鈣鈦礦奈米晶體之激態緩解動力學與電場引致光譜

Abstract

於本論文中，將藉由電場引致吸收光譜 (Electro-absorption measurement) 與飛秒螢光上轉換光閘系統 (femtosecond photoluminescence optical gating system) 來針對有機鈣鈦礦材料之激發態緩解機制作一系列的探討。 第一部分為探討不同大小之鈣鈦礦晶體於玻璃基材或二氧化鈦奈米晶體基材上的電致吸收光譜來了解其光譜精細結構以及探討固態晶體當中激子的史塔克效應。由於在室溫之下作量測往往有複雜之光譜展寬，因而正常情況下無法直接由吸收光譜獲得激子吸收峰以及相干電子電洞對之躍遷譜線，透過對電致吸收光譜作二次積分並以展寬函數擬合，能夠做到對原光譜的拆解及擬合，並作為精確測定激子結合能之新穎測定方法。 第二部分為鈣鈦礦激態緩解動力學。藉由量測鈣鈦礦在不同激發光強度之下於氧化鋁、氧化鈦、以及氧化鎳的熱載子緩解機制，提出四能級模型並引入歐傑載子能量傳遞之概念，吾人可了解透過相干之電子電洞間庫倫作用所導致不同激發強度下能量移轉與傳統之熱載子緩解速率的對比。最後透過量測鈣鈦礦之能帶邊緣螢光緩解，我們能夠得到激子與載子共同存在於螢光訊號之比例變化，並能以包含高次緩解過程之激子或載子模型擬合並得到對應的電子傳輸生命期最短為12.3奈秒與電洞傳輸生命期最短4.3奈秒，因而造就氧化鎳基底之鈣鈦礦太陽能電池之高光電轉換效率。

Electro-absorption measurement (EA) and femtosecond photoluminescence optical gating system (FOG) were introduced to investigate the relaxation dynamics and field-induced spectrum change. By means of electro-absorption measurement, the field-induced spectral change is obtained from the perovskite with varied crystal sizes on both of FTO substrate and TiO2 scaffold. If we integrate EA spectrum twice, the hidden peak can be resolved from second derivatives components of spectrum. Therefore the presence of exciton has been confirmed and separated from the direct band-to-band transition. Under the framework of Elliott theory of optical transition, not only the field induced change in dipole moment and polarizability but also the exciton binding energy ~25 meV can be resolved. The second part is the femtosecond relaxation dynamics of hot carriers inside the perovskite sample on different metal oxide scaffold with varied incident photon flux. By introducing a four-level carrier relaxation mechanism and the concept of Auger type energy transfer between the correlated electrons and holes, we found that the energy transfer rate is even faster than the conventional cooling rate of hot carriers in high photon flux density. Finally the time-resolved photoluminescence decay profiles at 770 nm at different excitation photon flux was fitted by using carrier model or the exciton model containing higher order decay processes. The extracted electron transport lifetime and hole transport lifetime are around 12.3 ns and 4.3 ns, respectively. Such fast hole-transporting process is the reason that makes NiO-based perovskite so efficient