對晶粒尺寸和轉化率表現出顯著的促進作用。我們確定了鈣鈦礦的晶體取向、晶格常數、晶粒尺寸和退火過程中的轉變速率與水分子之間的相關性，在當相對濕度為40%的氮氣中退火時，鈣鈦礦晶體顯示出較好的取向性和較大的晶粒尺寸。我們的工作對於鈣鈦礦薄膜在結晶過程中的水混入效果提出了深入地瞭解，更重要的是，透過簡易的水結合方式即可大幅提升元件的光穩定性和熱穩定性，使得原本較為不穩定的MAPbI3鹵化物鈣鈦礦可與他人所使用較為穩定的鈣鈦礦系統(CsFAMA)所提出的研究成果相提並論。

關鍵字

鈣鈦礦太陽能電池 ； 奈米蒙脫土 ； 氟化富勒烯電子傳輸層 ； 長效穩定性 ； 原位掠入射廣角X射線散射 ； 定向晶體形成

並列摘要

The perovskite solar cells (PSCs) with organic ammonium lead iodide as photoactive material have been highly regarded to have potential for commercialization because of their high-power conversion efficiency (PCE) and low processing cost. However, long-term stability still needs to be addressed. Montmorillonite (MMT) is a natural smectic clay of multiple stacked nanoplatelets (the thickness of the layer is approximately 1 nm, and the lateral dimensions of these layers vary from 300 angstroms to several microns) Once MMT is exfoliated to fabricate the polymer composite film, the exfoliated MMT (exMMT) function like a barrier blocking the moisture from penetrating into the films. Moreover, exMMTs are anionic in nature and can interact with organic ammonium iodide through cation exchange. In this dissertation, we found they formed as a shell on the surface of perovskite crystalline grains when incorporated into the MAPbI3 perovskite precursor solution after spin-coating into a film. The as-fabricated unencapsulated PSCs are extremely stable, with the PCE of 17.29% barely changed after half a year of storage in the controlled ambient environment (relative humidity, RH 50%). Notably, exMMT can be easily fabricated by sonicating the MMT in aqueous solution after swelling in water, the process of which is manifestly supported by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and is also low cost.
Next, we introduce a new fluorinated fullerene to serve as a robust electron-transporting layer (ETL) in perovskite (CsFAMA) photovoltaic cell that deliver high PCE of 21.27% with substantially improved durability against humidity, heat and ion migration. The hydrophobic nature of the new fullerene protects the un-encapsulated perovskite cell in 85% relative humidity for over 1,400 hours. Notably, the device maintained 70% of its original performance when immersed under water for 600 seconds. Moreover, we found the fluorinated fullerene can immobilize the organic ions in the perovskites and passivate the surface traps. As a result, the device exhibits excellent tolerance to long-term light soaking as well as high-temperature aging, which retained >95% of the peak PCE under constant 1-sun illumination or heating at 85oC for 1,000 hours in a nitrogen atmosphere. It is expected that the success of fluorinated fullerene-based ETL in PSCs can stimulate further research and bring real momentum to future photovoltaic technologies.
We further utilized in situ synchrotron X-ray characterization to explicitly reveal the material transformation and crystallization process of perovskite films during annealing over various RH and time scales. The water incorporation exhibits a significant promoting effect on grain size and transformation rate. We identified the water-content-dependence of the perovskite crystal orientation, lattice constant, grain size, and transformation rate during annealing. The perovskite crystal displays a preferred orientation, the fast-annealing rate, and the largest grain size when annealing in nitrogen with 40% humidity. Clearly, our work presents an insightful understanding in the water incorporation effect of perovskite films during the annealing process. More importantly, it is explicit to see that our simple water-incorporated approach (at mild RH = 40%) sustains the device photo- and thermal- stability as well as those nontrivial instability-mitigation approaches (e.g. ion insertion, composite additive, grain boundary reduction, passivation layer) for the well-known unstable MAPbI3 halide perovskite.

並列關鍵字

perovskite solar cells ； montmorillonite ； fluorinated fullerene electron transporting layer ； long-term operation stability ； in situ grazing incidence wide-angle X-ray scattering ； orients crystal formation

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