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大連化物所納米晶熱載流子弛豫動力學研究取得新進展

文章來源:大連化學物理研究所   發布時間:2019-05-10  【字號:     】  

  近日,中國科學院大連化學物理研究所光電材料動力學特區研究組研究員吳凱豐團隊采用飛秒瞬態光譜技術系統地研究了量子限域的鈣鈦礦納米晶的熱載流子弛豫動力學,發現該體系呈現出亞皮秒級別的熱載流子壽命與之前理論預測的“聲子瓶頸”機制不符,進一步研究發現熱載流子能量耗散通道由表面配體分子誘導的非絕熱弛豫機制所主導。相關成果發表于《化學科學》(Chemical Science)上。

  半導體中的光生載流子在與晶格(聲子)碰撞弛豫之前,具有顯著高于帶邊的能量,通常被稱為熱載流子。熱載流子的應用可使光伏器件的效率突破Shockley-Queisser極限,理論上可達到66%。然而,由于常見半導體中光生載流子與聲子間的散射發生于亞皮秒級別,熱電子的提取極為困難。有鑒于此,科學家們對可能產生長壽命熱載流子的各種機制進行了長期的探索。其中,半導體納米晶(或稱量子點)被預測具有長壽命的熱載流子,原因在于量子限域效應使得納米晶出現類原子的分立能級,這些能級間的能量差異高達幾百meV,使得熱載流子很難通過發射聲子的形式進行弛豫,被稱為“聲子瓶頸”現象。此外,近期也有報道稱鈣鈦礦光伏材料具有長壽命的熱載流子,因為這類材料“動態”的晶格結構可導致極化子的形成,從而屏蔽熱載流子與聲子間的散射。然而這一現象的存在與否尚存較大爭議。

  在該工作中,研究團隊合成了一系列量子限域的CsPbBr3鈣鈦礦納米晶,其中最小尺寸為2.6nm,是目前文獻報道限域效應最強的鈣鈦礦納米晶。科研人員采用態-態選擇激發的飛秒瞬態光譜,測定了不同尺寸納米晶中從第二到第一激發態的熱載流子弛豫時間,發現均處于亞皮秒量級,并未出現理論預測的“聲子瓶頸”現象;采用變溫和表面配體交換的實驗,也排除了多聲子發射和能量轉移等機制作為主要能量耗散通道的可能。研究團隊提出該體系的熱載流子弛豫可能由表面配體誘導的非絕熱躍遷機制所主導,并通過納米晶表面波函數計算進行了印證。具體而言,量子限域效應使得納米晶的電子波函數強烈地離域至納米晶表面,與表面配體分子的核運動發生非絕熱作用,從而誘導熱載流子在不同能級間的弛豫。納米晶尺寸越小,電子波函數在表面的幅度越大,與配體分子的非絕熱作用越強,從而導致越快的熱載流子能量耗散速率,這與實驗所測數據十分吻合。

  該工作首次研究了量子限域效應對鈣鈦礦納米晶熱載流子弛豫動力學的影響,揭示了非絕熱弛豫機制在其中的主導作用。同時,該工作也表明鈣鈦礦材料雖然表現出優異的光伏性能,但在熱載流子應用領域相比常見半導體材料并無明顯優勢,這對鈣鈦礦光伏器件具有指導意義。

  該工作得到中科院戰略性先導項目、國家重點研發計劃、國家自然科學基金等的資助。

 

大連化物所納米晶熱載流子弛豫動力學研究取得新進展




(責任編輯:葉瑞優)

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