利用混合陽離子方法提升二維錫鈣鈦礦穩定性

Abstract

有機-無機鈣鈦礦憑著其優異的光電性質在近十年受到各界大量的關注，但是常見的鈣鈦礦材料中含有鉛，存在環境汙染與毒性之風險，尋找替代元素即成為重要的研究議題。然而，錫鈣鈦礦具有高吸收係數與低能隙的優點而成為相當好的替代元素，不幸的是錫鈣鈦礦穩定性極差容易受環境影響而氧化，因此本研究專注於錫鈣鈦礦的穩定性提升。 本篇成功將合成二維錫鈣鈦礦晶體BA2FASn2I7 (n = 2)，再利用混合陽離子方法將甲脒與銫共同摻入鈣鈦礦製成BA2FA0.5Cs0.5Sn2I7 (n = 2)，藉由粉末X光繞射鑑定結構得知銫確實有進入錫鈣鈦礦晶格且具有清楚的等間距繞射峰，測量吸收光譜得知能隙僅有略微變化依序為1.74 eV和1.72 eV，又發現BA2FASn2I7經過雷射激發後會出現錫鈣鈦礦主體以外的放射光，而在BA2FA0.5Cs0.5Sn2I7沒有發現此現象，僅有螢光強度的衰退，證明混合陽離子確實能夠抑制其他晶相的產生提升錫鈣鈦礦穩定性。最後將二維錫鈣鈦礦晶體置於大氣中觀察其結構變化，發現BA2FA0.5Cs0.5Sn2I7經過12小時候仍維持其二維層狀結構，表示混合陽離子的二維錫鈣鈦礦結構比BA2FASn2I7穩定。 未來可嘗試合成更高層數的混合陽離子二維錫鈣鈦礦，應用於光伏元件，將有機會提升效率以及穩定性。

Organic-inorganic perovskites have received a lot of attention from different field in the past decade due to their excellent photoelectric properties. However, perovskite materials usually contain lead element, which has the risk of environmental pollution and toxicity. Finding alternative elements has become an important research issue. Tin halide perovskites have the advantages of high absorption coefficient and low energy gap and become a good substitute candidate. Unfortunately, the stability of tin halide perovskite is very limited and it is easily oxidized by ambient. In this article, we successfully synthesized two-dimensional tin halide perovskite crystals BA2FASn2I7 (n = 2), and then mixed formamidine and cesium into perovskite to form BA2FA0.5Cs0.5Sn2I7 (n = 2) by mixed cation method. The powder X-ray diffraction revealed that cesium did induce to the tin halide perovskite lattice and had clear repeating unit diffraction pattern. The band gap is 1.74 eV and 1.72 eV, respectively. After laser excitation, BA2FASn2I7 will emit the unknown photoluminescence, which is not same as tin perovskites’ original emission. This phenomenon has not been found in BA2FA0.5Cs0.5Sn2I7 crystals, also proves that mixed cations indeed suppress the production of other crystal phases. The two-dimensional tin halide perovskite crystals were placed in the ambient to observe the structural changes. It was found that BA2FA0.5Cs0.5Sn2I7 still maintained its layered structure after 12 hours, indicating that the mixed cation method can stabilize the structure. In the future, a higher layered of mixed cation two-dimensional tin halide perovskite have the potential to improve photovoltaic devices efficiency and stability.