We investigate hot carrier (HC) cooling in two-dimensional (2D) perovskite single crystals by applying two complementary ultrafast spectroscopy techniques – transient absorption (TA) and time-resolved two-photon photoemission (TR-2PPE) spectroscopies. TR-2PPE directly maps the hot electron distribution and its dynamics in the conduction band to the detected photoelectron distribution. While TR-2PPE selectively probes the upper layer of the material, TA provides information on the whole bulk. Two cooling regimes are resolved in both techniques. The fast timescale of 100–200 fs is related to the electron scattering by longitudinal optical (LO) phonons and the slow timescale of 3–4 ps corresponds to the LO phonon relaxation. The HC cooling dynamic of TA measurement has faster initial stage and higher starting temperature for the slower stage than in TR-2PPE measurements. Conclusions about spatial sensitivity of the cooling dynamics across the 2D perovskite single crystals constitute valuable information that can guide the future development of HC solar cells and thermoelectric applications based on 2D perovskites.

中文翻译：

---

结合双光子光电子发射和瞬态吸收光谱来解决二维钙钛矿单晶中的热载流子冷却：表面层的影响

我们通过应用两种互补的超快光谱技术——瞬态吸收 (TA) 和时间分辨双光子光电发射 (TR-2PPE) 光谱来研究二维 (2D) 钙钛矿单晶中的热载流子 (HC) 冷却。TR-2PPE 直接将热电子分布及其在导带中的动态映射到检测到的光电子分布。TR-2PPE 选择性地探测材料的上层，而 TA 则提供有关整个体积的信息。两种技术都解决了两种冷却方式。100-200 fs 的快速时间尺度与纵向光学（LO）声子的电子散射有关，3-4 ps 的慢时间尺度对应于 LO 声子弛豫。与 TR-2PPE 测量相比，TA 测量的 HC 冷却动态具有更快的初始阶段和较慢阶段的更高起始温度。关于二维钙钛矿单晶冷却动力学的空间敏感性的结论构成了有价值的信息，可以指导基于二维钙钛矿的 HC 太阳能电池和热电应用的未来发展。