Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics
Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics
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DOI：
10.1038/nmat3500
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发表时间：
2013-01-01
期刊：
影响因子：
41.2
通讯作者：
Zhu, X-Y.
中科院分区：
文献类型：
--
作者：
Jailaubekov, Askat E.;Willard, Adam P.;Zhu, X-Y.
有机光致发光器件（OPV）中的光电流产生依赖于激子在供体/受体异质界面处解离成自由电子和空穴。有机半导体的低介电常数导致电子-空穴对之间的强库仑相互作用，其原则上应对抗自由电荷的产生。电子和空穴克服这种库仑陷阱的确切机制仍然没有解决，但越来越多的证据表明热电荷转移（CT）激子在协助这一过程中起着关键作用。在这里，我们提供了一个实时查看热CT激子的形成和弛豫使用飞秒非线性光学光谱和非绝热混合量子力学/分子力学模拟酞菁富勒烯模型OPV系统。对于酞菁上的初始激发，热CT激子在10（-13）s内形成，随后在10（-12）s时间尺度上弛豫至较低能量和较短电子-空穴距离。这种热CT激子冷却过程和电荷分离的崩溃为导致有效光电流产生的竞争性电荷分离通道设定了基本的时间限制。
Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.