Kang Hyun Suk, Peurifoy Samuel, Zhang Boyuan, Ferguson Andrew J, Reid Obadiah G, Nuckolls Colin, Blackburn Jeffrey L
National Renewable Energy Laboratory, Golden, CO 80401, USA.
Mater Horiz. 2021 May 1;8(5):1509-1517. doi: 10.1039/d0mh01810d. Epub 2021 Mar 10.
The primary photoexcited species in excitonic semiconductors is a bound electron-hole pair, or exciton. An important strategy for producing separated electrons and holes in photoexcited excitonic semiconductors is the use of donor/acceptor heterojunctions, but the degree to which the carriers can escape their mutual Coulomb attraction is still debated for many systems. Here, we employ a combined pump-probe ultrafast transient absorption (TA) spectroscopy and time-resolved microwave conductivity (TRMC) study on a suite of model excitonic heterojunctions consisting of mono-chiral semiconducting single-walled carbon nanotube (s-SWCNT) electron donors and small-molecule electron acceptors. Comparison of the charge-separated state dynamics between TA and TRMC photoconductance reveals a quantitative match over the 0.5 microsecond time scale. Charge separation yields derived from TA allow extraction of s-SWCNT hole mobilities of ca. 1.5 cm V s (at 9 GHz) by TRMC. The correlation between the techniques conclusively demonstrates that photoinduced charge carriers separated across these heterojunctions do not form bound charge transfer states, but instead form free/mobile charge carriers.
激子半导体中的主要光激发物种是束缚电子-空穴对，即激子。在光激发的激子半导体中产生分离的电子和空穴的一种重要策略是使用供体/受体异质结，但对于许多系统而言，载流子能够逃脱其相互库仑吸引的程度仍存在争议。在此，我们对一组由单手性半导体单壁碳纳米管（s-SWCNT）电子供体和小分子电子受体组成的模型激子异质结进行了泵浦-探测超快瞬态吸收（TA）光谱和时间分辨微波电导率（TRMC）的联合研究。TA和TRMC光导率之间电荷分离态动力学的比较揭示了在0.5微秒时间尺度上的定量匹配。由TA得出的电荷分离产率使得通过TRMC能够提取出约1.5 cm² V⁻¹ s⁻¹（在9 GHz下）的s-SWCNT空穴迁移率。这些技术之间的相关性确凿地表明，在这些异质结上分离的光致电荷载流子不会形成束缚电荷转移态，而是形成自由/移动电荷载流子。
