利用奈秒時間解析近紅外光光譜研究鈣鈦礦薄膜

Abstract

鈣鈦礦 (perovskite CH3NH3PbI3) 在近幾年來，被發現作為光捕捉材料具有非常大的發展性。perovskite有許多的優點，像是高效率的能量轉換效率(PCE)和低製造成本，使其對於太陽能產業更是未來的潛力材料。探其它的光物理特性，像是高流動性的電荷載子、光誘發自由電荷載子的長擴散距離，都吸引許多化學與材料研究者的興趣。雖然有許多研究都致力於超快時間範疇內電荷分離動力學行為的了解，但只有少數研究是關於時間稍慢的動力學模式像是分離電荷的再結合。 在本論文中，作者利用高靈敏度的奈秒時間解析近紅外光光譜，去解析在perovskite中的分離電荷載子的再結合機制。利用532 nm雷射去激發perovskite薄膜，研究從12000到3800波數所產生的瞬態種類，並且比較不同濃度的perovskite之間的差異。研究結果顯示，在此區間內有兩種瞬態種類：一種出現在波數大於10000 cm-1的區間，並且快速衰減，其時間與儀器反應的時間相當(20 ns)。另一種則涵蓋了10000以下到3800 cm-1，衰減的時間結合了快速衰減和緩慢衰減的分量，其中快速衰減跟高波數區間所呈現的衰減時間相當。這些瞬態種類將會在接下來的章節中討論，除此之外，將環己烷覆蓋於perovskite上的實驗也會被測試。由目前的研究結果發現，導入溶劑之後將會顯著地改變電荷載子的光學特性。因此，以時間解析近紅外光光譜學作為研究的方法，將會對於perovskite的研究提供全新的洞見。

Organic-inorganic lead halide perovskite CH3NH3PbI3 has emerged as a promising light harvesting material for the industry of solar energy. There are many advantages of the perovskite to be utilized, such as efficient energy conversion properties and low manufacturing cost. Photophysical characteristics of the perovskite such as a high charge carrier mobility and a long diffusion length of photoinduced free charge carriers have also attracted great interest among chemists and materials scientists. Although the kinetic behaviors of charge separation in the ultrafast time regime have been a focus of intensive research, little has been studied on much slower dynamics such as recombination of the generated charge carriers. In the present study, high-sensitivity nanosecond time-resolved near-infrared (TRNIR) spectroscopy was used to elucidate the mechanisms of the recombination dynamics of the charge carriers (electrons and holes) in CH3NH3PbI3 perovskite thin films. A series of transient NIR spectra (12000–3800 cm−1) of the perovskite films with different concentrations of CH3NH3PbI3 were measured after photoexcitation at 532 nm. Results revealed two types of radiative decays in the NIR region. One transient appears above 10000 cm−1 and decays almost instantaneously with a time constant comparable to the apparatus response of ~20 ns. The other transient has a very broad feature that covers the entire NIR window of our observation and seems to exhibit a slower decay together with a rapid decay similar to the higher-wavenumber transient. The origin of these transient species is discussed in terms of trap states in the perovskite thin films. Furthermore, the effects on the TRNIR spectra of the presence of cyclohexane on the perovskite films are also examined. It was found that the presence of a solvent could significantly affect the NIR optical properties of the charge carriers. The present study demonstrates that TRNIR spectroscopy provides new insight into the carrier dynamics in the perovskite.