論文標題:Ferroic domains regulate photocurrent in single-crystalline CH3NH3PbI3 films self-grown on FTO/TiO2 substrate
期刊:npj Quantum Materials
作者:Boyuan Huang, Guoli Kong, Ehsan Nasr Esfahani, Shulin Chen, Qian Li, Junxi Yu, Ningan Xu, Ying Zhang, Shuhong Xie, Haidan Wen, Peng Gao, Jinjin Zhao & Jiangyu Li
發表時間:2018/06/22
數字識別碼:10.1038/s41535-018-0104-5
原文連結:https://www.nature.com/articles/s41535-018-0104-5?utm_source=Other_website&utm_medium=Website_links&utm_content=JesGuo-Nature-npj_Quantum_Materials-Energy-China&utm_campaign=NPJ_USG_JRCN_JG_npj_Ferroic_Aug_sciencenet
最近十年,鈣鈦礦結構最耀眼的明星,無疑是以CH3NH3PbI3為代表的有機無機雜化鈣鈦礦。自2009 年日本Miyasaka 研究組首次報導以來,鈣鈦礦太陽能電池的轉換效率,在短短十年不到的時間從約3 % 大幅度增長至22 % 以上,刺激了全球範圍內激烈競爭。
究其優異光電性能背後的物理根源,材料結構-性能關係則是回答這一問題更基本、關鍵的基礎。目前普遍接受的是CH3NH3PbI3在高溫下是立方結構、室溫下是四方結構。四方結構的空間群又有兩種可能性:一種是屬中心對稱的 I4/mcm 結構;另一種是屬非中心對稱 I4cm 結構。遺憾的是,這兩種晶格結構差別細微,非常難以分辨。CH3NH3PbI3的鐵電是非也一直是國際爭論的焦點。
最近黃博遠等人對CH3NH3PbI3單晶疇結構進行了深入分析,他們通過極性光學顯微鏡、掃描電鏡、AFM形貌、和PFM,都可以觀察到疇結構。這些疇很可能是立方-四方相變所產生的變體所形成的結構。
其次,他們觀測到:對電致正應變而言,存在高響應疇和低響應疇,如圖1 所示。有意思的是,高響應疇中一階線性效應佔主導,而低響應疇中二階非線性疇佔主導。進一步分析揭示極性與非極性疇共存。如圖2所示,高正應變響應的疇在合適取向下具有高的剪切應變,與極性疇匹配;低正應變響應的疇剪切應變響應低,與非極性疇匹配。
圖1:高低響應疇一階二階應變比較。
此外,他們還發現極性疇光生電流低,非極性疇光生電流高。更有意思的是,無論是疇結構、壓電響應、還是光電流,在升溫降溫過程中,都隨相變產生明顯的記憶效應,如圖 3 所示。這進一步證明了光電流與疇結構之間的緊密關聯。為此類體系的疇控制研究找到了夾縫中成長與擴張的理由!
圖2:高低響應、剪切應變與品質因子,表明極性與非極性疇共存。
圖3: AFM 形貌(上),PFM 響應(中),光電流(下) 相變過程中的記憶效應。
摘要:Photovoltaic conversion efficiency (PCE) of halide perovskite solar cells has risen spectacularly, yet the very crystalline structure of CH3NH3PbI3 remains ambiguous after extensive researches, and its polar nature remains hotly debated. Here we present compelling evidences that CH3NH3PbI3 crystals self-grown on FTO/TiO2 substrate consist of ferroic domains with alternating polar and nonpolar orders, in contrast to previous experimental and theoretical expectations, and polar domains possess reduced photocurrent. It is found that polar and nonpolar orders of CH3NH3PbI3 can be distinguished from their distinct lateral piezoresponse, energy dissipation, first and second harmonic electromechanical couplings, and temperature variation, even though their difference in crystalline lattice is very subtle, and they possess two-way memory effect through cubic-tetragonal phase transition. We hope these findings resolve key questions regarding polar nature of CH3NH3PbI3 and its implication on photovoltaics, reconcile contradictory data widely reported, and point a direction toward engineering ferroic domains for enhanced PCE.
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期刊介紹:
npj Quantum Materials is an online-only, open access journal, publishing original research results and reviews on broad coverage of quantum materials, their fundamental properties, fabrication and applications.
來源:科學網
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