Sub-10-fs observation of bound exciton formation in organic optoelectronic devices.
Sub-10-fs observation of bound exciton formation in organic optoelectronic devices.
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DOI：
10.1038/s41467-022-32478-8
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发表时间：
2022-08-23
影响因子：
16.6
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中科院分区：
文献类型：
--
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有机半导体中激子形成的基本机制是复杂和难以捉摸的，因为它发生在超短的亚100毫微秒的时间尺度上。这个过程的一些基本方面，如激子结合能的演化，还没有在实验上及时解决。在这里，我们将亚10fs泵浦-推光电流、泵浦-推光发光和泵浦-探测光谱的组合应用到聚荧烯器件中，以跟踪激子的超快形成。泵浦-探测对激发态的总浓度敏感，而泵浦-推动-光电流和泵浦-推动-光致发光只对束缚态敏感，从而提供了研究激子束缚动力学的途径。我们发现，由近吸收边光子产生的激子是本征束缚态，或者在激发后10 飞秒内变为束缚态。同时，具有适度超额能量的激子可以在50 飞秒内自发解离，然后获得束缚特性。这些结论得到了激发态分子动力学模拟和定量再现实验数据的全球动力学模型的支持。有机光电子器件的超快作用谱表明，束缚激子态的形成最快可达10 飞秒。具有超额能量的激子在获得束缚特性之前可以在50-飞秒内自发解离。
Fundamental mechanisms underlying exciton formation in organic semiconductors are complex and elusive as it occurs on ultrashort sub-100-fs timescales. Some fundamental aspects of this process, such as the evolution of exciton binding energy, have not been resolved in time experimentally. Here, we apply a combination of sub-10-fs Pump-Push-Photocurrent, Pump-Push-Photoluminescence, and Pump-Probe spectroscopies to polyfluorene devices to track the ultrafast formation of excitons. While Pump-Probe is sensitive to the total concentration of excited states, Pump-Push-Photocurrent and Pump-Push-Photoluminescence are sensitive to bound states only, providing access to exciton binding dynamics. We find that excitons created by near-absorption-edge photons are intrinsically bound states, or become such within 10 fs after excitation. Meanwhile, excitons with a modest >0.3 eV excess energy can dissociate spontaneously within 50 fs before acquiring bound character. These conclusions are supported by excited-state molecular dynamics simulations and a global kinetic model which quantitatively reproduce experimental data. Ultrafast action spectroscopies of organic optoelectronic devices reveal that the formation of bound exciton state occurs as fast as 10 fs. Excitons having excess energy can dissociate spontaneously within 50-fs before acquiring bound character.
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