透過使用小分子添加劑理解鈣鈦礦太陽能電池的缺陷物理及晶體成長機制

由於低成本的鹵化鉛鈣鈦礦同時具備優秀的性能與潛力，因此成為光伏應用元件的主要熱門人選之一。近十年來，鈣鈦礦太陽能電池的轉換效率已經從3.8%提高至25.7%；然而鈣鈦礦太陽能電池本身的不穩定性是造成無法商業化的主要瓶頸。在鈣鈦礦太陽能電池中，缺陷的存在是不可避免；同時為延長元件的壽命，因此必須了解鈣鈦礦的缺陷物理特性。為了瞭解溶液製程的CH3NH3PbI3 （MAPbI3）薄膜的成核和晶體生長，透過以路易斯鹼（Lewis base）的尿素（urea（CH4N2O））和硫脲（thiourea（CH4N2S））與路易斯酸（Lewis acid）的富勒烯衍生物（PCBM）和富勒烯（C60）作為添加劑，可同時研究在鈣鈦礦光伏電池當中的缺陷形成和鈍化。我們的實驗結果中表明：不同的添加劑其鈍化機制也不盡相同，主要是取決於添加劑與 MAPbI3晶體之間的化學交互作用所致。透過路易斯鹼作為添加劑，在晶體生長過程中會增加MAPbI3薄膜中的晶粒尺寸以及結晶性將會有所提升，而富勒烯作為添加劑則是會促使MAPbI3薄膜當中的晶粒進行合併。從拉曼光譜的結果表明：尿素分子主要分佈於MAPbI3薄膜的頂部區域，藉此鈍化薄膜界面上MA+陽離子缺陷；而當使用C60分子則是會形成MA+-C60-MA+的陽離子來鈍化MAPbI3薄膜中的表面缺陷。在傅立葉轉換紅外光譜（Fourier transform infrared spectroscopy）的結果表明：硫脲與鈣鈦礦前驅物形成穩定的中間態，這會促使整個生長晶粒在（110）結晶面方向上具有優先取向。另外從光致發光光譜的結果可以預期到富勒烯分子會透過凡德瓦爾力（Van der Waals force）在MAPbI3薄膜的晶體邊界中進行遷移。此外，一種利用反溶劑混合間接成核（ASMEN，Anti-solvent mixture-mediated nucleation）的方式可用來抑制MAPbI3薄膜當中的點缺陷形成，並且利用下轉換機制所製作的硼摻雜石墨烯量子點：PMMA的薄膜來對元件進行保護，以避免MAPbI3太陽能電池造成不可逆的照光降解效應發生。這是為了呈現來自環境當中的不利影響，針對MAPbI3太陽能電池所採用一種簡易型封裝方法。在封裝的MAPbI3薄膜樣品中可以觀察到鹵化物離子的遷移行為，此現象導致電子會具有更高的費米能階。為了解釋封裝對MAPbI3光伏電池的穩定性的影響，須在MAPbI3薄膜的缺陷形成、鈍化和遷移等行為進行研究分析。本論文透過系統性的研究，利用添加劑來輔助鈣鈦礦晶粒的晶體生長和鈣鈦礦薄膜當中的缺陷鈍化，並藉此改善光伏的性能和穩定性。值得注意的是，這些研究對於解決鈣鈦礦太陽能電池的低穩定性是至關重要的。

關鍵字

鈣鈦礦；缺陷鈍化；遷移；穩定性； MA+-C60-MA+ ；添加劑

並列摘要

Low-cost lead halide perovskites have emerged as a potential candidate for applications in photovoltaics due to their extraordinary performance. The power conversion efficiency for perovskite solar cells has increased from 3.8% to 25.7% in the recent decade. However, the low stability of the perovskite solar cells is the major bottleneck for their commercialization. The inevitable existence of defects in the perovskite solar cells leads to poor long-term stability. To increase the device lifespan, it is essential to understand the defect physics of perovskites. To explore the nucleation and crystal growth of the solution-processed CH3NH3PbI3 (MAPbI3)　thin films, Lewis bases urea (CH4N2O) and thiourea (CH4N2S), and Lewis acids PCBM and C60 were used as additives, which provides a comprehensive way to investigate the formation and passivation of defects in perovskite photovoltaic cells. Our experimental results indicate that the passivation mechanism from different additives is different and is dependent on their chemical interaction with the MAPbI3 crystal. Lewis bases increase the grain size and crystallinity of the MAPbI3 films by participating in the crystal growth process, while fullerene additives promote the merged grain morphology for MAPbI3 films. Surface-sensitive Raman spectra results show that the urea molecules are mainly present in the top region of MAPbI3 films and thereby passivating interfacial MA+ cations. While the use of C60 molecules passivates the surface defects in the MAPbI3 films via the formation of MA+-C60-MA+cations. Fourier transform infrared spectroscopy results indicate the formation of the stable intermediate state of thiourea with the perovskite precursor which promotes the formation of monolithic grains with (110) preferred crystallographic orientation. Besides, the long-term migration of fullerene molecules via Van der Waals force at the grain boundaries of MAPbI3 films is predicted from the photoluminescence spectra results. In addition, a facile method named ASMEN (Anti-solvent mixture-mediated nucleation) is proposed to prevent the formation of point defects of the MAPbI3 films. Furthermore, downconverter boron-doped graphene quantum dots:PMMA thin-film-based shield is also proposed to prevent the irreversible photo-induced degradation of MAPbI3 solar cells. To present adverse effects from the environment, a facile encapsulation method for MAPbI3 solar cells is employed. The migration of halide ions in MAPbI3 films is observed in the encapsulated samples which results in a higher quasi-Fermi level for electrons. The formation, passivation and migration of defects in the MAPbI3 films are also investigated in order to explain the stabilities of the encapsulated MAPbI3 photovoltaic cells. This thesis presents a systematic study on the additives-assisted crystal growth of perovskite grains and the passivation of defects in perovskite thin films for improved photovoltaic performance and stability. It is noted that these studies are crucial for addressing the low stability issue of perovskite solar cells.

並列關鍵字

Perovskite ； Defect passivation ； Migration ； Stability ； MA+-C60-MA+ ； additives

參考文獻

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