利用烯丙胺合成高品質二維Ruddlesden-Popper鈣鈦礦單晶

Abstract

近年來，三維有機-無機鹵化物鈣鈦礦材料因高吸收係數、低激子束縛能和高載子擴散長度與載子遷移率等諸多優點，使得鈣鈦礦材料在各個領域上都有突出的表現，尤其在太陽能電池領域下才短短的幾年內，效率已由3.8 %[1]提升至24.2 %[2]。但是它們對於水氣、光和高溫表現出差的穩定性，使其無法達到商業化的門檻。為了獲得更加穩定的鈣鈦礦，科學藉由長碳鏈有機胺將鈣鈦礦分隔開來，降低了水氣與鈣鈦礦反應降解的機率，並形成具有多量子井效應的二維有機-無機Ruddlesden-Popper鈣鈦礦(OIRPP)。然而，因為有機絕緣層的關係，使其載子傳輸受到限制。為了獲得高穩定性和較好的載子遷移率，在本研究中我們使用烯炳胺(Allylamine)來代替現有的長碳鏈間隔物，希望藉由烯丙胺的短碳鏈和雙鍵效應改善絕緣有機層所帶來的影響，並搭配我們團隊所提出的恆溫緩慢揮發溶劑成長法(SECT)，合成出高品質毫米級大小的Ruddlesden-Popper鈣鈦礦晶體，此外我們還藉由調控反應中甲胺(Methylamine)與烯丙胺之間的比例，成功的獲得不同層數(n = 1, 2 )。由X-光粉末繞射圖中清楚看到單一層數的等間距繞射峰，為了確認其晶格結構為Ruddlesden-Popper相，我們還通過密度泛函理論獲得理論的X-光粉末繞射圖譜並進行比對。此外，由螢光光譜得知其放射波長分別為512.4 nm (n = 1)和 579.9 nm (n = 2)，並由Tauc圖計算出其能隙大小為2.32 eV (n = 1)、2.09 eV (n = 2)。接著我們還將其與相對應的三維鈣鈦礦和正丁胺、苯乙胺間隔物的單晶做比較，發現含有烯丙胺間隔物的二維Ruddlesden-Popper鈣鈦礦同樣具有極高的穩定性。最後，為了後續元件的製程，我們開始嘗試將我們所合成的晶體回溶至二甲基甲醯胺和二甲基亞碸中並進行塗膜，由X-光粉末繞射圖，證實此方法得到之薄膜具有高度均質性，改善了以往利用莫爾數比來塗膜時常發生混相之缺點。In recent years, three-dimensional organic-inorganic halide perovskite materials shown several promising properties, such as high absorption coefficient, low exciton binding energy, large carrier diffusion length and high carrier mobility. Based on these outstanding properties, they are considered as next generation materials for solar cells (power conversion efficiency achieved 24.2%[2]). However, the commercial perovskite solar cells are lacking due to they displayed poor stability under moisture, illumination, and high thermal. To obtain long term stability perovskite, scientists slice organic long carbon chain molecules into 3D perovskite slabs to demonstrate phase stable two-dimensional organic-inorganic hybrid Ruddlesden-Popper perovskite (OIRPP) with quantum well structure. However, the electrical properties in OIRPP are displayed limit due to long carbon chains are insulator. In order to obtain long term stability and better carriers transport, in this study, we mainly use the short carbon chain and π-π interaction of allylamine to replace original long carbon chain spacers. We use slow evaporation at constant temperature solution-growth method to synthesize high quality millimeter-sized OIRPP crystal with allylamine, and obtained different n-value ( n = 1, 2 ) by different ratio of methylamine to allylamine. In the XRD measurements, the patterns show clear repeating unit peak without any mixed n values. To confirm the structure OIRPP with allylamine, we also obtain similar XRD patterns from simulation by density functional theory (DFT). Moreover, the bandgaps of OIRPP are 2.32 eV (n = 1) and 2.09 eV (n = 2), and the emission peaks are at 512.4 nm (n =1) and 579.9 nm (n = 2). In XRD measurements, the stability of OIRPP with allylamine display superior stability than 3D organic inorganic hybrid perovskite. Finally, in order to fabricate optoelectronic devices, we also dissolve the single crystals in DMF and DMSO to fabricate pure phase OIRPP thin films with similar repeating unit. The thin films exhibit pure n value compared to traditional method which dissolve from the precursor powder.