太陽能電池材料之載子動力學

Abstract

本論文利用時間解析光譜討論太陽能電池材料之載子動力學。在披覆AlxGa1-xAs0.8Sb0.2層的InAs量子點材料中，x = 0 的第二型量子點電洞透過熱能的吸收，有機會躍遷至由披覆層與量子點共同形成的更高能階，與量子點中的電子復合後產生類似第一型能帶排列的躍遷。另外，隨著鋁含量的增加，會有發光峰值藍移與載子生命期縮短的現象，此二現象顯示出掺雜鋁對能帶排列的調變。鋁含量增加同時也減少了價電帶能量差，使得電洞由披覆層被推回量子點中，進而提升電子電洞的波函數重疊率。由實驗結果以及八能帶k ⃑∙p ⃑理論計算，我們認為x > 0.2的樣品為第一型能帶排列。而在CIGS太陽能電池元件中，SRH的非輻射復合使得高轉換效率的元件將具有較強的發光強度以及較長的載子生命期等特性，我們可透過三者的正相關性，建立元件效率的即時檢測。此外，利用高載子濃度注入的方式，可屏蔽元件中內建電場，降低對載子生命期量測的影響。

Carrier dynamics in materials for solar cell applications have been investigated by time-resolved photoluminescence. One is InAs self-assembled quantum dots (QDs) covered with a thin AlxGa1-xAs0.8Sb0.2 layer. There is a type I-like transition in type II InAs/GaAsSb QDs due to the recombination of electrons from QDs and holes residing in extended levels composed by the capping layer and the QDs, which is activated by thermal excitation. With the increasing Al content, a blueshift in the QD emission peak and a shortening of the PL decay time are observed, indicating that the band alignment can be controlled by varying the Al content in the AlGaAsSb capping layer. Increasing the valence band offset tend to push the hole wave function into QDs, which in turn improves the overlap between the electron and hole wave functions. According to the experimental results and the theoretical calculations based on eight-band k ⃑∙p ⃑ model, we demonstrate that the AlxGa1-xAs0.8Sb0.2 covered InAs QDs exhibit a type-I band alignment when the Al content exceeds 0.2.Another one is CIGS solar cell. High conversion efficiency in CIGS solar cell is associated with stronger PL intensity and longer carrier lifetime, which is caused by the domination of SRH recombination at room temperature. Besides, radiative recombination in CIGS device is strongly affected by the built-in electric field. Therefore, the intrinsic carrier lifetime can be obtained by injecting higher carrier density.